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2.
Detection and spectrometry of ionizing radiation
2.1.
Introduction - methodology of ionizing radiation detection
2.2. Photographic
detection of ionizing radiation
2.3. Ionization
detectors 2.4. Scintillation
detectors
2.5. Semiconductor detectors
2.6. Detection and
Spectrometry radiation a and b . Liquid
scintillators. Neutron detection
2.7. Measurement of
radioactivity of samples (in vitro)
2.8. Absolute
measurement of radioactivity and radiation intensity
2.9. Measurement of
radioactivity in the organism (in vivo)
2.10. Calibration and
inspection of spectrometric instruments
2.11. Statistical
fluctuations and measurement errors
2.1.
Introduction - methodology of ionizing radiation detection
Ionizing radiation is invisible to the eye , so
in order to be convinced of its existence, it is necessary to detect
it using appropriate physical methods and
appropriate instrumentation , which converts
invisible radiation into other visible or measurable quantities.
In addition to "visibility",
detection allows us to investigate the properties of
this radiation and use it in a number of
scientific, technical, industrial and medical applications. It
provides us with quantitative information on intensity, energy,
spatial distribution and possibly. other properties of radiation.
In this chapter we will describe the individual methods
and devices for the detection of ionizing radiation and
for the measurement of its energy - spectrometry. In the
introductory §2.1 we will give a basic division of detection
methods and devices and summarize some common methodological
aspects of ionizing radiation detection. In the next §2.2-2.10
we will describe in more detail specific types of detectors and
spectrometers, their principles, properties and technical
construction.
Types of
ionizing radiation detectors
A number of ionizing radiation detectors have been developed
which (in addition to the common basic phenomenon of ionizing
radiation effects) use various principles and technical
constructions. Instruments for detecting ionizing radiation are
sometimes collectively referred to as radiometers
. They work either independently or are part of devices for
measuring certain quantities and monitoring certain events using
radiation methods.
Dosimeters A special type of radiometers are the so-called dosimeters
. They are usually simple detection devices that are calibrated
in radiation dose units
(Gray, Sievert) or dose rate. They are used in radiation
monitoring to assess the effects of radiation, especially on
living tissue (see §5.1 " Effects of radiation on matter. Basic quantities of
dosimetry . "). For measurement of dosimetric characteristics
of radiation, see also §2.8 " Absolute measurement of
radioactivity and radiation intensity ", section "Measurement of radiation
intensity and dose rate".
Ionizing radiation detectors can be divided according to three
criteria: the time course of detection ,
the physical-technical principle of detection
and the complexity of the measured radiation
information.
× 1. According to the time course of detection, we distinguish two basic groups of detectors:
× 2. Different
types of detectors provide a response to the interaction of
ionizing particles by different, often very different,
mechanisms. They therefore differ in their properties and thus in
the possibilities and areas of their use.
According to the detection principle, we
distinguish three groups of detectors :
![]() Fig.2.1.1. Basic block diagram of an electronic continuous radiation detector. To some extent, the block diagram is similar for a radiometer with an integral detector. The difference is that the cumulative detector and the evaluation part are separated, while in the case of continuous detectors they are built into one apparatus. |
Electronics
- optoelectronics - photonics
Some special scientific and technical fields
deal with the transmission of energy and information :
¨ Electronics
is a scientific and technical field dealing with the transmission
of energy and information through electrical signals
- especially electron currents and electromagnetic fields and
waves excited by them. ¨ Optoelectronics , also called photonics
, is a scientific and technical field dealing with the
transmission of energy and information through photons
, especially visible light. It deals with photon sources
, such as lasers and light emitting LEDs, light
transmission techniques (eg optical fibers), methods
detection of photons and their
conversion into electrical signals (photodiodes and
phototransistors, CCD, photomultipliers) and processing of these
signals in electronic circuits, including computer software.
Emissions, interactions and detection of photons take place at
the quantum level , so there is sometimes talk of quantum
photonics .
Electronics and optoelectronics play a key
role in the detection of ionizing radiation (§2.4 " Scintillation detection and gamma-ray
spectrometry ", §3.2
"X-rays - X-ray diagnostics", section " Electronic
X-ray imaging detectors
") , in electronic sources of ionizing
radiation ( X-rays, accelerators) as well as in the relevant
measuring and control technology.
Significant opto-electronic components or
devices are LASERs ( Light Amplification by
Stimulated Emission of Radiation ). They are electronic
sources of very intense coherent rays of light (or infrared or
ultraviolet radiation), which arises on the principle of stimulated
emission , in which excited atoms move en masse to lower
energy levels, which is accompanied by light emission with an
avalanche increase (chain effect - emitted light stimulates more
and more deexcitation with photon emission). The use of lasers
in nuclear physics is mentioned in several places in our
dissertation - eg §1.3, section "Fusion of atomic
nuclei ", passage " Inertial
thermonuclear fusion", or §1.5, part" Charged
particle accelerators ",
passage" Laser plasma accelerators LWFA
"and" Ion sources ". However, a more
detailed explanation of the physical principles and technical
design of lasers lies completely outside the scope of our nuclear
and radiation physics (and in addition
laser technology author is not an expert) .
× 3. The ionizing
radiation we need to detect often consists of particles and
quanta of different kinds and energies that come from different
directions and places in space, from different radioactive,
electronic or cosmic sources.
According to the complexity of the measured information
, measuring instruments of ionizing radiation can be divided into
4 groups :
In terms of a specific type
of sensor sensitivity, we can label simple detectors
(and all radiation detectors in general) as radiation-sensitive
sensors, spectrometers are also energy-sensitive
, imaging and trajectory detectors are position-sensitive
radiation sensors.
The basic physical properties
of detectors include:
¨ Sensitivity and efficiency of
the detector
¨ Temporal resolution of the
detector (its dead time)
¨ Energy resolution of the
spectrometer
¨ Spatial (or angular) resolution
of imaging detectors
Furthermore, it is the background of the
detector, linearity and homogeneity of
response, accuracy and time stability , " resistance
" to radiation overload, calibration parameters .
These properties of detectors, as well as their quantification,
are described in more detail below in the section " General physical and instrumental effects in
detection and spectrometry
", for imaging detectors in Chapter 4 " Radionuclide scintigraphy ", §4.2 " Scintillation cameras ",
section " Imaging camera features "and
§4.5" Quality control and phantom
scintigraphic measurements ".
Spectrometry
- a powerful tool for physical knowledge and applications
We consider it useful to recall here the key role of
spectrometric methods of analysis of electromagnetic and
corpuscular radiation for physical knowledge and applications of
physical methods in various fields of science, industry and
medicine. The measurement of energy spectra is
the main source of knowledge about stars and galaxies in outer
space, the composition of matter, the properties of atoms and
molecules, the structure of atomic nuclei, the nature and
interactions of elementary particles. Most of this knowledge is otherwise
inaccessible to us - whether for long distances (in
space) or submicroscopic dimensions deep inside the microworld.
Without spectrometry, we would know much less about the world. A
number of analytical methods such as X-ray
fluorescence analysis, activation analysis, nuclear magnetic
resonance, Mössbauer spectroscopy, and indirectly scintigraphy,
Doppler and interferometric methods are also directly based on
spectrometry .
Shielding, collimation and filtration of
detected radiation
In many cases it is not enough to place the "bare"
detector of the required radiation in a certain place and
register the incoming quantum. In addition to the analyzed
radiation itself, there is almost always other unwanted
and interfering radiation at the measuring point . It
is, on the one hand, natural radiation (natural
radiation background - cosmic radiation, radioactivity of the
environment) , radiation from possible
other surrounding sources, sometimes even undesirable components
in the measured radiation itself. To eliminate or reduce these
interfering radiation effects, the detector is equipped with
other suitable mechanical or electronic parts, whereby the beam
or field of the detected radiation is basically adjusted
in three ways:
× 1. Shielding of the detector
To suppress unwanted radiation coming from the environment, it is
necessary to surround the detector itself with a sufficiently
strong envelope made of a substance that absorbs
radiation well - place the detector in a suitable shield
. The most common construction material for shielding g radiation is lead
, in special cases tungsten and other materials are also used.
Sometimes we also use partial shielding of the primary
detected radiation - especially in the case of strong
radiation (high fluence), which would overwhelm the sensitive
detector.
Influence of detector
shielding and radiation collimation on the shape of the spectrum
In the shielding material, the detected radiation interactions
with the atoms of the substance, which can lead to the formation
of secondary radiation. In addition to Compton
scattering, which generates radiation with a continuous spectrum,
it is also a photo effect, accompanied by the formation of
characteristic X-rays with a line spectrum. In Fig.2.1.2 we see
the influence of different types of scintillation detector shielding
(scintillation detectors are
described in detail below in §2.4 " Scintillation
detectors ",
scintillation spectrum in the section " Gamma radiation scintillation
spectrum ") on the shape of the measured sample of radionuclide 99m Tc emitting gamma
radiation with an energy of 140keV. For a detector without
shielding ( a ), there is a rather indistinct
monotonic Compton continuum in front of the photopeak. To measure
low activities, it is necessary to place the detector inside a
massive lead shield ( b up). A side effect of
this useful measure is the interaction of gamma-photons with
shielding atoms, among other things, by the photoeffect
, which produces secondary characteristic X-rays
(lines K a, b ) of lead with an energy of about 70-80keV, which is
applied in the spectrum ( b below). A special
type of shielding are collimators , used as a
primary imaging element in scintigraphy (§4.2, section " Scintigraphic collimators ") . The interaction of
gamma-photons with a photoeffect with lead baffles between the
collimator orifices also produces a characteristic X-ray (lines K
a, b) lead with an energy of about 70-80keV. If the
collimator has relatively thicker baffles (approx. 0.5 mm), the
characteristic X-ray of lead is effectively absorbed and we can
see a faint X-peak in the spectrum of transmitted radiation (Fig.
C ). In collimators LE UHR with small holes and
very thin baffles are significantly radiography
gamma and characteristic X-rays of lead, so that the spectrum can
be rentgenovský photopeak even more pronounced than the primary
photopeak 140keV ( d ) (scintigraphy,
however, the window of the analyzer set to photopeak 140keV , so
that X-rays are not registered, see the spectrum image in the
section " Amplitude analyzer " §4.2 " Scintillation cameras
") .
Fig.2.1.2 Influence of different detector shielding geometry on
scintillation spectrum.
a) Basic scintillation spectrum of the 99m Tc sample measured
by a detector without shielding. b) Spectrum
measured by a detector inside a lead shield (7cm
Pb) .
c) Spectrum of the 99m Tc sample measured through a lead scintigraphic
collimator type HR. d) Spectrum through a UHR
collimator with small holes and very thin septa.
× 2. Collimation of detected
radiation In case we need to
detect only radiation coming from a certain direction
, we provide the detector with a collimator -
such a mechanical and geometric arrangement of materials
absorbing a given type of radiation, which transmits only
radiation coming from certain desired directions (angles), while
radiation from absorbs and does not let go in other directions.
The simplest collimators have the shape of various tubes
and orifices. Special intricately configured imaging
collimators with a large number of holes play a key role
in scintigraphy - §4.2 "Scintillation
cameras", section " Collimators ". Different types of specially shaped collimators
are used in radiotherapy ; the most important is
the multi- lamellar multi-leaf collimator MLC (§3.6 " Radiotherapy
", part " Modulation of irradiation beams IMRT, IGRT").
Electronic radiation collimation
In addition to the above-mentioned straightforward
"physical" radiation collimation, some special
detection systems use another method of directional radiation
selection, so-called electronic collimation ,
without the use of a mechanical collimator. It is based on the
specific behavior of quantum ionizing radiation in the detection
system - the propagation of pairs (or more) of quantums in
certain precisely given directions , their coincidence
detection by a system of a large number of detectors and
subsequent positional and angular reconstruction of the
direction of quantum propagation. This analysis makes it possible
to select for further processing only those quantities of
radiation that have the desired direction - to
perform electronic collimation and display the
distribution of radiation in a given field. The
electronic collimation method is used in positron
emission tomography PET (see §4.3 "Tomographic
cameras, part" PET cameras ")
and in some complex detection systems such as ring imaging
Cherenkov RICH detectors (see ....), trackers and
muon detection systems for accelerators (see ....)
× 3. Filtration of detected radiation
is used in special cases where the measured radiation itself
contains quantum or particles of different types and energies,
while we need to measure only one of the components of the
primary radiation and we want to get rid of the others.
An example is the measurement of radionuclide
purity of preparations in case a given basic
radionuclide emitting low energy radiation g (eg 99m Tc, g 140keV) is contaminated with a small admixture of a
radionuclide emitting higher energy g (eg 99 Mo, g740keV). In direct measurement, the detector would be
flooded with basic energy of lower energy, in the
"flood" of which the infrequently coming high-energy
photons would be "lost". In this case, it is possible
to use the method of filtration with a shielding
absorbent insert : place the vial with the investigated
preparation in a lead shield of suitable thickness (approx. 2-5
mm), which almost completely absorbs intense low-energy radiation
g of the basic radionuclide g
-radiation of the contaminant. This method
is described in more detail in §4.8 " Radionuclides and radiopharmaceuticals
for scintigraphy ", section
"Quality and purity of radiopharmaceuticals".
When
using collimation and filtration, we must be aware that a certain
part of the incoming radiation will not be detected, the detection
efficiency is reduced . For
quantitative measurements, it is therefore necessary to make an
appropriate correction for this circumstance or
to include it in the calibration of the detection system
.
Arrangement and configuration of radiation
detectors
Individual types of ionizing radiation detectors are used in
various configurations for their own measurements :
¨ One separate detector In most radiation applications, one detector is
sufficient , which we choose according to the type of radiation,
its energy, intensity, geometric distribution. We pay particular
attention to the optimization of detection
efficiency, energy response, linearity and other parameters;
sometimes also prices ... One detector is used, for example, in
personal dosimetry, in some industrial applications, when
measuring radioactive samples (Fig.2.1.3 top left).
The geometric design of the detector can be of
two types :
l Planar
detectors
cylindrical or square in shape, measuring radiation coming from 2
p -polorine ; some may also be sensitive to radiation coming from
other angles ( 4 p ).
l Well
detectors designed in the shape of a vessel - a " well
" into which the measured sample is inserted. The sensitive
volume of the detector forms the walls of the well surrounding
the sample. Most of the radiation thus passes through the
sensitive area, which leads to a higher detection efficiency (it can be close to 100%) . These detectors are listed below in §2.7 " Measurement of
radioactivity of samples (in vitro) ".
¨ Multidetector
systems
To measure more complex radiation processes, we usually need to
measure radiation in different places of the monitored system Þ the need
for the simultaneous use of multiple detectors
(Fig.2.1.3 top right). Most of them are detectors of one type,
or. two species (eg for g and b ). Several detectors are used, for example, in
monitoring systems or in multi-detector sample meters. In §4.2
" Scintillation cameras " and §3.2 "X-ray diagnostics", part
" Electronic X-ray imaging
" we will see that for electronic radiation imaging
systems of a large number of elementary detectors and
opto-electronic members (sometimes several thousand) are used. In
addition to the requirements applicable to stand-alone detectors,
the concurrence and harmonization of the
parameters of the individual detectors is important here
.
![]() |
Fig.2.1.3. Arrangement and
configuration of ionizing radiation detectors. Above: Use of one detector, multiple detectors of the same type. Bottom: A complex system of a large number of detectors of various types for the analysis of high-energy particle interactions. |
¨ Detection systems for high-energy particle
interactions
The most complex detection systems are used to study the
interactions of high-energy particles in large accelerators
. Here, during the interactions of accelerated primary particles,
a large number of secondary particles and radiation of various
kinds arise, which need not only to detect but also to measure
their energies, momentum, charge, trajectories (§1.5, section " Analysis of dynamics of particle interactions ") . This requires very complexly
configured multidetector systems, consisting of a large
number (tens and hundreds of thousands) of individual detectors
of various kinds, in a complex electronic circuit, often with the
participation of strong magnetic fields. These systems of a large
number of electronic detectors gradually replace the previously
used bubble chambers (described below).
A typical arrangement of such
an electronic detection system is simplified schematically in the
lower part of Fig.2.1.3. Its task is to capture, if possible, all
particles and quantum arising during the studied high-energy
interaction. During these interactions, a large number of
secondary particles fly out in all directions .
The whole detection system is mostly cylindrical in shape
and surrounds the place where the accelerated
particles interact with the target or in the opposing beams. It
consists of several axially symmetrical parts - cylindrical
layers or "shells" of detectors, whose functions
complement each other:
l Internal trajectory detector - tracker
The inner part of the detector starts very close (usually a few
cm) from the interaction and consists of a large number of
elementary detectors ( detection "pixels" and channels)
- semiconductor and ionization chambers, which serve as so-called
trackers - electronic path detectors
charged particles. They are deposited in several layers so that
it is possible to electronically reconstruct the paths of the
particles flying out of the interaction site. The best positional
resolution is pixel semiconductor detectors, which are placed in
the innermost layer, closest to the site of primary interaction.
Semiconductor strip and drift detectors are
also used (see §2.5 " Semiconductor
detectorsThe trajectory,
charge and momentum of the particles can be further determined in
the next layer of position - sensitive multi - wire drift
ionization chambers; the curvature of the trajectory in the
magnetic field determines the charge and momentum of the
particle.
l Spectrometer - calorimeter This is followed by a spectrometric layer, called a calorimeter
, where the energy of the flying particles is
measured by its orbital detector . absorb all the energy of the
particle and provide an output signal proportional to this
energy.This layer is formed absorbent material
interspersed with detectors. In the so-called hadron
calorimeter, high-energy hadrons (protons, neutrons, pions), when
passing through absorbent material through
"photonuclear" fragmentation reactions
with nuclei, produce a larger number of other secondary hadrons,
which further interact with material nuclei in a similar way -
creating a gradually expanding spray of ionizing
particles. which are registered by interleaved detectors. The
so-called electromagneticThe calorimeter is
optimized for the detection of particles interacting by
electromagnetic interaction (photons, electrons, positrons and
other leptons). When high-energy electrons and positrons pass
through the material, bremsstrahlung radiation is produced in the
form of high-energy photons. And high-energy photons produce
electron-positron pairs as they pass through a substance. The
result of the interaction of high-energy photons, electrons and
positrons with the material of the calorimeter is an electron-photon
spray , formed by a larger number of electrons,
positrons and photons. Scintillation detectors are suitable for
the detection of energetic particles (see §2.4 " Scintillation detectors "), especially high-density materials such as BGO,
LSO or PbWO 4 crystals., which have good conversion efficiency even
for higher energy photons.
l Muon spectrometer The last, outer cylindrical layer detects penetrating
particles, muons m , which
fly out. It consists of a large number of large ionization
chambers, located in a magnetic field, which evaluate the curved
paths of muons to determine their charge (+, -) and momentum. The
detection system further comprises coils (often superconducting)
generating a strong magnetic field , curving the
paths of the charged particles; it is used to measure the
momentum and charges of these particles. As already
mentioned, the task of these complex detection systems is to
capture as many particles as possible that fly
in all directions from the point of interaction. The detection
system therefore has the shape of a cylinder, a kind of
"pot", surrounding the site of interaction. In order
not to escape even particles flying at small angles along the
axis, the detectors are also distributed in a circle at both ends
of the cylinder - they form a kind of "lids" (they are
not drawn in Fig.2.1.3 below). Since the particles formed during
interactions at large accelerators generally have high energies,
this implies the large size and weight of the detection system so
that no particles escape without detection. In each case,
however, neutral, weakly interacting particles such as neutrinos
escape.
The most complex detection systems of this kind (called ATLAS,
ALICE, CMS ) are built at the largest LHC
accelerator at CERN (§1.5, part "Large
Accelerators"). The complex detection systems a large number of
detectors are also used in experiments for detecting
neutrinos (§1.2, section" neutrinos "), and cosmic radiation , e.g.
observatory AUGER (§1.6, section" Cosmic ray
"passage" Detection and Spectrometry space radiation ").
Electronic
circuit and processing signals from the detectors
electronic detectors are connected to respective electric
circuits , which provide two important functions:
¨ Power supply detector
for the proper function of the detector must be supplied adequate
power supply to the detected ionizing radiation
could detector cause a corresponding electrical changes causing
output electrical signal - detector response to radiation. We
recognize two types of power supplies:
- Low voltage sources of about 5-24V, used
to power electronic circuits equipped with semiconductor
components: amplifiers, discriminators, coincidence circuits,
counters, indicators, etc.
- Sources high voltage approx. 100-2000V, which
is needed for the function of photomultipliers, some
semiconductor detectors, ionization chambers.
For more complex detection devices, an additional
supply voltage is required for electromagnets or motor movement
of the detector components.
¨ Electronic signal processing and
evaluation of results
The primary electrical signal from the detector output is usually
very weak (it has a small amplitude), so in the
first phase it is necessary to amplify it
(Fig.2.1.1). Amplification may also be in two stages: at the very
output of the detector is sensitive preamplifier
, partially amplified signal is then in the evaluation apparatus
amplified in the amplifier to a desired level.
This is followed by further processing - signal analysis
and its recording or registration in a counter
or computer memory. Signal processing may include appropriate
pulse shaping and amplitude sorting
. For systems of two or more detectors, the signals from the
individual detectors are processed either independently
(for monitoring systems or multi-detector sample meters) or
together. The simplest joint processing is simple signal summation
- the system then behaves like one "larger" detector.
When detecting more complex structured radiation, especially
correlated pairs of quanta, the connection of detectors in consensus
oranticoincidences . In the case of coincidence
connection, a signal appears at the output only if the detection
occurred simultaneously in both detectors *) - it is used, for example, in PET positron emission tomography .
Conversely, in an anticoicidal circuit, the signal only passes
if, at a given moment, detection occurs only on only the detector
and not on the other (simultaneous detection is excluded). An
advantageous feature of the concurrent circuit is the substantial
reduction of noise and other disturbing
impulses.
*) In special cases, the so-called delayed
coincidence is also used - cases are detected when a
preset short time (usually less than m s) elapses between the
detection on one and the other detector .
In complex systems of many
detectors, so-called trigger circuits are
included , which trigger the detection process in the system of a
large number of detectors only for particles of selected
properties (eg trip angle, energy). This helps reduce the large
number of "ballast" pulses that would overwhelm the
system and make it difficult to find "useful" signals.
There is often a very complicated processing - processing
- including arithmetic operations between the magnitudes of
individual signals, weighing processes and other manipulations,
according to the physical-mathematical model of the investigated
radiation process.
General physical and
instrumental effects in detection and spectrometry
The task of radiation detection and spectrometry is the objective
measurement of the number of quanta, energies, intensities and
other characteristics of ionizing radiation. However, a
completely accurate measurement with 100% efficiency is only an
ideal assumption; in fact, the measuring process manifests itself
in a number of unfavorable physical and
technical influences, limiting the possibilities
of measurement or distorting the results. For
individual types of detectors, these effects will be specifically
discussed below. Here we will mention some common physical and
instrumental influences, which we will discuss mainly for the
general case of spectrometry, where the situation is the most
complicated; some of these effects are then applied in simpler
cases of simple radiation detection.
Detection
efficiency and sensitivity
The task of radiometric detection devices is to objectively
measure the intensity of radiation or the number of its
quantities at a given location, or emitted from a radioactive
sample. The optimal situation of "100% efficiency",
where the device will register every quantity of analyzed
radiation, is seldom met - a certain part of the radiation for
physical or design reasons is usually not detected. An important
parameter of the radiometer is its detection efficiency
, sometimes called the sensitivity *) of the instrument.
*) However, it is not entirely correct to
confuse the sensitivity and efficiency of detection. The word
" sensitivity " can also express other
properties of the detector. In general, sensitivity
means the ability of a sensor to respondon certain
stimuli. The sensitivity of the detector
expresses the ability of the detector to react to radiation - to
generate a processable signal when a given type of radiation
enters. The quantitative measure of this sensitivity is then
expressed as the detection efficiency .
Sometimes the sensitivity of the detector also means the
smallest detectable radiation intensity , or the smallest
detectable activity of the sample, etc., which the detector
is still able to measure (distinguish it from the radiation
background in the context of statistical fluctuations and other
measurement errors).
We distinguish two types of detection efficiency :
¨ Absolute (total) detection
efficiency of measurements
is the ratio of the number of pulses recorded by the detector to
the number of quanta emitted by the source in a given time, or
the ratio of the frequency of pulses from the detector to the
total flux in the field or beam of radiation. The absolute
detection efficiency depends on the geometric arrangement of the
source and the detector (see below §2.7
" Measurement of radioactivity of samples ", geometry 2 p
, 4 p ) , on resp. absorption of
radiation in the environment between the source and the detector
and, of course, also on the intrinsic internal efficiency
of the detector used.
¨ Internal detection efficiency h detector
is given by the probability of registration of quantum radiation
passing through the sensitive area of ??the detector. It is
expressed as the ratio of the number of pulses recorded by the
detector to the number of quanta of a given type of radiation
that entered the detector (its input
window) ; is given as a percentage (0% <
h <100%).
The number of pulses registered
at the detector output is always slightly smaller
(often significantly smaller) than the number of quanta of
radiation flying into the detector. On the one hand, some
particles arriving at the detector may not reach a sensitive
volume at all because they have been absorbed by the material of
the package or inlet window. Other quanta, such as high-energy
gamma photons, can in turn fly through the sensitive volume of
the detector without interaction (and thus without response and
registration). Even if there is an ionization interaction in the
detector material, the generated electrons and ions may combine,
or the energy of the electrons may be transferred to other atoms
and molecules outside the luminescent centers, for example ... -
leads to only a slight increase in the temperature of the
detector material. These "parasitic" phenomena reduce
the number of pulses at the output - they reduce the detection
efficiency.
The internal
detection efficiency, which is a characteristic of a given
detector (its type and even a specific piece), is given by a
number of physical and technical circumstances. Above all, it is
an effective cross section of the interaction of
a given type of quantum with the detector material. Furthermore,
it is the size of the sensitive volume, the absorption properties
of construction materials - mainly the input window,
"competitive" interaction processes without the
production of a useful signal, dead time, electronic processing
and signal analysis. We will deal with these phenomena
specifically for individual types of detectors.
For most
applications of ionizing radiation, we naturally require the
best possible detection efficiency . However, in the
case of high intensity of the measured
radiation, high detection efficiency could lead todetector
overload , high dead time loss, cumulative effects and
other phenomena leading to violation of the linearity of the
response, deterioration of the measurement accuracy
, in extreme cases even damage to the detector .
In such cases, we prefer a detector with less detection
efficiency, or we artificially reduce the overall detection
efficiency by suitable collimation or filtration of radiation in
front of the detector, or by increasing the distance between the
source and the detector. We then measure a lower signal flow
(response), but correctly . With suitable
calibration, we measure a certain representative sample
of the analyzed radiation.
Time
resolution and dead time
There is a certain time delay between the moment
of interaction of the quantum of radiation in the sensitive
volume of the detector and the electrical impulse at the output
of the detector . It is caused mainly by two factors:
1. Physical processes in the detector itself -
energy transfer, formation of ionization, propagation of
electrons and ions in the detector material, charge collection by
electrodes, deexcitation time, duration of scintillation, etc.
The response time of the detector itself depends on the type of
detector, the size of the sensitive area (smaller detectors are
faster), the material and the design. It ranges from tens of
microseconds to G.-M. detectors per unit of nanoseconds for
scintillation detectors.
2. Electrical processes in electronic circuits
- steepness of rise and fall of electrical impulses, charging and
discharging of capacitors, response speed of semiconductor
components. The decisive factors here are mainly the circuits of
the input preamplifier circuits . Current electronic
circuits are quite fast, their response time is in the nanosecond
range.
Individual
quantities of radiation come to the detector with irregular
"time intervals", at higher radiation intensities the
particles come in very fast succession, with insignificant time
intervals. No electronic detector works "infinitely
fast", it has a finite time resolution.
¨ Time resolution is the time that the detector needs to process and
register the response signal from one quantum of radiation.
¨ Dead time
detector is the time interval from the detection of one
quantum, during which the detector is not able to correctly
detect another quantum. During this time, the detector is either
insensitive to radiation or the second response signal would be
composed of the first (eg pile-up
effect , see §2.4, section " Scintillation spectrum "
below) .
The dead time of the detector causes some quantums
that come "too fast in succession" not to be
detected . This leads to a reduction in the
detection efficiency , and (worst of all) this detection
efficiency is not constant , but depends
on the intensity of the analyzed radiation - a non-linearity
of the response arises . This can lead to significanterrors
in measurement procedures.
The issue of dead time, its
measurement and correction, will be discussed in more detail
below in §2.3, section " Dead time
").
Background
of the detector
for each of the real measurement device, and therefore the
radiometer, over the measured signal and translates superimposed
"zero" signal - the so-called. Background
( background ). The background of a radiometric
detection device generally has three origins :
1. External radiation from the surrounding space
- a small amount of ionizing radiation is always present in our
environment and in the laboratory. This radiation comes from many
sources, such as cosmic radiation and its interaction with the
atmosphere, terrestrial radiation from radioactive minerals in
soil and rocks, from building materials of buildings (there is mainly potassium 40 K), from solid radon, from
fallout during tests of nuclear weapons and nuclear accidents.
Furthermore, it can be radiation from surrounding insufficiently
shielded sources, radioactivity of spectrometer components and
the like. The background from the outside can be significantly
reduced by thoroughly shielding the detector.
The background of the unshielded and shielded detector is
compared in detail below in Fig.2.1.5.
2. Internal radioactivity of the detector material
, which may be of dual origin :
¨ Sensitive
detector material may contain long- lived natural
radionuclides . These natural radionuclides are
ubiquitous and difficult to clean the detector material
completely from them. In organic scintillators, a small amount of
radiocarbon 14 is presentC. Of the commonly used detectors, the LSO
scintillator has the highest internal radioactivity (approx. 250Bq / cm 3 of the natural radioisotope 176 Lu - see the section " Scintillators ...",
section " Internal radioactivity LSO ") .
¨ Nuclear
reactions and activation of the detector material
may occur inside the detector due to external radiation . Both
short-term and long-term radionuclides can be formed, which
internally contaminate the detector (these
phenomena are discussed in more detail below in the section
" Aging and radiation wear of detectors ",
section " Nuclear reactions and induced radioactivity
inside detectors ") .
Radionuclides (natural and induced)
contained in the detector material during their radioactive
transformations emit quantum radiation, mainly beta and gamma,
which are detected with high efficiency. This internal
radiation background contributes to the overall
background of the detector.
3. Electrical noise of the device arising
due to quantum fluctuations in the movement of electric charges
in the detector itself, in the photomultiplier (in the case of scintillation detectors) and amplifying electronic circuits. Radiometer noise can
be significantly reduced by cooling detector and
preamplifier to the temperature of liquid nitrogen or even
helium. Electronically, the resulting noise can be reduced by
appropriately setting the lower discriminant level or by using a
coincident connection of two or more detectors.
For correct measurement, the background
must be subtracted from the resulting measured
values. Problems occur when the measured radiation is so weak
that it is comparable to the background intensity. The background
adversely increases our minimally detectable
radiation intensity or minimally detectable
radionuclide activity - it reduces the sensitivity of
detection.
Combinatorial
background
In more complex multi-detector systems (as above in Fig.2.1.3),
where the resulting measurement response is created by a certain combination
signals from individual detectors, when simultaneously detecting
a large number of particles, coincidence or anticoincidence
signals originating from different, unrelated processes may be incorrectly
paired . False data generated in this way is sometimes
referred to as combinatorial background . It
plays a negative role, especially in complex detection systems
for accelerators, where a large number of quanta are created
almost simultaneously during interactions and the investigated
rare phenomena can then be lost in the combinatorial background.
Spectrum
General physical and instrumental influences have
their specifics in radiometers operating in spectrometric mode.
Ideally, the measured (instrumental) spectrum n = n (E) should coincide
with the actual (physical) spectrum N = N (E) of the emitted
radiation. In reality, however, the measured spectrum differs
from the actual one due to some distorting physical and
instrumental effects :
![]() |
Fig.2.1.5.
The spectrum of the natural radiation background
measured by a scintillation detector NaI (Tl)
with a diameter of 5 cm and a height of 5 cm. Above: Free-standing detector without shielding. Bottom: Detector surrounded by a robust 7 cm thick lead shield. The basic spectrum (middle part of the figure) in the energy range 0-3 MeV was measured with an acquisition time of 12 hours. The figures on the right show a reduced section of 3-6 MeV from the adjoining high-energy part of the spectrum acquired by 48-hour acquisition. |
When the detector was
equipped with a massive shield (7cm lead, lower part of Fig.2.1.5) , the background decreased more than 10 times
and the 1460keV photopeak of 40 K potassium almost disappeared (the rest is potassium contained in the glass,
from which the photomultiplier flask is and a
scintillator window) . The 2614keV
peak almost disappeared (because 208 Tl in the
air inside the shield disintegrates quickly and the new 208 Tl does not
get into the enclosure) , the
3185keV 214 peak is at the resolution limit.Bi. The
continuous spectrum is formed by the interactions of
high-energy quanta from cosmic rays and the internal
radioactivity of the scintillator. At the beginning of
the spectrum, a clear peak of the characteristic X-ray of
lead can be seen, which arises during the photoeffect of
high-energy radiation in the lead atoms of the shield.
The continuous spectrum continues up to the highest
energies (for high-energy and muon
radiation, lead shielding acts as a "target" in
which interactions cascade) .
The continuous part, corresponding to beta radiation,
with or. gamma peaks, we also observe in the background
spectrum from the internal radioactivity
of the detector material (see,
e.g., the spectra of the internally activated NaI (Tl)
scintillator in Fig. ..., or the spectrum of the LSO
crystal containing the natural radionuclide 176 Lu in Fig.
...) . Careful measurement of the
background spectrum should precede the
spectrometric work itself, as increased background and
background peaks could be misinterpreted in the
measurements. The background spectrum can reveal or.
contamination of the detector or its surroundings. For
correct spectrometric measurements, the background
spectrum must be subtracted from
the measured spectrum. Problems occur when the measured
radiation is so weak that it is comparable to the
background intensity. Then the "useful" peaks
in the spectrum of the measured radiation are lost in the
noise and are difficult to detect.
Other influences that may affect the accuracy of radiometric (and especially spectroscopic) measurements are, if necessary. mechanical and thermal instability , influence of external fields (especially photomultipliers in scintillation detectors are highly sensitive to magnetic field ) and detector selectivity (ratio of detector sensitivity for registration of required type of radiation with respect to sensitivity for registration of other types of radiation) .
Specific properties and
use of different types of detectors
The specific properties of different types and designs of
detectors must be carefully considered when using them in
different applications of ionizing radiation. Strong radiation
fluxes (eg in radiation beams in radiotherapy) are best measured
with an ionization chamber, which has a low detection efficiency,
but shows a linear response even for high radiation intensities.
Operational monitoring of weaker and medium-strong radiation for
the purposes of radiation protection, where in principle high
accuracy cannot be achieved, is most often performed by GM or
proportional detectors. Measurement of radioactive samples,
natural materials and environmental radiation is performed using
highly sensitive scintillation and semiconductor detectors, with
a low background(For example, there is no
suitable LSO scintillator with relatively high intrinsic natural
radioactivity) . High-energy semiconductor
spectrometers are used in neutron activation analysis, X-ray
fluorescence analysis, and nuclear physics research. Accurate
spectrometry of charged particles - electrons (beta radiation),
protons, alpha-particles - can be performed only on magnetic (or electrostatic) spectrometers (in which, however, a non-spectrometric detector can
serve as a sensor). Detection of
high-energy cosmic rays and neutrinos is performed experimentally
mainly using Cherenkov detectors. In nuclear medicine, where we
need high detection efficiency without the need for high energy
resolution, scintillation detectors and their imaging systems -
gamma cameras - are used. For hybrid PET / MRI combinations (positron emission tomography + nuclear magnetic
resonance) , special semiconductor
photodiodes are used in PET camera scintillation detectors
instead of photomultipliers that would adversely affect the
magnetic field.
All these aspects of
the suitability of using different types of detectors will be
discussed in more detail below in this chapter (§2.3 - 2.8), in
the relevant parts of Chapter 3 " Applications
of ionizing radiation ",
Chapter 4 ".Radionuclide scintigraphy ", for neutrinos in §1.2, part" Neutrinos - "ghosts" between particles ", for cosmic radiation in §1.6, passage" Detection and
spectrometry of cosmic radiation ".
Radiation
detection by type, energy and intensity
The choice of detection methods and devices naturally depends
primarily on the properties of the radiation we want to analyze -
the type of radiation, the energy of its quanta and their
frequency (radiation intensity). We will mention here some of the
problems we generally encounter in detecting
radiation of different types, energies and intensities. We first
notice the type of radiation :
¨ Photon g and
X radiation are relatively easiest to detect
with the help of ionization chambers (including GM detectors),
scintillation and semiconductor detectors. This applies in
particular to radiation of medium energies of tens to hundreds of
keV and intensities of approx. 10 ¸ 10 4 photons / second. Details below §2.4 " Scintillation
detection and gamma-ray spectrometry
", §2.5 " Semiconductor detectors ".
¨ Corpuscular radiation a, b , p + is more difficult to detect - due to
its low penetration in the substance it is
difficult to get into the sensitive volume of the detector, it is
often absorbed in the material of the sample itself - see below
" Detection of alpha, beta radiation ". Effective detection can be achieved using
special methods, such as the use of liquid
scintillators .
¨ Neutrinos radiation is
the most difficult to detect of all known types
of radiation, due to the extremely weak interaction of neutrinos
with matter. It is only possible to detect with limited detection
systems - see §1.2, section " Neutrinos
".
It also depends on the intensity of the detected
radiation :
l Radiation of medium intensity , approx.
10 ¸ 10
5
particles per second, is again relatively easily detected if we
have a detector sufficiently sensitive to the given type of
radiation (with sufficient detection efficiency).
l Low intensity radiation
, significantly weaker than 1 particle / second, is difficult to
measure accurately. It is usually overexposed by the natural background
and noise in the detector, the measured values ??are
significantly affected by statistical fluctuations
. It is desirable to use detectors with a high detection
efficiency and a low level of self-background, well shielded from
external radiation, including natural radiation background. To
reduce the effect of statistical fluctuations, the measurement
times are quite long - to accumulate a sufficient (statistically
significant) number of useful pulses.
l High intensity radiation , such as tens
of millions of particles / second, can overwhelm the
detector
(dead time, cumulative processes) and prevent accurate
measurements. When exposed to strong radiation, radiation-induced
chemical reactions can occur in the detector material,
deteriorating the detector's properties - reducing detection
efficiency and deteriorating resolution. Immediately after such
overexposure, the detector may exhibit an increased intrinsic
background, caused by deexcitation of metastable levels and
chemiluminescence of molecules released by radiation in the
detector during intense exposure. Extreme radiation intensity can
damage the detector irreversibly ! Suitable
detectors with low detection efficiency and linear response, such
as ionization chambers, are used to measure strong radiation.
Furthermore, with the help of shielding or collimation, we can
define a certain defined "sample" of the analyzed
radiation and measure it correctly with the help of a sensitive
detector.
The
radiation detection methodology is significantly dependent on the
energy of radiation quanta :
×
Medium energy radiation
, keV units up to tens of MeV, can be detected in the case of
usual types of radiation ( g,
b, a, p + , ...) without major problems using ionization
chambers, scintillation and semiconductor detectors. × Low energy radiation , less than about
1keV, is very difficult to detect . Due to the high
absorption in the substance (low permeability), it is
difficult to penetrate into the sensitive volume of the detector
and elicits a low response in it , often covered
by quantum noise. Low-energy corpuscular radiation is often
present undetectable
(this is absolutely true for neutrinos due to their slight
effective cross-section of the substance interaction).
×
Radiation of high energies
, higher than hundreds of MeV, of the order GeV and TeV, often
shows a low effective cross section of the
interaction with the detector substance, which reduces
the detection efficiency - most quanta can pass through
the detector without response. In particular, the spectrometry of
such radiation is difficult because high-energy quanta lose only
a small part of their energy in conventional detectors. Special
robust detection systems, called calorimers ,
are used here, composed of massive absorption layers interspersed
with detectors (these detect emerging sprays of secondary
particles). High energies can be encountered in large
accelerators or in cosmic rays. In addition to the technical
difficulties of measuring high-energy radiation, it is necessary
to draw attention to the risk of radiation-induced
internal radioactive contamination of the detector
material, discussed in more detail in the following paragraph,
section " Nuclear reactions and induced radioactivity inside
detectors ".
Aging and radiation wear of detectors
Like any device (and object and material in
general ...) , the ionizing radiation
detector is not immutable and eternal . Its
properties with time during use and changing .
Changes in properties can be short-term or long-term, reversible
and irreversible. Short-term reversible changes - device
instabilities - were mentioned above. Sudden irreversible
changes belong to the category of device failures . Here
we will briefly touch on longer-term irreversible changes
- the "aging" of detectors. In practice, we can
distinguish two types of these irreversible time changes of the
detector properties:
l Spontaneous temporal changes in the
properties of the detector
due to physical and chemical influences inside or by exposure to
the external environment. A typical example is yellowing of
scintillation crystals NaI (Tl) (see
Fig.2.4.8 in the section " Inorganic scintillators
") , deterioration of optical contact
between scintillator and photomultiplier, or gradual changes in
gas pressure in ionization chambers (eg gas leakage through
chamber leaks) .
l Radiation
damage and depletion of detection material due to
physico-chemical processes during interactions with detected
radiation. When detecting radiation g and b
lower and medium energies, low-density excitations and
ionizations occur, with subsequent deexcitation and recombination
usually leading to the restoration of the original
properties of the material. Scintillation or ionization
detectors g and b can work for many years without significant wear. When
heavy charged particles and neutrons are detected, massive
ionizations occur, or even nuclear reactions, in which
considerable energy is transferred locally. There is damage to
the crystal lattice, chemical changes, or even transmutations of
atoms of the detector material (see below
" Nuclear reactions and induced
radioactivity inside the detectors ") . The active area of
??the detector is therebyradiation damages and depletes
.
All these phenomena can lead to a gradual reduction
of detection efficiency and in more complex detectors to
deterioration of other parameters such as energy resolution,
spatial resolution, homogeneity and linearity of the image. Aging
and radiation wear of detectors are significantly reflected in
systems where detectors are exposed to high energy fluxes for a
long time. These are, for example, monitoring detectors in
nuclear reactors, detection systems (trackers and calorimeters)
of secondary particles on accelerators, dosimetric devices on
irradiators in radiotherapy. This leads to a limited
lifetime of the detectors.
In normal conditions, detection,
spectrometry and scintigraphy of gamma radiation with energies of
tens and hundreds of keV and pulse flows up to about 3 . 10
5 cps, but
no radiation "aging or wear" no . Radiation damage to the detector by
intense flux of radiation Strong
flux of ionizing radiation in the detector causes radiolysis and dissociation
of molecules of the detector material, eg in the NaI
(Tl) of the NaI ® Na + + I - detector . Dissociated molecules can recombine
immediately or with a certain time delay. During recombination,
light photons are often emitted - chemiluminescence
occurs. If the dissociation of the scintillator molecules
persists for a long time, these photons, registered by the
photomultiplier, can significantly increase the internal
background of the detector in the region of
low-amplitude-energy pulses. This phenomenon lasts a transient
time, after the recombination is completed, the internal
background quickly returns to the original value.
Ionization chambers are highly "resistant"
to strong radiation fluxes, which can measure over a wide range
of intensities with an almost linear response. Scintillation
detectors in the "on" mode (with high voltage on the
dynodes) must not be exposed to high radiation fluxes (>
approx. 10 7 kvat / s) in order to avoid overloading the
photomultiplier photon multipliers and their irreversible
damage !
Nuclear reactions and induced
radioactivity inside detectors
If high energy radiation (> 10MeV) or neutron
radiation enters the detector , (photo) nuclear
reactions may occur in the detector material (as well as
in the construction material of the housing, shield and
collimator) (see §1.3 " Nuclear
reactions and nuclear energy
"and §1.6" Ionizing
radiation ", part" Interaction of gamma and X - rays "and" Neutron radiation and its interactions "). In some of these reactions, radioactive
nuclei may be produced - induced radioactivity
is formed inside the detector, internal radioactive contamination of the
detector . Such an internally activated detector after
exposure to high-energy or neutron radiation may have impaired
detection properties : it has a higher internal
background and spectrometric analysis shows a disturbing
artificial spectrum - continuous beta, or gamma or
characteristic X-ray peaks. Fortunately, this phenomenon lasts
for a transitional periodbefore the induced
radioactivity is emitted with the appropriate half-life. However,
this circumstance must be taken into account if we want to
measure with a detector that was previously in the field of
high-energy or neutron radiation (either during the actual
measurement or even switched off). The induced activity can be
many kBq, which can invalidate particularly sensitive
measurements of weak radiation fluxes and low activities.
Internal activation of
the NaI (T1) scintillation detector
An example of this phenomenon is the NaI (T1)
scintillation detector (discussed in detail below "
Scintillation detection and spectrometry "). Due to neutrons, nuclear reactions can occur
inside the crystal (n, g ): 127 I + n ® 128 I + g , in which radioactive iodine-128 is formed from the
original inactive iodine-127, which with a half-life of 25 min. by - -
decay (E b max = 2,12MeV) it
converts to stable 128 Xe and from 6.5% by electron capture to 128 Te. In addition to
beta radiation with a continuous spectrum, 441keV gamma radiation
and characteristic X-radiation are also emitted. This internally
induced 128
I activity produces an artificial continuous b- spectrum
extending up to over 2 MeV and a characteristic X-ray peak of 26
kV tellurium. The phenomenon lasts with a decreasing tendency for
several hours, until the decomposition of iodine-128.
Similarly, when irradiated with high-energy photon
radiation (energy greater than about 15MeV)photonuclear reactions ( g , n) 127 I + g ® 126 I + n can occur
inside the NaI (Tl) crystal , which produces radioactive
iodine-126, which with a half-life of 13 days is converted by 44%
b - decay (E b max = 1.25MeV) to
stable 126
Xe and 55% by electron capture to 126 Te. In addition to electron beta radiation with a
continuous spectrum (extending over 1 MeV), gamma radiation of
386keV and 667keV is emitted, as well as characteristic
X-radiation Te with an energy of 26keV. In this case, the
detector is internally contaminated for several weeks!
The scintillation spectrum of this internal
contamination, measured by the same detector, differs
significantly from the spectra of the respective radionuclides
from external sources. Some quantum b, g, X are detected coincidentally
, so the resulting pulses are summed. As a result, some gamma
peaks are not present in the spectrum (the apparent "mystery
of lost gamma") - their pulses summed by the signals from
the electrons fall into the continuous beta spectrum; for this
reason, we do not see in the spectrum, for example, the g- peak 411keV 128 I or the peak
386keV 126
I. Other peaks have shifted energies due to superposition with
characteristic X-rays; eg peak g 667keV 126We also see an
extended and in a higher position around 690keV, caused by a
coincidence sum with a characteristic X-ray of 26keV.
For germanium -based semiconductor
detectors , the radionuclide 71 Ge (decays with electron capture, T 1/2 = 11.4 days) can be formed
either by neutron capture from a stable 70 Ge, or by a photonuclear reaction ( g , n) from 72 Ge. Some gallium
radioisotopes can also be formed here by neutron capture, eg 72 Ga (T1 / 2 = 14
hours, b - 3.15
MeV, a number of g lines from 0.6 to 2.5 MeV) and to a lesser extent
several other short-term ones. For silicon
semiconductor detectors can induce only a small number of
short-term isotopes - neutron or g activation by aluminum
isotopes, eg 28 Al (T1 / 2 = 2.3min., b - 2.85MeV, g 1.78 MeV).
In ionization chambers filled with xenon (which
is a mixture of a number of stable isotopes from 124-136 Xe), neutron or
gamma activation can produce various radioisotopes of iodine
(including the known 131 I) and xenon, but due to the low density of gaseous
xenon only in small amounts usually does not affect the
properties of the chamber.
2.2.
Photographic detection of ionizing radiation. Material detectors.
All photographic and material detectors operate in a
cumulative (integral) mode in the sense of the classification in
§2.1. We will first describe photographic detection, which is
the most common.
In photographic detectors, the detection
medium is photographic material . When ionizing
radiation enters this photographic material *) containing silver
halides (such as silver bromide AgBr), a photochemical
reaction occurs at the ionization sites .
*) The use of photographic materials was
the first and oldest method of indicating nuclear radiation;
using it, after all, H. Beckerel discovered the radioactivity of
uranium ore.
Photochemical
reaction
Below the photochemical reaction
generally we mean any chemical reaction caused by the incidence
of light or other radiation - interaction radiation
quanta (photons, electrons, protons, and particles
capable etc.) with atoms and molecules of the substance. The most
important photochemical reaction in nature is photosynthesis in
plants. Photochemical reactions are used in photography, leading
to such chemical changes in the light-sensitive material
, which can be used to visualize the spatial distribution of
radiation - for photographic imaging .
Photographic (light-sensitive) materials are formed
by small crystals of silver halide (now it is almost
always silver bromide, crystal size approx. 1 m m, density approx.
10 9 / cm2 ), which are
dispersed in a gelatin layer. This so-called photographic
emulsion is applied on the surface of a plastic foil - film
; glass photographic plates were also used in the past .
In the silver bromide molecule AgBr, the silver and bromine atoms
are bound by an ionic bond - Ag + Br - , which is relatively weak; the AgBr crystal lattice
forms a cubic system.
The classical photochemical reaction
is caused by the absorption of a light photon f, whose energy h. N releases an
electron from the bound bromine atom (bromide ion Br - ): Br - + f ® Br + e -.
The released electron can be absorbed by some silver ion Ag + bound in bromide: Ag + + e - ® Ag, thus forming
a neutral silver atom. This is the primary photochemical
reaction in which the energy of the photon must be higher
than the binding energy of the molecule that is cleaved during
photolysis. Due to these processes, the
disintegration ( photolysis ) of silver bromide occurs .
A similar photochemical reaction occurs when irradiation of
photographic material with ionizing radiation ,
which causes the decomposition - radiolysis
- of silver bromide. The result is the release of silver
atoms from its bond from the AgBr compound and formationlatent
image .
At first we would not see anything on the exposed eye
with the naked eye, the image is "hidden" (latent),
formed only by sparsely distributed silver atoms. The
physicochemical change in the silver bromide crystals is only
visible during development . The development
process is an electrochemical reaction in which electrons are
primarily transferred from the developing agent to AgBr via the
silver atoms in the latent image. Developers (usually methanol
and hydroquinone) cleave bromine (which passes into the
developer) from exposed bromosilver particles, reducing
the original silver bromide to metallic silver..
From the hydroxyl group OH bound to the cyclic hydrocarbon
(benzene nucleus) of the developing agent, a hydrogen ion is
cleaved off, which combines with a bromine ion to form HBr
(dissolved in the developer) and a silver atom of Ag. The
developing agent penetrates (in dissociated form) to the nuclei
of the latent image, transfers AgBr to its electrons, and silver
is reduced . By encountering the transferred electrons
with other silver Ag + ions in AgBr, the reduction process is further
transferred to the silver bromide crystal and further silver is
released. The effect of the microscopic latent image is initiating
and catalytic - the chemical process of reduction
gradually captures the whole grain of silver bromide, a large
number of silver atoms is formed (multiplication factor of about
10 8).
This process occurs only on those AgBr crystals which already
contained several atoms of photolytically precipitated silver
before development. The different degree of exposure of the
photographic emulsion is thus made visible by
the density of the colloidal silver grains. The remaining unlit
silver bromide is removed from the sensitive layer by dissolving
in a stabilizer (aqueous sodium sulfate solution).
After exposure to ionizing radiation, the blackening
rate of the developed photographic material is proportional
to the ionization density at that location, and thus the
amount of ionizing radiation energy that has been absorbed at
that location. By macroscopic observation or measurement of the total
blackening of photographic material, or its individual
places, we can determineintensity of radiation
in dosimetry and X-ray diagnostics or defectoscopy, microscopic
observation of grains of released silver in the
photoemulsion can then observe and evaluate the paths of
charged particles in special nuclear emulsions (see
below).
Film dosimeters , X-ray
films
Simplest use of photographic detection of ionizing radiation, are
film dosimeters . The basis of the film
dosimeter is a field of photographic film, lightly wrapped in
black paper (it differs from ordinary
photographic film in that it has a thicker emulsion with a higher
content of silver bromide) . The ionizing
radiation passes through the film coating and creates a latent
image in the photoemulsion, which is visible upon development.
The optical density of the graying or blackening of the film,
which can be evaluated photometrically, is then a measure of the integral
amount of radiation that has passed through the film
during exposure; this also indicates the doseradiation
that would be absorbed in the substance exposed to this exposure.
For small doses of radiation, there is an approximately linear
relationship between the dose of radiation and the
blackening of the photographic material, at higher doses the
blackening grows more slowly and then reaches a state of
saturation .
Note: For the
objective determination of the radiation dose, a standardized
development procedure should be used and the blackening should be
compared with a reference film irradiated with a known radiation
dose.
Fig.2.2.1. Personal film dosimeter used to monitor workers.
(Thermoluminescent TLD and photoluminescent
OSL dosimeters are discussed below ,
Fig.2.2.2)
Film dosimeters are mainly used for
personal dosimetry of workers with ionizing
radiation. The actual field of the film is inserted into a plastic
case (Fig.2.2.1), provided with several small rectangles
of copper and lead plates of different thicknesses, which serve
as filters absorbing radiation g depending on its
energy. These filters are used to correct the dependence of
blackening on the energy of radiation, and by comparing the
blackening under individual filters, it is possible to estimate
the type and roughly the energy of radiation (of course, the film itself does not have spectrometric
properties) . The film dosimeter is worn by
workers at the reference point (left pocket on the shirt) and
regularly (usually once a month)is exchanged, developed and photometrically
evaluated ; using a suitable calibration factor, the
resulting measured value is the radiation effective dose
in mSv.
A similar type of film (usually of much larger
dimensions) is used for X-ray imaging in planar X-ray diagnostics (§3.1 " X-rays - X-ray
diagnostics ") , as well as in defectoscopic measurements (§3.2 and
3.3). Once developed, the images on these films are evaluated
either visually or photometrically.
Autoradiography
This laboratory radiographic method consists in photographically
depicting the distribution of the radio indicator in the
examined objects in close contact *) of the
photographic emulsion with the sample (the
prefix " auto " indicates that the radioactive
substance is inside the sample) . The
radioactive sample is attached to the photoemulsion, which is
then exposed to darkness (in a light-tight
housing) for some time . Sensitive
photographic film photochemically captures the emitted beta or
alpha radiation (gamma radiation
contributes only slightly to blackening) .
Upon induction, differences in the local concentration of the
radioactive substance are manifested by varying degrees
of blackening
emulsion on which we can see the distribution of structures with
higher or lower accumulation of radioindicator.
*) Image quality
Close contact of the photoemulsion with the sample is
necessary to achieve good sharpness and spatial resolution of the
autoradiographic image. Image "collimation" is achieved
by each site of the photographic emulsion receiving the largest
contribution of radiation from the area of ??the sample that is
closest, while from other - more distant areas - the beta / alpha
flux density decreases rapidly (even faster
than the reverse inverse law). powers of distance, because this
radiation is strongly absorbed in the substance and has a short
range). It is obvious that the image of the
radioactivity distribution will be sharper the closer we press
the photographic film to the sample. With increasing distance of
the photoemulsion from the sample, or its thickness, the
sharpness and resolution deteriorate rapidly.
The quality of the image also depends on the energy of the
emitted beta radiation. For lower energies with shorter electron
range we can achieve better resolution (eg
with low-energy phosphorus 33-P we can achieve better resolution
on autoradiograms than with high-energy 32-P - see §1.4, passage
" Phosphorus, Sulfur ") . Beta-radionuclide-
labeled radioindicators , such as tritium 3 H , radium carbon 14 C , phosphorus
32.33 P ,
sulfur 35 S , are most often used for autoradiography of biological
preparations. If selective pharmacokinetics are appropriate,
radioiodes labeled with radioiodine
131 I (or 125 I ), yttrium 90 Y , in the 1970s also 198 Au, and other suitable radionuclides are also used. The
required applied activity depends on the radionuclide used, the
degree of accumulation of the radioindicator in the investigated
structures, the sensitivity of the emulsion, the exposure time.
These parameters are influenced by a number of factors, they are
optimized empirically(according to
experience to obtain sufficient blackening is per 1cm 2 photoemulsions need exposure of about 5-10 million beta
electrons, or about 2 million alpha particles) .
In addition to classical autoradiography,
electronic digital autoradiography is now used
in larger specialized laboratories , where instead of a
photographic emulsion, the imaging is performed using a flat
system matrix of semiconductor detectors (simpler similar to so-called flat panels in
radiology, see " Electronic X-ray imaging ") . sample. They allow
online quantitative autoradiographic analysis to be performed
operationally in a short time. And if the
distribution of the radio indicator does not need to be
registered in image form, it is enough to use a simple radiometric
detector (possibly
several parallel detectors) , which passes
over the sample and measures the profilogram of the
radioactivity distribution. This simple method is routinely used
especially in radiochromatography (§2.7
" Measurement of radioactivity of samples ", passage " Radiochromatography ") .
Autoradiography is
often used in laboratory methods of molecular biology and
genetics. According to the size of the analyzed tissue samples,
we have two groups, the third group concerns the imaging of
electrophoretic and chromatographic samples :
1. Macro-autoradiography
, which shows the distribution of radioactivity in structures the
size of millimeters and centimeters (so we
can assess differences in blackening with the naked eye). Thus, after previous application of the
radioindicator, sections of organs or their parts can be
displayed - Fig.2.2.2 a.
2. Micro-autoradiography
shows histological tissue sections or cytological samples (smears on a microscope slide) obtained
after previous application of a biologically targeted
radioindicator. The structure of blackening after exposure is
evaluated under a microscope - Fig.2.2.2 b (and
can be confronted with the classical observation of a stained
specimen under a microscope) . By using
thin sections and fine-grained photoemulsions, a high resolution
of 3-5 micrometers can be achieved, down to the subcellular
level.
Fig2.2.2. Examples of three types of autoradiography . a)
Macro-autoradiogram of a kidney section of a
laboratory animal after application of the radioindicator 131 I-hipuran.
b) Micro-autoradiogram of a histological section
of the liver with a radiolabeled colloid which is taken up in
Kuiper cells.
c) Autoradiography of 4 samples of the sequence
of DNA fragments labeled with 32 P radiophosphorus and separated by gel electrophoresis.
3. Molecular
separation analysis of sequenced samples
Very important is the use of autoradiography sample analysis, in
which the desired radiolabeled molecules divided - sequenced
- by the length of their chains separated in a chromatography
column, or more usually by electrophoresis in a gel -
see below " Radio-electrophoresis ."
Gels form a dense network - a " molecular sieve
" through which larger molecules pass more slowly than
smaller molecules. The analyzed molecules are thus gradually
divided according to their size (fragment
length) at a distance of several
millimeters to centimeters. If they are labeled with
beta-radionuclide, we can display them autoradiographically
after applying the photoemulsionin the form of
"strips", where the separated molecules traveled
according to their size (in terms of
geometric dimensions it is a macro-autoradiography ) - Fig.2.2.2 c. The read positions of individual
fragments are compared with the standard sample analyzed in
parallel.
Note: Current
routine biochemical DNA sequencing methods use fluorescently
labeled nucleotides, which are analyzed by capillary
electrophoresis.in a plurality (several dozen) of parallel
capillary sequencers, the fluorescent light being registered by
means of a sensitive optoelectronic detector. These new
sequencing techniques allow very fast and relatively inexpensive
"reading" of entire genomes. A huge amount of sequence
data obtained in this way is processed by computer - it becomes
the subject of bioinformatics .
Material detectors
Material detectors use some permanent small physical and
chemical changes that ionizing radiation causes as they
pass through suitable substances (materials). These can be
excitations, structural changes in the crystal lattice,
polymerization, changes in optical properties (such as color),
electrical conductivity. The rate of these effects is
proportional to the amount of radiation (number of radiation
quanta) transmitted by the detection material - it is therefore
proportional to the radiation dose at the
detector site. They are therefore mainly used for dosimetric
purposes.
Thermoluminescence
and photoluminescence (OSL) dosimeters
Material radiation detection is based on the phenomenon of metastable
excitation of some dielectric materials: electrons
released by ionizing radiation pass from the valence band to the
conduction band, from where they are captured at the fault of the
crystal lattice. ") *) and they remain there for a long time
- the levels are metastable. Electrons cannot get out of these
levels spontaneously, but are only released by supplying a
certain amount of energy (heating or light irradiation). In this
way, part of the energy absorbed during irradiation collects
in the material . By heating - thermoluminescence
(thermally stimulated luminescence) or by irradiation with
visible light - OSL (Optically Stimulated Luminescence)
deexcites and electrons return to lower energy
levels (and to the electron shells of the atoms of the material).
The released excitation energy is emitted in the form of visible
light photons - luminescence
(fluorescence) of the material occurs , mostly in blue-green
light. The higher the radiation dose of the material, the more
electrons accumulated in metastable levels and the more photons
emitted when evaluated by thermoluminescence or OSL-luminescence:
this light yield is therefore proportional to the
radiation dose in the irradiated material.
*) The mechanism is to some extent similar
to the origin of scintillation in scintillators - §2.4
"Scintillation detectors", part "Scintillators and their properties ", Fig.2.4.5 left. The main difference is in the
lifetime of excited electron levels: while for scintillators
almost immediate deexcitation with the shortest possible
afterglow time is desired, for thermoluminescent and OSL
materials the maximum (meta) is required. stability, with the
least fading . like scintillation materials, although
there may be a capture centers made by first base material,
first, a large number of these centers can additionally create
introducing ions of the activator in the crystal
lattice - these ions cause additional discrete levels in the
forbidden band. a common activator, or : dopant
impurities for TLD and OSL materials tend manganese,
dysprosium, carbon.
¨ Thermoluminescent dosimetry TLD Lithium
fluoride LiF (: Mg, Ti, Cu) *), calcium fluoride
CaF 2 (:
Dy, Mn), calcium sulphate CaSO 4 (: Mn, Dy), alumio-phosphate glass Al are most
often used as thermoluminescent substances . (PO 3 ) 3 -Mg (PO 3 ) 3 , Li 2 B 4 O 7 (: Mn) (low
sensitivity, suitable for high doses). A sample of a precisely
defined amount of a given TLD substance is encapsulated in a
thermoluminescent dosimeter, which is exposed to radiation at the
place where the radiation dose is to be determined.
(TLD materiál bývá
rùznì proveden, na prstovém dozimetru na obr.2.2.1b vlevo je
ve tvaru fólie - "chip" tlouky cca 1mm). Po skonèení expozice se
termoluminiscenèní látka vyjme z pouzdra a ve vyhodnocovacím
zaøízení se zahøeje na teplotu cca
160-300°C (podle druhu materiálu) a pomocí fotonásobièe
se snímá emitované viditelné svìtlo. Elektrický signál z
fotonásobièe se zaznamenává v závislosti na teplotì -
vzniká tzv. vyhøívací køivka, její integrál
(plocha pod køivkou) je úmìrná dávce v
dozimetru.
*) Pro dozimetrii neutronù
je místo pøírodního lithia (s pøevaujícím 7Li) pouito lithium obohacené izotopem 6Li.
Fig.2.2.3. Left: Thermoluminescent TLD
dosimeter. Middle: Photoluminescent OSL
dosimeter. Right: Principle of
operation of TLD and OSL dosimeters.
¨ Photoluminescent
OSL dosimetry
also called PLD ( Photo Luminescence Dosimetry ).
Optically stimulated luminescence is shown, for example, in the
richly distributed silica (quartz), but in dosimetric
practice, mainly aluminum oxide Al 2 O 3 (: C), activated by carbon, is used. A defined sample
of this substance is exposed to radiation in the dosimeter at the
place of radiation monitoring. Irradiation with LED light (longer
wavelength - yellow-green light) is used for evaluation, while
the resulting luminescence (shorter wavelength - blue light) is
detected by a photomultiplier. The total luminescence thus
generated is again proportional to the irradiation
dosimeter. Compared to TLDs, the evaluation
is simpler and more reproducible (LED irradiation is easier to
standardize than controlled thermal heating).
Thermoluminescence and OSL dosimetry is
schematically shown in the right part of Fig.2.2.3. These
dosimeters can be designed as finger dosimeters
for monitoring during laboratory work, or whole-body
dosimeters monitoring a sample of total irradiation at a
reference site. As with film dosimeters, several separate
detection elements, sometimes covered by different filters
, are sometimes used.- for the analysis of the type and energy of
ionizing radiation. Compared to film dosimeters, there is the
advantage of higher radiation sensitivity and accuracy, a larger
range of measured doses, low sensitivity to external influences
(temperature, humidity, chemical fumes), the possibility of
continuous operational evaluation and reuse of
material (exposure and evaluation is not destructive).
Luminescent
archaeological dating
Materials capable of long-term metastable (almost stable!)
Excitation of electrons in crystal lattices are commonly found in
nature. The excitation of these materials occurs slowly and
continuously due to natural radioactive radiation (whose
intensity is constant for a long time, but may vary for different
localities). This phenomenon can be used for archaeological
dating. The older the studied material of this kind -
the longer the time has passed since its last heating or light
irradiation - the more it was "excited" by the excited
electrons. It's a kind of "time counter".
Thermoluminescence can be used to
determine the age of fired ceramic objects or bricks
containing quartz- the most common mineral exhibiting
thermoluminescence. All electrons previously trapped in the
metastable levels of the trapping centers of the respective
material were released by high temperature heat treatment during
production. These empty levels are then gradually occupied by
electrons released by natural ionizing radiation (thorium and
uranium decay series, potassium-40, cosmic radiation). We heat
the investigated ceramics (to a temperature of approx. 300 ° C)
and measure the luminescence it emits. The intensity of this
thermally stimulated luminescence depends on the time
that has elapsed since its original firing (during manufacture)
and the current firing during analysis.
Similarly, small grains of quartz and feldspar, which
are commonly found in all layers of archaeological excavations,
showoptically stimulated luminescence (OSL):
metastable electron levels in crystal lattices are gradually and
long-term occupied by electrons released by ionizing radiation
from natural radioactivity, especially potassium 40 K. The longer the
quartz grain is exposed to this natural ionizing radiation, the
more electrons accumulate in these metastable "traps".
When irradiated with visible light, the captured electrons then
deexcite, measuring the luminescence that is proportional to the
time that has elapsed since their last illumination (when they
were covered in the landfill from daylight; samples must be taken
and processed without access to light! - otherwise premature
deexcitation of metastable levels would occur and the accumulated
"record" would be "reset").
These luminescence methods allow archaeological
dating in the time range of about 100 - 100,000 years.
Other material detectors
This includes, for example, trace detectors ,
based on the fact that after the impact of densely ionizing
particles, especially alpha, there are minor local defects in the
crystal lattice of certain materials (eg mica, special glasses,
organic polymers). These microscopic defects can be increased to
macroscopic dimensions by etching (the damaged
material is more chemically sensitive, so it dissolves). The traces
etched in this way are then observed under a microscope and their
density is calculated - either manually or automatically using
electro-optical methods. They are mainly used to measure the
long-term average concentration of radon in the field
(in building interiors) using the density of traces created and-particles from
the radioactivity of radon and its daughter products.
The so-called long base silicon diodes
(LBSD - Long Base Silicon Diode ), designed for
measuring the radiation dose (kerma) from heavy particles,
especially fast neutrons, can also be included in
this category . When a fast particle collides with a
silicon atom, this atom is ejected from its position in the
lattice. As a result of irradiation and ionization, the crystal
lattice of silicon is damaged, which changes the lifetime of the
minor charge carriers and thus the conductivity of the diode. The
voltage drop across the diode is measured in the forward
direction before and after irradiation, the change in voltage
drop after irradiation relative to the initial value being an
approximately linear function of the radiation dose (kerma).
Due to their relatively small use in nuclear and radiation physics and technology, we will not deal with material detectors in the next text of this chapter. For interest, here are just 3-dimensional gel detectors :
3-dimensional gel detectors
The most complex, but physically interesting variant of material
detectors are the so-called 3-dimensional gel dosimeters
, which allow to record and store the spatial distribution of the
radiation dose in the form of a certain dose pattern
. They are integral (cumulative) chemical detectors in which
information is fixed in a gel environment. The
measured volume (eg phantom), in which we want to determine the
spatial distribution of radiation intensity or dose, is
filled with a gel consisting mostly of a gelatin
carrier (matrix), in which the radiation-sensitive
substance is dispersed .. Upon irradiation of
this volume, free radicals are generated by the interaction of
radiation in the gel, mainly due to radiolysis of
water, which by chemical reactions induce permanent
physicochemical changes of the sensitive substance at
the irradiation site, the rate of which is proportional to the
local absorbed radiation dose. In particular, two types of
sensitive substances dispersed in the gel are used, differing in
the mechanism of the radiation effect:
w Radiochromic
gel dosimeters , where the sensitive substance changes
its color by irradiation . The most frequently
used iron sulfate [ferro ammonium sulfate (NH 4 ) 2
SO 4 .FeSO 4
.6H 2 O in the so-called Fick's
solution with sulfuric acid H 2
SO 4 ], where free radicals and
hydrogen peroxide, formed by irradiation of an aqueous solution
in a gel, cause oxidation of Fe +2
ions to Fe +3 . This leads to
increased light absorption, especially at a wavelength of around
300 nm. A certain disadvantage of this substance is the diffusion
of radiation products - iron ions - from the place of their
origin to the surroundings in the gel, which soon blurs the
information about the spatial distribution of the dose. To reduce
the diffusion effect, the chemical composition was improved by
the addition of xylenol, forming a color complex ( xylenol-orange
) with Fe +3ions. In this
so-called FXG dosimeter, in addition to reducing the diffusion
effect, an increase in the optical response is achieved by
absorbing the wavelength of 585 nm.
Note: Plastic 3-D dosimeter: Instead of
a gel, the radiochromic substance is sometimes fixed in a
low-melting plastic; with optical evaluation.
w Polymer gel detectors consist of a
monomeric substance dispersed in a gelatin carrier. Free radicals
formed in the gel upon irradiation induce local
polymerization of the monomer at the site of
irradiation, in proportion to the absorbed dose. The gel carrier
causes the radiation-formed polymer (with large molecules) to
remain permanently localized at the site of interaction. This
polymerization leads to changes in optical properties - the
originally transparent gel acquires turbidity at
the site of irradiation, the opacity of which is
proportional to the absorbed dose. The density
also increases slightly . The most commonly used sensitive
substance here is methacrylic acid , resp. a
mixture of acrylamide and N,
N-methylenebisacrylamide . Compared to ferro-sulphate gels,
polyacrylamide gels have the advantage that the polymerization
chains remain permanently fixed at the site of their formation,
so that the record of the spatial distribution of the dose is stable
for weeks and months. Therefore, polymer gel dosimeters appear to
be more promising.
A certain disadvantage of polymeric gel dosimeters
is their sensitivity to atmospheric oxygen, which can inhibit
polymerization. Their material must therefore be prepared in a
nitrogen atmosphere and filled into hermetically sealed
containers. Sometimes oxygen absorbers are added to the gel.
The resulting
irradiation effect of both types of 3D gel dosimeters is local
material changes proportional to the local absorbed
dose. These changes manifest themselves in three ways, which
allows the evaluation of the spatial distribution of the dose in
the irradiated gel dosimeter by three methods :
× Transmission
optical CT
The non -irradiated gel
is optically almost transparent . The sites with
the higher absorbed dose where the radiochemical reaction
occurred show a variety of
optical properties
- discoloration, turbidity - have
increased light absorption, higher opacity . By
irradiating the detector gel with light rays at different angles
0-360 °, with the measurement of the transmitted intensity,
resp. attenuation of the beam, using photodiodes (or CCD
detectors) and subsequent reconstruction based
on the Radon transformation by the method of filtered back
projection, we obtain images of opacity and thus the local
distribution of the dose in cross sections. The set of these
cross-sections will create a three-dimensional image of
opacity and thus the spatial distribution of the radiation dose
in the sensitive volume of the gel detector. This optical CT scan
and reconstruction is analogous to X-ray CT, described in §3.2
"X-ray diagnostics", part " Transmission
X-ray tmography (CT)The gel
dosimeter is mounted in a special holder, rotating by means of a
stepping electric motor. The irradiation is realized either by a
thin laser beam - an accurate but lengthy method, or by a wide
beam of light - a fast but less accurate method (usually sufficient in practice) .
optical contact is suitable for irradiation with a dosimeter
immersed in an aqueous medium
× Nuclear
magnetic resonance NMRI At the sites of radiochemical
reaction, chemical properties change, which leads to subtle changes
in magnetic behavior.There are changes in the
interaction of nuclear moments, manifested by changes in
relaxation times T 1 and T 2
magnetic moments of hydrogen nuclei when imaging a gel
detector by nuclear magnetic resonance (see §3.4, section "
Nuclear
magnetic resonance "). With appropriate
modulation (sequence and "weighing"), the intensity in
the NMRI image will be proportional to the local dose in the gel.
× X-ray
CT
The radiochemical reaction may lead to slight changes
in the density of the material. In places where
radiation-induced polymerization has taken place, the volume of
the sensitive substance decreases and thus the density
increases detector material. This leads to an increase
in the linear attenuation factor for X-rays, which can be shown
by CT transmission tomography. With an X-ray CT gel detector,
using the soft tissue CT settings, we obtain an image whose
density expressed in Housfield CT units will be modulated by the
spatial distribution of the radiation dose in the gel dosimeter
material.
3D gel detectors are
used in dosimetric measurements in radiotherapy
, especially in measuring intricately shaped spatial
distributions of radiation doses in advanced radiotherapeutic
techniques IMRT, stereotactic radiotherapy, high-density
brachytherapy or hadron radiotherapy - see §3.6 " Radiotherapy.
"Dosimetric gel may be filled vessel modeling anatomic
various organs and body parts for the purpose phantom
dosimetric measurements . A significant advantage of gel
dosimeters their energy independence , tissue
equivalence (density of the gel is approximately equal
to the density of soft tissues), and almost linear
response of the size the radiation dose and in high
doses . the disadvantage is, as all the material
detector, low sensitivity , to operate by
relatively high doses of about 4Gy.
3D dosimetry gel is
relatively complex and costly method both in the
stage of laboratory preparation(including high
demands on the homogeneous distribution of the sensitive
substance in the gel) , as well as the evaluation of the
response in the dosimeter material. It is therefore used only
occasionally. For comparison and interest, see
§3.6, passage " Make the invisible
visible " - display of
radiation beams ", where
pictures of the optical response of electron, photon and proton
radiation using Cherenkov radiation in water and scintillation
radiation in a liquid scintillator are shown.
Particle
trace detectors
Nuclear
photoemulsions for particle trace detection
To study the properties of particles, it is useful to photograph
the path of their movement in matter. To detect
traces of particles, a photographic emulsion
with a relatively large thickness (approx. 0.1-1
mm) and a high content of silver halide in gelatin is applied to
the film or glass plate . When a fast charged particle enters
this emulsion, it leaves an ionization trace
along its path of movement , in which a photochemical reaction
releases silver in the grains of silver halogens dispersed in the
gelatin of the emulsion. The particle thus leaves traces of loose
silver on its path in the emulsion from grain to grain, ie a latent
image of its path is formed.
; upon development, a visible trace of a more or
less dense sequence of black particles is formed , the density of
the silver grains along the path depending on the species and
energy of the particle.
Viewing and measuring these traces in exposed and
induced nuclear photoemulsions is performed using special
microscopes equipped with micrometric shifts. The range
of the particle (if its path ends in the emulsion) and
the density of the silver grains are
measuredalong its path. From these measurements, the energy and
mass of the particle that has flowed through the emulsion can be
determined. The greater the particle's energy, the greater the
range of the particle. The density of silver grains, i.e. the
relative number of grains per unit length of the path, is
proportional to the relative loss of energy of the particle in a
given section of the path. According to the so-called Bragg
curve (see §1.6, Fig.1.6.1), each charged particle
causes more and more ionization towards the end of its path with
a maximum just before the end of the path (stopping), so that the
end parts of the particle track appear the highest blackening
density number of silver grains).
When a nuclear emulsion is placed in a strong magnetic
field of a given intensity and direction upon exposure ,
the paths of the charged particles are curved by the action of
the Lorentz force; from the curvature of particle pathsit
is then possible to determine the ratio of charge and momentum of
the particle (images are similar to Fig.2.2.4, but negative - the
trace is black on a white background). If a fast particle
interacts with another particle within the emulsion, important quantitative
data on the dynamics of this interaction can be
determined from the angles of travel, curvature, blackening, and
trajectory of the individual secondary particles formed .
Another option is to stack several
photographic emulsions (films or plates) on top of each
other. The passage of the particle through this system causes a
small amount of blackening at each point (the intersection of the
path of the emulsion particle) on each of the plates. By
evaluating these traces from the individual emulsions, the spatial
trajectory of the particle can be reconstructed.
( Note: Now, electronic
position-sensitive detectors ( trackers ) are
spatially assembled into blocks or other structures .)
Nuclear emulsions were also used to study nuclear
reactions , while the emulsions were filled with compounds
of some elements - lithium, boron, uranium, etc., with which the
reactions were studied .
Nuclear photoemulsions played an important role in
the study of nuclear processes and elementary particles in the
first generations of accelerators and in cosmic rays; a number of
elementary particles were discovered using these emulsions. The
disadvantages of nuclear emulsions are their small dimensions
(especially thickness) and low operability of use - particle
paths are only visible later, after development, their evaluation
is slow and laborious *). Therefore, they were gradually pushed
out primarilybubble chambers , which provide
significantly faster and more complete information about the
motion and interactions of elementary particles. And now the
bubble chambers are being pushed out by electronic
detection systems , see below.
*) ECC photoemulsion
chambers
However, nuclear photoemulsions are still used in some particle
experiments, where a very high spatial resolution of
registered particle paths , on the order of micrometers,
is required. Often a "sandwich" arrangement of
photoemulsions is used in a number of layers of
films applied close to each other, with or. interleaving with
layers of target material (eg lead). This arrangement is
referred to as the Emulsion Cloud Chamber ( ECC)
- a kind of " photoemulsion fog chamber
" which, after evaluation, provides similar images of
particulate traces as a conventional fog chamber (described
below). The evaluation of photoemulsions in modern
detection systems is fully automated, performed by microscopic
scanning, electronic digitization and computer evaluation. The
largest detection system based on nuclear photoemulsions is
currently OPERA in the Gran Sasso underground
laboratory for the detection of neutrinos and the study of their
oscillations (see §1.2, section " Neutrinos
- " ghosts "between particles
", passage " CNGS + OPERA
").
Mist and bubble
chambers for the detection of trace particle
Wilson cloud chamber
Frst type of detector, providing continuous visible
traces of flight of charged particles, is the Wilson cloud
chamber . It consists of a closed glass cylinder filled
with a gas (perhaps air) with saturated vapors of
suitable liquids - water vapors are used with an admixture of
vapors of organic substances, most often alcohol. Sometimes the
chamber space is also filled with rare gases, such as argon. The
cylinder is provided on one side with a piston or diaphragm, the
displacement of which allows a rapid change in volume and
pressure inside the cylinder. If we perform a rapid
expansion of the working space of the chamber (to about
1.2-1.4 times the original volume), it will occur due to adiabatic
expansion of the gas in the cylinder to drop the
temperature and the saturated vapors present with the
resulting cooling below the dew point become supersaturated
. Supersaturated vapors tend to precipitate in the form of
droplets (mist) on the walls of the vessel, on the other hand on
the dust particles and on the ions which are contained in the gas
and form condensation nuclei to form droplets.
If condensation nuclei are not present (dust-free environment in
the cylinder), supersaturated vapors will last for a short time
without condensation.
If a charged particle passes through such a working
space either just before expansion or during expansion, it will
form a number of ions along its path, which attract and thus
locally concentrate vapor molecules. Supersaturated vapors
precipitate on these ions as condensation nuclei - the path of
the particle is covered by a series of small droplets. With
suitable side lighting, the ionization paths are clearly visible
as light traces on a dark background and can be
photographed in this way.
The supersaturated mist chamber remains sensitive to
particle path registration for only tenths of a second. After
photographically capturing the traces of particles, the chamber
must be reset: the gas in the working cylinder is
back-compressed, the droplets evaporate or flow along the walls
of the cylinder, the steam becomes saturated again. A new one can
then occurexpansion - exposure - compression duty cycle
, which may be repeated periodically.
The length of the nebula and its
"saturation" is characteristic of different types of
ionizing particles and their energy. In order to be able to
derive quantitative parameters of particle motion from the
observed trajectory, stereoscopic images of the
trajectory are taken by two photovoltaic devices oriented at
appropriate angles. Then, the reconstruction of the captured
paths, their accurate measurement and evaluation of the
quantities characterizing the motion and interaction of the
detected particles are performed. To determine the electric
charge of the particles, the fog chamber is placed in a strong
magnetic field and the curvature of the particle paths is
evaluated.
Diffuse fog chambers
The disadvantage of the classical Wilson mist chamber is the
short sensitive registration time of the particles during the
working cycle. Therefore, types of mist chambers operating not
cyclically but continuously - diffuse mist chambers
have been developed . A vertical temperature gradient
is achieved in the working cylinder of this chamber by heating
the upper plate of the chamber with a heating element, while the
bottom of the chamber is cooled, for example, with solid carbon
dioxide (the opposite temperature gradient
can also be used) . The alcohol vapors
generated in the hot part of the chamber diffuse
into the cold part of the chamber. In a certain part of the
chamber space, a zone is formed in which a state
of supersaturated steam occurs, needed to
condense vapors on ions along particle paths. The vapors are
constantly replenished by drops of supplied alcohol (it evaporates in the upper part, the condensed alcohol
is discharged from below) , they diffuse
against the direction of the temperature gradient, so that the
diffusion mist chamber can operate continuously
in a steady state .
Note: The air in the chamber itself contains many ions
and charged dust particles, which would be disruptive (reduce the
contrast or even prevent the visibility of traces); other ions
and charged particles are formed during the actual measurement.
Therefore, a direct voltage (of the order of one
hundred to a thousand volts) is inserted between the lids of the
chamber - the generated electric field cleans the
working space of the chamber from disturbing charged particles.
Fig.2.2.4. Sample photographic image from a bubble chamber. The
primary particle (proton) from the accelerator, arriving from the
left, leaves an ionization trace and then collides to produce
other particles, of which the electrically charged ones again
leave ionization traces. The chamber is inserted into a magnetic
field, so that according to the sign of the charge, the particle
paths are curved to the left (here negative particles) or to the
right (positive particles).
The bubble
chamber for detecting trace particles
is based on a similar principle as the nebula chamber, but uses
the opposite states than the nebula chamber to make ionization
trace particles visible: the formation of gas
(or vapor) bubbles in superheated liquid
along the ionization stage of the particle. Compared to mist
chambers, in which the gas is too thin, the bubble chambers have
the advantage of a higher liquid density
with which high-energy particles can interact better.
Furthermore, it is a faster work cycle speed.
If we heat a clean liquid to a temperature slightly
higher than the boiling point, it may not start to boil
immediately, but it may remain in a superheated liquid
state for some time (a few seconds).; then it starts boiling
violently. If, during the unstable state of the superheated
liquid, before the onset of boiling, a charged particle passes
through the liquid, it forms a number of ions along its path. Microscopic
bubbles of steam begin to form on these ions , which, if
the liquid overheats sufficiently, can grow to macroscopic
dimensions - a sequence of visible small bubbles
is formed along the path . These bubble traces are photographed
, reconstructed and evaluated in a similar way as in fog chambers
under suitable side lighting (discharge flash) (the formation of bubbles along the tracks can also be
monitored in time - the growth of bubbles can be stroboscopically
filmed) .
The first types of bubble chambers were filled with
ether heated to a temperature of about 140 ° C and by regulating
the pressure (approx. 20 atm) a suitable state of superheating
was achieved. Today's bubble troughs are filled with liquid
hydrogen , or deuterium (for monitoring interactions
with neutrons), propane, freon, liquid xenon, etc., according to
the specific type of studied particles and their interactions.
They often reach considerable dimensions of several meters and
contain hundreds and thousands of liters of liquid gas. The
superheat state of the liquid gas is very precisely regulated by
pressure changes by mechanical movement of the piston
(electronically controlled). As the pressure increases, the
bubbles disappear, the boiling stops and the chamber is brought
to its initial quiescent state. By reducing the pressure, a
superheated liquid is created again, particle paths, etc. are
registered periodically. The cyclic phases of the bubble chamber
operation are synchronized with the accelerator duty cycle so
that the particles enter the chamber at a time when the pressure
is momentarily reduced and the liquid is in a superheated state.
Bubble chambers are almost always placed in a strong
magnetic field , so that by measuring the curvature
of the paths due to the Lorentz force, it is possible to
analyze the charge and some other dynamic parameters of the
registered particles - Fig.2.2.4. From the direction of
curvature, we determine whether the particles have a positive or
negative charge, the magnitude of the curvature
(Larmor 's radius) makes it possible to determine its momentum
; if the path of the particle ends inside the chamber, the energy
of the particle can be determined from the range .
For high-energy particles that
fly through the chamber and continue to move, the velocity
of the particle can be determined using two or more
detectors (preferably fast-response scintillators) located at
defined distances in the path of the particles. The particle
velocity can be determined from the time differences (delayed
coincidences) of the pulses from the individual detectors. From
the relationship between velocity and momentum (relativistic) it
is then possible to determine the mass of the
particle, which together with other parameters allows the
particle to be identified.
Photographic
emulsion mist and bubble chambers are in research practice now
squeezed by electronic particle detectors are expensive
, so-called. Trackers ( engl. = Trace track
lane ). The conceptual diagram of such a detector is above
in the section " Arrangement and configuration of radiation detectors " in Fig.1.1.2 below. They are significantly more
flexible, their advantage is high scanning speed and direct
electronic information processing.
2.3. Gas-filled ionization detectors
Ionization chambers
The ionization chamber is the simplest electronic detector of
ionizing radiation; it directly uses the basic property of this
radiation contained in the name - ionizing effects on the
substance . The basic scheme of a simple ionization
chamber is on the left in Fig.2.3.1.
Fig.2.3.1. Left: Schematic representation of the
ionization chamber principle for detecting the flow of ionizing
radiation. Right: Ionization chamber in a well
design as a meter of activity of radioactive preparations.
The ionization chamber consists of two metal
plates (or wires) - electrodes - anode and
cathode, located in a gaseous environment *) and connected in an
electrical circuit to a voltage of the order of hundreds of
volts. Usually a cylindrical arrangement resembling differently
shaped coaxial " cylindrical capacitors " is
used (the electronic function of the
capacitor is not used here!) , Where the
outer metal shell is one electrode and the inner wire or cylinder
is the other electrode
*) The gas filling of the ionization
chamber can in principle also be ordinary air, but special gas
fillings formed by inert chemically stable gases
show better properties, whose molecules are not subject to
chemical changes during radiation ionization and the passage of
an electric current. The most commonly used argon, krypton,
xenon .
Under normal circumstances (without the
presence of radiation) no current passes through the system - the
gas between the electrodes is non-conductive, el. the circuit is
not closed. However, when ionizing radiation enters the space
between the electrodes, it ejects electrons from the originally
neutral gas atoms and converts them into positive ions. Negative
electrons travel in the electric field immediately to the
positive anode, positive ions are set in motion to the negative
cathode - a weak electric current begins to flow through the
circuit caused by the ionic conductivity of the ionized gas
between the electrodes. The current measured by the microammeter
is directly proportional to the intensity of the ionizing
radiation; can be calibrated in units of radiation intensity or
dose rate (Gy / s). Thus, the detection of the
flow of invisible ionizing radiation is realized by converting it
into a measurable magnitude of the electric current
circumference of the ionization chamber. The electrical signal
from the ionization chamber is measured in the evaluation circuit
either as an ionization current - ionization
chambers with continuous ionization (these are discussed in this section) , or a short current or voltage pulse -
pulse ionization chambers (described
below in the section " Geiger-Müller
detectors"). "and in the
section" Proportional detectors ") .
The electric current flowing through the ionization
chamber is generally very weak (approx. 10 -16 to 10 -9
A) - the ionization chamber has a low sensitivity
(low detection efficiency), so it is not suitable for detecting low flux
radiation. However, its advantage is the linear dependence of the
current even in the area of ??high intensities of ionizing
radiation. The ionization chambers therefore have a very
good linearity of response to the intensity of the
detected ionizing radiation in a very wide range. It is therefore
used, for example, to measure the distribution of intensity (dose
rate) in radiation beams in radiotherapy. The most common use of
the ionization chamber is in dosimetry for
measuring the dose of ionizing radiation.
Activity meters with a well
ionisation chamber
Ionisation chambers in a well design are often used in activity
meters for radioactive preparations (these meters are sometimes incorrectly called
"dose calibrators") - Fig.2.3.1
on the right. It is a hollow cylindrical chamber filled with an
inert gas (mostly argon) , in the center of which is formed an inter-cylindrical
depression - a " well " for the
insertion of the measured radioactive sample. Inside the chamber
are electrodes to which a direct voltage (several
hundred volts) is applied. Until
radioactivity is present, the gas is not ionized and virtually no
electric current passes - except for a slight current caused by
the ubiquitous radiation background. There should be lead shielding
around the chamber to reduce unwanted background .
The vial or syringe with the measured
radioactive substance is inserted into the opening of the well
ionization chamber, which then registers the outgoing gamma
radiation in a geometry close to 4 p . The activity of the
measured radionuclide determines the intensity of gamma radiation
in the chamber space, and thus the density of gas ionization and
the value of the electric current between the electrodes. The
electrical signal I from this chamber is then proportional
to the activity of preparation A and G - constant of the given
radionuclide: I ~A. G . The gamma-constant is different for each radionuclide,
it depends mainly on the number of photons emitted per
radioactive conversion and on the energy of these photons. The
electronic circuits of the activity meter are calibrated
(via the G- constant) so that for the selected radionuclide the display shows
its activity directly in [MBq] *).
*) In order for this measured activity
value to be correct and accurate, a careful metrological
calibration is required for the activity meters . The result
of this calibration is the values ??of the " isotope
factors""individual radionuclides which
are multiplied by the measured ionisation current when obtaining
the activity [MBq]. These multiplication factors are stored in
the memory of the device and correct typing species measured
radionuclide apply.
measurers activity
studnovou ionization chamber have very good linearity
to high activity hundreds of TBq (at our workplace we measured the linearity of several
types of activity meters up to about 40GBq, we did not have
higher activity .) Common activity meters
(such as Curiementor or Bqmetr) with measuring
times around 5-10 sec. are able to reliably measure activities up
to about 100kBq. Only those activity meters that have the option
of extending the measurement time for low activities (such as Mediac
by Nuclear Chicago )
are able to measure activities of the order of kBq units - but
the measurement can take several minutes. Well scintillation
detectors must be used to measure samples of even lower
activities (see below §2.7 " Measurement
of sample radioactivity
") .
The detection efficiency of the well
ionization chamber, and thus the accuracy of measuring the
activity of the preparations, significantly depends on the
position of the sample in the well, the sample volume and the
absorption of gamma radiation inside the sample, in the vial
walls and the inner wall of the ionization chamber. Problems of
positional and volume dependencemeasurement in
the well detector is discussed in §2.7, passage " Detection efficiency t",
Fig.2.7.2. For these circumstances, it is necessary to apply the
appropriate correction - multiplication of the measured value by
an experimentally determined correction factor .
Well ionization chambers are basically
used to measure the activity of radioisotopes emitting g radiation - either
pure gamma emitters (such as 99m Tc ) or
mixed b + g (such as 131 I ) or a + g . Even pure beta
emitters with higher energies (such as 90 Y) can be measured in an emergency despite
the resulting braking radiation, but with significantly worse
accuracy, significantly dependent on geometric influences. Also
for radionuclides with low gamma radiation energy (such as 125 I ) , the measurement accuracy
deteriorates due to absorption. The main use of activity meters
with a well ionization chamber is for the measurement of radiopharmaceuticals
applied to patients in nuclear medicine for
scintigraphic diagnostics and radionuclide therapy (§4.8 " Radionuclides and
radiopharmaceuticals for scintigraphy ") .
Electroscope
The historical predecessor of the ionization chamber was the leaf
electroscope used in electrostatics. This simple device
consists of a vertical insulated metal rod to which a thin metal
sheet is conductively attached. If we apply an electric charge to
this system, due to the repulsive electric force (the rod and the
ticket are uniformly charged) the ticket will deviate from the
rod; the angle of deflection depends on the size of the charge.
If the air were a perfect insulator, the charge of the
electroscope would not change and the angle of deflection of the
leaf would remain the same for an indefinite period of time. In
reality, however, a certain amount of ions is contained in the
air, so that the electroscope slowly discharges and its leaf
gradually returns to its original suspended position. If the air
around the electroscope is exposed to ionizing radiation, the ion
concentration will increase significantly and the electroscope
will discharge significantly faster:
With the help of these electroscopes, many important
experiments were done in the early researches of ionizing
radiation and radioactivity. Until recently, the simple principle
of discharging the electroscope was maintained in pencil
personal dosimeters , where the ticket was replaced by a
thin fiber ("fiber electroscope"), which also served as
a hand for immediate reading of the radiation dose.
Electret detectors
This is a simple variant of gas ionization chambers, in which the
electric field is not excited by an external electronic voltage
source, but by a so-called electret *). The
charge of the electret creates an electrostatic field in the
chamber. Radiation entering the chamber (or generated by
radioactivity directly inside the gas charge - air) ionizes the
gas and the resulting negative electrons are attracted to the
electret, which is gradually discharged. The
rate of discharge of an electret is directly proportional to the
amount of radiation (radiation dose) that has entered the chamber
and been absorbed there. The discharge of the electret (ie the
difference in polarization charge before and after exposure) is
measured via the electrical voltage between the electrodes of the
respective evaluation device ("reader"), where the
electret removed from the exposure chamber is inserted. Electret
detectors work in a cumulative (integral) mode in the
sense of the classification in §2.1 .
*) Electret is
such an electrically non-conductive substance - dielectric
, which maintains a permanent electrical polarization
even after the removal of the external electric field. It is the
electrical equivalent of a permanent magnet .
These simple detectors are mainly used to
measure the average concentration of radon in the field
(in the interiors of buildings) - in the air that forms the
"gas filling" of the chamber. After several days of
exposure, the discharge of the electret is evaluated, which is
directly proportional to the concentration of radon in the
object.
Electrical properties of
the ionization chamber
For a better understanding of the operation of individual types
of ionization detectors, we will only briefly mention the
electrical properties of the ionization chamber. The dependence
of the ionization current on the voltage between the electrodes
of the ionization chamber is schematically shown in Fig.2.3.2 -
we assume a constant intensity of flux of quantizing radiation.
![]() |
Fig.2.3.2.
The dependence of ionization current I to the chamber on the applied voltage U . |
This dependence, sometimes called the "volt-ampere characteristic" of the ionization chamber, can be divided into three areas:
Geiger-Muller
detectors
The Geiger-Müller (G.-M.)
detector is an ionization chamber, hermetically sealed, filled
with a gas with a pressure usually lower than atmospheric
(however, there are also chambers with a higher pressure); works
in pulse mode . The electrodes of this chamber
are connected in the electrical circuit to such a voltage that
the chamber works in the area III B of the characteristic in Fig.2.3.2 (voltage is about
600-1000V), in the so-called Geiger mode . Schematic
chart of G.-M. detector is in Fig.2.3.3.
Fig.2.3.3. Schematic diagram of Geiger-Müller detector. Some shapes and designs of G.-M.
detectors ; Right:
G.-M. (or proportional) detectors in a planar arrangement as a
meter of radioactive contamination ("iron" shape).
Upon entry of a quantum of ionizing
radiation, ionization occurs in the gas , after
which the electrons begin to move toward the anode and positive
ions toward the cathode. Because the gas is diluted or the
voltage at the electrodes is high enough, the mean free
path of each electron is long enough to gain such kinetic
energy in the electric field that it is able to eject
additional electrons (and ions) when it strikes
a gas atom. These secondary electrons then emit other secondary
electrons, etc. This process of secondary ionization is avalanche
(up to 10 10 secondary electrons are formed from one primary
electron) - an electric discharge is generatedin
the space between the electrodes. A relatively strong current
pulse passes through the circuit and a relatively high voltage
pulse is generated on the working resistor R ,
which is processed via a separating capacitor C in the
appropriate electronic unit (amplifier,
counter, integrator) - the quantum of the
respective ionizing radiation is detected by
conversion to electric impulse . The resulting
electrical impulses have the same size and
shape, regardless of the type and energy of the detected quantum
- the GM detector has no spectrometric properties.
The discharge that occurs when a particle
is detected in the space between the electrodes must be
interrupted as soon as possible, because no other particles can
be registered during the discharge (prolonged
discharge could also damage the gas charge of the detector and
the electrode itself!) . Two circumstances
are involved in the interruption of the discharge
. The first is the voltage drop across a
relatively high operating resistance R (of the order of M W ), which reduces
the voltage at the electrodes and reduces the production of
secondary electrons. However, in the ionized gas charge, ions
recombine and deexcite the excited atoms, emitting ultraviolet
photons. Photons of UV radiation are able to ionize and eject
additional electrons from the cathode from the cathode, which
tends to prolong the discharge. Therefore, a quenching
agent (usually methyl alcohol, bromine vapors, etc.) is
added to the gas charge , the molecules of which absorb
ultraviolet photons and thus contribute to the rapid interruption
of the discharge.
GM gamma detectors are most often designed
as cylindrical tubes filled with a suitable
inert gas. The inner metal wall of the cylinder serves as a
negative electrode, a positive electrode in the form of a wire is
guided in the middle of the tube. GM beta detectors have a
different design with a thin inlet window at one
end of the tube.
Detection efficiency
G.-M. detectors
it generally depends on the walls (or inlet window) of the
detection tube and on its gas filling. They differ diametrically
for charged particle radiation and for photon radiation.
For heavier charged particles
(eg for alpha radiation) and for electrons, the detection
efficiency is close to 100% , provided that they
reach the gas charge, ie the sensitive volume of the detector. In
order to penetrate there, the entrance window must be made of the
thinnest possible light material; there is talk of
"windowless" detectors.
For photon radiation X
and especially gamma, the detection efficiency of the gas charge
itself is very low , due to its low absorption
in the gas. The vast majority of g-photons pass through a
sensitive volume of gas without interaction. Photons with higher
energies can be detected by a gas-filled detector practically
only if they interact with the wall material of the
detection tube. Then some electrons released by
Compton scattering or photoeffect penetrate the gas charge, where
they are already effectively detected. The detection efficiency
of GM photon radiation detectors therefore depends on the
material and wall thickness of the tube (most
often a few tenths of a millimeter thick aluminum is used) , of course in relation to the radiation energy. For
medium energy photon radiation, the detection efficiency is
usually about 0.1-10%.
The use of G.-M. detectors
G.-M. detectors played an important role in the development of
nuclear and radiation physics - it was the first type of
detectors that could register individual quanta of
ionizing radiation, not just the intensity or flux of radiation,
as is the case with ordinary ionization chambers. Even today,
G.-M. detectors used for their simplicity , but
mostly only for less demanding measurements. E.g. in radiation
protection they are contamination meters ,
radiation detectors, monitoring systems , etc.
For more accurate and demanding measurements, they were replaced
mainly by scintillation and semiconductor detectors, which are
many times more expensive, but have significantly better
parameters in all respects (see below §2.4
" Scintillation detectors",
§2.5" Semiconductor detectors ") .
Dead
time of detectors
It is clear that for the duration of the avalanche discharge in
G.-M. the detector is insensitive to other incident quanta.
Similarly, for other types of detectors (scintillation,
semiconductor) there is a certain period of insensitivity
, during which the device is "busy" by processing the
response from the currently registered quantum. The time from the
registration of one pulse during which the detector is not able
to register further pulses is called the
detector dead time , denoted by t or DT ( Dead
Time ) and is measured in microseconds *). An alternative
name is the time resolution of the detector
("computer" of quantum radiation). In G.-M. detectors,
the dead time is of the order of 10 -4 seconds, ie t @ 100 m s (which is a relatively long dead time!), For
scintillation detectors it is often shorter than 1 m s.
*) For radiometers and spectrometers, the
dead time, resp. total dead time loss - Dead
Time is often expressed in % of
the total measuring time.
Dead time in general (also applies to other types of detectors - scintillation, semiconductor)
The
time interval from the detection of one quantum during
which the detector is unable to detect another quantum is called the detector dead time . |
Dead time causes not all interacting quantum of
radiation to be detected, but there is some loss of
detected pulses, which loss due to dead time increases
with the frequency (flux) of the measured radiation
quanta. This violates the linearity of the response of
radiometric instruments in the higher frequency range. Instead of
the actual average input (theoretical) frequency N
[imp./s] of the incoming radiation quanta, we measure the
registered pulse frequency n [imp./s], where n <N.
Functional dependence of n = n (N) registered pulse frequency n
on actual (theoretical) frequency N expresses response
functiondetector in terms of the measured number of
pulses. According to the nature of this dependence, dead time is
sometimes divided into two types: non-paralyzable and paralyzable
(cumulative).
¨ The above dead time is so-called non-paralyzable
, characterized in that during this dead time the detector does
not register incoming particles, these particles have no effect
on its operation and after the dead time the detector is
immediately ready to detect another pulse. The dependence between
the registered n and the actual (theoretical) pulse
frequency N is given by the relation *)
n
= N / (1 + N. t ) .
With a linear increase in radiation intensity Nthe
registered frequency of pulses n first increases
practically also linearly (in practice with
the coefficient given by the detection efficiency, which we
consider here as 1) , then the growth
begins to slow down and at high frequencies N >> 1 / t almost no longer
increases and reaches saturation: lim N
® ¥ n (N)
= 1 / t (Fig.2.3.4
left).
Fig.2.3.4. Influence of detector dead time on detector response
function. Left:
Non-paralyzable; Right: Paralyzable.
¨ Paralyzable dead time (also called cumulative
) is such that during it the detector not only does not register
other particles, but each such particle that flies into the
detector during the dead time, prolongs its insensitivity by the
same time - "paralyzes" the detector, dead time
"cumulates". In other words, each pulse entering the
detector generates a dead time t , regardless of whether or
not it is registered. The dependence between the measured and the
actual pulse frequency is here *)
n
= No - N. t .
As the input frequency of the particles increases, the response
first increases linearly again, then slows down and reaches a
peak, after which, as the input frequency increases further, the
response of the detector begins to decrease (Fig. 2.3.4 on the
right). For too high frequencies N , the detector even
stops counting pulses completely: lim N
® ¥ n (N)
= 0 - the detector is "paralyzed" (flooded).
*) The general mathematical
derivation of the influence of dead time on the
functional dependence of n = n (N) between the input
(theoretical) and measured (registered) pulse frequency is based
on a statistical analysis of the time
distribution of incoming pulses according to the Poisson
distribution. If the detected quantum comes with the actual
(theoretical) average frequency N, then the probability of
occurrence of the pulse in the time interval dt is N.dt, while
the probability that the pulse falls outside the time interval t is P ( t ) .dt = No -N.
t .dt. More complex manipulations with integrals t ò ¥ P ( t ) dt and time intervals can then be used to derive the
corresponding functional dependences n = n (N).
In the case of a non-parillable
detection system, this derivation can be substantially simplified
by a simple consideration from the "opposite side":
Each registered pulse is accompanied by a detector insensitivity
time equal to the dead time value t . Therefore, if we register
with the non-paralyzable detection system npulses per
second, then for input (theoretical) pulses of frequency N
there is an effective shortening of the measuring time
from 1 second to (1-n. t ) - seconds. This means that instead of theoretical N
pulses we measure n = N. (1-n. T ) pulses per running
second. From this, a simple modification gives the resulting
functional dependence n = N / (1 + N. T )
.
For a paralyzable detection system, the dependence is
nonlinear, so a differential analysis must be used. As the
incoming frequency of quanta increases by D N, the registered frequency
changes by D n = D N- D N. t. ........ ....... .............
Dead time
sources
In general, all parts of the
detection system contribute to the dead time effect: own detector
- GM tube, scintillator, photomultiplier, semiconductor detector;
preamplifier or amplifier; amplitude analyzer; pulse counter;
analog-to-digital converter. For scintillation detectors, one of
the causes of pulse loss is the "dead time" so-called pile-up
effect (discussed below in §2.4 " Scintillation
detectors ", section " Scintillation spectrum ").
With a GM detector, the relatively long dead time is
given by the detection principle and cannot be shortened too
much. However, for scintillation and semiconductor detectors, technical
developments in the field of electronics and materials
have led to a significant reduction in dead time..
While in the past (60s) the dead time of scintillation detectors
was about 5-10 m s, in the 80s and 90s this value was reduced to about 1 m s due to the use
of fast electronic components . The slowest link in the detection
chain gradually becomes the scintillator itself
. For this reason, in some devices, NaI (Tl) or BGO scintillation
crystals are replaced by faster scintillators
based on rare earth doped silicon oxides, especially LSO (see
" Scintillators and their properties
" below ). Gradually, the photomultiplier
becomes the limiting factor of dead time- however, it will be
replaced in some applications by special photodiodes (see below
" Photomultipliers ", fig.2.4.2G ).
................. forced dead time ...- add ....
?? ........
Measuring dead time
There are basically three ways to
measure the dead time of a detector (if we have a shielded
detector, we can neglect the background) :
Note:
For all these dead time measurements, there is a risk of large
systematic errors due to other physico-electronic
influences that can simulate dead time ! Before
measuring the dead time and interpreting the results, it is
recommended to examine the following two circumstances:
a) Dependence of the position of the photopeak
on the frequency of pulses - determine the position of the
photopeak for the range of input frequencies about 10 2 -10 6 imp./s.
b)Temporal stability of photopeak position at
high frequencies. Some scintillation detectors show a kind of
"fatigue effect" of the photomultiplier: at high
electron fluxes, the photomultiplier gradually decreases its gain
(mostly reversible), which is manifested by a gradual decrease in
the position of the photopeak. We recommend loading the detector
with a pulse flow of approx. 10 6 imp./s. and for about 60min. monitor the position of
the photopeak.
Partial leakage of the photopeak position from the
analyzer window leads to a reduction in the detected pulse
frequency, which simulates the effect of dead time.
Dead time
correction
If we measure such high frequencies of radiation quantums that
the dead time of the detector is significantly applied, it is
necessary to make a correction for this dead
time in order to obtain objective and accurate results. To
perform this correction, it is of course necessary to know the
specific value of the dead time for the given detector, ie the
measurement must be performed according to the previous
paragraph. If it is a non-paralyzable detector, we perform a
correction for the dead time, ie determining the actual frequency
N based on the measured frequency of n pulses,
according to a simple relation N = n / (1 - n. T ). In general,
dead time correction can be performed by applying an inverse
relationshipbetween theoretical and actually registered
pulse frequency; we can also use the measured dependence between
the theoretical and registered frequency, obtained by the
above-mentioned method of continuous change of pulse frequency.
In the case of a paralyzable dependence, the correction can in
principle only be performed for lower frequencies in the
ascending area of ??the graph, for the descending area it is
usually not possible to make the correction (the correction
becomes ambiguous).
Proportional
detectors
Proportional detectors also use secondary ionization,
but due to the lower voltage there is no avalanche
microdischarge, but they work in the IIIA region on the
volt-ampere characteristic according to Fig.2.3.2 - in the
proportional region. The gain coefficient is about 104-104,
the dead time is usually of the order of 10-6s.
The connection and design is similar to G-M detectors, they work
in pulse mode. The output voltage pulses are proportional
to the energy of the detected radiation (more precisely,
the energy absorbed by the interaction of the quantum of
radiation with the gas charge), so that these detectors can in
principle be used for spectrometry, although their resolution
does not reach to scintillation or semiconductor detectors.
...............
Spark
detectors
For some applications, detectors operating in the last (highest)
area of ??the volt-ampere characteristic of the ionization
chamber according to Fig.2.3.2 - in the area of ??the spark
discharge - are also seldom used . If a voltage is
applied to the electrodes of the ionization chamber only slightly
lower than the breakdown voltage leading to spontaneous discharge
and spark jump, nothing will happen at rest. However, if a
quantum of ionizing radiation enters the space between the
electrodes, it will immediately initiate a jump of the
spark , which is manifested both by the passage of a
strong electrical impulse through the circuit and by the
appropriate light and sound of the spark discharge.
Spark detectors in an arrangement with many
electrodes distributed over a larger area form so-called spark
chambersfor the registration of traces of
charged particles (ie for a similar purpose as, for
example, bubble chambers - cf. §2.2). The spark chamber consists
of a large number of electrodes in the form of plates or wires,
to which a voltage of approx. 10 kV *) is applied. The charged
particle, which passes through the spark chamber, by its
ionization gradually causes spark discharges in
the individual sections of the chamber between the electrodes ,
which follow the trace of the particle in the
chamber. On the one hand, these discharges are visible
and can be captured photographically (eg stereoscopically by two
cameras); however, they are also audible and can
be registered electroacoustically, for example, via piezoelectric
sensors. After registration, an electric field is applied to
remove the generated electrons and ions, and the measurement
cycle can be repeated. The spark chambers are able to operate in very
fast cycles , while high voltage can be applied to the
electrodes for a correspondingly short time via gate circuits
triggered by synchronization detectors monitoring the primary
particle beam before entering the spark chamber (so-called triggering
).
*) High voltage is applied either between
adjacent electrodes (with opposite polarity of even and odd
electrodes) or between electrodes forming eg a cathode and a base
plate forming opposite polarity, eg anode.
Drift
ionization chambers
When an ionizing particle passes through a gas-filled chamber,
the released electrons and ions do not reach the collecting anode
and cathode immediately, but "penetrate" the gas - they
drift at a certain speed (depending on voltage,
distance from anode and gas density) to anode or cathode . The
time of electron drift to the anode carries information about the
position of the place where the particle passed
in the tube and where it caused ionization. The drift chambers
have several electrodes in the shape of wires or strips, and
according to the place - electrodes - where the electrons travel
and the time of drifting, the course of the path of the ionizing
particle in space can be determined. The most perfect detectors
of this type are multi-wire drift chambers ,
composed of a large number (even several thousand) wires -electrodes
, stretched in a gas filling in several layers (in each layer the
wires are stretched in a different direction - they form a
spatial grid). As the charged particle passes, ionization occurs
along its path. Electron clouds drift at each location to the
nearest electrodes, where an electrical signal is generated. The
intersections of the electrodes from the different layers that
have thus received the signal indicate the passage points of the
detected particle. The ionization cloud of electrons can reach
several nearby electrodes; the evaluation electronics then
determine the coordinates using the weighted averages of the
signals from the various electrodes. The location of the charged
particle can be determined with an accuracy of about 0.1 mm.
Multi-wire drift ionization chambers are now used in
complex detection systems (such as above in Figure 2.1.3 below)
in accelerators. As a rule, several layers of chambers placed in
a strong magnetic field are used. The magnetic field changes
(curves) the paths of charged particles, depending on the charge
and momentum of the particle. From the changes in the path of the
particles registered in the individual layers of the chambers, it
is then possible to determine the momentum of the
respective particle (if we know its charge). The more accurately
we can measure the momentum of secondary particles flying out of
the point of interaction, the more accurately we can determine
the rest masses and other characteristics of the resulting
investigated particles.
2.4.
Scintillation detectors
Scintillation detectors of ionizing radiation are based
on radioluminescence - the properties of some
substances react with light flashes or scintillations
(lat. Scintilla
= spark, flash ) to absorb quantums of ionizing radiation; these flashes
are then electronically registered using photomultipliers
. Substances exhibiting this property are called scintillators
. The oldest used radioluminophore is zinc sulfide
activated with silver ZnS (Ag), from which the screens of
sciascopic X-ray devices were used, in the past platinum-barium
cyanide was also used.. However, for the purpose of
detecting g radiation , thallium - activated sodium
iodide - NaI (Tl) , in the form of a single
crystal , is most often used . Other scintillators (including
liquid scintillators) will be listed below, where the mechanism
of scintillation formation will also be discussed (see section " Scintillators and their properties ") . Fig.2.4.1 schematically
shows the basic principle of operation of a scintillation
detector / spectrometer.
Fig.2.4.1. Schematic diagram of a scintillation detector with a
fixed scintillator. On the left , the
formation of scintillations in the crystal, the emission of
electrons from the photocathode and their multiplication by the
dynode system are shown.
In the middle part is the electronic
processing of the generated signals. The upper branch of the
scheme represents simple detection using a
"single-channel" amplitude analyzer and pulse counter.
The lower branch of the diagram shows spectrometric measurements
using an analog-to-digital converter and computer acquisition of
the energy spectrum ("multichannel" analyzer). The
typical shape of the scintillation spectrum of gamma radiation is
plotted on the screen - compared to the actual line spectrum
above.
Right is assembled a scintillation
detector (probe) with a crystal, light-tightly encapsulated
photomultiplier and a resistive divider.
The quantum of the measured invisible radiation
penetrates into the scintillation crystal material, where it
interacts with the substance (eg the most
frequently detected gamma radiation is a photoeffect, Compton
scattering or electron-positron pair formation, as explained in
more detail in §1.6., Section " Interactions gamma radiation and X ") . Due to these
interactions, the ionizing quantum is partially or completely absorbed
and part of its energy is converted in the scintillator into a flash
(scintillation) of visible light (Fig.2.4.1 top left).. The
resulting scintillation consists mostly of several hundred of
these secondary photons, depending on the absorbed energy of the
primary detected quantum and the conversion efficiency of the
scintillator. The total number of scintillation photons is directly
proportional to the energy of the detected quantum
absorbed in the scintillator.
A photomultiplier is
optically attached to the scintillation crystal - a special
optoelectronic component that converts scintillation light into
an electrical signal with high sensitivity. The construction and
properties of photomultipliers are discussed below in the section
" Photomultipliers ". In the classic design, the photomultiplier is a
vacuum tube, on the entrance window of which a thin metal layer
is applied from the inside - a photocathode (thickness approx. 10 -7cm, the material is cesium and antimony with low
electron output) , which converts light
photons into electrons. Furthermore, the photomultiplier contains
a system of electrodes - the so-called dynod
(their number is about 8-12), which acts as an electron
amplifier . There is, of course, a high vacuum inside the
entire tube. Positive voltage is applied to the individual
dynodes - they gradually increase and increase for each dynode.
The photons from the scintillator light flashes are incident on
the photocathode from which fotoefelektrickým phenomenon
sprout electrons e -. Each such electron in an electric field begins to move
rapidly to the first (nearest) dynode, to which a positive
voltage of, say, about 100V is applied. It hits this dynode with
a kinetic energy of about 100eV, which causes at least 2 or more secondary
electrons to be ejected from the metal surface of the
dynode. These electrons set out on the path to the next dynode,
on which there is a higher positive voltage - about 200V. The
energy to which it accelerates (given by
the voltage difference, here again about 100eV) , again ejects 2 or more secondary electrons for each electron - so we
already have at least 4 electrons, which move to the next dynode,
where twice more electrons, etc. . Thanks to this repeated
multiplication was originally a small number of
electrons released from the photocathode too is
multipliedand about 10 5 -10 8 electrons fall on the last dynode (already anode) ,
which is already a sufficient number to induce a well-measurable electrical
pulse of amplitude A on the working resistance R (has a value of the order of magaohma) in the electrical circuit. This pulse is fed via an
isolating capacitor C to an amplifier and other electronic
circuits for processing.
Special photodiodes or
arrays of matrix photodiodes (" semiconductor
photomultipliers ") can also be
used to register and convert light scintillations into electrical
pulses., or hybrid combination of a tube photomultiplier with a
photocathode and subsequent semiconductor registration of
photoelectrons - see the section " Special
types of photomultipliers " below .
Thus, a scintillation detector works in
this way, which can be used either separately
(one detector) or can be part of multidetector systems
(cf. the section " Arrangement and configuration of radiation
detectors " above,
Fig.2.1.3) . Systems of a large number of
elementary detectors and opto-electronic elements (sometimes
several thousand) are used for electronic radiation imaging
- §4.2 " Scintillation cameras " and §3.2 "X-ray diagnostics", part
"Electronic display of X-rays
", specifically it is a so-called flat-panel with
indirect conversion of X-rays.
The design of the scintillation
crystals and photomultiplier
Scintillators
can be inorganic crystals, organic plastic materials, liquid
solutions of organic substances, respectively. and rare gases.
Here in the basic text we will consider inorganic
scintillators , plastic and liquid scintillators will be
described below (§2.6 " Detection and
spectrometry of radiation a and b . Liquid scintillators
"). The mechanisms of scintillation formation and the
properties of scintillation materials will be discussed in the
section " Scintillators and their properties
". The most commonly used inorganic scintillators are
thallium-activated sodium iodide crystals - NaI (Tl)
.
For general
detection and spectrometry of g-
radiation , planar scintillation crystals of
cylindrical shape with a diameter of about 2-7 cm and a height of
about 2-8 cm are used . For the
detection of soft g and X radiation, thin
crystals 1-5 mm thick with a thin aluminum or beryllium
entrance window. On the other hand, large-volume
scintillation crystals (approx. 20 ´ 15 cm and larger)
are suitable for the detection of high-energy radiation g . In addition to the generally used planar
cylindrical scintillators, well or transversely
drilled scintillation crystals with a hole for measuring samples
in test tubes are also produced (see below §2.7 " Measurement of sample radioactivity").
For the measurement of larger volumes of sample
are used in bulk studnové scintillation
detectors with a diameter of 18 cm and a height of about 12 cm
with a volume measuring the well area of approximately 250 ml.
Scintillators for particular purposes (such as positron emission
tomography, PET or spectrometers-calorimeters detection systems
for accelerators) will be given at the appropriate places in the
description of its methods.
Scintillator NaI (Tl) is placed in
a light tight aluminum housing which protects
the crystals particularly against the penetration of external
light into the photomultiplier and also from moisture from the
air (NaI is hygroscopic and may cause its
hydrolysis direct humidity, see below " Inorganic scintillators
", fig.2.4.8). At the bottom of the housing is a
transparent glass exit window through which
scintillation photons penetrate the photomultiplier
. Photons from scintillation flashes are emitted isotropically in
all directions, often outside the output window and photocathode
of the photomultiplier. The inner sides of the scintillator
housing are therefore provided with a white reflective
layer which reflects light photons on the photocathode
of the photomultiplier.
By simply applying a scintillation
crystal to the photomultiplier, scintillation photons would be
lost by total reflection in the air layer between the two
glasses, the crystal and the photomultiplier. The space between
the scintillator outlet window and the photocathode is therefore
filled with light guide material, silicone
grease (with a refractive index approximately the same as that of
glass) is most often applied, ensuring good optical
contact of the photocathode with the crystal. If the photomultiplier and the scintillator are further
apart, they are connected by a light guide , in special
cases optical fibers are also used .
Photomultipliers
are special opto-electronic components for sensitive
detection of low light fluxes and their conversion into
electrical signals. Conventional photomultipliers are vacuum
glass tubes containing a photocathode inside and several dynodes,
to which a voltage of several hundred volts is applied. High
sensitivity is achieved by the small number of electrons emitted
by the impact of photons on the photocathode (due to the
photoelectric effect) being multiplied by the
repeated punching and acceleration of the secondary electrons.
The signal is thus amplified so that even the impact of a single
photon of light can cause a well-detectable electrical impulse.
Photomultipliers are used not only in scintillation detectors,
but also in spectrophotometry, luminescence detection(of physical, chemical or biological origin) , detection of Cherenkov radiation, electron and mass
spectrometry and in other technical applications.
Note 1: The name
" photomultiplier " is somewhat misleading, it
does not multiply photons. Rather, it is an " electron
multiplier " in which the secondary electrons
released by the photoeffect from the photocathode are multiplied.
Note 2: First we will deal with
"classic" types of photomultipliers with photocathode
and dynodes. Special photomultipliers, including semiconductors,
will be mentioned below. Classic
photomultipliers are special vacuum tubes in
which electrons are generated by photoemission from a photocathode
. Such a photomultiplier - PMT ( PhotoMultiplier
), consists of a glass bulb equipped at one (front) end with an
optical input window with a photocathode, inside
it contains a series of electrodes - dynodes -
connected to the terminals on the socket at the other end of the
photomultiplier. Photomultipliers with a side input
window are seldom used . The
photocathode consists of a very
thin layer (approx. 10 -7 cm thick, optically semi-transparent) vapor-deposited
on the inside of a transparent input window, working in transmission
mode (unlike the photocathode of a
photon, which works in emission mode; photocathode mode
even with photomultipliers) . The
photocathode must be sufficient thin so that the
electrons released by the photoeffect can easily fly out and not
be absorbed in the material. The material of the photocathode is
substances with low electron output for the photo effect. The
most common are antimonides of alkaline elements
, eg cesium and antimony Sb-Cs (SbCs 3 ), bialkaline
materials Cs-K-Sb, Cs-Rb-Cs, Na-K-Sb, then Ag-O-Cs, or
multialkaline Na -K-Sb-Cs. Photocathodes of p- type
semiconductor materials with a suitable band structurehave
also been developed, the surface of which has a negative electron
affinity, so that light-excited electrons penetrate the
conductivity band very easily out into the vacuum.For
example, a gallium-arsenide photocathode GaAs or InGaAs is used.
An important parameter is the so-called quantum
efficiency of the photocathode, which indicates the
percentage ratio of the number of emitted electrons to the number
of incident photons of light. This efficiency depends on the
material of the photocathode, and also significantly on the
wavelength l of light (photon energy h. N = hc / l ) - spectral
sensitivity *) of the photocathode. For optimal
detection, it is desirable that the luminescent spectrum of the
scintillator sufficiently overlap with the maximum spectral
sensitivity of the photocathode. If part of the spectrum extends
into the ultraviolet region (as is the case, for example, with
Cherenkov radiation, see below), it is desirable to make the
input window of the photomultiplier from quartz glass.
*)The spectral sensitivity of the
photocathode is limited from below (for larger
wavelengths l of light) by the condition that the energy h. N = hc / l of the photon is
higher than the output energy of the electrons from the
photocathode material. From above (for shorter wavelengths - UV
radiation) it can be limited by the optical transmittance of the
window; therefore, this window is sometimes made of quartz
glass with better UV transmittance. For high
photon energies (X and g ), the probability of a
photoeffect in the thin film layer of the photocathode is very
small - the photomultiplier is unusable for direct
detection of this radiation . Using the conversion of
hard radiation into flashes of light inscintillators
, on the other hand, detection is very effective
.
Dynodes
The weak electron flux from the
photocathode is further amplified by the secondary
emission of electrons on the dynodes . A thin layer of
metal with a low electron output work (most often SbCs or BeO)
and thus a high secondary emission factor S , like a
photocathode material, is deposited on the surface of the dynodes
. The mean number of ejected secondary electrons from a dynode is
proportional to the energy of the incident electron. The
current gain D I of one multiplication stage (one dynode) is thus D I = S. D U, where D U is the interdynode
potential . At the usual value of the coefficient S » 0.04-0.06 and the voltage difference used between
dynodes D U » 80-100V, the gain of one stage is approx. D I » 3-6. By repeating
the electron multiplication process between the dynodes, a large
total gain G (up to 10 8 ) of the initially very weak current from the
photocathode can be achieved (for the number N of dynodes,
the total gain G = D I N ). The multiplication system, consisting of approx.
8-12 dynodes, is terminated by a collecting electrode - anode
- with the highest positive potential, from where the output
signal is taken via the working resistor R.
The electrical leads of the photocathode and dynode are
not led "sideways" (with a few exceptions) , as shown in Figures 2.4.1 and 2.4.2 for clarity, but
are all led "down" (at the
opposite end to the photocathode) in many
pin socket . A high-voltage source
(with a voltage of approx. 1000-2000V) and a resistive
divider (exceptionally a diode
cascade multiplier) are used to supply
the dynodes and anodes of the photomultiplier . In the
last three dynodes, the resistors in the divider are usually
bridged by capacitors to ensure voltage stability in pulse
operation. See also " Electronic power supply for
photomultipliers ... " below for
some electronic aspects .
The photomultiplier is drawn on the left in Fig.2.4.1
only schematically and simplified. The dynodes in a given
arrangement actually have a concave curved shape
(suitable shaping of the spatial distribution of the electric
field potential), ensuring the focusing of
electrons on the next dynode. Between the photocathode and the
first dynode, a grid ( diaphragm ) is
sometimes placed , the positive voltage of which accelerates the
emitted electrons and directs them to the dynode. In addition to
the linear arrangement of the dynodes ( Fig.2.4.2.A
), a compact circular arrangement of the dynodes
(again in a focused shape) into a cylindrical surface ( Fig.
2.4.2.B ) is sometimes used . Lamellar
dynodes are often used in photomultipliers (in the shape of
"blinds" - unfocused), are placed in layers on top of
each other; through the gaps between the lamellae
("blinds") the ejected electrons pass to the next
dynode with oppositely oriented lamellae and a higher positive
voltage (Fig.2.4.2.C). Furthermore, dynodes in the shape of a
wire mesh, coarser or finer. Rarely is used a linear arrangement
of quarter-circle dynodes (somewhat similar to Fig.2.4.2.A),
between which there are grids - sometimes referred to as a box-grid
arrangement. The last dynode - the anode - is common to most
photomultipliers. However, there are special multianode
photomultipliers ( Fig.2.4.2.D and partly also
Fig.2.4.2.H), listed below in the section " Special
types of photomultipliers ".
Continuous channel
dynodes
The above "classical" electron multiplier systems in
photomultipliers consist of individual separate dynodes
with secondary electron emission, supplied with voltage from a resistive
divider . It can be said that it is an electron multiplier
with discrete dynodes .
However, there is also a completely
different design solution of the electron multiplier - a continuous
dynode ( Fig.2.4.2.H ). It consists of a glass tube
(channel), which is coated on the inside with a thin layer of
semiconductor material, which has a high resistivity and good
secondary electron emission properties. Between the input and
output ends of the tube is connected high voltage, about
1000-3000 V, which along the inner wall it splits
and creates a large onevoltage gradient . The
functions of the secondary emission and the voltage divider are
thus combined.
The photoelectrons emitted by the light
from the photocathode enter the channel, where they cause the emission
of secondary electrons when they hit the inner wall .
These electrons are accelerated by a voltage gradient
and, on further impact, more secondary electrons emerge from the
wall. This process is repeated many times along the channel - the
secondary electrons bombard the walls of the tube and produce
more and more more electrons. The channel thus behaves as a
" continuous-dynode " electron
multiplier , the gain of which G is determined by
the voltage on the tube (at a voltage of 3
kV the gain of approx. 10 6 -10 is achieved8
, comparable to classical dynod systems) .
Finally, a large number of electrons fly out of the output end of
the channel, which impinges on the anode and create the resulting
current signal.
This Continuous Electron Muttiplier (CEM ) system is sometimes called a channeltron.. The
channeltron tube is often initially wider and then narrowed and
bent into the shape of a "ram's horn". Its advantage is
small dimensions, when using a suitably shaped twisted channel,
it can be just centimeters - Fig.2.4.2.H top right. Due to the
high intensity of the electric field and the smaller transit
distance of electrons, these photomultipliers have better
resistance to magnetic fields. Channeltrons are used in
optoelectronics, in electron and mass spectrometers. The complex
combination of channel multipliers - multi-channel plate
photomultipliers MCP, are listed below in the section "
Special types of photomultipliers
".
Fig.2.4.2. Design of various types of
photomultipliers. In the upper part there is a photograph, in the
middle and lower part of the picture there is a diagram of the
construction and operation of the respective photomultiplier.
Special types of photomultipliers
imaging position sensitive photomultiplier multianodové PSPMT
For special viewing position light sources,
especially in the position of scintillation radiometric detector,
a photomultiplier construct more complex systems, referred to as PSPMT
( Position Sensitive photomultiplier ) - position
sensitive photomultipliers . Such a photomultiplier
consists of a photocathode with a larger area
(approx. 5 ´ 5cm), an intricately configured system of dynodes
(lamellar or channel design) and a larger number of
anodes arranged on the other side of the evacuated flask
(rectangular shape) into a matrix 4 ´ 4, 8 ´
8, or 16 ´ 16 independent metal elements (Fig.2.4.2.D). At the
location of the photocathode, where the detected light photon
hits, a photoelectron is released, which is attracted and
accelerated between the lamellar dynodes. Secondary electrons are
released, which ultimately fall preferentially on the one of the
anodes which lies opposite the point of emission of the
original photoelectron - i.e. opposite the point of impact of the
detected photon. By electronic evaluation of the amplitudes of
signals from individual anodes, it is possible to
determine the place of impact of the detected photon (or
photon flash) on the photocathode - the photomultiplier has imaging
properties. One such larger, position-sensitive
photomultiplier can replace several separate smaller
photomultipliers in some applications (such as special
scintillation cameras or Cherenkov imaging detectors).
The microchannel plate
photomultiplier
MCP ( Multichannel
Plate Photomultiplier ) differs fundamentally from other
types of photomultipliers in its shape and design. A plate (disk)
is placed under the surface photocathode, in which a large number
of glass capillaries (channels) with an inner
diameter of about 5 - 100 m m are guided across . A semiconductor layer with a
secondary emission property of a suitable considerably high
electrical capacity is applied on their inner wall. resistance. High
voltage is applied between both ends approx. 1000-3000V,
which creates a large voltage gradient along the inner wall. The
electrons emitted by the light from the photocathode enter the
channels, where they cause the emission of secondary electrons
when they hit the inner walls. These electrons are accelerated by
a voltage gradient and further secondary electrons emerge on
further impact. This process is repeated many times along the
channels - secondary electrons bombard the walls of the channels
and produce more and more more electrons. Thus, each channel
behaves as a separate electron multiplier
("continuous-dynode"). Finally, a large number of
electrons fly out of the output end of each channel, which
impinges on the anode and creates the resulting current signal -
Fig.2.4.2.H. In addition to photoelectronics, MCP detectors are
used in electron and mass spectrometers. INmultianode
design can also have imaging properties , similar to the
above PSPMT - Fig.2.4.2.H top center.
Hybrid
Photon Detector HPD
An interesting type of sensitive "photomultiplier" is
the so-called hybrid photon detector HPD ( Hybrid
Photon Detector ). It combines the principle of a vacuum
tube and a photocathode with semiconductor detection of
photoelectrons. It is again a vacuum tube with an optical input
window, on the inner wall of which a thin layer of photocathode
is applied . However, photoelectrons, released by the impact of
detected light, do not fall on the dynodes, but are accelerated
by a voltage of + approx. 10 kV
(a negative voltage of approx. -10 kV is
connected to the photocathode, the anode is at zero ground
potential) . Accelerated electrons impinge
on the semiconductor detector CCD (forming the
anode), where they release electron-hole pairs, the sensing of
which generates electrical impulses - in Fig.2.4.2.E. Each
electron accelerated by a voltage of U produces about U /
3.5 electrons and holes (at a voltage of 8 kV the
"multiplication effect" is about 2500), which can be
further multiplied 10-100 times by an avalanche effect
when using an avalanche semiconductor diode APD (see below). Even
the detection of one photon creates a relatively high output
pulse, the level of several mV / 1 photoelectron. The device has
a high signal-to-noise ratio and is suitable for detecting
weak light signals(single-photon measurement), such as
Cherenkov radiation.
The latest type of this technology is an imaging
(position sensitive) pixel hybrid photon detector
. The optical input window has larger dimensions and is provided
on the inside with a highly homogeneous photocathode layer. The
photoelectrons released by the impact of the detected light are accelerated
by a voltage of 10-20kV applied to the ring
electrodes . There are several of these electrodes
(2-5), with a voltage gradually increasing from 0 at the anode to
about -20kV at the photocathode. In addition to acceleration,
these electrodes also serve as focusing " electron
optics"", projecting the surface of the
photocathode to the anode equipped with a pixel detector
(inverted reduced projection). Accelerated and directed electrons
then impinge on the (silicon) pixel detector ,
formed by a matrix of a number of elementary detectors ( pixels
) - about 32 ´ 32 = 1024 pixels. -hole pairs, the sensing of which
generates electrical impulses from individual pixels, corresponding
in position to the respective point of impact of light
on the photocathode (Fig.2.4.2.F) .The device therefore functions
as a sensitive imaging light detector.Suitable
especially for position-sensitive registration and display light
signals such as Cherenkov radiation (RICH imaging detectors, see
below).
SPM
semiconductor photomultipliers
In addition to tube (vacuum) light detectors - photons,
conventional photomultipliers and hybrid HPDs, pure
semiconductor light detectors have also been developed .
For higher luminous fluxes, where extremely high sensitivity is
not required, photoresistors , photodiodes or phototransistors
are commonly used , operating in (approximately) linear mode. At
very low intensities, light already consists of individual
separate photons. The detection of such light then consists in
"counting" the individual photons. Semiconductor
so-called avalanche photodiodes APD have been
developed for "single photon" light detection (which is
a standard photomultiplier).
(Avalanche Photodiode). They are silicon semiconductor diodes
connected in the inverse direction to a suitable voltage
(slightly higher than the breakdown voltage ) so that
they work in the so-called Geiger mode . The
photoelectron, released by the impact of light on a thin surface
layer " P " (thickness <1 m m - serves as a
"photocathode"), is accelerated in a strong electric
field to cause further ionization, as well as the released
secondary electron, etc. - is initiated an avalanche of
electron-hole pairs in the depleted layer " i
" (thickness approx. 200-500 m m), impinging on the
opposite layer " N " - temporary electrical breakdowndiodes
(fig.2.4.2.G below). This achieves a total electron gain of
approx. 10 5 -10 6 . A so-called quenching resistor R Q (values ??of tens to W ) is connected in
series with the diode , on which the resulting voltage drop
causes a rapid interruption of the discharge in the diode. During
an avalanche breakdown of a diode, a well-detectable electrical
pulse is generated in the circuit , the height of which
is constant, independent of the number of input photons that
caused the avalanche detection; this property is analogous to a
conventional gas-filled Geiger-Müller detector (see " G.-M.
detectors " above).
For spectrometry, a certain disadvantage of photon
detection is a single APD photodiodea uniform
magnitude of the output signal (which is given by the
capacitances and resistances in the diode circuit), which does
not allow to distinguish between the detection of one or more
photons. This disadvantage has been overcome by using an entire array
of multiple small APD elements , densely spaced on one
small semiconductor wafer, including quench resistors for each
element. The individual elements (APD + quenching resistor) are
connected in parallel, so the output signal is the sum of the
signals from the individual elements - the upper part of
Fig.2.4.2.G. At low luminous flux, there is a low probability of
simultaneous impact of multiple photons on a single cell (the
impact of multiple photons will result in the activation of
multiple independent cells), so the output signal will be proportional
to the number of photonsincident on the detection field.
The output signal is taken as standard from the working resistor
R (approx. 100 W ) via the isolating capacitor C (approx. 0.1 m F).
Optoelectric components of this type are sometimes
called " silicon photomultipliers
" and are abbreviated SiPM ( Silicon
Photomultiplier ), or SPM ( Semiconductor
Photomultiplier ), also SSPM ( Solid-state
Photomultiplier ), or MPPC ( Multi-pixel
Photon Counter).). Due to its small size and weight,
mechanical and magnetic field resistance, ease of installation
and wiring, it can be expected to replace conventional
photomultipliers in a number of applications in the
future , such as Cherenkov imaging detectors or hybrid PET + MRI
imaging combinations.
Physical properties of
photomultipliers
Basic parameter photomultiplier is its overall
sensitivity S , which is the product of
sensitivity photocathodes S F and multiplying dynod gain G ( gain ) :
S = S F . G.
In general optoelectronics, the total sensitivity of a
photomultiplier is given in current units [Amperes / lumens], it
is about 10 to 100 A / lm (however, the
maximum permissible output currents of photomultipliers are only
tens of mA - see below " Adverse effects with
photomultipliers ") . In
radiometry, the sensitivity of a photocathode is
expressed as [electron / photon] - the number of electrons
emitted per incident photon, or as the quantum efficiency,
expressing this circumstance in [%]. The sensitivity of a
photocathode significantly depends on the wavelength
(energy of photons) of the incident radiation - we are talking
about the spectral sensitivity of the photocathode
, which is expressed graphically. Relative spectral
sensitivity in [%] is often used , which is related to the
maximum spectral sensitivity value.
Response linearity photomultipliers are characterized by a direct
relationship between the intensity of the incident light and the
output current. Photomultipliers are usually linear over a wide
range of radiant flux per photocathode, about 10 2 -10 10 photons / sec. In
the region of larger radiant fluxes, the linearity begins to
decrease (the output signal is lower) due to the fatigue of the
dynodes and due to the large spatial charge of the electron
cluster at the last dynodes.
The time constant of the photomultiplier
characterizes the duration (width) of the current pulse at the
output of the photomultiplier at the impact of an instantaneous (almost infinitely short) light flash on the photocathode. The duration of the
output current pulse from the anode is influenced mainly by the
velocities (velocity distribution) of the secondary electrons and
the different lengths of the electron paths from the photocathode
between the dynodes. Due to the time scattering of the transit
time, not all secondary electrons come from the instantaneous
photoeffect to the anode at the same time, but their time
distribution forms a pulse (approximately Gaussian) with a
half-width of about 10 -8 sec. The time constant of the photomultiplier, the
duration of scintillation in the crystal and the time constant of
the electronics determine the speed of the
scintillation radiometer - its dead time , or
time resolution (analyzed above).Dead time detectors ").
The signal-to-noise results mainly when
measuring very low luminous fluxes on the photocathode. Noise
photomultiplier is caused by a" dark current
"- output current that flows through the photomultipliers
unlit photocathode(see"
Adverse effects with photomultipliers "). the dark current is formed mainly by thermal emission
of electrons from the photocathode and dynod, leakage currents
between electrodes and between the terminals in the socket
photomultiplier. at room temperature, the dark current of about
10 -15 a
photomultiplier cooling it can be greatly reduced. the
signal-to-noise ratio sometimes expressed by the so-called
light equivalent of dark noise (ENI - Equivalent Noise Input )
, which is the value of the luminous flux
incident on the photocathode that causes a signal equal to the
dark current to be output at the photomultiplier.
Adverse effects of
photomultipliers
Although photomultipliers are very sensitive and perfect light
detectors, as with any more complex instrument, there may be
several adverse effects that impair the detection properties.
One of the unfavorable phenomena in photomultipliers
is the so-called dark current: it is an electric
current flowing through a photomultiplier, even when the
photocathode is not irradiated. The dark current is caused by the
thermoemission of electrons from the photocathode (which occurs
to a small extent even at normal temperatures), or thermoemission
from the first dynodes participates in it; the multiplier effect
on the other dynodes is then amplified. Statistical fluctuations
of the dark current cause the detected noise pulses. The dark
current of the photomultiplier is temperature dependent, by
cooling the photocathode or the whole photomultiplier we will
significantly reduce it.
Another problem affecting the energy
resolution of the entire detection system is the inhomogeneity
of photoelectron collection.; especially from the
peripheral parts of the photocathode, the efficiency of
photoelectron collection on the first dynode of the
multiplication system is reduced. Statistical fluctuations in
quantum efficiency and dark current also contribute to the
deterioration of energy resolution, which are superimposed on the
useful signal and blur the amplitude of the output pulses.
The use of photomultipliers in the
presence of a magnetic field , which deflects
electrons in their path between dynodes by Lorentz
force, is also problematic. In a stronger
magnetic field, the photomultipliers stop working!
At stronger luminous fluxes, there may be
a current overload of the photomultiplier - an
enormous flux of electrons falls on the last dynodes, which
causes " fatigue " of the dynodes.,
leading to a reduction in their emissivity. In the case of a
short-term overload with a total current of several tens or
hundreds of milliamperes, this is a reversible
phenomenon, when after a certain period of "rest" the
photomultiplier returns to its original state. However, in case
of large or permanent overload, there is great fatigue or permanent
damage to the last dynodes (in the case of large
currents, also thermal damage - "burning" of the dynode
surface)!
Note: These
properties apply to "classic" photomultipliers with
photocathode and dynodes. With appropriate modifications, these
parameters and effects can also be applied to semiconductor
photodetectors.
Scintillation probe
Scintillation crystal and photomultiplier with socket and
resistive divider for powering dynodes, or with preamplifier, are
housed in a light-tight housing *); this unit
forms a so - called scintillation probe or
scintillation detection unit , generally called a scintillation
detector - Fig.2.4.1 on the right and Fig.2.4.3 a
. In the next we will deal with scintillation detectors with
"classical" vacuum photomultipliers with discrete
dynodes powered by a resistive divider - these are so far most
often used for radiation detection and spectrometry.
*) Warning:
Light
tightness scintillation detector
is an essential condition for its function. The scintillation
probe must not be disassembled in the light when the high voltage
on the photomultiplier is connected, as a high luminous flux
would cause such a strong current of electrons on the dynodes
that it could be irreversibly damaged and destroy the
photomultiplier ! - was discussed above in the section
" Adverse effects of photomultipliers ".
In general purpose detectors, this housing
is generally easy to disassemble so that the crystal and
photomultiplier can be replaced to assemble a
scintillation probe with the desired properties for each
application. Single-purpose scintillation probes are usually
firmly assembled into a so-called scintiblock ( Fig.2.4.1 on the right). One of the tasks
of the metal housing of the scintillation probe is the magnetic
shielding of the photomultiplier (or
the external magnetic field could affect the movement of
electrons in the photomultiplier and thus change its
amplification) .
The basic scintillation probe consists of
a " planar " cylindrical scintillation crystal
in optical contact with a photomultiplier (Fig.2.4.3 a
). The thickness of the crystal is chosen according to the gamma
radiation energy used: for soft gamma and X it can be only 5 mm,
for medium energy radiation of tens to hundreds of keV the
thickness of the crystal is about 5-10 cm. By installing such a
probe in a shielding case, equipped with a tube- collimator
at the front , a collimated scintillation probe
is created (Fig.2.4.3 b), which is used to
selectively detect gamma radiation from a desired direction,
defined by a collimator. Collimators are usually replaceable, the
entire probe is mounted on a robust tripod with
the possibility of vertical and horizontal displacement and
angular rotation of the probe. Collimated detection probes are
used in many applications of nuclear and radiation physics. In
nuclear medicine, they have been used for the targeted detection
of gamma radiation from examined organs to assess the
accumulation of radiopharmaceuticals. V 60.-80. In the 1930s
, pairs of collimated probes were widely used for radisotope
nephrography (later it was displaced
by far more perfect dynamic scintigraphy of the kidneys
on a gamma camera - see §4.9.2 " Nephrological
radionuclide diagnostics
"). However, the collimated detection
probe is still used to measure the accumulation of
radioiodine in the thyroid gland (§4.9.1
" Thyrological radioisotope diagnostics ") .
To measure radioactive samples in test
tubes , a well scintillation crystal
(Fig.2.4.3 c ) is used in the scintillation
probe , into the opening of which a test tube for measurement in
4 p -geometry
is inserted (see below §2.7, Fig.2.7.1) . This probe is built into a robust lead shield
(Fig.2.4.3 d ), it is mainly used for measuring
low-activity samples in laboratory radiochemical analyzes, in
nuclear medicine in in vitro methods (see below §2.7 "Measurement of radioactivity of
samples (in
vitro) "), mainly by radioimmunoassay. For the measurement of
radioactive samples with larger volumes (approx. 250 ml.)
Large-volume well crystals are used in the scintillation
probe(Fig.2.4.3 e ).
Fig.2.4.3 Some constructions of scintillation probes for
measuring gamma radiation.
a) Basic scintillation probe with planar
(cylindrical) scintillation crystal. b)
Collimated scintillation probe for measuring radioiodine
accumulation in the thyroid gland. c)
Scintillation probe with well (cavity) scintillation crystal. d)
NKG314 well scintillation detector for measuring radioactive
samples in test tubes. e) High-volume well
detector NKG315 (for illustration, the
large sensitizing probe placed above is actually located inside a
robust lead shield - it is indicated by a white arrow) .
A very special complex scintillation detector is a scintillation camera containing a thin large-area scintillation crystal equipped with many photomultipliers - see chapter 4 " Radionuclide scintigraphy ", §4.2 " Scintillation cameras ". For various detection and spectrometric purposes, very complex systems of many scintillation detectors are often constructed in various geometric arrangements, or in combination with other types of radiation or particle detectors. Extensive detection systems are being built for large accelerators , containing, among many other detectors, several thousand individual scintillation detectors, connected in a coincidence or anticoincidence mode to complex electronic evaluation apparatus.(as mentioned above in the section " Arrangement and configuration of radiation detectors ") .
Electrical supply
of the photomultiplier and processing of output pulses from the
scintillation detector
For the correct "multiplication" operation of the
photomultiplier, it is necessary to apply a correspondingly high
voltage to its individual electrodes - photocathode,
dynodes, anode . The basic supply voltage of approx. 500 ¸ 1500V, supplied
from a high voltage (HV) source, is divided into gradually
increasing voltages by a set of resistors (resistor divider
) and led to individual dynodes (the last
dynodes before the anode are sometimes blocked by capacitors
to increase the pulse current did not cause voltage fluctuations
on the divider, which would lead to changes in gain in the dynod
system) . As a rule, it is introduced
between the photocathode and the first dynodehigher voltage
than between other neighboring dynodes (the distance of the first
dynode from the photocathode is also greater than between the
other dynodes). A full voltage is connected between the
photocathode and the anode via a working resistor
(values ??of the order of M W ) , on which a current pulse generated by the passage of
electrons through a photomultiplier causes a voltage
pulse (by the effect of
"voltage drop" according to Ohm's law) . This output voltage pulse is fed via an isolating
capacitor to the amplifier for further
electronic processing.
The simplest
connection of a photomultiplier is a single-core power
supply with positive polarity , where only one
coaxial cable . Its "live" conductor supplies
a positive voltage of 500 ¸ 1500V both through the anode and through the resistive
divider of the individual dynode. The cable is connected to the
HV source via a working resistor (which is a
part of the HV source), from which voltage pulses generated on
this resistor during the passage of electrons through the
photomultiplier are taken via a separating capacitor. This
connection is simple and elegant, but it cannot take full
advantage of the photomultiplier gain. It is suitable for the
detection of higher energy radiation (from tens of keV), where
the scintillations are sufficiently intense and the
photomultiplier signal is relatively high.
For low energies, a more complex multicore connection
with negative polarity is more suitable . A full
negative voltage is applied to the photocathode ( - 500 ¸ - 1500V), the
negative divider then gradually decreases the negative voltage to
the individual dynodes; the anode is connected to zero ground
potential via a working resistor. The output pulses from the
working resistor are routed through a separate
"signal" coaxial cable to the amplifier. A pulse preamplifier
can then also be installed in the photomultiplier socket .
Negative polarity with the anode at "zero" ground
potential (in idle mode) is particularly advantageous when more
complex low-current electronic processing of the output signals
is performed and an excessive overall potential difference could
be disruptive; this is the case, for example, with the
above-mentioned hybrid photon detectors HPD. When radiation is detected, a large amount appears at
the output of the photomultiplier
electrical
impulses of various sizes, which must be further processed
and evaluated . The relevant electronic circuits are
used for this, which amplify the pulses , sort
them according to their size - they analyze ,
calculate, register, display. The processing of these pulses can
be performed in two basic ways, according to the purpose of
scintillation measurement - the upper and lower branch in the
diagram in Fig.2.4.1 :
In special cases of multi-detector systems , further electronic analysis of signals is performed - coincidence, anticoincidence and comparison of signals from individual photomultipliers - in order to display, evaluate the position of sources, particle trajectories or angular correlations.
The technical development
of radiometers and spectrometers
is closely connected with the development of low-current
electronics and its component base. From an electronic point of
view, radiometers and spectrometers can be divided into 3
generations:
1. Generation
Until the 1960s, radiometric instruments were equipped
with tubes - they had larger dimensions and very
limited possibilities of processing electrical signals from
detectors, mostly single-channel measurements
with one (lower) discriminant level - fig.2.4.4 a
. The display of the measured number of pulses was by means of
analog hand gauges, or digitally by means of light bulbs
or incandescent lamps placed vertically in
columnar decatrons .
This display consisted of 10 light bulbs or
incandescent lamps placed in a row (column) above each other,
which illuminated the enclosed narrow glass rectangle with the
numbers 0,1,2, ...., 9. These columns were placed side by side in
several sets (4-6) according to the required number of decimal
places. During the measurement, the numbers in the individual
decatrons "ran" from bottom to top, and after the end
of the preset measuring time, the respective digits remained lit
in the individual decimal places (Fig.2.4.4 and
left).
In the short transitional period of the
late 1960s, circular glow decatrons were used to
display the registered number of pulses.. They were glow plugs
with 10 wire circular electrodes, marked with the numbers 0,1,2,
..., 9. In addition to the display of the registered number of
pulses used also as an electronic timer to tick
pulses: When appropriate electronic participation of each
incoming pulse switch lighting up the side of the electrode (1 or
higher). And when the value "9" was exceeded, when the
electrode "0" lit up, a pulse was sent to the
neighboring decatron to increase the number there. During the
measurement, the lighting electrodes on the decatrons at the
circularly arranged digits "circled" in a circle.
However, the reading of the measured number of pulses was not
very clear, the decatrons were soon replaced by digital digitrons
.
2. Generation
of radiometric devices (60.-70. years- Fig.2.4.4 b
). It could be implemented multichannel analysis, using the
digital display was doutnavkových
digitronek . The measured
numbers of pulses could be printed on an electro-mechanical
printer.
Digitrons are small
circular or elliptical glow plugs , whose
electrodes are thin wires shaped into the numbers
0,1,2, ...., 9. The electronic circuits of the pulse counter at
each digit - a decimal place - light up and display the
corresponding digit (Fig.2.4.4 b above). They
were later replaced by semiconductor 7- segment LEDs
. By lighting combinations of different segments, the necessary
digits of the measured number of pulses are modeled (or other
alphanumeric characters) - Fig.2.4.4 b down.
3. Generation
Great progress has been made since the 1980s with the development
of digital electronics with a high density of
integration and computer recording and control
of measured pulses (Fig.2.4.4 c , d
). They enable accurate complex processing of measured data and
their mathematical analysis using special software. Including
multichannel spectrometric analysis using an
analog-to-digital converter ( ADC ).
Fig.2.4.4 Technical development of electronics of radiometers and
spectrometers (examples of some instruments at KNM
Ostrava) .
a) Electron spectrometer NZG 319 (
TESLA VÚPJT Pøemylení ) with a columnar
glow decatron display. b)
Transistor spectrometer NZQ 717T ( TESLA
Thinking ) with glow digital digitron display. c)
Digital radiometer MC1256 (TEMA) with 256-channel analyzer. d)
Computer 4096-channel Genie 2000 spectrometer ( Canberra Packard ) for
scintillation and semiconductor spectrometry.
All these generations of radiometric and spectrometric instruments have been gradually used at our workplace of nuclear medicine in Ostrava since 1973. It is interesting that the electronic evaluation apparatus underwent significant technical changes, while the detectors themselves - ionization chambers, scintillators, photomultipliers remained essentially the same for many decades. Only conventional vacuum photomultipliers have recently been seldom replaced by semiconductor photodetectors (they are described above in the section " SPM semiconductor photomultipliers ") .
Advantages
of a scintillation detector
If we compare the situation with G.-M. detector, we see that we
achieved a similar result with the scintillation detector -
again, the quantum of invisible ionizing radiation was detected
by converting it into electrical pulses at the output of
the photomultiplier. The question may arise: Why so complicated?
The answer is: Scintillation detectors have, compared to G.-M.
detectors have three main advantages (we will list them here for the most common case of
gamma radiation detection) :
These three properties make the
scintillation detector an almost ideal device
for the detection and spectrometry of ionizing radiation,
especially gamma radiation. High detection sensitivity allows its
use for the detection of even very weak radiation or low
activities. The short dead time, in turn, allows lossless
measurement of even relatively higher radiation intensities or
higher activities. *) The scintillation detector therefore has a
very wide range of detectable intensities of ionizing radiation,
with the possibility of spectrometric selection and analysis.
*) The same dependence applies to the dead time of the
scintillation detector as shown in Fig.2.3.4 on the left in the
paragraph about G.-M. detector, the value of the dead time being
mostly t @ 1 m s. The same principles apply to the measurement of dead
time and event. dead time correction.
Gamma scintillation
spectrum
Let's stop in more detail at point 3 - spectrometry
. Scintillation detectors are by far the most commonly used for
gamma radiation. Let us imagine initially that we irradiate a
scintillation detector with mono-energy gamma radiation
*) of energy E g . The real ("physical") spectrum of this
radiation will have a simple shape according to Fig.2.4.1 at the
top right - the only narrow peak g 1 in the spectrum (we do not consider the other two lines
yet). Photons of gamma radiation will interact with the
scintillator either by a photo effect - then in
a single interaction the photons are absorbed and give all their
energy to the ionizing electron, or by Compton scattering , when they give up
only a part of their energy to the electron (and then either
escape or cause another interaction), at high energies also by
the formation of electron-positron pairs (with a number of subsequent interactions including
annihilation of the positron with the electron to produce
additional radiation g ) . The generated secondary
electrons then transfer their energy in the detector
material in a series of collisions until they
are completely braked (to thermal energy) . The height of the pulse at the output of the
photomultiplier will always be proportional to the energy
that the gamma photon actually lost in the crystal.
*) Gamma-photons come from nuclear
transitions between discrete levels with
precisely given energies, so they are basicallymonoenergetic
, their ideal physical spectrum represents a sharp line
on the energy E g . Very small fluctuations in energy values ??are caused
by quantum uncertainty relations and backscatter of
nuclei during gamma-photon emission. Furthermore, since
these photons practically never come from free nuclei, but are
emitted from radioactive atoms contained in a substance (in a
certain material), some photons interact with the substance
before leaving the sample. It can also blur their energy a bit.
For e + e -
annihilation radiation , the 511keV peak is
somewhat blurred (by about 1.2keV) due to the Doppler broadeningat different
residual rates of positron braking in the substance. However, the
magnitude of these extensions is generally very small compared to
the effects of self-radiation detection. The real, physical,
spectrum of gamma radiation can therefore be considered
practically linear (monoenergetic) - it is
discussed in §1.2., Passage " Spectrum
of gamma radiation ".
If we plot the
magnitude of the amplitude A of the output pulses from the
photomultiplier on the horizontal axis and the number of pulses n
with this amplitude A on the vertical axis , we get the
curve of the characteristic shape in Fig.2.4.1 at the bottom
right - gamma radiation scintillation spectrum .
The horizontal axis of the amplitude can be calibrated
so that the individual divisions correspond directly to the energy
of the detected radiation in keV (see below). The energy
scintillation spectrum of gamma radiation consists of two main
parts - a sharp photopeak and Compton's
continuous spectrum . The structure of the scintillation
spectrum is shown in more detail in Fig.2.4.5 on the left.
Fig.2.4.5. Scintillation spectrum.
Left: Scintillation spectrum structure. Right:
Dependence of the shape of the scintillation spectrum on the
scattering medium.
Photopeak
- energy resolution of scintillation detector
on the curve scintillation spectrum is seeing a significant peak
- the so-called. Photopeak or peak total
absobrbce corresponding photons g which were crystal
completely absorbed (especially the
photoelectric effect, or. Multiple scattering, or a combination
of several interactions) and surrendered
all their energy.
The question arises, why is the
photopeak relatively wide, when the actual spectrum of
monoenergetic gamma radiation is very narrow - discrete, linear?
There are several reasons for this "blurring" of the
photopeak:
1. The primary limitation on energy
resolution is caused by statistical fluctuations
in the number of scintillation photons released.N . This
gives a basic theoretical limit of what the best
resolution can be obtained with a given crystal of known light
yield - for the resolution defined below as the half-width of the
photopeak in percent it is R = (2.35 / Ö N) .100%. In the NaI (T1)
scintillator, an average of about 40 scintillation photons are
released per keV; according to the laws of quantum statistics it
will be 40 ± 6 photons, ie the relative statistical fluctuations in
the number of photons emitted during scintillation will be about
16% for this energy 1keV and R @37%. Incomplete
scintillation transfer to the photomultiplier and the imperfect
efficiency of opto-electrical photon detection further reduce the
number of detected photons and worsen the statistical
fluctuations of the actually detected photons. These statistical
fluctuations in the number of detected light photons blur
the amplitude of the output pulses - and thus the
photopeak.
2. Nonlinearity light yield of the
scintillator (disproportionate light output) - the number of
photons of light emitted per unit of energy absorbed is not
constant, but depends on the energy of the particles exciting the
crystal *). Scintillation in the crystal occurs by cascades of
secondary electrons of different energies, so if the light yield
is disproportionate, the number of emitted photons will vary
(according to different energy distributions of the primary
detected quantum to a cascade of photons and electrons of
different energies), although the total absorbed energy is the
same. This causes degradation of the energy resolution.
*) Violation of the proportionality
of the scintillation yield is manifested mainly in the
area of low energies(keV units). The main cause
is the high concentration of charge carriers created by the
interaction of ionizing radiation with scintillation material. As
the kinetic energy of an electron moving in a matter of matter
decreases (according to Bethe's relation), the linear energy
transfer dE / dx increases and the
ionization density increases along the path. This leads
to higher non-radiation recombination of
electrons with ions - there is a " quenching
" of potentially scintillating electron contributions and
thus a reduced scintillation yield in the field
of low energy. However, some of the charge carriers may diffuse
into the environment with a lower ionization density, where they
may cause scintillation. This depends on the mobility of the
charge carriers in the material. Significant changes in the
effective cross section of the interaction in the areas of
binding energies on the K and L shells of the scintillation
material also contribute to the proportional anomalies (K- and
L-edge effects).
The nonlinearity of the scintillation yield can be characterized
as a function of the energy of photons or electrons. We recognize
the photon disproportionality of the response,
which affects the linearity of the energy calibration function of
the detector. In terms of the effect on energy resolution,
however, the nonlinearity of the response is more important as a
function of electron energy - the electron
disproportionate response.
3. Imperfect
(inhomogeneous) efficiency of scintillation photon collection on
a photomultiplier photocathode. If scintillation occurs in the
peripheral parts of the crystal remote from the photocathode, a
slightly smaller number of photons strike the photocathode than
when scintillation occurs in the middle and near the
photocathode; the amplitude of the output pulses will differ even
with the same delivered primary radiation energy. This effect is
exacerbated if the scintillation crystal is not sufficiently
optically transparent and homogeneous. The resolution therefore
also depends on the perfect optical transparency of the
scintillation material - so that, if possible, the same
percentage of scintillation photons, regardless of the
scintillation site, can fall on the photocathode of the
photomultiplier.
4. Inhomogeneous photoelectric
sensitivity of the photocathode - the impact of the same number
of photons in different places of the photocathode can lead to
the emission of a slightly different number of photoelectrons.
There is also an inhomogeneity in the collection of
photoelectrons; especially from the peripheral parts of the
photocathode, the efficiency of photoelectron collection on the
first dynode is reduced.
5. Statistical fluctuations of
quantum efficiency and dark current of the photomultiplier, which
are superimposed with the useful signal and blur the amplitude of
the output pulses.
These effects cause "blurring"
of the photopeak and deterioration of energy resolutionscintillation
detector. From points 1.-3. it follows that the best energy
resolution can be expected for scintillators with high and energy
proportional scintillation yield, high optical transparency and
good optical contact with the photomultiplier.
Energy resolution R
of the detector mean smallest difference energy of the detected
radiation in the spectrum yet distinguish such two peaks, or
equivalently called. Half-width photopeak D 1/2 - its width at half height (FWHM). The resolution is
expressed either absolutely in keV or relatively (percentage) as
the ratio of the half-width D
1/2 to the energy
value E g of the center of the photopeak: R = ( D 1/2 /
E g ) .100 [%]. The measured value of energy resolution
depends on the energy E g ; it is customary to give it for E g = 662keV of radionuclide 137 Cs (Fig.2.4.5
left). In our spectrometric measurement of
the 137 Cs radionuclide on a scintillation NaI (Tl) detector, the
half-width of the FWHM gamma peak at 662keV was 55keV (8.3%),
while with the semiconductor HPGe FWHM detector it was only
1.4keV (0.2%) - significantly better energy resolution ! For the NaI (Tl) scintillator, the theoretical
resolution limit, resulting purely from the statistics of the
number of scintillation photons emitted, for an energy of 662keV
would be about 1.4%. In reality, however, the resolution is much
worse (this is due to
the significant disproportionate light output in the low energy
region and other influences discussed above at the beginning of
this paragraph). For NaI (T1)
scintillation detectors of conventional designs, the energy
resolution is around 6-10%; it is better for small thin
scintillation crystals, while for large-volume and well detectors
it deteriorates to about 15-17%. The best energy resolution of
about 3% is provided by the lanthanum bromide
scintillation crystal doped with LaBr 3 (:
Ce) cerium ; this is due to its high light
yield of 63,000 photons / MeV and good energy proportionality .
In Fig.2.4.1 on the right we see how the
imperfect energy resolution of the scintillation detector causes
the photopeaks of the two spectral lines of radiation g 2 and g 3 with close energies
to partially merge into one peak; if the two energies were even
closer, a single photopeak would be formed from which these
energies could not be distinguished. In this case, a
semiconductor detector must be used (see
below §2.5 " Semiconductor detectors
" , Fig.2.5.1) .
Terminological note:
In the older literature we can find the terms " differential
spectrum " and " integral spectrum".
This comes from a time when amplitude analyzers were not yet as
perfect, it was either just the lower discrimination level or two
independent levels. The" integral spectrum "was created
by measuring the integral pulse rate as the lower discrimination
level gradually shifted upwards; The descending curve with the
largest gradient of decrease at the photopeak Derived the
"integral spectrum" to create a real spectrum, then
called "differential." The term "integral
spectrum" has not been used for a long time
, each spectrum is "differential" with a certain
analyzer window width. the name spectrum or energy
spectrum , or with the adjective " scintillation
" or "semiconductor "...
Continuous Compton
scattered spectrum
In front of the photopeak, a continuous
spectrum corresponding to photons, which have lost only
part of their energy in the crystal by Compton
scattering, stretches to the left to the beginning of
the graph (Fig.2.4.5). The continuous Compton scattering spectrum
has a characteristic shape resulting from the laws of Compton
scattering (see §1.6 "Ionizing
radiation", section " Interaction
of gamma and X-rays ", passage
" Compton scattering ") .
Just before the photopeak, the Compton continuum ends with a
relatively rapid decrease called the Compton edge
- it corresponds to the maximum possible energy transferred to
the electrons in one Compton scattering of a given gamma
radiation (at a total reflection of 180 °). With multiple Compton scattering of the photon g , the transmitted
energy is higher - part of the Compton scattering also extends
into the photopeak region, which is disturbing in spectrometry.
The shape and relative representation of the Compton spectrum
with respect to the photopeak somewhat depends on the geometric
conditions at detection. In the continuous Compton continuum, a
low and wide backscattering peak corresponding
to photons that were scattered in the surrounding material and
then detected is sometimes observed ...... Since Compton
scattering also occurs in the measured sample itself and in the
material around the detector, it can be in the presence of a
larger amount of scattering medium a substantially increased
representation (height) of the continuous part of the spectrum,
as can be seen in Fig.2.4.5 at the bottom right.
X-escape spectrum
Upon absorption of the primary photon g in the scintillator
material by the photoeffect, a characteristic X-ray (K-series) is
produced, which is usually absorbed and then contributes to the
photopeak, but some of it can escape from the
detector. If this occurs, then a response is generated reduced by
the energy of this photon X. In the case of the NaI (T1)
scintillator, it is a characteristic iodine X-ray with an energy
of about 28keV. At energies g
higher than about 200keV, when the
photopeak is wide, the corresponding impulses fall into the
photopeak and cause only some expansion of its leading part.
However, at low energies (around 60-80keV, when the photopeak is
narrower), a so-called escape peak may appear in
the spectrum , lying about 28keV lower than the main photopeak.
Annihilation
spectrum
When g is
detected with an E g energy higher than 1.022 MeV, electron-positron pairs
are produced when interacting with the detector material, with
the positrons then annihilating with the electrons to form two
gamma quanta of 511 keV. Therefore, an annihilation
photopeak corresponding to this 511keV energy appears in
the spectrum . If both annihilation photons are detected by
complete absorption, they contribute to the primary photopeak.
Furthermore, some of the 511 kV annihilation photons may escape
from the detector, which will reduce the response by this energy
- an escape peak corresponding to the energy E g -511 kV
appears in the spectrum . If both annihilation photons escape,
this will result in a peak in the energy range 1022keV lower than
the primary photopeak.
The sum (peaks)
peaks
are discussed in the following section "Gamma-ray
spectrometry".
Pile-up effect
If two quanta of radiation fly into the scintillator almost
simultaneously, the respective scintillations are not detected
separately, but the light and electrical response from the two
quanta is added ( pile-up ) and gives
rise to a single the resulting pulse at the
output of the photomultiplier, the amplitude of which corresponds
to the sum of the amplitudes from the two scintillations. If one
or both scintillations correspond to a photopeak, then the
resulting summation pulse will be above the photopeak; when
measuring in the analyzer window set to the photopeak, the pulse
will fall outside the window and the corresponding pair of quanta
will not be detected. This situation occurs mainly at high
frequencies of radiation quanta - the pile-up effect contributes
to the loss of dead time , it can simulate the
so-called paralyzable component of dead time. However,
if there is a pile-up effect on two simultaneous Compton
scattered photons, the resulting pulse may fall into the
photopeak with its amplitude. This situation can be adversely
applied to scintigraphy at high pulse frequencies - see §4.2
" Scintillation cameras"Section" Adverse effects for scintigraphy
- Comptonovský scattering g . "
Noise
very beginning spectrum appearing pulses of low amplitude (but
unfortunately high frequency) corresponding to the noise
- spontaneous thermo photocathode noise in electronic circuits.
Noise is a fundamental limiting factor and These noise pulses
can, if necessary, be substantially reduced by cooling
the photomultiplier and the preamplifier, for example to
the temperature of liquid nitrogen.
Gamma
and X-ray spectrometry
Here we briefly analyze some methodological principles of spectrometric
analysis , which are common to all types of
spectrometric detectors, not only for scintillation
but also semiconductor (described
in the following §2.5 " Semiconductor
detectors ") and
magnetic (" Magnetic spectrometers ") .
The basic task of gamma
radiation spectrometry *) is to determine the energy
and intensity of individual discrete groups of
gamma radiation photons emitted by the investigated
radionuclide or mixture of radionuclides, or photons of
characteristic or braking X-rays arising in the
electron shell of atoms as a result of nuclear or electrical
processes.
*) Gamma radiation is by far the most
common subject of spectrometric analysis. Beta spectrometry will
be briefly discussed in the following paragraph. Spectrometry of
other types of ionizing radiation is performed only sporadically;
appropriate references to the methodology of such measurements
are given in various parts of the text as appropriate.
In most
spectrometers, the energy and flux of gamma photons are not
determined directly, but by measuring the energy and current
intensity of secondary charged particles.,
especially electrons, which are produced by the interaction of
primary radiation with the substance - sensitive material of the
detector. The only exception is crystal diffraction
spectrometry , where the energy (wavelength) of X - ray
or soft gamma radiation is determined directly - goniometrically
according to the scattering angle (§3.3,
section "X - ray diffraction analysis") .
The individual energy groups of gamma
photons (or characteristic X-rays) are displayed in the spectrum
as respective peaks called photopeaks , the
radiation energy determining the position of the
photopeak on the horizontal axis of the spectrum and the
intensity determining the height of the peak ,
resp. area(integral) under the photopeak.
Braking X-rays and Compton-scattered gamma rays have a continuous
spectrum. The energy and efficiency calibration of the
detector must be performed to accurately determine the energies
and intensities of the radiation g . *)
Gamma radiation is by far the most common subject of
spectrometric analysis. Beta spectrometry will be briefly
discussed in the following paragraph. Spectrometry of other types
of ionizing radiation is performed only sporadically; appropriate
references to the methodology of such measurements are given in
various parts of the text as appropriate.
Calibration of the spectrometer
In order for the measured spectrum to objectively express the
distribution of energies and intensities of photon radiation, an
accurate calibration must be performed energy
responses and the dependence of the detection efficiency of the
spectrometer on energy.
The energy calibration of a
spectrometric detector consists in determining the correct scale
on the horizontal axis, based on the fact that the amplitude of
the output pulses is proportional to the energy of the radiation
absorbed in the detector. For energy calibration, it would in
principle be sufficient to measure the position of the photopeak
for one known radiation energy g and to guide a straight
line passing through the origin (direct proportionality) through
this calibration point. However, the exact linearity of the
entire electronic chain may not be ensured a priori; the
calibration dependence also sometimes does not go through a
"zero" origin (different voltage
levels in electronic circuits play a role here) . It is therefore advisable to use it for reliable
energy calibrationseveral lines of radiation g of different known
energies. The most common standard radionuclides
for energy calibration of gamma spectrometers are: americium 241 Am ( g 26.3+ 59.6 keV + X
11.9 + 13.9 + 17.8 +20.8 keV) , cobalt
57 Co ( g 122 +136 keV) , cesium
137 Cs ( g 662 keV - the most important standard ever) , cobalt 60 Co ( g 1173 +1322 keV) , europium 152 EU (g122, 245,
344, 779, 867, 964, 1086, 1112 and 1408 keV). The
scintillation and semiconductor spectra of these radionuclides
are given in §1.4, section " Properties of some of the most important
radionuclides ".
For energy calibration, we should
therefore measure the spectrum of at least three radionuclides in
the region of energies Eg ofinterest (or one radionuclide with a larger number of
gamma radiation energies) and determine the positions of
photopeaks Ag. Using the calibration points [Eg, Ag] obtained in this way,
weinterpolate the calibration line (or the curve
with larger deviations from linearity), by projection of their
values Eg we get the energy calibration of the horizontal axis -
Fig.2.4.6 on the left.
Fig.2.4.6. Energy calibration dependence
(left) and efficiency calibration dependence (right) of a gamma
radiation scintillation spectrometer (a 10 mm thick NaI (Tl)
scintillation crystal was used, an Al cover of 1 g / cm 2 ).
Calibration of the
detection efficiency of a spectrometric detector is much
more complicated. This is because the detection efficiency is
significantly dependent on the gamma radiation energy
according to the curve shown in Fig.2.4.6 on the right: for low
gamma radiation energies, the detection efficiency is low because
these photons are absorbed through the input window and have
difficulty penetrating the sensitive detector volume. Therefore,
first the detection efficiency increases with energy and reaches
a maximum for energies of about 60-100 keV. Then, again, the
detection efficiency slowly decreases with increasing energy,
because at higher energies an increasing part of the photons g flies through the
sensitive volume of the detector without being absorbed by the
photoeffect. The dependence of the detection efficiency on the
energy can be roughly expressed by the biexponential function:
h (E g) = P. (1- e -R 1 .E g ). e -R
2 .E
g ,
where the coefficients R 1 and R 2 ( > 0), indicating
the rate of rise and fall, depend on the size and material of the
detector and on the absorption properties of the window.
For absolute calibration of the detection efficiency h is necessary to
measure the spectra of several spectrophotometric standards
of precisely known activity and the intensity I g of gamma radiation
of different energies E g , zintegrováním determine the area under the photopeak
S g and
then formed, calibration points [e g , Sg ] interpolate the
efficiency calibration curve h (E g ), as shown in
Fig.2.4.6 on the right. If only a relative calibration of the
detection efficiency is sufficient, a suitable radionuclide with
several g radiation lines (such as europium
152
Eu ) can
be used and the calibration curve can be interpolated based on the known intensity ratios of the individual
peaks. If we have already calibrated the spectrometer, the actual
spectrometric analysis begins by preparing the
sample and measuring its spectrum with a sufficiently high
"statistic" - a sufficiently large number of n
registered pulses to allow statistical fluctuations 1/Ö n
were low enough. The following is an analysis in which we find
individual photopeaks in the spectrum, determine their energy
and use the integral ( area )
under the peak to determine the intensity of the
respective gamma radiation line. For this purpose, it is usually
necessary to mathematically separate the actual photopeak curve
from the continuous spectrum and, if necessary. decompose
the composite photopeak into individual components -
photopeaks of energetically close lines g . Photopeaks are usually
approximated by Gaussian curves. This mathematical
analysis of the spectra is now performed using special
computer software, and on the basis of the measured energies and
intensities of the gamma radiation lines, this spectrum isthey
assign the corresponding radionuclides - interpretation
of the spectrum.
Scintillation and
semiconductor gamma radiation spectra of a number of important
radionuclides, together with decay schemes, description of their
properties and applications, are shown in §1.4
"Radionuclides", section " Properties of some of the most important
radioactive isotopes ".
Summation (coincidence) peaks
In gamma radiation spectrometry we can encounter an interesting
phenomenon of false so-called summation
peaks , which do not correspond to any of the actual
energies of the emitted radiation g or X. This phenomenon
occurs at the detection level when the measured radionuclide
emits two or more groups of radiation photons g or X at
the same time (none of the respective levels is
metastable). If both such photons g 1 and g 2 are detected
simultaneously, the light or electrical responses from both
quantums are summed in the detector to give a
single resultant pulse whose amplitude corresponds to the sum of
the energies E g 1 + E g 2 . The resulting summation peak mimics
gamma radiation of total energy, which does not actually exist.
The relative intensity of the summation peak
crucially depends on the detection efficiency.
With low detection efficiency, simultaneous detection of both
quanta is unlikely, so when planar measurements in geometry 2 p and lower have a
summation peak of negligible intensity and we usually do not even
observe it. The higher the detection efficiency, the more
pronounced the summation peak; with a detection efficiency of
100%, we would no longer observe the primary peaks of both real
quanta, only a false summation peak would appear in the spectrum!
- (provided, of course, 100% of radiation emission, without
internal conversion, etc.). This dependence can even be used to
determine the absolute detection efficiency of measuring a given
radiation or activity of a sample. Under these simple
circumstances, the detection efficiency h can be determined from the
simple relation h = 4. I 2 Sg 1+
g2 / (I g 1 + I g 2 + 2.I Sg 1+
g 2 ) 2 , where I g 1 and I g 2 are the intensities
of the primary peaks and I Sg
1+ g 2 is the intensity of the summation peak. The
summation peak often manifests itself in radionuclides decaying
by electron K-capture (accompanied by the
emission of characteristic X-rays when electrons jump from the L
shell to the K shell) followed by the emission of g from the excited
level of the daughter nucleus. This is where coincidence g is detected
and the characteristic X-rays (line K a , K b ), and form a summation
peak corresponding to energy E g + E X . A typical example
is the 125
I radionuclide , which is converted to 125 Te by K-capture , emitting g with an energy of 35 kV and
X with an energy of 27 kV. With sufficient detection efficiency ( 125 I samples are often measured in tubes in a well
scintillation detector) , a significant
summation peak corresponding to an energy of 62keV is observed -
see §1.4. "Radionuclides", passage " I-125 ". Summation peaks can also be observed in the
gamma spectra of lutetium 176 Lu or india 111In .
The summation peak can occur even at a high flux of
photons of radiation g , when two photons can fall into the detector at the
same time and cause the resulting scintillation with the sum
intensity - random coincidence.
Analysis,
evaluation and interpretation of spectra
The analysis of the measured spectrum is used primarily to
determine the energies and intensities of
individual components of the detected radiation. The
radiation energy is determined by subtracting the position
of the peaks of the photopeaks in the measured spectrum,
of course, assuming the correct energy calibration of the
spectrometer (Fig.2.4.2 on the left). The intensity of
the respective line g
is determined from the integral of
the photopeak
(area under the photopeak curve), in the first approximation can
also be easily determined using the height of the photopeak
. This value then needs to be recalculated
(corrected) according to the energy efficiency
curve (Fig.2.4.2 on the right).
A significant problem in the analysis of spectra is
the imperfect energy resolution of the
spectrometer. Photopeaks of nearby energies then partially (or in
the worst case even completely) merge, it is difficult to
separate them from each other. Special " filtration
or deconvolution is sometimes used to"
additionally improve "the energy resolution when evaluating
spectra(eg application of Laplace filters, Metz or Wiener
filters). However, these procedures are only applicable with a
sufficient number of accumulated pulses (good
"statistics"), as they are very sensitive to data
fluctuations - their application significantly amplifies
statistical fluctuations. The issue is largely similar to the spatial
resolution of photographic or gammagraphic imaging (cf. the
work " Filters
and filtration " in
scintigraphy).
In the mathematical analysis of spectrathe
positions of the vertices are determined, the photo peaks are
interspersed with suitable functions (mostly Gaussian curves),
the background curves are also fitted, the photo peaks are
separated from the background and from each other. The integrals
of the resulting photopeaks are then determined and corrected for
the energy dependence of the detection efficiency. This creates
values ??of energies and intensities of individual components of
the measured radiation, which is the final result from the point
of view of spectrometry. Depending on the application used, these
results are then further interpreted - for
example, they are assigned the appropriate radionuclides
(their type and quantity) contained in the analyzed sample.
Spectrum analysis used to be done manually, which was a very
laborious matter. Powerful spectrometry computer software is now
available that allows instant online analysis of spectra
, including evaluation and interpretation of results - such as
assignment using a radionuclide spectrum database
.
Adverse
events with scintillation detectors
were discussed in more general terms in more detail in the
conclusion of §2.1, passage " Aging and
radiation wear of detectors " and " Nuclear
reactions and induced radioactivity inside detectors.
In addition, we will mention the risks of very intense radiation.
Exposure to strong radiation can cause radiation-induced chemical
reactions in the detector material, deteriorating the detector's
properties - reducing detection efficiency and deteriorating
resolution. Immediately after such overexposure deexcitation of
metastable levels and chemiluminescence of molecules released in
the detector during intense exposure.Extreme radiation intensity
can irreversibly damage the detector !
- strong electron flux in the photomultiplier can damage the
surface of the dynodes.
Scintillators and their properties
Mechanism of scintillation formation
Inorganic
scintillators
Solid state physics describes the electrical and optical
properties of these substances using the so-called band
theory , according to which electrons in matter are
combined into energy bands separated from each other by
unoccupied bands of "forbidden" energies. Discrete
energy states of electrons orbiting individual atoms in solids in
orbits propagate into energy bands due to
interaction with other atoms in the solid , but there are certain
gaps between these bands - the so-called bands of
forbidden energies , which electrons cannot acquire. The
highest energetically occupied belt is the valence
belt, followed by a forbidden strip and above it lies the conductivity
strip , which is completely unoccupied
in the equilibrium (basic) state of the insulators . However, this is only the case
with an ideally formed crystal lattice. In reality, however,
various changes and defects occur in the
regularity of the crystal lattice (they
occur both spontaneously and can be created by activation
of ions of suitable elements) ,
which lead to local discrete energy levels in
the forbidden area between the energy bands. Electrons from other
bands can jump into these new energy levels. This creates excitation
centers , which are of three
types according to the nature of the process in which the
transition of electrons between energy levels occurs: luminescent
centers, centers of metastable states and quenching
centers - Fig.2.4.7 on the left (there is
only one luminescent center marked) .
If ionization occurs in the
crystal by a charged particle or photon and the released electron
has a sufficiently high energy, it jumps to an empty conduction
band. During movement in this band, the electron can be captured
by the luminescent center at a higher energy
level (excited state). When switching from this higher level to a
lower level, the emission of luminescent
(fluorescent) radiation occurs. However,
electrons from the conduction band can first be trapped in free metastable
levels and only after returning to the conductive band
will they cause light radiation to pass through the luminescent
centers - delayed luminescence radiation , or phosphorescence
, is emitted . The activated alkali metal
halides there are metastable energy level and at the luminescent
centers, so there occurs immediate fluorescence and delayed
phosphorescence in the same range (from a
metastable state of electrons transferred after obtaining the
energy initially on arousing luminescent energy level from which
occurs the transition to the ground state as well as in direct
luminescence) . Finally, electrons can be
trapped in the levelsextinguishing center ,
where there is no emission of light, but non-
radiative energy transfer by electromagnetic interaction with
surrounding atoms.
In inorganic scintillators
, the luminescent centers can be formed by the
base material, and a large number of luminescent centers can be
additionally formed by introducing activator
ions - dopants *) into the crystal lattice -
these ions cause said additional discrete levels in the band gap
(Fig.2.4.7 left ). Silver ions in zinc sulfide
crystals ZnS (Ag) or thallium ions (in an amount
of 1-2%) in crystals of alkaline element iodides such as NaI (Tl)
can serve as activators .
*) Activator labeling convention:
In the literature on ionizing radiation detection, it is
customary to indicate the activator or doping element in parentheses
after the chemical label of the main carrier - eg NaI (Tl), Lu 2 SiO 5 (Ce), etc. However,
chemists prefer the convention of colon labeling
- eg NaI: Tl, Lu 2 SiO 5 : Ce, Al 2 O 3 : C and the like. The designation with parentheses in
chemistry could be confused with indexed groups in chemical
samples of compounds, eg SO 4 (NH 4 ) 2 . Here in our physical treatise, we mostly use
designations with parentheses, sometimes compromise designationscombined
- Al 2 O 3 (: C), CaF 2 (: Dy) etc.
Fig.2.4.7. Symbolic representation of the mechanism of
scintillation formation in inorganic and organic substances.
Left: In inorganic scintillators, scintillation photons
are formed by electron jumps captured at higher levels of
luminescent centers formed by perturbations in the scintillator
crystal lattice (activator T1 in the NaI crystal lattice).
Right: In organic scintillators, scintillation occurs by
deexcitation of the excited molecules of the scintillator itself.
For both cases, the resulting scintillation consists of several
hundred of these secondary photons, depending on the absorbed
energy of the primary quantum detected (and thus the number of
ionization electrons) and the conversion efficiency of the
scintillator.
Organic
scintillators
In organic scintillators , the
mechanism of scintillation formation is different from that of
inorganic substances - it is the excitation and deexcitation of
the energy states of the scintillator molecules
themselves . Organic molecules exhibiting scintillation
properties are mainly benzene nuclei of cyclic
"aromatic" compounds. The
energy states of a molecule , which are quantized, arise
in three ways: by rotating the molecule as a whole, by
oscillating motion (vibrations) of atoms in the molecule, and as
a result of changes in its electronic configuration. Rotational
states are separated only by very small energy intervals
(approx. 10 -3 eV) and the radiation that arises during transitions
between these states has a spectrum in the microwave region with
wavelengths of 0.1-10 mm. The vibrational states
are separated by slightly larger energy intervals (approx. 0.1
eV) and the vibrational spectra lie in the infrared region with
wavelengths of approx. 1 m m-0.1 mm. The electronic states of the
molecule have higher energies, the distances between adjacent
energy levels of valence electrons are several electron volts,
and the respective spectra are in the visible and ultraviolet
regions. It is the excited electronic states that are important
for the formation of scintillations; these are mostly excitations
and deexcitation of electrons forming interatomic bonds in the
aromatic molecule ( p -electrons). A molecule in an excited
electronic state can lose energy and return to its ground state
in various ways (if this energy is too high, the molecule may
even dissociate and disappear). One possibility is simply a
direct transition to the ground state with the emission of one
photon (if the excitation occurred by irradiation with light, the
emitted photon has the same energy as the absorbed photon).
Another possibility is fluorescence : a molecule
can give off part of its vibrational energy in collisions with
other molecules, so that the radiant transition occurs from a
lower sub-level of the electronic state (fluorescent radiation
has a lower frequency than the radiation originally absorbed).
Radiant transitions between some levels are "forbidden"
by selection rules, so such transitions occur for a very long
time - it arisesphosphorescent radiation , which
can be emitted for minutes or even hours after the molecules have
been excited. Many transitions during collisions lead to non-
radiative energy transfer. In general, however, the electron
transitions are accompanied by radiation in the visible or
ultraviolet part of the spectrum, with each transition having a fine
structure - appearing as a series of closely spaced
lines due to the presence of different rotational and vibrational
states in each electronic state.
Thus, when energy is
absorbed, the molecules transition from the ground state to a
higher energy level, from which the molecule returns to the
ground state by radiating thermal energy and the fluorescent
quantum - Fig.2.4.7 on the right(again,
only the luminescent deexcitation of the scintillator molecule is
marked there) . Fluorescence molecules
appear mainly in aromatic hydrocarbon molecules with double or
multiple benzene nuclei (specific species will be mentioned
below).
Different
scintillation in inorganic and organic scintillators
It is appropriate to emphasize the different mechanism of
scintillation in organic and inorganic scintillators:
¨ In inorganic
scintillators, the scintillation effect is a property of a
suitably arranged crystal lattice with
luminescent centers - they are always solid -based
detectors . When the inorganic detection
substance (eg in water) is dissolved, the crystal lattice
disappears and the scintillation effect disappears
.
¨ In organic
scintillators, scintillation occurs by deexcitation of its
own molecules suitable organic compounds. When
dissolving such a scintillator in a suitable organic solvent, the
organic molecules remain unchanged and the scintillation effect
is usually preserved - a liquid
scintillator is formed (the
properties of liquid scintillators and their use will be
discussed in more detail below in §2.6, section " Detection of beta radiation by liquid
scintillators ") .
Properties
of scintillators
We will now introduce some physical parameters
that can characterize scintillation materials and which are
important for their practical applications.
¨ Conversion
efficiency
The basic parameter of the scintillation material is the conversion
efficiency , which is the ratio of [%] of the total
energy of the emitted light and the absorbed energy of the
incoming quantum of ionizing radiation. In practice, the
so-called light yield is used more often than
the conversion efficiency , given as the number of emitted light
photons per 1 MeV of absorbed energy of the detected quantum of
primary radiation.
¨ Luminescence
spectrum
describes the spectral composition (wavelengths) of the emitted
light. It is important to compare this luminescence spectrum with
the maximum spectral sensitivity of the photocathode
, which in most photomultipliers is in the blue region of the
spectrum about 600-700nm.
¨ Scintillation
afterglow
Another important characteristic, describing the
temporal properties, is the duration of scintillation
, or so-called scintillation afterglow - the
time during which the fintillation photon flux drops to 1 / e.
This parameter co-determines the speed of the whole scintillation
detection process - the dead time of the
detector and the time resolution when coincidently
using two or more scintillation detectors.
¨ Density
and the proton (atomic) number of the scintillation material is
particularly important for the detection of gamma
radiation . It determines the degree of absorption
of radiation g in the scintillator and thus the resulting detection
efficiency . For light materials, most of the g- radiation passes
through the photoeffect (or multiple Compton scattering) without
absorption. Scintillators with a density of about 3-9 g / cm 3 are suitable for the
effective detection of especially harder radiation g (with energies of
hundreds of keV up to MeV units) . ¨ Mechanical,
chemical and optical properties of scintillator material
are important for practical implementation and construction of
scintillation detectors. Of particular importance is how big single
crystals can be grown from a given material, while
maintaining good homogeneity and optical transparency (see next
point). This is especially crucial for applications in
scintillation cameras (see §4.2 " Scintillation cameras ") . Some scintillation
materials, especially NaI (Tl), are hygroscopic
, so they must be hermetically encapsulated. Some larger
inorganic single crystals can be quite brittle ,
so they need to be protected from mechanical pressure and also
from larger temperature gradients (different
thermal expansion can cause mechanical stress inside the crystal
and lead to its cracking) . Solubility
is important for organic scintillators in
organic solvents and other chemical properties important for the
application of liquid scintillators (see below).
¨ Energy
resolution
characterized by the ability to distinguish two photons of gamma
radiation with close energy values. The resulting resolution
depends on several factors, discussed above in the " Scintillation Spectrum "
section. The internal resolution of a
scintillation crystal depends on the light yield of the
scintillator and the nonlinearity of the scintillation response
for different electron energies. Furthermore, also on the perfect
optical transparency of the scintillation
material - so that, if possible, the same percentage of
scintillation photons, regardless of the scintillation site, can
fall on the photocathode of the photomultiplier.
Scintillation
materials
There are a number of substances that exhibit scintillation
properties. It can even be said that almost every optically
transparent substance, when interacting with ionizing radiation,
emits a certain amount of photons of visible light, but the light
yield is usually small, the wavelength of this light may not
correspond to the spectral sensitivity of the photomultiplier
photocathode and scintillation time may not be short enough.
Therefore, substances that have optimized these
properties are referred to as true scintillators
.
Inorganic scintillators First we will mention inorganic
scintillation materials . As already mentioned, the longest known
scintillator is zinc sulfide activated by silver
atoms ZnS (Ag). However, the most used inorganic scintillator is
a crystal: ¨
NaI (Tl) - sodium iodide ,
activated with 1-2% thallium. It is suitable for the detection of
low and medium energies of gamma radiation (see
the curve of energy dependence of the detection efficiency in the
right part of Fig.2.4.6) . Its disadvantage
is that it is hygroscopic. It must therefore be
mounted in airtight housings with a glass outlet window, the
inner walls of the housing are provided with a reflective coating
such as magnesium oxide to increase the light yield. If the case
is not perfectly hermetic, the crystal absorbs moisture from the
air, it is hydrolyzed (manifested by yellowing -
dissociation of NaI), loses perfect transparency, deteriorates
energy resolution and conversion efficiency (Fig.2.4.8 b, c).
Fig.2.4.8. Influence of encapsulation hermeticity on NaI (Tl)
scintillator properties.
a) Perfectly hermetic encapsulation of the
crystal ensures its long-term transparency and good spectrometric
properties.
b) The NaI (T1 ) crystal,
yellowed due to imperfect hermeticity, shows impaired resolution
and reduced conversion and detection efficiency.
c) A more serious violation of the hermeticity
leads to a browning of the NaI (Tl) crystal, which not only
completely loses its spectrometric properties ,
but to a large extent also loses its simple detection properties!
Note: All three scintillation detectors
have the same dimensions (diameter 45 mm, height 25 mm) and their
age from production is 43 years.
For the detection of higher energies of gamma
radiation, scintillators with a higher density are more suitable
from the point of view of detection efficiency :
¨ Bi 4 Ge 3 O 12 (bismuth-germanium oxide, abbreviated BGO
);
¨ Lu 2
SiO 5 (: Ce) (cerium-activated lutetium
orthosilicate - LSO );
¨ Lu 1.9 Y 0.1 SiO 5 (lutetium yttrium silicate LYSO );
¨ Y 2 SiO 5 (: Ce) (cerium-activated yttrium
orthosilicate - YSO);
¨ Gd2 SiO 5 (: Ce) (cerium activated gadolinium
orthosilicate - GdSO);
¨ further LuAlO
3 (Ce) (LAO).
¨Based on LSO , the LFS (
Lutetium Fine Silicate ) scintillator was further developed,
which has a finer crystal structure and, in addition to
basic lutetium, silicon and oxygen (LSO) with doping Ce, also
contains carefully tested small impurities of some other elements
such as Ca, Gd, Sc, Y, La, Eu, or Tb. Depending on the type and
content of alloying elements, there are several types of numbered
LSF scintillators, such as LFS-3. This results in slightly better
energy resolution and shorter scintillation afterglow.
These high-density scintillators are mainly used for the
detection of 511keV energy annihilation e - e + gamma
radiation in positron emission tomography (PET)
cameras , where it is necessary to achieve high detection
efficiency with not too large crystal thickness (in order to achieve high spatial resolution of
scintillation localization by a system of photomultipliers) - see §4.3 "Tomographic cameras", section
" PET cameras ".
¨ The heaviest scintillators, such as PbWO
4 , are used as part of complex detection systems that
register high-energy radiation
from the targets of large accelerators of elementary particles (eg the new "Large Hadron Collider" at CERN
uses almost 80,000 PbWO 4 crystals in the detection part ) -
see §1.5 "Elementary particles", section " Charged particle accelerators ".
For some special purposes, such as the
detection of harder X-rays in CT, other scintillation materials
such as Lu 1.9 Y 0.1 SiO 5 (LYSO), CdWO 4 are used . Scintillators based on ceramic materials (silicon
oxides), doped with rare earths (such as gadolinium or yttrium)
and possibly and other elements, sometimes referred to as UFC
(Ultra Fast Ceramic) - ultra-fast ceramic detectors
. Scintillation flashes are detected by either photomultipliers
or phototransistors (significantly simpler and
cheaper phototransistors or photodiodes are sufficient if it is
only a simple detection, not spectrometry; another advantage is
the miniature size) - see §3.2 "X-ray diagnostics",
section " Transmission X-ray tmography (CT) ". The use of so-called silicon semiconductor
photomultipliers described above is promising (section " Photomultipliers ", Fig.2.4.2g ) .
The
following table lists several inorganic scintillation materials
more commonly used in scintillation detectors. They are sorted by
increasing density (which increases the detection efficiency for
higher energy gamma radiation) :
Scintillator: | NaI (Tl) | CsI ??(Tl) | Y 2 SiO 5 (Ce) | BaF 2 | LaBr 3 (Ce) | Gd 2 SiO 5 (Ce) | Bi 4 Ge 3 O 12 | Lu 2 SiO 5 (Ce) | CdWO4 | PbWO4 |
Density [g/cm 3] | 3.67 | 4.51 | 4.53 | 4.89 | 5.1 | 6.71 | 7.13 | 7.41 | 7.9 | 8.23 |
l max [ nm ] | 415 | 400/565 | 420 | 220/310 | 360 | 440 | 480 | 420 | 470/540 | 410/500 |
scint. afterglow [ms] | 0.23 | 0.6 / 3.4 | 0.07 | 0.008 | 0.016 | 0.06 | 0.3 | 0.04 | 20/5 | |
h [photon/MeV] | 4.10 4 | 5.10 4 | 4.6.10 4 | 1.8.10 3 | 6.3.10 4 | 1.10 4 | 8.10 3 | 3.10 4 | 5.10 3 | 3.10 2 |
Note: In terms of
spectrometric properties, it is worth noting the
lanthanum bromide crystal doped with LaBr 3 cerium(: Ce 5%),
which thanks to its high light yield of 63000 photons / MeV and
good energy proportionality in the scintillation detector
provides very good energy resolution (approx. 4
% at 137-Cs). It also has a very short scintillation
afterglow of 16ns, which makes it promising for the
coincidence detection of annihilation photons in PET
positron emission tomography (§4.3
" PET cameras ", TOF
analysis).
Internal radioactivity
of LSO scintillators
A certain disadvantage of lutetium -based
scintillators (such as LSO or LYSO) is their relatively high
background due to the internal radioactivity
contained in the scintillator. In addition to the basic stable
isotope 175
Lu (97.4%), natural lutetium also contains an indelible
admixture of the long-term radioisotope 176 Lu (2.6% - a natural radionuclide of primary origin ), which decays with a half-life of 3.8.10 10 years b - converted to a stable hafnium 176 Hf, emitting beta radiation with max energy E b max = 596 keV (99.6%) and a prompt cascade of gamma
radiation with energies E g88keV (15%), 202 keV (78%), 307keV (94%) and 401keV
(0.4%) - see §1.4, passage " Lutetium
".
The mass specific activity of 176 Lu in the LSO
material is about 39 Bq / gram LSO *) and the resulting internal
radiation background (due to the 100% efficiency of internal
detection) reaches values ??of about 250 imp./s./cm 3
LSO ; medium scintillation detector 100cm 3The LSO would
therefore have an internal radiation background of about 25,000
imp./s.! Therefore, LSO scintillators are not suitable for
measuring weak radiation fluxes and low activities. However, when
used in medical positron tomography PET, for which they are
primarily intended, this disadvantage does not apply in practice.
On the one hand, coincidence measurements are performed, and on
the other hand, the internal background of LSO / LYSO
scintillators is negligibly small compared to the fluxes of the
measured annihilation radiation of about 10 6 photons / s in clinical scintigraphy. However, certain
problems may arise in experimental studies of PET with low
activities and long measurement times (animal PET) - it is
discussed in more detail in §4.3 "Tomographic
cameras", section " PET cameras ". *) Mass specific activity
can be determined using the formula derived in §1.2. "
General laws of atomic nucleus
transformation ", passage" Relationship
between half-life and activity ": A 1g @ (6.10 23 /N).ln2/T 1/2 @ 4,16.10 23 / (NT 1/2 ). Substituting the
mass number N and half-life T 1/2 [s] for 176 Lu is 51.23 Bg / 1 g of pure 176 Lu and after conversion to a content of 2.6% 176 Lu in lutetium and
a lutetium content of 76% in LSO we get a final value of about 39
Bq / 1 gram LSO . Just a small interesting
thing is that the above-mentioned lanthanum bromide
The LaBr 3 (: Ce) scintillator
has an internal natural radioactivity: natural lanthanum, in
addition to the stable basic isotope 139 La, also
contains 0.09% of the long- lived radioisotope 138 La,
which is converted to barium 138 by electron capture (70%) with a
half-life of 1.12.10 11 years. , or b - radioactivity (30%) at cer 138; it is accompanied by
the emission of gamma photons 1426 and 790 keV. However, the
volume specific activity here is only about 1 Bq / cm 3
LaBr 3 .
How did lutetium 176 Lu form?
The answer is given by nuclear astrophysics - a
fascinating scenario of cosmic nucleogenesis (see "
Cosmic
alchemy"or" We
are the descendants of the stars! "). 176 Lu was nuclear systetizováno, along with a stable of 175 Lu and all the
heavier elements, more than 5 years milardami during a
supernova explosion , the ejected gases which formed the
solar system, including planet Earth ...
Organic scintillators
There are also a number of organic
substances that have scintillation properties (see Fig.2.4.7 on the right for the mechanism) . It is mainly naphthalene , which
emits scintillation radiation with a very short flash time of
0.08 m s
and a wavelength of around 345 nm (this
wavelength is shorter compared to the maximum spectral
sensitivity of the photocathodes of most photomultipliers, so a
"spectrum shifter" is sometimes used - see § 2.6) . An important organic scintillator is anthracene
, which is used as a standard to compare the properties of all
other organic scintillators. Anthracene emits scintillation with
a flash time of 0.03 mwith a wavelength of 450 nm, its conversion efficiency
is about half that of NaJ (Tl). Other organic substances include stilbene
, which emits scintillation with a duration of 0.08 m s and a wavelength
of 380-410 nm; its advantage is the ability to form large
crystals. Organic scintillators have too low a density to detect
gamma radiation, so the detection efficiency would be low.
However, they are very suitable for the detection of beta
electrons, alpha particles, protons, deuterons and also fast
neutrons (neutrons emit protons from the molecules of organic
matter, which cause scintillation and are thus registered).
Organic scintillants usually retain their
scintillation properties even when dissolved in suitable organic solvents *) (toluene, xylene,
benzene, dioxane, phenylcyclohexane, phenyl ether, etc.) - liquid
scintillators are formed . Liquid scintillators have the
advantage that they can be adjusted to a suitable shape even by a
simple filling into a suitable container, even to a size which is
not achievable with solid (crystalline) scintillators; they are
therefore used, for example, in the detection of cosmic rays.
However, the main use of liquid scintillators is in the method of
detecting beta emitters directly in solution
with these scintillators (§2.6, section
" Liquid
scintillators ") . The use of organic and liquid scintillators will be
described below in §2.6 in connection with the beta and alpha detection
methodology.
*) This is due to the mechanism of
scintillation formation (which was outlined above - Fig.2.4.7).
In inorganic scintillation crystals, where scintillations occur
during deexcitation of energy levels in the luminescent centers
of the crystal lattice , when the scintillator
dissolves, the crystal lattice disappears and the scintillation
effect disappears. In contrast, in organic scintillators, where
scintillation occurs during excitations and deexcitation of the
energy levels of the molecules of the organic
substance itself, the scintillation effect persists
even after the scintillator is dissolved.
Cherenkov detectors
In addition to the scintillation mechanisms described above,
there is another process of light formation during the
interaction of ionizing radiation with matter: Cherenkov
radiation . As stated in §1.6 " Ionizing radiation
", the passage " Cherenkov radiation ", a charged particle that flies through an
optically transparent medium with a refractive index n at
a speed higher than the speed of light c '= c / n in this medium,
produces "shock" electromagnetic waves - visible light
called Cherenkov radiation. The physical mechanism of this
radiation was clarified in the mentioned §1.6, passage " Cherenkov radiation", where the table also shows the threshold
energies for the generation of this radiation for different types
of particles in different media environments. This radiation can
be used to detect charged high-energy particles
or hard gamma radiation, which is previously converted to
electrons by interacting with The
Cherenkov
detector , in its simplest configuration, consists of a transparent
dielectric with a high refractive index (eg plexiglass),
in which the passing charged particles excite Cherenkov radiation
which impinges on the photocathode. photomultipliers
, where it is converted into electrical pulses
similar to scintillation detectors. Different sizes and shapes of
dielectric are used, the detection medium is sometimes liquid (eg
water) or air, lens or mirror optical systems are sometimes used
to concentrate Cherenkov radiation on the photocathode of one or
more photomultipliers. Since the number of emitted photons and
the angle of the direction of their emission with respect to the
direction of motion of the primary particle depend on its energy
(superluminal velocity), the energy of the detected charged
particle and the direction of its motion can be determined.
When detecting Cherenkov
radiation, the problem of a small number of emerging
photons is encountered . According to the relations
given in §1.6, the passage " Cherenkov radiation", about 200 photons per centimeter of
ultrarelativistic electron trajectory are produced in water,
under less optimal conditions it is less. Therefore, high demands
are placed on the properties of photomultipliers - high quantum
efficiency of the photocathode for the Cherenkov spectral field,
low noise hereinafter also low absorption of radiation in the
environment. the issue is somewhat similar to the detection of
low energy b -záøení tritium 3 H in liquid scintillators (see
below §2.6 passage " scintillation
") .
The
ring imaging Cherenkov detectors - RICH
For special purposes of directional detection of high-energy
particles, more complex detection systems have been developed,
using the properties of Cherenkov radiation. They are called RICH
( Ring Imaging Chrenkov detector ). They consist of a
system of mirrors (spherical and planar) that
reflect light photons of Cherenkov radiation, created along the
path of a particle in an optical medium (eg CF 4 , C 4 F 10 , ....), and direct
them to a system of a large number of imaging
photomultipliers . Electronic analysis of impulses from
these photomultipliers can reconstruct the trajectory of the
particle and determine its energy. The device therefore works asimaging
detector-spectrometer of fast charged particles. Cherenkov detectors have their main use for the detection
of high-energy particles - they are used in large
accelerators and in the detection of cosmic radiation (see also
the passage " Neutrinos " in §1.2 " Radioactivity " or "
Cosmic radiation " in §1.6).
2.5.
Semiconductor detectors
By the mechanism of direct
electrical use of ionizing radiation effects, the semiconductor
detector is somewhat similar in principle to the
ionization chamber, but the sensitive medium is not gas but a
suitable semiconductor material. From an electronic point of
view, the semiconductor detector is basically a diode
connected in a high voltage electrical circuit (approx. 1000-2000
V) via a large ohmic resistor in the closing
(non-conductive) direction (Fig.2.5.1), so that no current flows
through the circuit at rest.
Fig.2.5.1. Diagram of a semiconductor
detector. On the right is an example comparing the semiconductor
spectrum of gamma radiation with the scintillation spectrum.
If a quantum of ionizing radiation enters the
active layer of the detector (it is a "depleted" layer
or volume region without free charge carriers), the ionizing
energy causes the semiconductor to jump a proportionate number of
electrons into the conductive band and form electron-hole
pairs . These electrons immediately start moving in the
electric field to the positive electrode (and the holes to the
negative) - a short current pulse passes through
the electric circuit , a voltage drop occurs on the working
resistor R and through the capacitor C the electric pulse leads
to a charge-sensitive preamplifier. The
amplitude (or time integral) of the pulse at the output of the
amplifier is directly proportional to the total charged charge,
and thus the energy of the detected radiation (more precisely,
the energy that was absorbed when the quantum of radiation passed
through the active detector layer). Thus, by amplitude
analysis of the output pulses, we can perform a spectrometric
analysis of the energy of the detected radiation,
similarly to scintillation detectors. The amplified pulses are
fed to an analog-to-digital converter and from
there to the memory of a " multichannel analyzer
", now realized in a computer, in the memory of which the
resulting spectrum is stored.
Semiconductor g radiation detectors have a very good energy
resolution (usually better than 1 keV), about 30 times better than
scintillation detectors - see comparison of spectra in Fig.2.5.1
on the right. Two basic factors in particular contribute to this
:
1. The collection of the charge
created in the semiconductor by ionization is relatively perfect
from the entire sensitive volume.
2. The small width of the band gap
leads to the low energy required to form one electron-hole pair.
The number of these charge pairs generated by the detection of a
quantum of a given energy is therefore high (more
than 10 times higher than with gas or scintillation detectors) and thus the relative quantum-statistical fluctuations
in their number are low.
Semiconductor detectors also have a
high ratio of photopeak to continuous Compton background.
However, compared to scintillation detectors, they usually have
somewhatlower detection efficiency for gamma
radiation and also longer dead time (dead time is given by the capacity of the detector +
preamplifier system and the value of the working resistance) . Semiconductor detectors are used wherever we need the
best possible energy resolution, eg in nuclear physics, neutron
activation analysis, X-ray fluorescence analysis, detection of
radionuclides in ecology or measurement of radionuclide purity of
preparations. Otherwise, all the principles and principles of
gamma-ray spectrometry (§2.4,
section " Scintillation spectrum ") , including energy and
efficiency calibration, which have been discussed in the
scintillation detector, also apply to the semiconductor detector.
In some applications, the great
advantage of semiconductor detectors is their independence
from the magnetic field (unlike
photomultipliers used in scintillation detectors) .
Differences in
scintillation and semiconductor gamma spectra
If we look at the example of gamma spectrum in Fig.2.5.1 on the
right (enlarged section from the spectrum
of radionuclide 123 I) , we can see at first glance
two significant differences between the spectra
measured by scintillation and semiconductor detector:
¨ The photo peaks on the scintillation
spectrum are round and gradual , as if
"scattered" - the energy resolution is relatively
imperfect here (approx. 10% for the 662keV
test line 137 Cs) , the nearby gamma lines
merge into one photopeak. The semiconductor
spectrum, on the other hand, consists of very sharp and
narrow peaks - the energy resolution is about 30 times
better. Some compact (
"over-extended") peaks of
scintillator spectra on a semiconductor spectrum spread over two
or several y-lines (obr.2.5.1 right) ...
¨
The scintillation spectrum is seen
distinctly represented continuous component
Comptonovsky diffuse radiation, especially in lower energy areas.
This continuous background is disturbing (especially
in the "peak" area of ??the backscatter, which can
interfere with the actual gamma peak of the measured
radionuclide). In semiconductor
spectra, the continuous component is strongly suppressed
because better energy resolution leads to narrow and high
peaks (while maintaining the same
area under the peak) , which automatically
leads to a reduction in the relative height of the continuous
background in the spectrum plot normalized to the peak peak.
Germanium and silicon semiconductor detectors
Semiconductor detectors are mostly
made of germanium single crystals , either with
a trace amount of lithium, so-called drift - Ge (Li)
detectors , or more recently of superpure germanium HPGe ( High Purity Ge ), or
Si * silicon ). Germanium detectors are constructed either in a coaxial
arrangement of nip layers (for the detection of higher gamma
energies) or in a planar shape with a thin input window
(for the detection of soft gamma and X). Silicon detectors Si
(Li) are mainly intended for the detection of soft gamma and X
radiation with high resolution, often a beryllium input window
with low absorption of soft gamma and X radiation is used.
*) Other semiconductor materials such as Ga (As) are also used. ,
Cd (Te) .... For the detection of gamma and X-rays, detectors
based on CdZnTe (CZT) are also used, which have a high detection
efficiency for photons of energy of tens of keV and work even at
room temperatures, see below.
Upon absorption of a gamma photon of
energy 1MeV in germanium HPGe, approximately 3 x 10 is formed5 electron-hole pairs.
Compared to silicon, germanium is more effective for detection
than silicon because its atomic number is much larger. The
average energy for electron-hole pair formation is 2.9eV for
silicon and 3.6eV for germanium. Due to its higher atomic number,
germanium has a significantly higher absorption coefficient for
gamma radiation, so it is suitable for the detection of gamma
radiation of higher energies up to several MeV. Silicon
detectors, which are several millimeters thick, are suitable for
low-energy spectrometry of gamma and X photons, keV units.
Scintillation and semiconductor detector when used in
gamma-spectrometry of radionuclides.
Left: Scintillation probe - scintillation
crystal NaI (Tl) + photomultiplier with shielding. Middle:
Analog-to-digital converter (ADC) and computer (CPU) multichannel
analyzer. Right: Semiconductor Ge (Li) / HPGe
detector with preamplifier and Dewar vessel with cooling liquid
nitrogen.
Semiconductor spectrometric detectors usually
need to be cooled to liquid nitrogen (LN 2 - Liquid Nitrogen) *) to function
properly to reduce the closing current and electronic noise. In
low-energy detectors, the preamplifier is often cooled, the input
element (field-effect transistor) of which is located in the cryostat together with the detector in
order to minimize the preamplifier noise. For some new types of
semiconductor spectrometers with HPGe, it is no
longer necessary to use liquid nitrogen added to the Dewar for
cooling, as an electronic cooling system is
used., working on the basis of Joule-Thomson expansion of
compressed gas (in addition to nitrogen, helium or other suitable
cryogenic gases are also used), with a miniaturized compressor
with a Stirling cycle and possibly also with Peltier
thermocouple .
*) Ge (Li) detectors even have to be cooled
permanently during storage; interruption of cooling
leads to diffusion of Li drift and destruction of the detector!
The advantage of detectors made of very pure germanium HPGe is
the possibility of thermal cycling - during the
measurement it is cooled to the temperature of liquid nitrogen,
but they can be stored at room temperature .
However, semiconductor detectors
have been developed that operate at room temperature
, using semiconductors with a large bandwidth, such as alloys of
two(GaAs, CdTe, InP) or
three (CdZnTe, InAlP) different
semiconductor elements. Although these detectors do not achieve
such a perfect energy resolution, they have a higher detection
efficiency against photo radiation (see CZT
detectors below ) .
Semiconductor detectors with a surface
barrier are used to detect corpuscular radiation (alpha,
beta, protons - see below), which has a short range in
substances.. They are designed so that a very thin metal layer of
eg gold (thickness of only a few atoms - less than 1 micrometer)
is applied to the front side of the polished silicon wafer of the
"n" type semiconductor, which serves both as an
electrode and an input window of the detector; the back wall is
nickel-plated and serves as the second electrode. A voltage of
approx. 100-200V is connected to such a detection diode with a pn
junction via a working resistor, pulses generated by particle
detection are taken from the working resistor for amplification
and further processing.
Diamond
detectors
Crystalline carbon - diamond can also serve as a
suitable material for electronic detection of low-energy
radiation . Here, the carbon atoms are bonded to each other by a
strong covalent bond and are arranged in a cubic crystal lattice.
The width of the band gap is 5.45eV. Pure diamond is electrically
non-conductive (dielectric, high resistivity » 10 16 W cm). A small amount of defects or dirt causes the
diamond to discolor; such a diamond also behaves like a
semiconductor. Diamond detectors are somewhat similar to silicon
detectors.
The
diamond crystal is placed between two electrodes to which voltage
is applied - the connection is the same as in Fig.2.5.1. An
ionizing particle or photon of electromagnetic radiation releases
electrons (and positive ions - "holes") in the crystal
lattice upon their impact and passage, which move under the
influence of an electric field in the conduction band in the
crystal and cause an electric impulse at the electrodes. The
principle is very similar to a classic gas ionization chamber - a
diamond detector is a kind of " solid ionization chamber
".
As with other semiconductor detectors, diamond detectors can be
made either as single single crystals or as layers
formed by Chemical Vapor Deposition (CVD) technology . ; lath. vapor
= steam ). With this CVD technology, even more complex
structures of a larger number of detection elements can be
created - multi -detector systems or polycrystalline
active surfaces in the strip-detector mode .
Diamond detectors have some advantageous properties:
× Mechanical resistance and good thermal
conductivity.
× Insensitivity to visible light (they are
sensitive only from harder UV radiation).
× They work at room temperature, no
cooling is required. × High signal-to-noise ratio.
× Fast
signal response. Diamond detectors are very fast, have a time
resolution in the order of tens of picoseconds. They are able to
work even at high intensity particle flow. They are therefore
used as internal " trigger " detectors, which
define the exact moment of interaction in complex detection
systems at accelerators (" Arrangement and configuration of radiation detectors ").
× High
radiation resistance to damage in strong radiation beams.
They are used, or are promising, in a number of areas :
- detection
of high energy particles, detection of neutrons and a-particles;
- monitoring
and dosimetry of photon and particle beams for radiotherapy;
-
measurements of cyclotrons and synchrotrons;
- trackers
for complex detection systems examining the interactions of high
- energy particles in accelerators (" Arrangement and configuration of radiation detectors ").
Cadmium-Zinc-Teluride ( CZT
) Detectors
Spectrometric germanium and silicon semiconductor detectors
provide excellent energy resolution, so they play an
irreplaceable role in the precise analysis of gamma radiation
from natural samples and laboratory preparations. However, their
technical disadvantage is the need to cool
to liquid nitrogen temperature. This is a serious, often
insurmountable, obstacle in many laboratory and
all practical technical applications of radiation . Therefore,
there is a need to develop semiconductor detectors that have at
least "slightly good" spectrometric properties, but
could operate at the usual room temperature (in the laboratory, natural terrain).
From this point of view, some
compounds of tellurium ( tellurides
) with metals such as zinc and cadmium (used in photovoltaic cells of solar panels) have proved successful . For detection and gamma
spectrometry is used in particular cadmium telluride and zinc
CZT ( Cadmium Tellurium-Zinc- ium)
- Cd x Zn (1-x) Te in different
ratios, usually x = 0.1-0.15. Sometimes small admixtures of other
elements are added to improve the crystalline-electrical
properties of the alloy (usually 0.05-0.07
selenium) .
This alloy acts as a semiconductor
detector operating at room temperature,
which converts gamma and X radiation into electrical impulses
with high efficiency. Semiconductor CZT detectors can in most
applications serve as a ( better ) replacement for
conventional scintillation NaI (Tl) detectors, with significantly
smaller dimensions.
The comparison of the basic parameters of the semiconductor
detectors HPGe, CZT and scintillation detector NaI (Tl) is in the
table (approximate practical - average,
typical - values are given) :
Detector type | Energy resolution (FWHM at 662keV 137 Cs) |
Density | Detection efficiency | Pulse reverberation time | Max. pulse frequency / s . |
HPGe | 1.4 keV (0.2%) | 5.32 g / cm 3 | % | ... [ns] | 1.5 . 10 5 cps |
CZT | 33 keV ( 5% ) | 5.82 g / cm 3 | ... | 2 . 10 6 cps | |
NaI (Tl) | 55 keV (8%) | 3.67 g / cm 3 | 230 ns | 2 . 10 5 cps |
Compared to spectrometric germanium GeHP
detectors, CZT detectors have poorer energy resolution (which is suitable for many
applications ) , but higher detection
efficiency and shorter dead time.
Semiconductor DHW detectors are
often mounted on printed circuit boards, which on the other hand
have connected integrated circuits containing amplifiers,
analyzers and other electronics for processing the detected
pulses. This creates miniaturized compact integrated modules - detection
blocks (analogous to scintiblocks) , which can be fitted to more complex detection systems.
The use of suitably arranged pixel DHW detectors for semiconductor
gammagraphy is particularly interesting and beneficial. (Planar and SPECT "scintigraphy" - §4.2, part
" Alternative physical principles of scintillation
cameras " , passage " Semiconductor multidetector gamma
cameras ") .
Multidetector semiconductor systems
Advantageous electro-mechanical properties of semiconductor
detectors enable their miniaturization and
integration of individual semiconductor elements into multidetector
systems . These multidetector systems can provide
information both on the energy of the registered
radiation and on the point of impact of the
individual ionizing quanta, or on the paths of the flying
particles - they can therefore have imaging properties
. The most commonly used semiconductor multidetector systems are
of three types :
¨ Array of semiconductor detectors arranged on a suitable surface, usually at regular
intervals. Allows a single measurement chart e.g. geometric
progression of the intensity of radiation beams (........
¨ Pixel semiconductor detectors ( SPD - Semiconductor Pixel Detector ) The
semiconductor thin plate (typically silicon, N-type) are applied
to the electrodes (P) in the form of an output electrical signal,
they dissipate the charge created by the passage of ionizing
particles.The electrodes are distributed in a dense regular grid
, forming cells - pixels - with dimensions of a
few micrometers to tenths of a mm. Electronic circuitry for
evaluation can also be integrated on the board - preamplifiers,
discriminators, multiplexers, counters, analog-to-digital
converters (ADC - allow to evaluate the energy of particles
absorbed in individual pixels). By processing the pulses from
such a detector, we get a planar image of the
distribution of the positions of the incoming particles and
possibly. and their energies. Such imaging detectors are used,
among others, in radiography , especially X-rays
- so-called flat panels with direct conversion,
§3.2, part " Electronic X-ray imaging ". Detectors of this type are also beginning to be
used in gamma cameras of nuclear medicine -
§4.2, part "Alternative physical principles scintillation camera "passage" Semiconductor multidetector gamma
camera . "
Pixel detectors can be spatially
stacked in many layers , in blocks or other
units, allowing spatial display tracks the
passing particles - this is called. Tracker
(tracking of trace particle). Systems These detectors are used in
complex analyzers of high energy particle interactions
, the typical arrangement of which is given above in §2.1,
section " Arrangement and configuration of
radiation detectors ",
Fig.2.1.3 below - forms the innermost part of the detection
system, the tracker .
A simpler variant of the
position-sensitive semiconductor detectors are called. Strip
detectors (strip-shaped), which also consist in systems
constituting trackers ............
¨ Semiconductor drift detectors ( SDD - Semiconductor Drift Detector )
On the surface of the N-type silicon wafer with a high
resistivity, regions P are implanted, forming PN transitions.
Anodes are placed on the edge of the plate, collecting charge
from the detector. During the passage of the ionizing particle,
electron-hole pairs are released, after which the electrons are
moved in a drift motion to the region of the anodes, where they
are captured and generate an output electrical signal. If we know
the rate of electron diffusion, we can determine the position of
the place where the particle flew through the detector from the
time of the pulse at the anode (from the time of the drift
motion). .................
LBSD detectors
A uniquely used type of semiconductor detector
is the so-called long base silicon diodes (LBSD - Long
Base Silicon Diode ), designed to measure the radiation dose
(kerma) from heavy particles, especially fast neutrons. As a
result of irradiation and ionization, the crystal lattice of
silicon is damaged, which changes the lifetime of the minor
charge carriers and thus the conductivity of the diode. The
voltage drop across the diode is measured in the forward
direction before and after irradiation, the change in voltage
drop after irradiation relative to the initial value being an
approximately linear function of the radiation dose (kerma).
Microcalorimetric detectors
During the interactions of radiation with a substance, a certain
part of its energy is converted into heat (the
thermal effects of radiation have already been mentioned in §1.2
" Radioactivity " and in §1.6 " Ionizing radiation ", section " Thermal and
electrical effects of radiation ") . Based on this knowledge, detectors were developed that
use the thermal effects of energy transferred to
the substance to absorb quantum radiation. These detectors can to
some extent be classified as semiconductor detectors ,
either in terms of the primary sensor or for semiconductor
technology used in the electronic processing of measured signals.
The methodology for measuring physical quantities through heat is
generally called calorimetry; In a figurative
sense, all detectors measuring the energy of radiation quanta are
sometimes referred to as calorimeters .
Calorimeter (lat. Calor = heat )
is generally an arrangement or device that
in thermodynamics is used to measure heat
quantities based on heat exchange between test specimens
located in an insulated system, where the law of conservation of
heat-energy applies. Heat, temperature, heat capacity, heat of
reaction, heat conversion to other types of energy, etc. are
measured. For simple calorimeters, the temperature of the
examined environment is measured with a conventional mercury thermometer
. With modern calorimeters, the temperature is registered
electronically using thermistors - electronic
components whose resistance is strongly temperature dependent.
The so-called Isothermal
calorimeters , operating at normal temperature, are
sometimes used for absolute measurement of the radioactivity of
high-activity preparations - bridge temperature thermistors are
used to compare the temperature difference between a reference
sample and a sample containing radioactive material (in which
radioactivity causes the material to heat).
Bolometer (from the Greek bole
= incident, beam )
is generally a device or element that
measures radiation based on its thermal
effects . These thermal effects are mostly measured thermoelectrically
, using the pyroelectric effect - the ability of some
materials to generate a temporary electrical potential when the
temperature changes Depending on the heating, the electrical
resistance of the detector changes. The basic principle of the
bolometer is simple: the absorber converts the
radiation into heat and the thermistor converts
this heat into an electrical signal. In modern bolometers, the
absorber and the thermistor are usually combined into one element
- the detector .
The detector consists of a suitably
shaped thin metal strip or semiconductor or superconducting
material - thermoresistor, with electrical
outlets to the evaluation circuit. By absorbing the incident
radiation, the temperature increases and the resistance of the
thermoresistor changes, on the basis of which the evaluation
apparatus determines the amount of absorbed energy. Bolometers
are sensitive to radiation of any frequency, they respond to
different types of radiation.
An interesting special type of
microcalorimeter is the bolometer on the edge of
superconductivity TES ( Transition
Edge Sensor ). It uses a very steep course
("edges") of the dependence of the resistance of a
suitable material on the temperature around the phase transition
between normal conductivity and superconductivity (Fig.2.5.2 on
the right). The bolometer sensor itself consists of a small,
suitably shaped thin metal strip (eg a tungsten film several tens
of nanometers thick, deposited on a silicon substrate) - Fig.2.5.2 on the left, cooled just below the
superconductivity temperature , so that its resistance
is practically zero. The impact of a quantum of radiation
(photon) slightly heats the tape material beyond the edge of the
superconductivity, thus transitioning to the normal conductivity
mode, the resistance rises sharply (Fig.2.5.2 on the right) and
in the electrical circuit it causes a fast voltage pulse, which
is amplified. 2.5.2 in the middle) and registered in the
evaluation device. After detection, the sensor cools down quickly
to superconducting temperature and is ready to detect another
quantum. The current pulse in the bolometer
circuit (its integral value) is proportional to the change in
temperature and thus the energy absorbed
detected quantum. Instead of a metal film, superconducting
nanofibers with a thickness of about 100 nm are
sometimes used in the TES bolometer .
![]() |
Fig.2.5.2. Highly sensitive bolometer
working on the edge of superconductivity TES ( Transition Edge Sensor ) - principle
diagram. Left: Cooled superconducting sensor. Middle: Electronic signal acquisition. Right: Steep temperature versus TES resistance curve. |
In the most demanding applications, to achieve
the highest possible sensitivity , the signal in
the electrical circuit of the TES bolometer is sensed and
amplified first using the so-called SQUID
magnetic detector *), magnetically connected via a coil
L connected in the bolometer circuit (Fig.2.5.2 in the middle).
The magnetic flux F from the coil L sensitively modulates the current
through both branches of the SQUID ring. Only such a preamplified
signal is fed to a standard electronic amplifier and then to
evaluation.
*) SQUID ( superconducting
quantum interference device )
is a highly sensitive magnetometer used for
measuring very weak magnetic fields. Is based on Josephson
junction , in which an electric current passes between two
superconductors separated by a thin layer of insulator. This is
due to the quantum tunneling of electron Cooper
pairs across this seemingly impermeable barrier. The most
commonly used superconducting material is niobium, or an alloy of
lead with 10% gold or indium. The insulating layer is usually
made of alumina. Two types of SQUID elements are used in
low-current electronics:
- Radio frequency RF SQUID consists of one Josephson
junction connected to a superconducting ring. It is used in the
circuit of a high-frequency oscillator, where the measured
external magnetic flux modulates the frequency of the signal ...
- DC SQUIDconsists of two Josephson junctions arranged
in a superconducting ring. It is much more sensitive to weak
magnetic fields: The
highly sensitive quantum
magnetometer DC SQUID consists of a small superconducting
ring interrupted by two Josephson tunnel layers
(contacts). The input current I is divided into two
parallel branches in the ring. The wave properties of electrons
are manifested here. In the SQUID ring, the electron waves split
into two, passing through the tunnel effect through the
insulating layers, and then the two meet again. In the absence of
an external magnetic field, the two waves meet without a phase
difference. In the presence of a magnetic field, it will interact
with the electric current in the ring and a phase
difference will occurin the wave functions of the
electrons between the two tunnel transitions. The electrons will
show quantum interferences depending on the
strength of the magnetic field (magnetic flux F through the SQUID
ring). The current through the superconducting ring with tunnel
layers then very sensitively depends on the intensity of the
magnetic field (the electrical resistance of the DC SQUID shows a
response even to very small changes in the magnetic field). Using
a suitable electronic circuit (Fig.2.5.2 in the middle), this can
be used to detect very small changes in the magnetic field.
Small note: The current I through DC SQUID is a
periodic function of the magnitude of the magnetic flux F (with the period
given by the elementary quantum of the magnetic flux F o= h / 2e). However, for our purposes of detecting slight
changes in the magnetic field, this periodic course is not
important.
Cryogenic microcalorimeters
are bolometers made up of elements of a
suitable thermoelectric material that electronically register a
slight increase in temperature caused by the absorption of
quantum radiation in the material. The element then cools down
again and is ready to detect another quantum. The smaller the
working element of the microcalorimeter, the smaller its heat
capacity and the faster it is enough to cool to the initial
temperature - the shorter the dead detection time
. Microcalorimetric detectors are therefore formed by a larger
number of individual small bolometric elements -
microchips FET ( field-effect
transistor ) or
SET ( single-electron tunneling
transistor ) , or the above-mentioned
highly sensitive TES bolometers. The whole system is cooled
by liquid helium, additional active cooling to an
operating temperature of the order of 0.1 ° K is achieved by the
method of adiabatic demagnetization (the most commonly used paramagnetic magnetocaloric
material are gadolinium alloys ) .
The great advantage of microcalorimetric
bolometers is their extremely wide spectral
rangesensitivity - in the electromagnetic field it
ranges from millimeter radio waves to high-energy gamma
radiation. It reacts similarly to particles of various energies.
Cryogenic microcalorimeters are used experimentally to detect
soft radiation b and X, and are used in some unique sensitive experiments
such as energy analysis of radiation b to determine the mass of
neutrinos (§1.1, part " Neutrinos ") . Their use in astronomy
for measuring the intensity and polarization of microwave
relic radiation is interesting (see
§5.4, passage " Microwave relic radiation - messenger of early
space news ", in the
monograph "Gravity, black holes and space - time physics
") .
Scintillation
and semiconductor gamma-spectrometry
The basic criterion for the choice of scintillation or
semiconductor detection and gamma-ray
spectrometry is the required detection efficiency
and especially energy resolution . Where an
energy resolution of about 10% is sufficient, scintillation
spectrometry is more advantageous, due to the somewhat higher
detection efficiency. For accurate energy measurement and
resolution of nearby gamma lines, spectrometry on a semiconductor
detector with about 30 times better energy resolution is
required. These differences can be seen in §1.4
"Radionuclides", section " The most important
radionuclides ", on a series
of radionuclide spectra measured simultaneously by a
scintillation and semiconductor detector.
2.6. Detection and spectrometry of a, b, protons and neutrons. Magnetic spectrometers. Liquid scintillators.
Detection of charged
particles - alpha, beta, proton
radiation Detection and spectrometry of alpha and beta radiation
is more difficult than with gamma radiation, because this
radiation is less penetrating, easy to absorb
and difficult to penetrate into the sensitive area of ??the
detector. Detectors for such radiation must have very thin
entrance windows , or are "windowless".
Otherwise, essentially
the same types of detectors as described above are used to detect
charged particles of corpuscular radiation. The simplest are ionization
chambers , G.-M. or proportional
detectors with appropriately adjusted thin windows (eg
mica), or flow ionization detectors (without
window).Scintillation detectors for radiation a and b are usually plastic
(high density is not required as for radiation g ) with or without
a thin metal window (insulating against light) - such windowless
scintillation detectors must be operated in light-tight chambers.
Furthermore, the semiconductor detectors
described in the previous paragraph are used, for higher energies
Cherenkov detectors can also be used . The use
of liquid scintillators is very common and
effective , as described in more detail below in a separate
paragraph.
Detection and
Spectrometry cosmic rays
within this category and falls partly detection of cosmic
rays (see
§1.5, section " Cosmic radiation "), consisting primarily of high-energy protons,
particles and , heavier nuclei. Secondary cosmic radiation, created by
interactions in the atmosphere, also contains electrons +
positrons, muons, gamma radiation, often in large sprays.
However, the detection of cosmic radiation has some significant
specifics, so we have already discussed it in the discussion of
cosmic radiation - §1.5, section " Detection and spectrometry of
cosmic radiation ".
Magnetic spectrometers
The most advanced instruments for accurate measurement of the
energy of charged particles are magnetic spectrometers
*). The magnetic spectrometer is based on the force of a magnetic
field on moving charged particles. Permeating particles of charge
q moving with velocity in the magnetic field
intensity (induction) B , it will be
(perpendicular to the direction of movement) to act Lorentz
force F = q. [ In ' B]
, causing bending the particle pathway . When
moving perpendicular to the direction of the homogeneous magnetic
field B , the charged particles are acted upon
by a radial Lorentz force, which is in equilibrium with the
centrifugal force: Bqv = mv2 / R. The particles will therefore move around the circle
of radius R = mv / (QB) = Ö (2E to .m) / (qB) wherein p = mv is the momentum of the
particles, Q charge and E to = 1 / 2 mv 2 is the kinetic particle energy. By measuring the radius
R of curvature of the path of a particle of known mass and
charge in a magnetic field of a given intensity B,
we can determine the energy of the particle E k . ...? ... give a formula for the relativistic case ...?
...
*) Electrostatic
spectrometers
For accurate radiation spectrometry bin the field of low
energies, electrostatic spectrometers based on
the curvature of the particle path in an electric field, or
combined spectrometers (for example, an electrostatic
spectrometer with a magnetic collimation).
Spectrometer with a
transverse magnetic field,
magnetic spectrometer with transverse magnetic field is
formed by a strong electromagnet , whose pole
pieces forming an almost homogeneous magnetic field *) is a
vacuum measuring chamber. Particles fly into the chamber through
the inlet orifice and fall along a path curved in a magnetic
field through another orifice to the detector ,
where they are registered by conversion into electrical pulses -
Fig.2.6.1. This detector does not have to have spectrometric
properties - the spectrometry is "taken care of" by the
magnetic field + geometric arrangement. Only good detection
efficiency is required for the analyzed charged particles in a
sufficiently wide range of energies.
*) Focusing: To achieve
better detection efficiency, or "luminosity" of the
spectrometer, special shapes of pole pieces are sometimes used,
creating in the main homogeneous magnetic field certain
additional gradients causing the effect of a magnetic lens to
focus an otherwise diverging beam of charged particles ......... For a given geometric configuration of the input window,
apertures and detector and a certain specific value of magnetic
induction B , only particles of a certain energy
E k = E fok can fall into the
detector , which is curved in the magnetic field so that it
"hits" the detector location of position Rfok . By increasing the
excitation current I by winding the electromagnet, we
increase the magnetic induction B and thus the energy of the particles that selectively
impinge on the detector and are registered.
Fig.2.6.1. Schematic representation of the operation of a
magnetic spectrometer with a transverse magnetic field ( left
) and a longitudinal magnetic field ( middle ). On
the right is the use of a magnetic spectrometer for
gamma radiation.
Each value of current I through the coil of the electromagnet thus corresponds to a certain energy E k of charged particles, which will be registered by the detector. The magnetic spectrometer operates cyclically in a dynamic mode, during which the current I increases continuously with the electromagnet, while the pulses from the detector are registered. Particles of different energies gradually fall into the detector, depending on the instantaneous intensity of the magnetic field. In the next cycle, the el. the current increases again from zero to the set maximum value. If we plot the appropriate calibration multiple of the current Ö I on the horizontal axis and the registered number of pulses for each value I on the vertical axis , we obtain a graph of the energy representation of the measured charged particles, ie.energy spectrum of corpuscular radiation. Magnetic spectrometers have a very good resolution, usually better than 1%, but their detection efficiency is relatively low (they compete with the "luminosity" of the spatial detection angle and the energy resolution) .
Magnetic lens spectrometers
In addition to transverse magnetic field spectrometers, smaller longitudinal
magnetic field spectrometers are also used , especially
for radiation spectrometry b . The focusing effects of the axial
magnetic field are used here , which, according to the laws of
electron optics, create an image of the source similar to a
contact lens. The principle of operation of such a spectrometer
is shown in Fig.2.6.1 in the middle. The spectrometer consists of
a coil *) through which a current I
passes and excites in the vacuum chamber inside the coil a
longitudinal magnetic field with a vector B
directed along the axis of the coil. Source of analyzed
radiation, eg radiation b
, is located on the axis of the magnetic
field. Particles (electrons) emitted by this source in the
direction of the coil at an angle J to the axis move under the
influence of magnetic forces for spatial spiral paths
whose projection into a plane perpendicular to the axis is a
circle with radius of curvature R = (mvsin J ) / (qB), and are focused
to one point on the axis, which is from the source at a distance
F = (2 p .p / qB) .cos J , where p = mv is the momentum of the particle. At a
constant angle J, which is defined by the aperture of the spectrometer,
we can focus particles with different momentum and thus different
energy by focusing the magnetic field B to one point on the coil
axis and register particles of respective energies with a
detector located at this point on the coil axis. Thus, depending
on the current through the coil winding, the particles passing
through the annular orifices are focused to the location of the
detector according to their energies - this dependence, after
appropriate calibration, creates an energy spectrum
.
*) It is either a long coil
- a solenoid, creating a homogeneous magnetic field inside, or a short
coil , creating an inhomogeneous axially symmetric
magnetic field, limited to a short space between the source of
measured radiation and the detector. If current I flowsa
coil with n turns, for particles with momentum pa and
charge q, this coil behaves in the direction of its axis as a magnetic
lens with a focal length f = k. (p / qnI) 2 , where the
coefficient k depends on the dimensions and construction
of the coil.
Mass spectrometers and
separators
Mass spectrometers and separators used in physical
chemistry and radiochemistry also work in a similar arrangement
as the charged particle magnetic spectrometer according to Fig.
2.6.1 on the left . The analyte is ionized in an ionization
chamber, the formed cations are accelerated by an electric field,
and ions with a constant velocity v are selected in a
velocity filter (consisting of, for example, a crossed electric
and magnetic field) . These then fly through the entrance slit
into the magnetic field, in which they describe a circle of
radius R = (v / eB) .m, proportional to the mass m . Ions
of different masses describe different paths and thus fall on
different places of the base - so the device separates
from each other ions of different masses (given by the weight of
the nucleus). By changing the magnetic field, ions of
corresponding masses are gradually focused into the detector - a mass
spectrum is created .
In the mass
separator , instead of the detector, a suitable target
is installed on the base , on which the incident ions of
the selected mass are absorbed. This method allows isotopic
enrichment of the target, or the creation of an isotopically
pure preparation, but only in small amounts.
Magnetic gamma spectrometers
An important use of magnetic spectrometers is also for accurate
gamma spectrometry . A thin foil called a radiator
is placed in the input window of the magnetic spectrometer (not very apt name, a converter would be better
...) , from which the incident photons g eject
electrons , whose energy is measured from the curvature
of the path in a magnetic field, as with a magnetic beta
spectrometer (Fig.2.6.1 right). If the radiator is made of a
substance with a low atomic number, the secondary electrons are
generated mainly by Compton scattering, in the case of using a
heavy substance (lead, tungsten) the secondary electrons will be
produced mainly by a photoeffect (at lower energies of quantum
gamma). High-energy gamma radiation (E g > 1.02MeV) also
produces electron-positron pairs; in this
case, paired gamma spectrometry can be used by
simultaneously measuring the energy of electrons and positrons
using two detectors located on opposite sides of the base.
Magnetic spectrometers have played a very important role in the accurate measurement of the spectra of b, g, a , protons, conversion and Auger electrons in radioactivity and nuclear reactions; they thus contributed to the specification of physical ideas about the structure of atomic nuclei as well as about the interactions of elementary particles. Most of the data in the nuclear tables (eg in the detailed tables of Lederer, Hollander and Perlman isotopes ) were obtained by measurements using magnetic spectrometers.
Spectrometric
analysis of radiation b
While the analysis of radiation spectra g , containing clearly
expressed photopeaks of discrete energy lines, is in principle
relatively easy, the analysis of radiation spectra b is quite
difficult. On the appearance of the continuous spectrum of
radiation b, we often do not even know visually whether the radiation
belongs to one maximum energy or is composed of several groups of
radiation. The so-called Fermi-Kurie graphs are
sometimes used for detailed spectrometric analysis of radiation b . The exact shape
of the curve of the radiation spectrum b follows from the analysis
of the energy distribution of the emitted electrons within
Fermi's theory of weak interaction. It follows from this theory
that the intensity of N (p) radiation b on momentum p
a energy E b is given by N (p) = (E b max -E b ) 2 .p 2 .F (Z, p), where E b
max
is the maximum decay energy b
and the constant F in itself includes the
relevant constants, including the proton number Z. From this
relation, for the spectrum b follows the equation E b max -E b = Ö [N (p) / p 2 .F (Z, p)]. If we plot the function Ö [N (p) / p 2 .F (Z, p)] on the
vertical axis depending on the energy E bon the horizontal
axis, we get a linear dependence called the Fermi-Kurie
graph . It is a descending line that intersects the
horizontal (energy) axis at the point indicating the maximum
decay energy b . If the studied radiation b is composed
of two or more energy groups, we get a graph composed of two or
more linear sections . By interpolating and
extrapolating these linear segments, we can determine the
energies and relative intensities of individual groups of
radiation b . Analyzes of this kind can now be performed by computer
using special spectrometric software.
Magnetic cyclotron radio-frequency
spectrometry
The spectra of low-energy beta radiation can in principle be
measured using coherent cyclotron radiation
emitted by individual electrons in a spiral motion in a magnetic
field. The evacuated detection chamber, located in a strong
homogeneous magnetic field, is equipped with microwave
antennas that register this cyclotron radiation and the
electronic apparatus evaluates its frequency. In the classical
approach the Larmor frequency f cyclotron
radiation f = eB / (2 p m e ) with the electron charge e and rest mass m e is given only by the
intensity of the magnetic field Band does not depend on
the velocity in the electron, and thus on its kinetic
energy E b . However, due to the effects of the special theory
of relativity , in reality the Larmor frequency will
slightly depend on the velocity of the electron v and thus
on its kinetic energy E b : f = [eB / 2 p .m e ]. Ö (1-v 2 / c 2 ) = (eB / 2 p m e ) / [1 + E b / (m e .c 2 )]. By accurately measuring the frequency, we can
determine the energy of the electron - perform beta
spectrometry. For a 1T magnetic field, the emitted
radiation will have a frequency of around 27GHz.
This method is not
suitable for higher electron energies in the relativistic region,
because cyclotron radiation changes into synchrotron
radiation , which has a continuous spectrum and no longer carries
direct information about the energy of the electron. The origin
and properties of cyclotron and synchrotron radiation are briefly
discussed in §1.6, section "Interaction of charged
particles", passage " Cyclotron and synchrotron radiation ".
The method is not yet
used by default. The prototype of this detector was assembled in
2009-2012 by J.A.Formaggio and B.Monreal.
Detection
of beta radiation by liquid scintillators
The difficulties encountered in detecting beta radiation on the
detector side, mainly associated with radiation absorption, were
mentioned above. However, even bigger problems occur on the side
of the measured sample! Significant (self) absorption of
beta radiation occurs directly in the sample itself -
beta electrons from the inner parts of the sample usually do not
penetrate at all, this radiation does not penetrate any packaging
of the radioactive sample (eg a bottle). To measure beta
radiation from such samples, it is necessary to make a relatively
difficult treatment of the sample into a very thin layer
(evaporator) and then try to measure it in a geometry of 2 p using GM window
tubes or plastic scintillators.
Accurate activity
measurement b-radioactive preparation, especially low-energy beta
radiation, is therefore not possible (due to the absorption of
beta radiation in the sample itself) even with the use of
self-improving radiation detectors b . However, there is an
interesting and effective method of accurately and with high
efficiency (approaching even 100%) to detect the radiation of b- radioactive
preparations: it is a method of liquid scintillators.
A liquid
scintillator is a substance in the liquid state which,
upon interaction with ionizing radiation, converts part of the
absorbed energy into flashes of light (scintillation), similar to
the solid state scintillators described above. These are suitable
cyclic hydrocarbons in organic solvents(Specific
compositions and properties of some types of liquid scintillators
will be given below) . The use of liquid
scintillators for measuring beta-radioactive samples is as
follows (Fig.2.6.2):
Fig.2.6.2. Left:
Schematic representation of the principle of beta radiation
detection by a liquid scintillator. Right:
Mark II instrument (Nuclear Chicago) with a sample exchanger for measuring beta-samples in a
liquid scintillator at KNM Ostrava.
In principle, one photomultiplier would be
sufficient to capture light flashes from a liquid scintillator.
The reason for using two photomultipliers in a coincidence
circuit according to Fig.2.6.2 is, in addition to
increasing the detection efficiency, to suppress unwanted
impulses coming from photomultiplier noise and
chemiluminescence. Some chemical reactions between the sample
material and the scintillator can lead to light emission -
chemiluminescence (see below), which in the photomultiplier produces false pulses not
originating in the detected beta radiation. In contrast to
scintillation, chemiluminescence is characterized by the fact
that only one photon (or a few photons) is emitted in one act,
while scintillation is a flash of several hundred or a thousand
photons. Therefore, if we use two photomultipliers according to
Fig.2.6.2 to detect light from a liquid scintillator, then noises
and photons from chemiluminescence always generate an impulse
independently in only one of the photomultipliers, while a large
part to both photomultipliers. Thus, at the output of the
coincidence circuit, we receive pulses only when beta-induced
scintillation is detected, while noise and chemiluminescent
pulses are not transmitted 3 H.
When measuring low-energy radiation b in liquid
scintillators, we encounter the problem of a small number
of emitted photons , which can be only tens of photons
per scintillation. Therefore, high demands are
placed on the properties of photomultipliers - high quantum
efficiency of the photocathode for a given spectral range (see
below for spectrum shifters), low noise (in co-production with
coincident photomultiplier connection), good optical contact of
photomultipliers with scintillator (incl. Reflectors), also low
absorption of radiation in the scintillator itself. The entire detection system with photomultipliers is of
course enclosed in a light-tight shielded box
, where the cuvette with
the measured sample, mixed with the liquid scintillator, is
lowered through a light curtain by means of an elevator and is
extended back after the measurement has taken place. Laboratory
instruments of this type for measuring series of a larger number
of samples are equipped with an electro-mechanical system of a sample
transducer with a capacity of several tens to hundreds
of vials, which gradually inserts individual cuvettes, after a
preset measuring time extends, moves and inserts another vial
(Fig.2.6.2 right - the principle is similar
to the one below in Fig.2.7.3 on the left ).
Some devices have the entire space of the detection system and
the sample converter tank cooled to a
temperature of about 4 ° C, which contributes to the reduction
of noise, chemiluminescence and possibly. evaporation of samples.
In Fig.2.6.2 on the right is the
deviceMark II ( Nuclear
Chicago ) with a cooled sample
exchanger for measuring beta samples in a liquid scintillator.
For many years (1974-1990), we have successfully used this
state-of-the-art instrument at our nuclear medicine workplace in
Ostrava to measure b- radioactive samples of 14 C radium , tritium 3 H, 32 P and others. It was later replaced by a newer RackBeta
( LKB ) device .
Measurement of
beta-radioactive samples using Cherenkov radiation We present this possibility as rather interesting. If
the radionuclide in the measured sample emits beta radiation of
sufficiently high energy (higher than about
300keV for water samples), we can use the described instruments
according to Fig.2.6.2 in principle to measure without
the use of liquid scintillator - using the emission of Cherenkov
radiation (§1.6., passage " Cherenkov radiation ") in the sample material.
It is applicable, for example, to 32 P or yttrium 90 Y samples . The conversion efficiency of Cherenkov
radiation production is significantly lower than for
scintillation radiation, so the method is not suitable for
samples of very low activities.
Use of
liquid scintillators for the detection of external radiation
In addition to the internal measurement of
radioactive samples mixed directly into the scintillator
described above , liquid scintillators can in some cases also be
used for the detection of external radiation .
We simply pour the liquid scintillator into a container of
suitable shape and size, so that we can obtain a detector of a
size not achievable with solid (crystalline) scintillators; they
are therefore used, for example, in the detection of cosmic rays
or neutrinos (.....) .
At our workplace, we used a liquid
scintillator (in a very unconventional way)
for mapping and visualization of
irradiation beams- electron, photon, proton - used in
radiotherapy. We filled a glass measuring cylinder
with a diameter of 6 cm and a height of 44 cm with 1 liter of
liquid scintillator (we used a dioxane
scintillator with a density of 0.95 g / ml) and
placed it under the irradiation head of the appropriate
accelerator - electron, photon CyberKnife, proton. From the side,
we observed and photographed the scintillation
radiation generated in the scintillator along the passage of the
irradiation beam - Fig.2.6.3.
![]() |
Fig.2.6.3 Scintillation radiation
generated in a cylinder (diameter 6 cm and
height 44 cm) filled with a liquid scintillator
during irradiation with electron, photon and proton
radiation beams. a), b): A cylindrical phantom filled with a liquid scintillator was irradiated with a wide electron beam of 6 MeV and 18 MeV from a linear accelerator. c), d): When irradiating the same phantom with a beam of photon radiation - max. energy 6MeV, beam with a diameter of 1.5 cm and 3.5 cm - they form secondary electrons along g beam scintillation radiation - with a deep decrease in intensity as the primary photon beam attenuates as it passes through the liquid. Acknowledgments: Irradiation of the scintillation phantom on TrueBeam and CyberKnife devices was performed in cooperation with colleagues: Ing.L.Knybel, Ing.L.Molenda and Ing.B.Otáhal. e), f), g): When irradiated with narrow (" pencil beam ") proton beams of energy 100, 170 and 226 MeV, the protons penetrate to different depths depending on the energy, with a significant Bragg maximum. Thanks: Irradiation with proton beams from the IBA cyclotron was performed in cooperation with colleagues: Ing.P.Máca, Ing.M.Andrlík, Mgr .L.Zámeèník , Ph.D. , Ing.M.Navrátil , Ph.D. (and consultations with colleagues Ing.V.Vondráèek and MUDr. MUDr. J.Kube, Ph.D.) from the PTC proton center in Prague. |
The results of these measurements are discussed in more detail in §3.6 , passage " Make the invisible visible " - display of radiation beams " .
Liquid
scintillators - composition and properties
The liquid scintillator consists
of two main components: the solvent and the scintillator
itself dissolved in it . The most commonly used solvents are toluene
and 1,4- dioxane . The actual scintillant is
some organic compound characterized by fluorescence. In
particular, 2,5-diphenyl 1,3-oxazole (PPO) and 1,3,4-oxadiazole
(PBD) are used in concentrations of about 5 g / liter, and naphthalene
in dioxane-based scintillators .
The energy of the beta radiation is first transmitted to the
solvent molecules. The excitation energy of these molecules is
then transferred to the molecules of the scintillant itself,
which emit photons of visible light during deexcitation. However,
the spectrum of this emitted light is distributed in the region
of shorter wavelengths than the maximum spectral sensitivity of
the photocathode of conventional photomultipliers. To increase
the detection efficiency, therefore, a spectrum shifter
*) is added to the scintillation solution , the molecules of
which absorb the primary energy of the scintillation and emit
subsequent light photons of lower energy, corresponding to the
spectral region of maximum photomultiplier sensitivity. The most
commonly used spectrum shifter is 1,4-bis- (5-p-tolyl-2-oxazolyl)
-benzene (POPOP) and its modifications, in concentrations of
tenths of grams / liter.
*) The spectrum shifter also provides
another positive effect. In one-component scintillators, the
optical emission and absorption spectra are identical, so that
the radiated scintillation photons in the surrounding
scintillator molecules self - absorb back . The
scintillator is thus not very transparent for the emitted
scintillation photons, from a greater depth many photons do not
penetrate the photomultiplier. This reduces the conversion
efficiency of the scintillator. The addition of a spectrum
shifter leads to the absorption of the original scintillation
energy and the emission of lower energy photons, for which the
environment of the organic scintillator is already well
transparent. For better miscibility
of the solvent and the sample, secondary solvents,
tissue solubilizers , are sometimes added to the
scintillation solution.
or gelling
agents. A dioxane scintillator that is up to about 10% miscible
with water is suitable for measuring samples in aqueous
solutions. Sometimes different heterogeneous mixtures of liquid
scintillator and measured sample are also used. E.g. the
microfilter film with the captured beta sample soaks up when
immersed in the scintillator and becomes transparent, so that the
resulting scintillations can also be measured similarly to a
conventional homogeneous arrangement. In all these non-standard
methods, due to the geometry of measurement, absorption and
increased quenching, the detection efficiency is significantly lower
than in homogeneous unquenched samples, but in many cases it is
fully sufficient and is sometimes the only applicable method.
Quenching
and chemiluminescence
Based on the principle of
detection by liquid scintillators, it can be expected that
practically every electron b emitted by the measured
sample inside the scintillator (except for a thin layer at the
surface and walls of the measuring cuvette) will cause
scintillation and be registered - this is true " 4 p -geometry
". detection efficiency should be close to 100% *). However,
there are two adverse events that can reduce detection
efficiency, increase background, and overall impair the accuracy
and reproducibility of liquid scintillator sample measurements:
quenching and chemiluminescence. *)
Radiation spectrum b
it is continuous and contains a significant
proportion of low-energy particles (the rest is carried away by
neutrinos - §1.2), whose scintillations cause low signal pulses
of the same magnitude in the photomultiplier as the noise pulses
generated by the thermoemission of the photocathode. These pulses
then do not pass through the lower discriminatory level of the
analyzer - a certain initial part of the spectrum is therefore
often lost for detection. Therefore, for soft beta radiation from
3 H, it is
generally not possible to achieve a better detection efficiency
than 50% in practice.
Quenching
A side effect of mixing the measured sample into the scintillator
is a series of chemical reactions that can cause a reduction in
the light yield (conversion efficiency) of the scintillator and
thus a corresponding reduction in the amplitude of the pulses
from the photomultiplier. These reduced pulses may fall outside
the discriminant levels on the analyzer and are not registered.
This phenomenon is called quenching . We
distinguish two types of quenching, which usually occur in the
samples at the same time.
Chemical quenching is caused by the fact that
the molecules or atoms of the measured sample, by their chemical
action, partially prevent the transfer of excitation energy
between the molecules of the solvent and the scintillator, so
that only weaker scintillation occurs .
Color quenching then it causes part of the
photons emitted during scintillation to be absorbed by
the substances contained in the sample. The designation
"colored" refers to the fact that these non-fluorescent
substances have a discrete absorption spectrum and absorb photons
in a certain energy (= color) range.
Some chlorinated hydrocarbons, such as chloroform or tetrachloro
CCl 4 ,
peroxides, as well as water and oxygen dissolved in the
scintillator, have strong quenching effects . The consequence of
quenching is that two samples containing the same activity but
showing different quenching give different numbers of pulses.
Quenching correction
Because quenching can vary the detection efficiency differently
for different samples, quenching correction is required for
accurate and reproducible measurements. The quenching correction
is based on the analysis of the measured
radiation spectrum b in the amplitude analyzer.
In Fig.2.6 .... on the right, the radiation spectra b of the same
radionuclide (eg 14 C) measured by a liquid scintillator at low quench and
at high quench are plotted . Extinguishing significantly affects
the shape of the spectrum b - the spectrum shortens and shifts to lower energies.
Thus, by analyzing the shape of the spectrum, we can determine
the degree of quenching. The analysis of the shape of the
spectrum is simply performed here by determining the ratio of the
number of pulses in the two analyzer windows suitably set to the
spectrum b.
Calibration or standardization is required to correct for
extinction. We will prepare several identical samples of the
given radionuclide b in a liquid scintillator. Gradually add an increasing
amount of quencher (eg chloroform) to each of them and perform
their spectrometric analysis on the instrument according to
Fig.2.6 .... The ratio of the number of pulses in two suitable
windows of the amplitude analyzer is plotted on the horizontal
axis, on the vertical axis efficiency (all samples have the same
activity). This gives the so-called quenching curve(usually
in the form of a parabola or hyperbola), which can then be used
to correct for extinction in unknown measured samples: from the
ratio of the number of pulses in the two analyzer windows on the
extinction curve subtract the correction coefficient by which we
must multiply the measured number of pulses to compensate for
loss by extinction. This is the so-called internal
standardization of extinction.
Some devices also have a
built-in so-called external extinction standardization
. It consists in that the measured sample with a liquid
scintillator, inserted in the measuring position between the
photomultipliers, is irradiated for a moment with
gamma radiation from an external source and the
correction coefficient is determined by analyzing the shape of
the spectrum thus obtained (ratio of two parts of the spectrum).
The external standard uses radionuclides (mixed emitters b+ g or a + g ) 137 Cs, 133 Ba, 241 Am, 226 Ra, sometimes a
pair of radionuclides, with an activity of tens of kBq. The
external standard is placed in a lead shield, from where it is
automatically extended to the measuring cuvette with a liquid
scintillator and pushed back again after standardization.
Chemiluminescence
Chemiluminescence is an event in which radiation is emitted as a
result of chemical reactions. Some chemical reactions between the
sample material and the scintillator can lead to this
chemiluminescence, which in the photomultiplier produces spurious
pulses not originating in the detected beta radiation.
Fortunately, however, chemiluminescence is limited in
time and expires exponentially within about 30 minutes
of placing the finished samples in the sample exchanger container
where they are not exposed to light.
Neutron detection
Because neutrons have no electrical charge and can not themselves
directly ionize the electron shells of atoms, their detection is
necessary to use the processes of interaction
which produce secondary charged particles which
already have an ionization effect, and can be detected. The
following methods are mainly used for neutron detection:
In principle, neutrons can be detected using
conventional g or charged particle detectors ( b, a , p), or provided with a
suitable converting material . This converting
material absorbs neutrons , creating secondary
radiation that can already be easily detected. For slow
neutrons, layers containing lithium 6 Li or boron 10 B are often used , in which neutrons are converted into
charged particles and g . For fast neutrons, for example, a polyethylene film is
suitable, from which fast protons are emitted by interaction. In
general, it should be noted that the layer of converting material
is depleted during interactions. Another
negative phenomenon in neutron detection may be internal
radioactive contamination of detector materials, induced
by nuclear reactions with neutrons inside the detector (see §2.1, section " Nuclear reactions and induced radioactivity
inside detectors ") .
Neutron spectrometry
Energy measurement, or neutron spectrometry , is
more difficult than with gamma or beta radiation. A so-called mechanical
selector is used to measure the spectrum of slow
neutrons. They consist of two disks made of a highly
neutron-absorbing fabric (cadmium) mounted on a rotating shaft.
The discs have a series of identical radial slots around the
circumference, the position of which is shifted at the second
disc by a very small angle relative to the first disc. The
measured neutron beam passes parallel to the shaft axis through
the slit of the first disk and is absorbed by the second disk,
except for those neutrons whose velocity is such that they reach
the second disk at a time when they can pass freely through the
shifted slit of the second disk. For a given shaft speed, disk
distance and angular displacement, neutrons are only transmitted
in a narrow speed range. By changing the speed of the shaft
speed, neutrons of different speeds are gradually transmitted,
their frequency is calculated by a neutron detector and thus
their speed spectrum is measured., from which
the energy spectrum is derived. In this way, the spectra of only
slow neutrons, of the order of eV, can be measured.
The spectrum of slightly
faster neutrons can be measured by their Bragg scattering
on the crystal lattice . Due to corpuscular-wave
dualism, the neutron of mass m n and kinetic energy E behaves as a wave
of wavelength l = h / (2m n E) 1/2. For slow neutrons, the wavelength corresponds to the
order of magnitude of the distance of the atoms in the crystals.
On such a grating, neutrons can be scattered by Bragg reflection
(similar to X-rays): neutrons of only one energy are reflected to
a certain angle between the plane of the grating and the
direction of the incident beam. By continuously changing the
angle between the neutron beam and the crystal on the goniometer
, the measured frequencies in the neutron detector give us an
angular distribution curve from which the spectrum is
determined. Crystal neutron spectrometers are suitable for
energies in the range of about 0.1-100 eV.
Reflected protons
can be used to measure fast neutron spectra, whose energy is
measured by a proportional or scintillation detector, or and by
measuring the length of the proton trace in the nuclear emulsion.
It is also possible to use the already mentioned scintillation
detector 6 LiJ (Eu), where neutrons in the reaction (n, a ) transfer to the
scintillator energy 4.78MeV + energy of the arriving neutron,
which can be determined by amplitude analysis of output pulses
from a photomultiplier with a resolution of about 10% ( at an
energy of 5MeV).
2.7.
Measurement of radioactivity of samples (in vitro)
One of the most common types of radiation measurements is the measurement
of radioactivity of samples - whether they are samples
in medicine and biology, samples from the environment, or samples
taken from various places in industrial systems. The specific
methodology for measuring the radioactivity of samples depends on
several circumstances - the type and energy of radiation, the
activity of samples, the size and shape of samples, the required
accuracy, whether it is a relative or absolute measurement, etc.
in §2.1-2.6, the most frequently used detectors for
gamma-samples in the passage " Scintillation probes ", fig.2.4.3. Here we will mention some practical
aspects of measurement geometry, on specific
methods for measuring series of samples and radiochromatographic
methods for analyzing samples.
Measurement geometry -
2 p
, 4 p
The overall measurement efficiency is given not
only by the detector's own detection efficiency, but also by the
mutual arrangement of the measured sample and the detector - the
so-called measurement geometry. By measurement geometry
we generally understand all aspects of the spatial
relationship and configuration of the measured sample or
beam of radiation with respect to the detector .
Fig. 2.7.1 shows typical geometric configurations when measuring
samples using gamma radiation :
Fig.2.7.1. Geometry of sample measurement. Planar detector for
geometry max. 2p and
two types of well scintillation detectors for measuring the
activity of samples in geometry close to 4p .
¨ Geometry 2p
The simplest configuration arises when the measured sample is
simply placed close to the detector (Fig.2.7.1
on the left). If we neglect the absorption and the effect of the
final size of the sample and the detector, ideally all radiation
is detected that goes from the sample to the half-plane in which
the detector is located, ie half of all radiation
emitted by the sample - we say that we measure in geometry
2 p (full spatial the 360 ??° angle expressed in radians
is 4 p ,
its half 180 ° then 2 p ) . The total detection efficiency
for the attached sample here can reach a maximum of 50%
. Detectors for this type of measurement are sometimes referred
to as
planar .
¨ Geometry less than 2p
If the sample is placed at a greater distance from the detector,
it is a measurement at a solid angle w < 2 p , with a
correspondingly lower detection efficiency
(0-50%).
¨ 4p geometry
If we want to increase the detection efficiency and measure all
radiation emitted by the sample to a full solid angle of 360 ° -
ie in 4 p
geometry , we use
the so-called well detector
(see Fig.2.4.3b in §2.4, passage " Scintillation probes " ) in the arrangement shown
in the middle and right part of Fig.2.7.1. A well detector for
measuring higher activities in the design of the ionization
chamber is shown in Fig.2.3.3 on the right in §2.3 dedicated to
ionization detectors. However, for sensitive measurements of low
sample activities, well scintillation detectors are used with a
hole drilled in the crystal either longitudinally
to a certain depth (middle part of Fig.2.7.1 - well
crystal ), or with a hole drilled transversely
through the whole crystal (Fig.2.7.1 on the right). ). The tube
with the measured sample is inserted into this hole, so that
almost all the radiation emitted by the sample (except the narrow
cone in the direction of the hole) must pass through the
sensitive volume of the detector and can be detected - a geometry
close to 4 p. In the 4 p geometry , the detection efficiency can theoretically
approach up to 100%, in practice it reaches approx. 80-90% for
well scintillation detectors.
Note: The real 4 p -geometry is the
above measurement of beta samples dissolved in a liquid
scintillator , where we can approach 100% efficiency ("
Detection of beta radiation by liquid scintillators ").
Detection
efficiency
Positional and volume dependence of the detection efficiency
The detection efficiency
of the measurement , i.e. the ratio between the measured
number of pulses and the number of quanta emitted by the sample,
crucially depends primarily on the geometric
configuration of the sample relative to the detector.
Each sample emits radiation isotropically in all
directions *), but only a certain part of this radiation enters
the sensitive volume of the detector and can be registered. In
the basic geometry of the planar detector
according to Fig.2.7.1 on the left, the detection efficiency
depends mainly on the distance of the sample
from the detector - it decreases approximately with the square of
the distance, reaching the highest values ??(but max. 50%) close
to the detector.
*) Angular (directional) correlations
of gamma photons
In some radionuclides, two or more gamma quanta are cascaded in
one and the same transformation event. In such a case, angular
correlations may occur between the emission direction of
these photons (see §1.2, section
"Gamma radioactivity", section " Angular
(directional) correlations of gamma radiation ") . This relative
"anisotropy" can have some effect on the geometric
dependence detection efficiency. If
the sample is close to the detector, the effect of angular
correlation is averaged over a wide range of angles (close to 180
°) and is practically non-existent. If the sample is at a
greater distance from the detector, the detection angle is
smaller and the effect of angular correlation on the different
efficiency of gamma photon detection from cascade deexcitation is
slightly increased. In some accurate measurements, even this
small dependence is corrected.
With a well
detector , the highest detection efficiency is when the
small sample lies at the "bottom" of the well detector
- then most of the radiation passes through the sensitive volume
of the detector and the smallest part comes out in the cone
through the hole without registration. The higher the sample is
placed in the well hole, the more radiation comes out
"useless"positional dependence of
detection efficiency. Closely related to this position dependence
is the volume dependence of the detection
efficiency: the larger the volume of the sample in the tube
inserted into the hole of the well detector, the larger part of
the sample is located near the hole where the detection
efficiency is lower - Fig.2.7.2 left. The self-absorption
of radiation in the sample also contributes to the
volume dependence of the detection efficiency .
Influence of radiation
absorption
In addition to geometric influences, the detection efficiency can
also be reduced by radiation absorption , both
in the sample itself ( self-absorption ) and in
the input window of the detector. Different glass
thicknesses can be used when measuring samples
tubes and ampoules, especially when measuring low-energy gamma
radiation (eg 125 I - plastic tubes are recommended here) .
Fig.2.7.2. Left: Volume dependence of the
detection efficiency of a well scintillation detector. Right:
Positional dependence of the photopeak on the location of the
sample in a well scintillation detector.
Positional dependence of
the photopeak in a well scintillation detector
In well scintillation detectors, we encounter an interesting
typical phenomenon: the broadening or doubling
of the photopeak and the dependence of its position on
the location of the sample inside the well. Gamma radiation from
a sample placed inside a well passes through two areas of the
scintillator:
1. A smaller portion of gamma-photons usually
passes through the well , but scintillations
from the appropriate area of ??the scintillator, closest to the
photomultiplier photocathode, are registered with higher
efficiency, ie higher amplitude output pulses.
2. The walls of the wellpasses a larger
proportion of gamma photons, with scintillations from these
scintillator regions further away from the photocathode being
registered with less efficiency, i.e. with a lower amplitude of
the output pulses.
In the resulting spectrum, the photopeak consists of two
parts . The main, dominant peak of the photopeak,
shifted to the left to lower amplitudes (energies), corresponds
to the majority detection of gamma radiation in the massive side
walls of the well crystal. Right (descending) part photopeak is
towards higher energies expanded a sort of bump
- " second photopeak ", corresponding to the detection of a smaller
fraction of gamma radiation passing through the bottom of the
well crystal - Fig.2.7.2 on the right. This effect is most
pronounced for a point source located at the bottom of the well,
where a significant part of the radiation passes through the
bottom of the well (" double photopeak "- lower
spectrum). higher rank source in the well, the effect diminishes
and ceases to be seen, with large-volume samples or sources
outside the wells, we see only one overall expansion photopeak.
Spectrometric setting
detection apparatus
on the detection efficiency has a significant influence and spectrometric
setting detection apparatus. the highest detection
efficiency would formally achieved in an integral mode
when we measure all impulses coming from the detector. We cannot
use the integral mode if we want to distinguish more
radionuclides with different energies in the measured samples -
then we have to use differential measurement
with the analyzer window set to the photopeak of the
required gamma radiation. However, even when the samples contain
only one type of radionuclide, photopeak measurement
is advantageous because we achieve the best signal / background
ratio, which is especially important when measuring samples with
low background-comparable activities. For accurate measurements
of the content of radionuclides in samples, mostly calibrated semiconductor
detectors with a multichannel analyzer are used, on
which careful spectrometric analysis is
performed measured gamma photopeaks.
Measuring
series of samples
Measuring large series of samples , numbering
tens, hundreds or thousands of samples, would be very laborious
and time-consuming when measured manually with one detector.
Therefore, special instruments are used that allow automatic
measurement of series of samples - sample converters and
multi-detector instruments.
Automatic sample transducers
Sample transducers, often also
called gamma-ray automates (usually gamma
radiation are measured *), are detection apparatus equipped with
an electro-mechanical device for exchanging samples. Prior to
measurement, the samples are placed in a container
with a capacity of about 100-500 samples, which has either a
chain or cassette arrangement. The electro-mechanical drive
unit uses an electric motor to move the individual
samples to the location of the detector, where a motor-operated elevator
inserts the given sample into the cavity of the well or drilled
detector. The measurement is then performed for a preset time,
the measured number of pulses is registered (printed
out, sent to computers, etc.), the elevator
extends the sample, the next sample moves and another measurement
takes place - Fig.2.7.3 on the left.
*) The same principle of electro-mechanical displacement of
samples in the tank is used by sample converters for measuring
beta radiation in liquid scintillators.
Fig.2.7.3. Measurement of larger series of samples. Left:
Gamma-automat with electromechanical sample converter. Right:
20-detector gamma sample meter.
Multi-detector systems
Samplers with one detector allow automatic measurement without
the need for manual manipulation, but measuring large series of
samples is time consuming - the total
measurement time N samples is T = N. (t + t m ), where t is
the average measurement time of one sample and t m is the sample
exchange handling time. E.g. The measurement of 300 samples with
a measuring time of 100 s takes about 8.5 hours. With such a long
time (sometimes measured overnight), there is also a certain risk
of power failure, instability or other failure.
The most powerful and fastest instruments for measuring large
series of samples are multi-detector systems .
They consist of 10, 12, 16 or 20 independent
well scintillation detectors placed right next to each
other in two rows (Fig.2.7.3 on the right). Each scintillation
detector has its own photomultiplier, usually firmly connected to
a crystal in a so-called scintiblock . The
samples are stored in trays (cases), which fit
exactly into the holes of the detectors. After the insertion of
such a set of samples, the measurement of each of them takes
place simultaneously (but independently) in well scintillation
detectors, the measured pulses being registered in the memory of
the instrument. At the end of the measuring time, insert the
container with another set of samples, etc. The above example of
measuring 300 samples after 100 s takes only less than 30 minutes
here when using a 20-detector device!
In addition to the high measurement speed of large series of
samples, the great advantage of multi-detector instruments is
that they have no mechanical parts (they are
purely electronic), so they do not require maintenance and have a
minimum failure rate.
The basic requirement for
multi-detector instruments is the same detection
efficiency of all detectors - the result must not depend
on which detector the sample was measured. This is achieved by
careful selection of scintillation crystals and photomultipliers,
the remaining differences are compensated by
numerical correction of detection efficiency -
results from detectors with slightly reduced sensitivity are
multiplied by an appropriate correction factor greater than 1,
pulse counters from increased efficiency by a factor less than 1.The
matrix of correction coefficients is obtained by either
measuring one sample sequentially on all detectors and
calculating the respective ratios to the mean value, or by
measuring a set of samples with the same activity. The correction
factor for each detector is then stored in the instrument's
memory and the measured pulse counts are automatically corrected.
Another requirement for
multi-detector measurement is that radiation from a sample
inserted in one detector does not radiate to the surrounding
detectors and does not affect the measurement results. Due to the
low energies of gamma radiation (devices of this type are most
often used for 125 J), this radiation is prevented by the lead shielding
in which the individual detectors are embedded. For higher
energies, where radiation can be actually applied, the devices
are equipped with radiation correction: the
numbers of pulses registered in the other (surrounding)
detectors, multiplied by certain weight transmission
factors, are subtracted from the measured number of
pulses in the given detector . The matrix of these transmission
factors is obtained by measuring the sample of a given
radionuclide gradually in individual detectors, registering not
only the number of pulses in the detector, but also the response
of other (empty) detectors and dividing it by the number of
pulses in the detector. The obtained factors are again stored in
the device's memory and the correction takes place automatically
during the measurement.
Hybrid sample converters
In addition to single-detector sample converters and
multi-detector systems, sample converters with several detectors
- approx. 3-5 detectors - are also rarely used. The
electro-mechanical device gradually moves the sample containers
and the elevators periodically insert 3-5 samples into the
detectors for the duration of the measurement. These instruments,
sometimes called "hybrid" instruments, measure faster
than single-detector sample transducers (as many times faster as
there are detectors), but generally do not achieve the
performance of multi-detector systems and are mechanically very
complicated.
Radiochromatography
Chromatography
is a physico-chemical separation method that
spatially separates the molecules of the analyte from
each other on the basis of their different mobility in
the carrier media. This is due to their different physical or
chemical properties, especially sizes - molecular weights
and shape, as well as polarity and chemical bonding. This
separation is performed primarily for the purpose of analyzing
the molecular presence in the test substance (occasionally also for the purpose of isolating the
required substances) . Terminological note: The
somewhat misleading name " chromatography " (formed by combining Greek words chroma = color, graphein = write,
draw) comes from the first experiments with
this separation method, which were performed with plant pigments
chlorophyll, carotene, xanthophyll (MSCvìt,
1903) . Their separation was observed
visually according to different colors - green, red,
yellow. The current analyzed substances are usually not colored.
The separation
process takes place in a chromatographic system , which
contains two phases: mobile and stationary . The
mobile phase (eluent, solvent) - liquid, gas or plasma, containing a sample of the
analyte, moves - flows, seeps - through the system and entrainscarry
the sample through the stationary phase (stationary, anchored in place) ,
which leads to separation of the various
molecular components. This creates a certain spatial
distribution of the analyzed substances along the
chromatographic system - chromatogram . The
analyte is applied ("drip") to a site called the " start
". The places where the individual molecular fractions of
the analyzed sample penetrate at a given time form more or less
sharp local concentration maxima - chromatogram
peaks .
The mobile phase itself (eg solvent) passes through the
system the fastest - it forms "face
"chromatogram (chromatography stop
when the face is reached, near the end of a chromatographic
system) . Passage of analyzed sample
components are then various ways retarded - retarded
according to the size or other characteristics of the molecule.
For quantification of the different speeds of the passage of
analytes in a sample is introduced so. retardation factor
R F - ratio of the distance from the start lead to the
distance of the center peak (
"spots") of the substance from
the start, ranges of values <0,1>. For substances that are
not carried by the mobile phase (heavy
poorly soluble macromolecules) . is R F = 0 - remain detained
at the start; for substances that the stationary phase does not
slow down at all and are freely entrained together with the
forehead, R F = 1.
For systems where a chemical chromatogram is not formed (such as liquid and gas chromatography) the so-called retention time is
introduced - the time from the start of flow so that the sample
fraction reaches the detector at the end of the chromatographic
column.
A number of
types of chromatographic methods have been developed, depending
on the arrangement of the separation system, the mobile and
stationary phase used, instrumental techniques (column
chromatography, liquid, gas, plasma chromatography, gel, paper,
thin layer chromatography). Three methods are more often used:
- Gelchromatography takes
place in a gel, which is placed in a vertical column. The gel
contains small holes (pores) inside, which act as a " molecular
sieve ". However, the gel is most often used in the electrophoresis
described below .
- Thin layer chromatography uses a plate covered
with a thin layer of sorbent - silica gel, to which the
analyte is applied to one starting point with a thin
capillary in a suitable organic solvent. The end of the plate is
then placed in ascending order of the solvent in front of the
starting point, which begins to rise with silica gel and entrains
the analyte with it, at a rate depending on the size of the
molecules.
- Paper chromatographyis the most common method, using
as a fixed phase "absorbent" - non - glued
chromatographic paper (thickness about
0.2-0.6 mm) , consisting of a compressed
layer of cellulose fibers, between which there is a large number
of gaps and pores; in them, the solvent leaks by capillary
forces. The analyzed sample is applied to the paper strip at a
certain "starting" point near one end and before this
point the strip is immersed in the mobile phase - solvent (the starting line must be above the surface!) . Depending on the type (solubility) of the analyte, the
mobile phase may be water or aqueous salt solutions, but
often organic solventsethanol or methanol, acetone,
methyl ethyl ketone, ethyl acetate, acetonitrile,
tetrahydrofuran, dichloromethane and the like. ... Chromatography
is performed in a vessel with saturated solvent vapors
to prevent the chromatographic strip from drying out during the
process.
Depending on the direction of movement
- leakage - of the mobile phase (solvent) in the chromatographic
strip, two embodiments are used, "descending" or
"ascending". In the descending design, a small
dish with solvent is attached in the upper part of the analytical
vessel, into which the upper end of the chromatographic strip is
immersed, which then hangs and the solvent seeps downwards
- Fig.2.7.4a above (second dish with
solvent, built on the bottom, ensures a saturated atmosphere of
vapors). In the ascending design,
a layer of solvent (approx. 5 mm) is poured at the bottom of the vessel and the
chromatographic strip is suspended so that its lower end is
immersed in the solvent, which then capillaries rises upwards
and carries fractions of the analyte with it from the starting
point - Fig.2.7 .4a bottom. In this ascending position, a plate
with a thin sicagel foil can be used instead of a paper
tape .
The mobile phase flows or rises
through a layer of paper or thin film due to capillary forces
and carries with it the individual molecular components of the
sample, which are divided according to their solubility
and mobility.. The low-molecular, well-soluble and
mobile components travel the furthest, larger molecules remain
near the starting point. When the front of the chromatogram
approaches the end of the paper tape, we end the process by
breaking the contact of the tape with the solvent (pull out the tape) . After the
mobile phase has dried , the chromatogram is detected
and evaluated .
Upon completion of the mobile phase,
the separated molecular components remain temporarily fixed
at the sites of the stationary phase where they have reached *).
This creates a basic chemical chromatogram ,
which is mostly latent (except for
the analysis of color substances) and it is
necessary to "make it visible" - "evoke" - detect
-evaluate . Several methods can be used
for this. In the chemical method of development, a
suitable indicator substance is sprayed onto the
chromatogram , which causes a color reaction with the
distributed molecules. Physical - radiation - methods of
detection use irradiation of the chromatogram with visible or
ultraviolet radiation, which elicits a fluorescent
optical response on distributed substances. From the point of
view of our field of nuclear and radiation physics, we will
describe the method of radiochromatography below
.
* ) This is especially true for paper and
thin layer chromatography. For some other methods (such as gas chromatography) no
chromatogram is created, but at the end of the column there is
one fixed detector, which gradually registers the arrival of
individual molecular fractions and evaluates the time of
their arrival - retention time ....
![]() |
Fig.2.7.4. Paper chromatography and
radiochromatogram evaluation. a) Descending (top) and ascending (bottom) chromatography on paper tape (in the ascending version, a plate with a thin silica gel foil can be used instead of chromatographic paper) . b) Simple method of radiochromatogram evaluation by measuring cut strips in a scintillation detector. c) Automatic radiochromatograph measuring the radiation profile of the chromatogram by moving the collimated detector along the strip. Note: Spots or stripes on the chromatographic tape are drawn only symbolically, they are not directly visible (- only after evaluation) ... |
Radiochromatography
is a method of chromatographic separation of radioactive
substances with subsequent use of ionizing radiation
detection for chromatogram evaluation. These are either primarily
radioactive substances (such as
radiopharmaceuticals) or substances labeled
with radionuclides specifically for the purpose of their
analysis.
Detection of the distribution of
analyzed radioactive substances on the chromatogram is performed
by radiometric methods i - using suitable
electronic radiation detectors (alpha,
beta, gamma) along the chromatographic
strip or column. The simplest (especially
previously used) procedure consists of cutting
dried chromatographic strip into narrow strips (width approx. 5 mm) , which are
then measured separately with a scintillation detector (usually in test tubes in a well detector) ; their radioactivity - the measured number of pulses -
is plotted graphically, resulting in a final chromatogram (Fig.
2.7.4b). It is a relatively lengthy and laborious procedure,
suitable only for occasional chromatography of simple samples.
Therefore, detection devices have been developed for frequent
routine use, which perform automatic profilographic
measurements and evaluation of radiochromatograms. The
collimated scintillation detector passes just above the
chromatographic strip or thin layer, registers the local
intensity of the emitted radiation - the frequency of detected
pulses - and outputs chromatographic curve (Fig.2.7.4c), with or.
quantitative computer evaluation. In the graphical record, the
individual chromatographically separated fractions of the
analyzed substance form "bell-shaped" curves - peaks
. The positions of the peaks are given by the
specific chemical properties of the fractions - molecular weight,
polarity, solubility. The area (integral) under
the curves of these peaks is proportional to the relative proportion
of the respective fractions in the separated mixture from
the analyzed sample.
Radiochromatography is
very often used to measure the radiochemical purity of
radiopharmaceuticals - §4.8 " Radionuclides and
radiopharmaceuticals for scintigraphy ", section " Radiochemical purity.
A special method of chromatogram
evaluation is autoradiography , based on the
action of emitted radiation on a photographic emulsion (or on an electronic imaging detector) - §2.2, passage" Autoradiography ".
Radio-electrophoresis
Electrophoresis (from the Greek electro
foresis - to be carried by electrons; - transmission by
electricity ) is a physico-chemical separation
method that spatially separates the molecules
of the analyte on the basis of different mobility of
charged particles - molecules - ions *) - by external electrical
field in a liquid, gel or porous medium. This
mobility of particles depends on the size of the charge, the
size, shape and weight of the particle (molecular weight), the
properties of the environment and, of course, the strength
(gradient) of the electric field. It is performed primarily for
the purpose of molecular representation analysis in the analyte (rarely used for preparatory purposes) . It is an alternative (often more
sophisticated) analytical method to the
chromatography described above.
*) Generation
of electric charge of analyzed molecules
Molecules of analyzed substances acquire electric charge by
interaction with the carrier medium. Conventional neutral
molecules within contain both positive and negative charges in
different parts of the molecule, with a total charge of zero
. By interacting with the environment, the molecule can gain or
lose hydrogen ions (H + - protons) and thus become more positive or negative.
This property depends on the pH value (acidity
or alkalinity) of the environment. IN
in an acidic environment (pH <7) there is an excess
of hydroxide cations H 3 O ( + ) , so molecules will generally tend to acquire protons
and become cations , while in a basic
environment (pH> 7) there is an excess of hydroxide anions OH ( - ) and molecules they
will more often lose protons and act as anions . The
isoelectric point pH (I) (often
abbreviated as pI) is the pH of the medium
at which the molecules of a given substance are electrically neutral
(on a statistical average) . If the pH value of the environment is lower
than pH (I), the molecules acquire an overall positive
electric charge, for pH values higher than pH
(I) the given molecules have a resulting negative
charge.
E.g. for proteins, pH (I) = 7.3 and
NH 3 groups have a
charge of "+" and COO (-) groups have a charge of "-", the molecules
are generally neutral. At lower pH values, eg around 6, positive
values ??prevail for NH 3 ( + ) , COOHs are without charge, the resulting charge is
positive. At a higher pH, around 8, the NH 2
groups are uncharged and a negative charge
predominates in COO ( - ) . Therefore, at the recommended pH = 8.6, the proteins
travel from the cathode (-) towards the anode (+).
The movement of
charged particles by electrophoresis
electrophoresis acting on a charged particle with a charge Q in
an electric field with intensity E two forces:
1. The electrostatic force F E
= E . Q, which tries to accelerate the
motion of a particle. The intensity E of electric field is
given by the voltage U [V] on the electrodes and their
vzdákeností L [m] (the length of
the chromatographic column) : E = U / L [V / m] , so that F E =
E . Q / L.
2. Environmental
resistance
(viscosity, impacts in the "molecular
network" - Fig.2.7.5a) , which tries
to slow down the speed of particle motion: force
F r = kR m . s , where k is the
material coefficient, R m is the effective diameter of
the molecule and s is the intrinsic resistance of the medium.
At the starting moment, when the velocity of
the particle is zero (except for thermal
movements) , the particle is set in motion
by the electric force F e . With increasing velocity,
the force F r of the environmental resistance
increases (in fact it is a slight moment,
microseconds) until both forces acting on
the particle equalize F e= F r . Now is the
stationary state , in which the particles will move at
a constant speed v = Q . U / (k . L .
R m . S ). The speed of particle
movement - electrophoretic mobility - is therefore
directly proportional to the charge of the particles and the
electrical voltage at the electrodes and indirectly proportional
to the distance of the electrodes, particle size and resistance
(viscosity) of the environment.
(Electrophoretic mobility is usually normalized to 1V voltage.)
The carrier,
medium or medium in which the electrokinetic movement of the
substances of the analyzed sample takes place can be either a
free liquid - an electrolyte(not commonly
used, except for capillary electrophoresis) ,
a porous support such as chromatographic paper or
cellulose acetate film (the channels of
which can serve as a "molecular sieve") , but it is usually a gel (agarose, polyacrylamide, starch) .
In terms of hydromechanical properties, the gel is
a kind of transition form between solid and liquid state. More
than 90% is made up of water, but in which a dense three-dimensional
network is formedfrom polymerized sugar or acrylic chains
connected by transverse hydrogen bonds. The size and density of
the "mesh" of this network depends on the concentration
and method of gel polymerization - the gel concentration
determines the size and density of pores through which the
analyzed molecules pass (typically around
100-300 nm, for selected gel species it is comparable to nucleic
acid molecules and proteins) . The
concentration of the gel can therefore influence the separation
rate and the size resolution of molecular fractions (proteins, DNA fragments) .
Usually a concentration of around 1-2% is
used (up to 4% gel is used for finer resolution of short DNA
fragments) .
The pores of this network function as a
" molecular sieve"To move the
molecules due to the electric field, which larger molecules pass
more slowly than smaller molecules - obr.2.7.5a. Analyzed
molecules thereby progressively separated
according to their size (molecular weight) at a distance of several millimeters to centimeters;
individual fractions are provided according to the size of
molecules The places where the individual molecular
fractions of the analyzed sample penetrate at a given time form
more or less sharp local concentration maxima -
peaks of the electrophoreogram.
![]() |
Fig.2.7.5. Gel electrophoresis and
evaluation of density and radio-electrophoresis. a) Principle of separation of molecular fractions by electric field through a "molecular sieve". b). Basic arrangement of electrophoresis in a gel environment. c) Stained electrophoreogram. d) Optical - photoelectric - evaluation of electrophoresis fractions. e) Radiometric evaluation of the electrophoreogram of the radioactive analyte. |
A small amount (drop - a
few microliters) of the analyzed sample - analyte
- is applied with a micropipette to the starting point
in the gel (it is a well
in the gel, usually several wells next to each other, extruded by
a special "comb") . Then an electrical
voltage of the order of tens to hundreds of volts (approx. 1-10 V / cm) is applied
to the electrodes between which the column is inserted *) ->
electrophoresis begins - Fig.2.7.5b. After a preset time, or when
the fastest molecular component approaches the end of the column,
the electrophoresis process is terminated by disconnecting the
electrodes from the high voltage.
*) Electrical
conductivity of the environment, pH, buffer
The electrical conductivity (resistivity) of the environment -
the gel - affects the passing current and the mobility of
molecules during electrophoresis. It is regulated by the
composition and concentration of the electrophoretic buffer
in the gel. Buffer (from the English buffer
= buffer, buffer buffer ) is an
agent regulating the alkalinity or acidity of the environment -
the pH value . The fundamental importance of the
pH environment for obtaining the electric charge and thus the
mobility of the analyzed molecules was discussed above in the
note " Origin of the electric charge of the
analyzed molecules". Without the presence of
buffer salts, the electrical conductivity is minimal and the
molecules move and separate hardly and very slowly. However, if
the salt content in the buffer is too high, undesired heating of
the gel may occur during electrophoresis. Typical parameters used
for conventional columns are: pH 8, 6, voltage 200-250 V, current
about 10mA per 1cm of gel width, separation time about 10 minutes
pH gradient,
isoelectric " focusing "
Depending on the pH value of the environment,
many important analyzed molecules (amino acids, peptides,
proteins) obtain positive or negative This is decided by their
so-called isoelectric point pH (I) - the pH value at
which they are uncharged (discussed above in the section)The formation of an electric charge of the
analyzed molecules ").
This regularity can be used for a certain" trick "to
improve the resolution of electrophoresis.
There are certain chemicals called
ampholites which, when an electric current passes through
the electrolyte (in gel) creates a stable linear pH
gradient , pH at the anode and high at the cathode. in
this situation, the assayed molecules, e.g., proteins deposited
on the gel, a pH gradient will move to a place where the pH of
the medium be equal to the isoelectric point pH (I).
There is stoppedbecause the molecule here
becomes electrically neutral; and it will remain firmly there,
because if it is deflected towards the anode or cathode, it would
gain a positive or negative charge and the electric force would
immediately return it . The result is very narrow
and sharp streaks - Zone - peaks in the gel,
where the molecule is exactly focused ( focused
for a ) , the high-resolution
separation. The method is called isoelectric focusing
( ISS ).
Electrophoreogram
After switching off the electric voltage, the movement of the
molecules stops and the separated molecular
components remain temporarily fixed in the
places of the gel where they reached. After electrophoresis, the
gel must be dried or fixed with a suitable reagent (such as acetic acid) to immobilize
the separated molecules in the support medium and prevent
their diffusion. This creates a basic chemical electrophoreogram
, which is usually latent and needs to be "made
visible" - detected - evaluated
. This can be achieved mainly by two methods:
- Optical
detection methods make use of colored or luminescent labeled
analyte molecules - gel was stained in order to
visualize individual molecular fractions
(sometimes the dye is added to the sample
before analysis) . Special organic dyes are
used (blue, red, green, similar to
microscopic observation of slides) .
Subsequent irradiation of the stained column with visible or
ultraviolet radiation will elicit a fluorescent optical
response on the distributed substances (Fig. 2.7.5c). It can be
evaluated visually or electronically by photodetectors .
In the density method, the electrophoreogram is shifted evenly
over the slit of the photometer through which light of
the respective color passes (wavelengths,
approx. 400-700 nm) . In the place of
higher concentrations of individual fractions there is a partial
absorption of radiation, which is registered by a photodetector -
densitometry recording the changing value of light
absorption according to intensity - density - color, Fig.2.7.5d.
In fluorescence mode, the photodetector directly measures the
intensity of the emitted fluorescent light.
In the graphical record of the
absorbance or fluorescence of the individual electrophoretically
separated fraction of the analyte, they form
"bell-shaped" curves - peaks . The
read positions and intensities of the individual fragments are
compared with the standard sample analyzed in parallel .
The positions of the peaks determine the molecular weight of the
fractions, the area (integral) under the curves of these peaks is
proportional to the relative representation of the
respective fractions in the electrophoretically separated mixture
from the analyzed sample - it allows quantitative
analysis. Into one of the wells in the gel
(in Fig.2.7.5b
it is the last well on the right) a standard
sample with a known representation of analyzed
substances, eg various types of proteins, is applied. By
comparing the positions of the peaks in the chromatograms of the
analyzed samples in other routes, it is possible to identify the
type and representation of the sought types of substances
(molecules). The position coordinate on the chromatogram ( Fig. 2.7.5d, e) - the distance of
the peak from the start in [cm] - is thus calibrated
to the molecular weight axis in [kDa].
- Nuclear
radiation methods use the detection of ionizing
radiation emitted by radiolabelled sample molecules and its
fragments. From the point of view of our field of nuclear and
radiation physics, we will briefly present the method below radio
electrophoresis .
The above-described method of
electrophoresis in "plate" gel columns according to
Fig. 2.7.5b, c is suitable for the analysis of a smaller number
of samples and their molecular fractions. Current laboratory
electrophoreographers have the option of about 12-60 sample
routes in the gel column. They are used in biochemical analyzes
of eg amino acids, peptides, proteins, nucleic acids. Serum protein electrophoresis is most commonly
performed. These proteins are divided into about 6 protein
fractions. The width of the zones shown depends on the number of
individual types of proteins with similar mobility(molecular weight)that are present
in the fraction. Albumin is the most common here
, which also travels furthest
towards the anode. By decreasing electrophoretic mobilities
follow a1, a2, b1, b2 - globulins , b lipoproteins, the
transferrin .... gammaglobulin (G,
A, M, D, E) form a wide "woolly"
near the strip start. The electrophoreogram of blood serum
proteins (unspecified example) was used above as an illustrative example in Fig.
2.7.5c.
Electrophoresis of proteins in
urine (mainly proteins of glomerular
and tubular origin) and cerebrospinal
fluid proteins are also used here. (An increased proportion of
different types of proteins is found due to the increased
permeability of the blood-brain barrier or the inflammatory
process in the CNS) .
However, for
analyzes of a large number (hundreds or
thousands) of samples, this standard method
is somewhat complicated and time consuming. There may preferably
be applied by capillary electrophoresis :
Capillary
electrophoresis
utilizes electrokinetic effect electrophoresis (and electroosmosis) to the
separation process materials within a thin capillary
. A quartz glass (silica) capillary with an inner diameter of
approx. 20-200 micrometers and a length of min. 20cm to 1 meter.
It is connected to high voltage of approx. 20-30 kV via tubes
with buffer. Such a high voltage
(causing a higher current density and thus
Joule heat) can be used due to the
efficient heat dissipation from the capillary to the
surroundings. High voltage leads to increased separation
efficiency and shortened analysis time to units
of minutes.
This capillary serves as an electrophoretic
chamber , in the end part of which a photoelectric fraction detector
is located . There is no "chemical
electrophoreogram" along the capillary (which
would have to be subsequently evaluated) ,
but the detection of fractions takes place continuously
- " on line ". The detector
continuously evaluates the insensitivity and retention
timearrival of a given molecular fraction - the time
from the beginning of the flow, necessary for the given fraction
of the sample to reach the detector at the end of the capillary
chamber. This directly results in electrophoreogram peaks
.
Current routine biochemical methods
of DNA sequencing use fluorescently labeled nucleotides,
which are analyzed by capillary electrophoresis in a
large number (tens or hundreds) of parallel
capillary sequencers., wherein the fluorescent light is
registered by a sensitive opto-electronic detector. These new
sequencing techniques allow very fast and relatively inexpensive
"reading" of entire genomes. A huge amount of sequence
data obtained in this way is processed by computer - it becomes
the subject of bioinformatics .
The development of advanced very
fast, highly parallel (approx. 10 6 ) sequencing techniques continues, using short-section
fragmentation, sequencing synthesis of complementary strands to
the analyzed fragment using DNA polymerase, implementation of
chemical luminescent labels signaling incorporation of
new bases into the DNA strand. Completely new possibilities of electronic
detection are also being testedduring sequencing, based on
electrical signals from a change in conductivity in the
"molecular sieve" environment, in which DNA fragments
are translocated ... However,
all these methods completely go
beyond our treatises on nuclear and radiation physics (as well as the professional focus of the author. .) .
Radioelectrophoresis
is a method of electrophoretic separation of radioactive
substances with subsequent use of ionizing
radiation detection to evaluate the distribution of
radioactive molecular fractions in columns. These are either primarily
radioactive substances (such as
radiopharmaceuticals) or substances labeled
with targeted radionuclides only for the
purpose of their analysis.
Detection of the distribution of
analyzed radioactive substances on the electrophoregram is
performed by radiometric methods i - using
suitable electronic radiation detectors (alpha,
beta, gamma) along the electrophoretic tape
or column (it is analogous to the
radiochromogram in Fig.2.7.4c) . The
collimated scintillation detector passes just above the
radioactive electrophoretic column, registers the local intensity
of the emitted radiation from individual places - the frequency
of detected pulses - and outputs the electrophoretic curve
(Fig.2.7.4e), with or. quantitative computer evaluation. In the
graphical record, the individual divided fractions of the analyte
form "bell-shaped" curves - peaks. The
area (integral) under the curves of these peaks is proportional
to the relative proportion of the respective fractions
in the electrophoretically separated mixture from the analyzed
sample.
Radioelectrophoresis
is sometimes used - in addition to the more frequently used radiochromatography
- to measure the radiochemical purity of
radiopharmaceuticals - §4.8 " Radionuclides and
radiopharmaceuticals for scintigraphy ", section " Radiochemical purity
". A special method of radioelectrophoresis evaluation is
the above-mentioned autoradiography , based on
the effect of radiation emitted from radioactive fractions on a photographic
emulsion (or on an electronic imaging
detector) attached closely to the
electrophoretic column - §2.2, passage " Autoradiography ".
Chromatography
<- versus -> electrophoresis
Both of the above analytical methods of chromatography
and electrophoresis have much in
common. They differ mainly in two aspects:
1. Chromatography is a passive method ,
where the molecules of the carrier and the analyzed substances
move and separate due to natural thermal motion and
intermolecular forces, manifested, for example, by capillary
forces.
2. Electrophoresis is an active method ,
where the molecules of the carrier and the analyzed substances
move and separate under the influence of an artificial
electric field and current. This controlled method provides wider
possibilitiesoptimized analysis of a large range of
substances, especially proteins, nucleic acids, DNA fragments.
Therefore, the technical design and
the course of the analytical process also differ. However, the
evaluation of the result - chromatogram and electrophoreogram
- is largely similar , including the possibility
of using radiometric methods - cf. Figures 2.7.4 and 2.7.5.
2.8.
Absolute measurement of radioactivity and radiation intensity
Like measuring methods in general, radiometric measuring methods
can be divided into absolute and relative
. For relative measurements, we are concerned with determining
the ratios of activities or radiation
intensities of individual samples either with each other or with
respect to a suitable standard ; in most
applications of ionizing radiation, such relative measurements
are sufficient for us. For absolute methods, we need to determine
the absolute value of the activity in [Bq] or
the absolute intensity of the radiation beam in
[number of quantum / cm 2 ] ( fluence) by direct measurement under
precisely defined conditions from the measured ionization current
or pulse frequency.) or in dose units [Gy] or dose rate [Gy / s].
Absolute measurement of radiometric quantities encounters a
number of fundamental and technical difficulties
, which will be briefly discussed below. Methods of absolute
measurement of radiometric quantities can be divided into two
categories:
Absolute
measurement of radioactivity
Several methods using physical and chemical manifestations of
radioactivity can be used for the primary absolute measurement of
the activity of radioactive emitters and preparations.
Absolute counting of emitted particles The
activity A of a certain radioactive emitter (ie the number
of nuclei that is converted per unit time) can be measured
directly using the frequency of particles
(quantum of radiation) that a given sample emits. Using a
suitable detector, measure the number of pulses N
for a certain time t , subtract the background
pulses N p
and multiply the result by the correction factor F
:
A [ Bq ]= F. (N - N p ) / t.
The total correction factor F includes all factors
affecting the detection of radiation from a given sample; is the
product of several partial coefficients: F = f g . f d . f a . Here f g = 4 p / w is a geometric
factor given by the ratio between the full spatial angle
4 p (into
which isotropic radiation from each source takes place) and the
actual angle w , in which the quantums emitted from the emitter fall
into the sensitive space of the detector (Fig.2.8. 1 top left). f
d is the
correction factor for the detection efficiency,
which depends on the type and size of the detector, the type and
energy of the detected radiation, or on the dead time of the
detector. f a is the radiation absorption correction
factor , which is the product of the self-absorption factors in
the sample, the radiation absorption in the detector window and
possibly radiation absorption in the environment between the
source and the detector. These correction factors must be
determined by independent measurement for each specific method.
GM tubes, scintillation and semiconductor detectors, proportional
detectors are used to detect quantum radiation. A special group
consists of methods with a measurement geometry of 4p,
when the active space of the detector completely surrounds the
radiation source. The measured sample is stored either inside the
ionization chamber - GM or proportional detector, or the
radioactive atoms are evenly dispersed in the gas charge, or the
radioactive preparation is dissolved in a liquid scintillator.
The measurement geometry is almost completely 4 p , only at the
edges and walls of the detection volume there is a reduction in
detection efficiency.
Absolute coincidence
methods
In certain special cases, the difficult and difficult
determination of the above correction factors F can be
circumvented. The elegant possibility of determining the absolute
detection efficiency (ie h = 1 / F), and thus
automatically the possibility of measuring the absolute activity
of a radioactive preparation, is offered for such radionuclides
that emit two quanta of ionizing radiation simultaneously
(or more quanta simultaneously). Here we can use methods of
simultaneous - coincidence - detection of these
two quanta of radiation emitted by a radionuclide.
Method of b-g coincidences
This method is suitable where the radionuclide emits beta
radiation accompanied by gamma photons (decay scheme in the left
part of Fig.2.8.1 above). In this case, we place a measured
sample of (sought) activity A between two detectors,
equipped with independent evaluation circuits (detection
"channels") and a coincidence circuit. Detector D b is
sensitive only to beta radiation and will measure the pulse
frequency n b = AF b , where F b is the geometric-efficiency factor for detecting
radiation b from the sample. Detector D g , sensitive only
to gamma radiation, will measure the pulse frequency n g = AF g , where
F g analogously characterizes the efficiency of g radiation
detection from the sample. A pulse will appear at the output of
the coincidence circuit only if pulses occur at both its inputs
at the same time. The probability that a particle b and a quantum g emitted during one
radioactive transformation will be registered at the same time is
F b .F g , so the frequency of coincidences will be n koin = AF b .F g . From
these three relations we can modify the unknown factors F b and F g by
adjusting , which gives the resulting relation: A [Bq] = (n b .n g ) / n koin, according to
which the absolute activity A can be
determined by the coincidence method only on the basis of
measuring the pulse frequencies n b and g in both
beta and gamma detectors.
Fig.2.8.1. Coincidence measurement of absolute activity and
detection efficiency.
Left: Coincidence detection of radiation b and g by two separate
detectors b and g . Right: Coincidence analysis of
measuring pairs of radiation quantum g .
Method of g-g coincidences
This method can be used if the investigated radionuclide emits
two photons simultaneously during each transformation - either
during deexcitation of nuclear levels in a cascade (eg 60 Co), or in electron
capture accompanied by simultaneous emission of a characteristic
X-ray photon from envelope and photon g from the excited daughter
nucleus (eg at 125 I) - Fig.2.8.1 at the top right. If both of these
quantums enter the detector and are registered, the detector will
not distinguish them from each other in time - they will coincide
and the transmitted pulse will be equal to the sum of the
pulses from both quanta. The summation peak n S thus
appears in the spectrumcorresponding to the sum of the energies
of both quanta (Fig.2.8.1 on the right).
If we measured 4 p in the geometry
and the detection efficiency was 100%, all pairs of coincidence
quanta would be detected simultaneously and only the
summation peak would be present in the spectrum . If the
detection efficiency is lower than 100% (this is practically
always the case), only one of the quanta is detected in a part of
the concentration pair, and in addition to the summation peak,
the actual peaks of both quanta with respective
energies appear in the spectrum . The lower the detection
efficiency (whether by deviation from the geometry of 4 p, or the lower the
efficiency of the detector itself), the lower the probability of
detecting both coincidence quanta and the lower the summation
peak with respect to the individual peaks of the individual
coincidence pairs. By evaluating the ratio
between the areas (integrals) of the summation peak n S and the
peaks of individual coincidence quanta n 1 , n 2 it is possible to determine the total detection
efficiency in a given measurement arrangement, which includes
all partial factors (geometric, absorption, detector
efficiency) and thus determine the absolute activity of
the measured sample: A [Bq] = (n 1 + n 2 + 2.n S ) 2/4.n S . This method works well in a geometry close to 4 p and with not too
low detection efficiency, when the summation peak n S is well
expressed (the accuracy of determining the absolute activity may
be better than 1%). With a geometry of 2 p or lower, the summation
peak in the spectrum almost disappears - the method is subject to
a large error or is not applicable at all.
Calorimetric measurement
of absolute activity
In principle, the thermal effects of the
energy released during radioactive transformationscan also be
used for the absolute measurement of radioactivity. Since the
amount of heat generated is relatively small from a macroscopic
point of view, this method can only be used for preparations with
relatively higher activity, of the order of hundreds of MBq and
GBq. The so-called Isothermal calorimeters ,
operating at normal temperature, are sometimes used for the
absolute measurement of the radioactivity of high-activity
preparations - bridge temperature thermistors are used to compare
the temperature difference between a reference sample and a
sample containing radioactive material. .............................
Electrostatic measurement
of absolute activity
can be used to measure the activity of radionuclides
emitting charged particles - a or b . When a charged particle is emitted from
a radionuclide (originally electrically neutral), this
radionuclide acquires an equally large charge of the opposite
sign. From the change in the total charge of the sample over a
certain time interval, the activity can in principle be
determined. As this creates very small charge values, it is
necessary to use sensitive electrometers. There are a number of
disturbing influences, coming from secondary electrons released
during ionization by radiation and from the absorption of charged
particles in the source itself ...
Calibration of detectors
for measuring activity
...............
Measurement of activity by
well ionization chamber
For approximate measurement of radioactivity, especially in the
field of medical applications of open radionuclides, ionization
chambers in well design are preferably used as activity
meters of radioactive preparations (these
meters are sometimes incorrectly called "dose
calibrators") - was mentioned above in
§2.3 " Ionization detectors ", Fig.2.3.1 on the right. The vial or syringe
with the radioactive substance is inserted into the opening of
the well ionization chamber, which in the geometry close to 4 p registers the
emitting radiation g . The electrical signal I from this chamber is
proportional to the activity of preparations A and G-constant of the
given radionuclide: I ~ A. G ; The G- constant is different for each radionuclide. The
electronic circuits of the activity meter are calibrated
so that for the selected radionuclide the display shows its activity
directly in MBq.
Measurement
of radiation intensity and dose rate
The intensity or fluctuation of radiation can be
measured directly using suitable detectors, sensitive to the
given type of radiation and located at the desired location of
the beam. The measured frequency of pulses N per unit area
of ??the detector must again be corrected by the factor F of
the detection efficiency. In the case of stronger beams of
radiation, used for example in radiotherapy, the intensity of
radiation is characterized by a dosimetric quantity of radiation dose
rate at a given location of the substance that is
exposed to radiation. Normalized and calibrated ionization
chambers , and more recently semiconductor detectors,
are most often used for these measurements .
For measuring doses and dose
ratesinside irradiated materials , eg inside
tissue in radiotherapy, the so-called Bragg-Gray method of
ionization chamber is used, located in a cavity inside the
material (or this chamber itself is the cavity). This measurement
is objective, assuming that the radiation intensity in the
surrounding material and in the chamber is the same, the
dimensions of the chamber cavity are much smaller than the range
of secondary electrons in the gas and the dimensions of the
surrounding material layer are much greater than the range of
secondary electrons in this material. Then the electron
equilibrium occurs in the system, the presence of the cavity
- the ionization chamber - does not disturb the electron balance
in the surrounding material. Then the ionization of the gas in
the chamber cavity is caused almost exclusively by electrons
coming from outside their walls (the contribution from the
interaction of primary radiation in the chamber cavity is
negligible) and these electrons lose only a small part of their
kinetic energy as they pass through the chamber cavity. The
transmitted energy at the location of the chamber and its
surroundings is practically equal to the kinetic energy of the
released electrons ( dose = kerma ). In this situation,
the number of ion pairs formed in the chamber cavity is proportional
to the dose of primary radiation in the material that
surrounds the chamber cavity. Thus the dose rate D ´ is
proportional to the current I through the ionization
chamber: D´ = LwI / ( r.V), where L is the ratio of the linear
ionization of the irradiated material and the chamber gas during
electron motion, w is the energy required to form one ion
pair in the chamber gas, r is the density (specific gravity) of the gas, V
is the volume of the chamber cavity. In practical measurements,
the ionization chambers are placed in the appropriate sites of
the phantoms , either aqueous or tissue
equivalent (which, by their composition, mimic the
irradiated tissue).
2.9.
Measurement of radioactivity in the organism (in
vivo)
A specific area of radiometric measurements is the detection of
radiation from radioactive substances deposited within
the organism - in vivo measurements .
Let us first ask the question: Under what circumstances can
radioactivity enter the body? There are basically two options :
In both of these conflicting situations, there may generally be a need to measure the amount or distribution of radioactivity in an organism .
Whole-body
measurement of radioactivity
Absolute measurement of the total amount of radioactivity
in the organism may be important in the mentioned case
No. 2 - internal contamination of radiation
workers.
We determine radioactivity in the organism on the basis of
external detection of outgoing gamma radiation. In order to
achieve approximately the same detection efficiency for all parts
of the body during whole-body measurement, several
scintillation detectors of larger dimensions are used,
distributed around the patient's body in the arrangement of the
so-called whole-body detector.. For some types,
the body passes evenly between the detector arrays. When
evaluating the number of measured pulses from individual
detectors, a number of corrections for absorption and geometric
conditions are used, as well as multiplication by calibration
coefficients expressing the relationship between the
measured pulse frequency and the activity of the monitored
radionuclide in the body.
To detect internal contamination by an
unknown radionuclide, or a mixture of different radionuclides,
high-energy spectrometric semiconductor detectors
are also rarely used in the whole-body detector , while the
spectra are measured and evaluated using a multichannel
analyzer .
In diagnostic
applications of radioactivity, in the 1970s and 1980s, whole-body
measurements were performed only very rarely in determining the
resorption of certain substances (eg vitamin B12 labeled with 57 Co); now this method
is mostly abandoned and by whole body measurements in nuclear
medicine we usually mean whole body scintigraphy
- chapter 4 " Radioisotope scintigraphy
".
Local
measurement of the distribution of radioactivity
In diagnostic and therapeutic applications of radioisotopes to
the body, we do not need to measure the absolute value of
radioactivity in the body (this was measured in a bottle or
syringe before application). Rather, we need to determine the distribution
of radioactivity in individual places and organs in the
body - this has direct diagnostic or therapeutic significance. It
is therefore a relative local measurement of
radioactivity in certain places of the organism on the basis of external
detection of the outgoing gamma radiation, which
penetrates the tissues out of the organism.
This local measurement of
the intensity of the emitted radiation can in principle be
performed by simply applying the scintillation detector to the
individual locations. However, a free detector (without
shielding) would register g radiation not only from the desired location, but also
from other locations in the body, with only slightly lower
efficiency (given the greater distance of these locations from
the detector). In order to achieve selective detection of
radiation only from the desired place (direction) in the body, it
is necessary to shield the radiation g coming from other
(undesirable) directions and measure only radiation from a narrow
cone in the desired direction - to equip the detector with a collimator
. By successively applying such a collimated detection
probeto individual parts of the body we can
"map" the distribution of the radioindicator in
individual organs within the organism. Or, with a collimated
detection probe aimed at a specific organ, we can monitor the
time course of distribution of the radioindicator in this organ
after the application of a radioactive substance to the organism.
Nuclear medicine
These detection methods have played an important role in the
relevant stage of development of nuclear medicine
- a field dealing with diagnostics and therapy using open
radionuclides (Chapter 4). The simplest method is to determine
the accumulation of 131 J in the thyroid gland , where after
application of a known amount of radioiodine, the radiation g from the thyroid
gland is measured with a
collimated probe g and the percentage of radioiodine taken up by the
thyroid gland is determined by comparison with the standard
value; repeated measurements for several days further determine
the half-life of iodine from the thyroid gland. The most used
measuring method in nuclear medicine in the 60s-80s was radioisotope
nephrography: Two collimated scintillation probes were
directed from behind into the right and left kidney, radioactive 131 J-hippuran was
applied, which was taken up in the kidneys, and the detection
probes registered the amount of radiation from the left and right
kidneys. The impulses from the nephrographic probes were fed via
integrators to the recorders, whose pens gradually plotted the
so-called nephrographic curves , showing the
accumulation of 131 J-hippuran in the left and right kidneys and its
gradual excretion into the urinary tract. From the shapes of
nephrographic curves (steepness of increases, times of peaks and
steepness of decreases) it is possible to deduce the functional
ability of the kidneys and their drainage.
Occasionally, g- registration methods have
been usedcollimated detection probes from the heart or brain -
the method of so-called radiocirculation in order to examine the
dynamics of heart activity or blood flow through the cerebral
hemispheres. However, these and similar methods were already
debatable at the time of their creation (due to large errors and
inaccuracies in the detection of "blind" radiation) and
were soon abandoned. All these methods, including nephrography,
have been replaced by significantly more perfect
and complex scintigraphic methods .
Radiation-guided surgery - sentinel
nodes
Local radiation measurements with a closely collimated miniature
gamma ray detection probe find important application in
radiation-guided surgery in the detection of so-called sentinel
nodes. In the surgical treatment of cancer, it is important to
remove not only the primary tumor, but also, if possible, other
tissues into which the tumor cells could be infiltrated. These
tumor cells spread from the primary site mainly through the
lymphatic pathways, so that the lymph nodes
around the tumor site are the first to be affected . If we apply
a suitable radioindicator of colloidal state to the peripheral
part of the tumor lesion (most often 99m Tc nanocolloid, particle size approx. 50-600 nm,
activity approx. 40-150 MBq), it will spread through the
lymphatic pathways and capture and accumulate in
those nodes that are lymphatically associated with the tumor
site. The first such node in the lymphatic "basin" of a
tumor foci is called the sentinel node . The
accumulation of the radio indicator in the nodes can be displayed
scintigraphically. However, the most important thing is to
monitor the radioindicator during the actual surgical procedure,
when using a collimated detection probe the surgeon can find a
sentinel node containing the radioindicator directly in the
operating field. perform a scintigraphic imaging with a plot of
the displayed nodes, then the patient goes to his own surgery
, during which a detection gamma probe is used.
*) Along with the radioindicator, a blue
dye (........) is applied at the same time, which also penetrates
the nodes, so that the surgeon can recognize the sentinel node by
its blue color.
Scintigraphy
The only viable and promising methods of analysis of the
distribution of radioactivity in the organism (apart from the above-mentioned method of sentinel node
detection) are methods of scintigraphic
imaging of static or dynamic distribution of the
radioindicator. These methods provide detailed mapping
and a clear viewdistribution of the radio
indicator, including all peculiarities and anomalies within the
organism, with the possibility of visual evaluation and detailed
mathematical analysis and quantification of the parameters of the
investigated processes within the organism. Methods of
radioisotope scintigraphy are described in detail in Chapter 4
" Radioisotope scintigraphy ".
2.10.
Calibration and inspection of detection instruments
In order to ensure accurate and reproducible results of
radiometric measurements, it is necessary to ensure the correct
setting and calibration of detection
instruments and to check these parameters regularly - thus
ensuring their stability .
Calibration of radiometric
instruments
The basic adjustment and calibration of these measuring
instruments is usually performed by the manufacturer upon
delivery of the equipment, it is a " company
calibration ". The need for self-calibration
of the instrument arises for the user when he intends to
use it for a different purpose than the one for which it was
calibrated. Possibly. it is a device for general use
, the final response of which is individual pulses, voltage or
current signal. Some methods of absolute calibration have been
mentioned above in §2.8. For the average user, usually only the
method of relative calibration is available -
calibration of the instrument using standards,
or by comparing it with another ("standard") device.
For instruments that are subject to high requirements for
accuracy and reproducibility of results, metrological
calibration or verification of the instrument is
performed using standards with the participation of an
authorized laboratory (metrological institute).
Stability of the measuring
instrument and its control
The instability of
radiometric instruments can be caused by various influences,
depending on the type of detection instrument. In simple devices
based on ionization chambers, instability is often caused by a
change in pressure and the composition of the gas charge in the
chamber due to a leak (charge leakage) - it usually leads to a reduction
in detection efficiency .
In the case of
spectrometric instruments, the instability of the detector and
the electronic circuits of the evaluation apparatus is manifested
by a shift of the spectrum and thus also by
a change in the position of the photopeak.; if the
analyzer window is correctly set symmetrically to the photopeak,
due to the instability, the photopeak will "pass" from
the window and the number of registered pulses will change
significantly. For multi-channel analyzers, instability can lead
to shifts and blurring of photopeaks. The cause of the spectrum
shift is a change in the height of the signal pulses
at the output of the detector, or a change in the
electronic gainevaluation apparatus. These changes can
be caused by high voltage fluctuations at the photomultiplier
dynodes (this is very sensitive!), Amplifier gain fluctuations,
changes in electrical values ??of electrical circuit components
either by temperature fluctuations or "fatigue" and
aging, photomultiplier "fatigue", changes in
scintillation crystal properties (such as yellowing due to water
absorption) or a semiconductor detector (drift diffusion, etc.).
To eliminate the effect
of supply voltage fluctuations, modern devices are equipped with voltage
stabilizers . In order to achieve high stability of the
detection devices and to reduce or eliminate the influence of
temperature on the response of the device, it is recommended to
ensure temperature stabilization the environment
or room in which the device operates (eg air conditioning).
Another principle to ensure accurate and stable measurement
results is that we never measure on the instrument immediately
after it is turned on. The amplification and other parameters of
the electronic apparatus, as well as the detector itself, may
change slightly after switching on the device and only in a few
minutes the device enters a steady mode , in
which it remains for a long time (unless there is instability due
to another cause or failure) . Some devices do not turn off at
all to ensure high stability.
Tests of quality, correct function and stability of measuring instruments are usually divided in time into two groups:
From an organizational point of view, each measuring instrument for detecting radiation have drafted legislation for setting , monitoring and measurement methodology , test results should be zapis Ovan in the technical journal of the instrument, and evaluated and archived computer.
Statistical check of instrument
response stability
If the radiometric instrument is properly calibrated and its
correct setting is verified, it will not show coarser measurement
errors, but in principle smaller deviations may occur due to
instabilities, which do not significantly affect the frequency of
measured background pulses, samples and standards, and "at
first glance" we do not know them. We can then verify the
correct and stable function of the device using a suitable statistical
criterion - whether the measured pulse frequency is
subject only to statistical fluctuations, or whether it is also
affected by other factors. This method of verification is based
on the knowledge of statistical fluctuations given in the
following §2.11; here we will only present the procedure.
One way to evaluate the
proper function of the device is torepeated measurement
of the same sample on the instrument, thus finding a set
of frequencies N 1 ,
N 2 , ..., N n . We calculate the average value
N´ = (N 1 + N 2 + ... + N n
) / for the standard deviation of the
measurement s = Ö (N 1 2 + N 2 2 + ... + N n 2 ) / n .. This we compare the standard deviation with
the standard deviation given exclusively by statistical
fluctuations of radioactive transformations Ö N´. If the standard
deviation s during the measurement significantly exceeds the
theoretical value Ö N´, the instrumentcontributes to error by its
instability .
For a quick orientation check, it is enough to measure the same
sample twice, which gives two values ??of the number of pulses N 1 and N 2 . To assess whether the difference between these two
values ??is still within the range of statistical fluctuations,
use the following criterion: if the difference exceeds | N 2 -N 1 | between the
measured numbers of pulses, approximately three standard
deviations of the statistical fluctuations, ie 3. Ö [ (N 1 2 + N 2 2 ) / 2 ] , this indicates a suspicion of instrument
instability .
2.11.
Statistical fluctuations and measurement errors
Like all other measurements of real natural quantities, radiation
detection and spectrometry methods are subject to certain errors
. However, the nature and origin of these errors have their own
specifics in nuclear and radiation measurements, which we do not
usually encounter in other areas. These are irreversible
statistical fluctuations . *)
*) In everyday life, we do not encounter
these flutterations because we mostly observe macroscopic objects
and the amount of photons of light with which we observe is so
large that the fluctuations are negligible. However, in
astronomy, for example, in the observation and spectrometric
measurement of the faint flow of light from distant galaxies,
statistical fluctuations are exactly the same as in the detection
of faint ionizing radiation. Quantum - statistical fluctuations
are generally encountered wherever the measured signal is so weak
that quantum fluctuations of the measured quantity are applied.
Statistical fluctuations
The emission of quantum ionizing radiation, as well as its
interaction with the atoms of the material environment (and thus
the mechanisms of radiation detection) takes place at the microscopic
level through events governed not by detrminist laws of
classical physics but by the laws of quantum mechanics
. These quantum regularities are in principle stochastic
, probabilistic (see §1.1). The transformation of radioactive
atoms is a completely random process and the
resulting ionizing radiation is emitted randomly, uncorrelated,
incoherently. The flow of ionizing radiation is therefore not
smooth but fluctuating . The response
will be just as volatile any device that detects
this radiation. In repeated measurements of the same sample under
the same conditions, we therefore measure somewhat different
values ??of the number of pulses, which fluctuate around
a certain mean value. These are deviations that can not
be eliminated by any improvement of the device or
method, these fluctuations have their origin in the very essence
of the measured phenomena.
The influence of statistical fluctuations on the results of
radiation detection and spectrometric measurements can be
expressed in a simplified (but concise) way by the following
rule:
If we measure N pulses on a radiation detector , we actually measured N ± Ö (N) pulses. |
More precisely, the standard deviation
s (also
called the standard deviation ) of an individual
measurement is given by the square root of the
measured number of pulses N: s
= ± Ö
(N). This means that when measuring
repeatedly, approximately 68% of the total number of measured
values ??lie in the interval (N- s, N + s) , 95% of the values
??lie in the interval (N- 2s,
N + 2s) and 99% of the values ??in the
interval ( N- 3s, 3s + N) . *)
*) The probability of occurrence of a
measured value of the number of pulses is generally called
expressed. Poisson law distribution (which is
zero in the areas around asymmetric), higher number of pulses is
then passed in a symmetricalnormal (Gaussian)
distribution (Fig.2.11.1 left).
Fig.2.11.1. Left: Distribution of the
probability of occurrence of the measured values of the number of
pulses according to the Gaussian normal distribution.
Right: Comparison of spectrometric measurements
at low (top) and high (bottom) number of detected pulses.
The relative error
(coefficient of variation) of the measurement is then D = s / N = Ö (N) / N = 1 / Ö (N), or ´ 100 if we want to
express it in%. Thus , the higher the number of
pulses we measure, the lower the measurement
error - and this is also the only way to reduce
the errors caused by statistical fluctuations! If we measure 10
pulses, the error is 1 / Ö (10) @ 33%, at 100 pulses the error will be 1 / Ö (100) = 10%, at
1000 pulses 1 / Ö (1000) @ 3%, and only when we measure 10000 pulses, the
statistical error will be only 1%: 1 / Ö(10 4 ) = 1%.
Thus, the only way to reduce statistical fluctuations is to
increase the accumulated number of pulses - the number
of "useful" photons g
from which the response in the detection
device arises. Thus, at low radiation intensity or radioactivity
of the measured sample, it is necessary to increase the
measurement time in order to accumulate the number of pulses
needed to achieve the required accuracy (relative error) of the
measurement. In Fig.2.11.1 on the right is an example of the
scintillation spectrum of a 131 I radioiodine sample measured by the same detector at a
maximum accumulated number of pulses N = 50imp / cell (measuring time 5 sec. - top) and
N = 15000 imp./cell (measuring time 300
sec. - bottom) . The
difference in the quality and accuracy of the spectrum is evident
- with a low number of registered pulses, the details in the
shape of the spectrum are "drowned out" by statistical
fluctuations, while with a high number of pulses the spectrum is
"drawn" smoothly and in detail with minimal
fluctuations. In the same way, statistical fluctuations are
manifested in all radiation measurements, eg in scintigraphic
images ( Chapter 4 - Radioisotope scintigraphy ).
Total measurement error
The resulting measurement error
generally consists of individual partial errors
, which can be divided into three groups :
If individual partial errors have a statistical character, the resulting measurement error according to the laws of mathematical statistics is given by their geometric sum : s = ( s 1 2 + s 2 2 + s 3 2 + .... + s n 2 ) 1/2 .
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