AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy | Physics and nuclear medicine |
1.
Nuclear and radiation physics
1.0. Physics - fundamental natural
science
1.1. Atoms and atomic nuclei
1.2. Radioactivity
1.3. Nuclear reactions
1.4. Radionuclides
1.5. Elementary particles
1.6. Ionizing radiation
1.6. Ionizing radiation
Radiation - an important natural phenomenon
Radiation : |
By radiation ( rays
) we generally mean processes in which energy
is transferred acros space "at a
distance" through physical fields or microparticles
*). In addition to energy, radiation also involves the transfer of matter and information . |
*) Thus, radiation is not the
transfer of kinetic energy, for example, by a stone thrown into
the distance or a bullet fired from a rifle. Radiation is,
for example, flying electrons, protons or neutrons, accelerated
nuclei of atoms, flying ions or whole atoms. And of course
electromagnetic waves and their quantum photons.
This radiative
energy transfer can be performed by two types of mechanisms :
Radiation can spread either :
Transmission
of information by radiation
The transmission of energy by radiation, due to its structure, is
accompanied by the transmission of information .
Radiation carries information about its source
(its nature, composition, "strength", or variability,
etc.), as well as about the substance environment through
which the radiation passes (density, thickness, chemical
composition of the substance environment). This information is
"coded" both in the intensity of the radiation
and in the energy spectral distribution . With
the help of detection and spectrometry ,
radiation can help us uncover the secrets of the composition
of matter , the structure and the evolution of
the universe. (especially stars and galaxies, as well as
global cosmological issues), in the biological field the anatomical
structure and physiological processes in living organisms
(§3.2 " X-ray diagnostics ", chapter 4 " Radioisotope
scintigraphy ").
Energy
and effects of radiation
In addition to the type of radiation (the type of particles that
make up radiation), the energy of the radiation
quantum determines its properties of propagation and interaction
with matter . From this point of view, we distinguish :
l "
Soft " radiation ,
the quantum of which has a low energy (<approx. 5keV) and is
not able to eject electrons from atomic shells.effects, in some
cases electrical effects (external and internal
photoeffect, changes in electrical conductivity) and photochemical
effects on more complex or weakly bound molecules (classical
photography, photosynthesis in plants).
l "
Hard " radiation ,
the quantum of which has a sufficiently high energy (tens of keV
and higher) and when passing through a substance, electrons are
ejected from atoms and ionize the substance .
Ionization then leads not only to electrical and photochemical
effects, but in the case of compounds to a number of chemical
reactions of decomposition of existing molecules and
possibly. formation of new compounds. Energy carried by hard
radiation, through the effects of radiation on matter,
can therefore be used in a number of so - called radiation
technologies ; in the medical field, the application of
radiation helps to treat certain diseases,
especially cancer (§3.6 " Radiotherapy ").
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 electrons and the 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 , especially lasers
and light emitting diodes, 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 , in
electronic sources of ionizing radiation (X-rays, accelerators),
as well as in the relevant measurement and control technology.
Ionizing radiation
In the study of radioactive phenomena and particle interactions,
we have repeatedly recognized that various types of emitted
radiation here usually have a very high energy ,
much greater than ordinary light. This high energy of quanta of
"radioactive" radiation, X-rays and some other species,
is an important property that determines the effects of
radiation on matter - it is ionizing
radiation :
Ionizing radiation: |
We call ionizing radiation such radiation, the quantum of which has such a high energy that they are able to eject electrons from the atomic shell and thus ionize the substance. |
To ionize the material environment by pulling
an electron out of the atom, it is necessary to transfer to the
electron energy greater than its binding
energy in the atom - output work, ionization energy. For
the lightest hydrogen atom, whose electron is in the ground state
on the K shell , it is 13.6 eV. For more complex atoms
with the proton number Z , the binding energy of the
electrons on the K shell is Z 2-times greater than hydrogen. For lead (Z = 82), the
ionization energy on the K-shell is about 85keV. For atoms with
more electrons, we have different values of ionization energies,
because electrons have different binding energies on different
shells. The value of the binding energy decreases rapidly with
the distance of the electron path from the nucleus - it is
inversely proportional to the square of the main quantum number,
which indicates the order of the electron path. The valence
electrons, which are furthest from the nucleus on the outer
shell, have the smallest binding energy.
For common types of photon (X and
g ), electron
( b - ) and alpha
radiation, the energy limit of ionizing radiation in practice
(radiation protection) is 5 keV. The situation
is more complicated with neutron radiation, where even
very slow neutrons enter the nuclei and can cause secondary
ionization through nuclear reactions (even delayed or longer-term
- activation of nuclei, formation of
radionuclides). Similarly, the ionization threshold energy for b + radiation is not defined , where even very slow
positrons annihilate with electrons to form hard ionizing
radiation g .
Terminological
note :
The names " nuclear radiation
" or " radioactive radiation
" are also sometimes used for ionizing radiation
. These names are not very appropriate and can be misleading.
"Nuclear radiation" includes only radiation a, b, g emanating
from the nucleus, but not braking and characteristic X-rays,
annihilation g- rays, nor radiation generated by accelerators and
particle interactions. The same is true of "radioactive
radiation", where the word "radioactive" may give
the impression that the quantum of this radiation is radioactive
- this is of course not true (except for neutron radiation (free neutrons are b
- radioactive)) and some "exotic" species. radiation, such as
muon or pion, consisting of short-lived decaying particles (but
this is not meant here). The term "radioactive
radiation" can only be understood in the sense of
"radiation generated by radioactivity" and even then
includes only certain types of ionizing radiation. Commonly used
name ""generally includes all types of radiation, not
just ionizing radiation.
Physics of
ionizing radiation - radiation physics - radiology - dosimetry
The physics of ionizing radiation is also known
as radiation physics or radiophysics. It covers
a wide range of issues :
¨ Mechanisms of radiation
¨ Physical
properties of radiation
¨ Interaction of
radiation with matter (including radiobiological
effects)
¨ Detection and
spectrometry of radiation
¨ Mathematical
analysis and evaluation of results
A special area of radiation physics is radiological
physics , dealing with physical aspects of radiation in
medicine. Dosimetry of ionizing radiation is a
field of radiation physics, which deals with the effects of
radiation on substances in relation to the types and properties
of radiation - substance interaction and the amount of radiation
absorbed in the substance (absorbed energy - "dose") -
§5.1 " Effects of radiation on matter . ". The studied substance is mainly in living
tissue , model measurements of doses and dose rates are
performed in water, air and special dosimetric phantoms
.
Radiology
-
radiation in biology and medicine
From the etymological point of view, the word radiology
generally means the science of radiation. However, historical
developments have narrowed and specified its significance. Radiology
is now a science of the importance and use of radiation in
medicine and biology , a medical field that uses
ionizing radiation for diagnosis and therapy. It mainly includes
three main special fields :
l X-ray
diagnostics , also called radiodiagnostics *) (§3.2
" X-ray diagnostics ")
l Radiotherapy (§3.6
" Radiotherapy ")
l Nuclear
medicine (Chapter 4 " Radioisotope
scintigraphy ")
*) During the development , they
vere in the field of radiodiagnostics included other diagnostic imaging
methods, that do not use ionizing radiation - ultrasonic
sonography (see Ultrasound
sonography ), nuclear magnetic
resonance (Nuclear magnetic resonance ) and thermography ( Thermography ).
Radiobiology deals with the biological
effects of ionizing radiation (see §5.2 " Biological effects of ionizing
radiation ") - a field on the
border of radiation physics and biology.
Types of ionizing
radiation
Directly
and indirectly ionizing radiation
Ionizing effects are therefore a common property of all types of
ionizing radiation. However, the specific mechanisms of
radiation-mass interaction are specific to each type of
radiation. In this respect, ionizing radiation is divided into
two groups :
In terms of physical, chemical and especially biological
effects of ionizing radiation on the irradiated
substance, especially on living tissue, the radiation is
sometimes divided according to the density of ionization
, which it causes in the substance during its passage :
¨ Sparsely
ionizing radiation - X, gamma, beta radiation. When
passed through water or tissue, it forms about 100 ion pairs / 1
micrometer.
¨ Densely
ionizing radiation - alpha radiation, neutron radiation,
proton radiation. It creates up to 2000 ion pairs / 1 micrometer
of tissue.
This occlusion is related to the so-called
linear energy transfer (introduced in §5.1).
X-rays, gamma and beta radiation have a relatively long range
in the fabric (especially gamma radiation) and therefore the ions
formed are sparsely distributed along the path
of the particle - low linear energy transfer. If the radiation in
the substance has a short range (neutrons, protons,
alpha radiation), the absorbed energy is distributed along a
short path - the linear energy transfer is high and the ions
formed are distributed very densely along the
path . For the purposes of radiobiology and radiation protection
(Chapter 5 "Biological effects of radiation. Radiation
protection"), a so-called quality factor Q is
introduced for each type of radiation, which indicates how many
times a given radiation is more biologically effective than
photon (X or gamma) radiation. For X, gamma and beta radiation,
the quality factor is Q = 1, for slow neutrons Q = 2-3, for fast
neutrons and for protons Q = 10, for alpha radiation Q = 20. This
issue is discussed in more detail in Chapter 5 " Biological
effects of ionizing radiation. Radiation protection ".
Wave and
corpuscular radiation
In the paragraph on corpuscular-wave dualism, we have
shown that waves can behave like a stream of particles and
particles, on the other hand, have wave properties. But this does
not mean that the difference between particles and waves is
completely erased! There is one important criterion according to
which we can reliably know whether radiation has a wave or
corpuscular nature: it is the rest mass m o quantum of
this radiation. The rest mass mo is the mass of the particle measured in the inertial
frame of reference in which the particle is at rest.
In addition to this basic division, specific
types of ionizing radiation are already considered, the names of
which are given by the type of particles or
quanta that make them up, or originated in a historical
context . For photon radiation , it is
X (X-ray) and gamma radiation -
depending on the energy of the photons and the mechanism of
formation. Of the corpuscular radiation, alpha
radiation (current of fast helium nuclei) and beta
radiation ( b - -
electron radiation, b + -
positron radiation).
This types of radiation are commonly known
.
Less common and
"exotic" types of radiation
In addition to the usual types of ionizing photon radiation (X, g) and corpuscular (
a, b ± ) in nature and
in artificial sources can sometimes be found with other less
common or even "exotic" types of radiation :
¨ Neutron
radiation , which occurs mainly during nuclear
reactions, in large quantities, especially in nuclear reactors.
Its properties are discussed in more detail below in the section
" Neutron radiation and its interactions ".
¨ Proton radiation is a
basic component of primary cosmic radiation in nature and is
artificially produced in accelerators. In addition to research in
particle physics, it is used for the production of artificial
radionuclides (together with accelerated deuterons; §1.4, part
" Production
of artificial radionuclides ") and in the
so-called hadron radiotherapy (§3.6,
section " Hadron radiotherapy ") .
¨ Radiation
of heavier ions , formed by fast-flying nuclei
of heavier elements than helium. They are trace in
primary cosmic rays and are artificially produced on accelerators
for nuclear research, radionuclide production (including
transuranium) and for the purposes of hadron radiotherapy
(eg accelerated 12 C carbon nuclei ).
¨ Muon radiation (current
of fast muons m + or m - ) is an important component of secondary cosmic
radiation in nature, falling on the earth's surface. It arises in
a number of high-energy interactions of particles in nature and
on accelerators.
¨ Pion
radiation (current of fast pions p + or p - , or p 0 ) occurs on Earth in the upper layers of the
atmosphere, where it arises from interactions of high-energy
primary cosmic radiation. At large accelerators, pions are formed
during nuclear interactions of protons accelerated to high
energies. In addition to basic research in nuclear physics, it is
experimentally tested as an effective tool in hadron
radiotherapy (§3.6, part " Hadron
radiotherapy ", passage " Radiotherapy
of meson p - ") .
¨ Antiproton radiation,
formed by a stream of fast antiprotons (antiparticles to
protons), arises during nuclear interactions of protons
accelerated to very high energies. In addition to basic research
in particle physics, its use for hadron radiotherapy is
being considered (§3.6, section " Hadron
radiotherapy ", passage " Antiproton
radiotherapy ") .
¨ Neutrinos
radiation is, in terms of particle fluence, one of the
most abundant and most intense radiation in nature
. However, its radiation significance is completely insignificant
(practically zero) and it is usually not even classified
as ionizing radiation . This
is due to the extremely small effective cross-section of the
interaction of neutrinos with the substance. The origin and
properties of neutrinos are discussed in detail in §1.2, section
" Neutrinos
".
Sources
of ionizing radiation
In §1.2-1.5 we have shown a number of phenomena in which
ionizing radiation is generated. Each object, device, material or
formulation which emits ionizing radiation is referred to as a
source of ionizing radiation or short emitter
. Emitters can be classified according to several criteria. The
division into natural and artificial
sources is not important to us here (however,
it is stated in §5.2, passage " Sources of irradiation by ionizing
radiation ") . According to the principle and mechanism of
radiation generation, we can divide the emitters into :
According to the energy of
emitted quantum radiation, we can divide sources of ionizing
radiation into two groups :
¨ Low-energy , producing particles or photons with energies of the
order of up to hundreds of keV . In radiological
jargon, they are also called kilovoltání
. These include X-rays and most radiosotopes.
¨ High-energy , producing quantum radiation of unit energies, tens or
hundreds of MeV , GeV to TeV
. In radiological jargon, they are also called megavoltages
. These are mainly high - energy accelerators ,
occasionally some radioisotopes (such as
60 Co).
According to the technical solution and constructional arrangement , the sources of ionizing radiation, especially radioactive emitters, are further divided into two groups :
Depending on their geometric shape
, radiation sources can be :
¨ Point - their size is much smaller than the distance at which
we examine the emitted radiation or where the irradiated object
is located.
¨ Line - the radioactive substance is filled in a thin tube or
contained in a thin wire (inside or applied
to the surface) . Line sources can be made linear
(line-shaped) or can be bent into a curve of any shape.
¨ Planar - the radioactive substance is applied in a thin layer
on a substrate, usually in the shape of a rectangle or circle .
¨ Volumetric (spatial) -
radioactive substance is distributed in the material of a certain
body, most often in the shape of a block, cylinder, sphere.
The distribution of the radiator in
length, area or volume can be homogeneous or inhomogeneous
. Point, line and area homogeneous sources are often used as
standard and calibration emitters for
radiometric instruments, including scintillation cameras (see eg " Phantoms and phantom measurements in nuclear
medicine ") .
Another more detailed division of sources of ionizing
radiation is according to the type of emitted radiation
(emitter a , b , g, X, neutron source, source of accelerated protons or
heavier ions), according to the application for
which the source is intended (industrial sources - eg
defectoscopic, medical irradiators, etc.), according to its
"strength" and thus the degree of radiation
risk in its use ( small source, insignificant,
significant or very significant source), see Chapter 5 " Biological
effects of radiation. Radiation protection ".
Physical
quantities characterizing ionizing radiation
Radiation energy
The basic physical quantity characterizing ionizing radiation is
the kinetic energy of its quanta (particles,
photons). The properties of radiation during interaction with
substances - ionization, range, or nuclear reactions, formation
of secondary radiation. The basic physical unit of joule
energy [J], generally used in all areas of macroworld science, is
impractically large for measuring the kinetic energy of
microparticles. The electron volt [eV] and its decimal multiples [keV, MeV, GeV, TeV] are
used here - see §1.1 " Atoms and atomic nuclei ", passage " Units
of energy, mass and charge in atomic and nuclear physics " . The conversion
relationship is: 1 eV = 1,6.10-19 J.
Sources of ionizing radiation are usually
not completely monoenergetic (they do not
emit radiation of only one energy) , but
emit quantum radiation of different energies . In such a
case, we characterize the radiation energy by
the energy spectrum , which indicates the
distribution (relative representation) of emitted particles or
quantum radiation according to their energy (it
is described in detail in Chapter 2 " Detection and spectrometry of ionizing radiation ") . It is expressed
graphically by plotting the energy in [keV or MeV] on the
horizontal axis and the relative number of quantum particles
having the corresponding energy on the vertical axis. The energy
spectrum can have two basic forms :
¨ Continuous spectrum ,
when a radionuclide or electronic source emits particles with all
energies in a certain interval (usually
from zero to a certain maximum energy) ,
while depending on the energy, their representation changes more
or less continuously . This is the case for beta
radioactivity (§1.2, section " Beta radioactivity ", fig.1.2.3 in the middle) ,
for bremsstrahlung from an X-ray machine
(§3.2 " X-rays
- X-ray diagnostics ",
fig.3.2.1 in the middle) , for synchrotron
radiation from accelerators (§1.5, part "") or
neutron stars (§4.2 " Final stages of stellar evolution.
Gravitational collapse ",
part " Neutron stars"Monograph" The gravity, black holes and the
physics of space ") . Graph is a smooth continuous curve in
some areas of energy increasing, in others decreasing or zero.
¨ Line spectrum when the source emits
particles of only one or a few specific - discrete
- energy This is the case with gamma radiation from
radionuclides (§1.2, part " Gamma radiation ", Fig.1.2.7) or with characteristic
X-rays from internal electron orbits of atoms (§3.2, part " Sources of X-rays
- X-rays ", the " characteristic
X-rays ") . Also in
conversion and Auger electrons (§1.2,
part " Internal
conversion of gamma and X-rays
") . The line spectrum is formed by
sharply expressed discrete maxima / peaks - peaks
- for precisely defined energies; in areas outside the peaks, the
values are zero (or in practice
significantly lower than in peaks) .
Note: In
the past (roughly until the 1960s), the spectrum of photon
radiation, especially X, was sometimes transmitted using the wavelength
on the horizontal axis. This method is very unsuitable
and misleading, especially in relation to the mechanisms of this
radiation, where energies emerge electron orbits
and nuclear levels in [keV], or accelerating voltage values in
[kV]. Now wavelengths long since abandoned ,
radiation spectra are plotted substantially in energy
units [keV and MeV] (wavelength is
sometimes still used for soft X-ray diffractometry radiation) .
Radiation
power - source emission, angular emission and radiation
characteristics
Another basic characteristic of each radiation source (not only
ionizing) is its "strength", intensity, radiance or
radiation power - the amount of radiation emitted by the source
per unit time. The emission quantity of
the source is introduced, defined as the number of
particles emitted from the source per unit time - the unit is
[number of quantums / second], abbreviated or dimensionally [s -1 ] *). Sometimes the radiant
power of a source is expressed in energy units of watts
[W], as the total energy of all quantum radiation emitted by the
source per unit time (1 second). Emitters for therapeutic use are
also sometimes characterized by dosimetric quantities of
dose or kerm yield [Gy.m 2 .s -1 ], which is the dose or kerm input at a reference
distance of 1 m from the center of the emitter.
*) The number of particles or energy must
be understood as an average value , time
averaged over statistical fluctuations radiation
flow from the source - is especially important for weak sources!
For radioactive sources, the emission is given by the activity
of the relevant radionuclide with appropriate
coefficients (see below " Intensity of radiation from radioactive
sources ") , indicating the number of emitted particles per 1
radioactive decay, as well as corrections for self-absorption in
the source and in the source housing. formation of secondary
particles during various interactions within the source.
In addition to
the total emission or radiation power of the source, the directional
distribution of the emitted radiation is also important
. Some sources, especially radionuclides, emit radiation almost isotropically
in all directions, others non-isotropically -
different radiation intensities are emitted in different
directions, they can also have a very narrow directional
radiation characteristic *). The quantity angular
emission of the source is introduced , which is the
emission related to the unit solid angle; the unit is [(number of
quantum / second) / 1steradian], dimension [s -1 .Sr -1 ]. The angular
distribution of the source emitted by the source - the angular
emission of the source - is expressed by the directional
radiation characteristic , which is a diagram in polar
or spherical coordinates, expressing for each angle the
corresponding flux of emitted quantums from the source to this
direction (angle).
*) They radiate from light sources almost isotropically,
i.e. they have a spherical radiation pattern, eg small light
bulbs (except for the direction of the base), many stars in space
(if they are in equilibrium and do not rotate quickly). Strongly non-isotropic
is the emission of lasers or semiconductor LEDs, in the universe
of neutron stars or accretion disks around rotating black holes (quasars - see §4.8 " Astrophysical
significance of black holes " of
the book "Gravity, black holes and space-time physics",
passage "Accretion disks" - there is also in the lower
right part of Fig.4.28 an example of a strongly non-isotropic
angular radiation characteristic of a thick accretion disk around
a rotating black hole, where the radiation takes place in the
form of jets along the axis of rotation) . Radionuclide
sources emit isotropic sources of ionizing
radiation(unless their geometric configuration or encapsulation
causes increased radiation absorption in some directions). X-rays
and generally bremsstrahlung radiation, which occur when charged
particles (electrons) hit a target, have non-isotropic
radiation. Very narrow radiation characteristics -
"pencil" beams - are present for accelerator particles.
Radiation is often intentionally shaped or collimated
into narrow beams (see below the section
" Radiation beam definition - collimation
") , eg on irradiators in radiotherapy
(§3.6, section " Modulation
of IMRT, IGRT irradiation beams
") .
All quantities
characterizing the intensity of radiation sources depend on a
number of circumstances within the sources and are generally functions
of time. For electronic sources, the time dependence is
related to the switching on and off of the source and to the
electronic regulation of energy, power and
intensity. In the case of radioactive emitters, it is mainly an exponential
decrease in activity with time (§1.2,
section " General laws of
transformation of atomic nuclei
", Fig.1.2.1) , depending on the half-life
of the relevant radionuclide.
Field and beam of radiation, intensity of
radiation
A quantity of radiation
propagating from a source of radiation creates a so-called field
of radiation (radiant field) in the surrounding space .
If the quantum of radiation in a given place in space moves
mainly in one particular direction, we speak of a beam of
radiation . In addition to the type
and energy of individual quanta of ionizing radiation, another
fundamental characteristic of a field or beam of radiation is the
intensity (strength) of this radiation, which
also determines the degree of effects of radiation on matter at a
given location. The intensity of radiation is quantified by a
quantity called fluence ( from
Latin fluentum = current, flux ).
Depending on the physical and application context, the intensity
of radiation (its flux - fluence
) quantifies in principle in two ways :
In the general case, the
radiation field is fully described if at each of its points (r, J , j ) the energy E and
the number of quanta of radiation N propagating in the direction
( J , j ) are known in
polar coordinates - energy and angular distribution of
radiation intensity I (E, J , j ). However, full knowledge
of this distribution is not necessary in practice (it would be
very difficult to measure it). By integrating the distribution
function over all directions (over all values ??of angles J , j from 0 to 2 p , ie over the full
spatial angle 4 p ) we obtain a spherical distribution I (E) = 0 ň
2p 0ň 2p I (E,
r, J , j ) d J d j , expressing the
total flow of particles (or energy flow) passing
per second of spheres with a unit main section in any direction -
fluence .
When using
ionizing radiation, there is usually a typical radiation
situation: we have a certain source of radiation that emits
radiation into its surroundings (creates a radiation field) and
this radiation acts on the substance present, which can be living
tissue. For some applications of radiation, especially in
radiotherapy (§3.6 " Radiotherapy
") and in radiation protection (§5.1 " Effects of
radiation on matter. Basic quantities of dosimetry. ") , the radiation field is also quantified using dosimetric
quantities - distribution of the radiation dose
or dose rate in a given irradiated substance, most often
in water or tissue. In practice, the radiation beam is never
homogeneous, so the spatial distribution of radiation intensity
and radiation dose is usually complex course (the highest dose is in the central part of the beam,
decreases towards the edges . ) The spatial distribution of the
radiation dose is often mapped using so-called isodose
curves - imaginary lines representing the joints
of points with the same dose. maximum dose, eg isodose
80%, 50%, 20% and the like.(reminiscent of
contours on the map) .
Geometric shape of
radiation beams
The spatial distribution of
radiation intensity - the geometric "shape" of the beam
- depends on a number of circumstances of radiation and the
propagation of radiation quanta in a vacuum or in a material
environment. Under normal circumstances, the radiation beam
generally has a diverging shape *). The primary
cause of this divergence is the different directions of
the velocity vectors with which the individual quantums
of radiation fly from their source to the surrounding spatial
angle. In the vicinity of the source, the divergence of the beam
of radiation is large, decreases with distance, and at great
distances, the radiation eventually becomes practically parallel.
In the case of copuscular charged radiation (alpha, beta,
proton), mutual electrical repulsion also
contributes to beam divergence uniformly charged particles.
During the passage of radiation through the
matter , the scattering of radiation quanta on
the atoms of matter (in the case of photon
radiation it is Compton scattering) significantly
contributes to the divergence and "blurring" of the
beam shape .
*) A certain exception is the parallel
light beams emitted by lasers and the secondary
radiation generated in the target by the impact of high-energy
(ultrarelativistic) particles, which is narrowly directed due to relativistic-kinematic
effects .
For many applications, parallel
or converging is required(convergent, focused)
shape of the radiation beam. In the optical field, this can be
achieved relatively easily by passing light through contact
lenses or by reflecting it from hollow mirrors. Unfortunately, no
refractive or reflective optics work for ionizing radiation. The
desired shape of the radiation beam, the (limited) only achieve collimation
(see " Definition of a
radiation beam - collimation
') , but at the cost of a large loss
of intensity of the initial radiation.
Intensity of radiation from radioactive
sources
Frequent sources of ionizing radiation in practice are radioactive
emitters (§1.2 " Radioactivity
") . The radioactive emitter emits its
radiation isotropically in all directions, up to a full
solid angle of 4p . With the distance r from the source, the
radiation "dilutes", it is distributed on an imaginary
sphere with an area S = 4p r 2 . The intensity of radiation I
( fluence of particle-quanta /s / m 2 ) emitted from a
radioactive source such, is therefore directly proportional to
the activity and preparation, and inversely proportional
to the square of the distance r from the source to the
measuring point (this is exactly the case
for a spot emitter, approximately in situations where the
distance is significantly greater than the dimensions of the
source) :
I
= G. A / 4 p r 2 .
The coefficient G indicates the number of quanta emitted
by the radionuclide in one radioactive conversion. In the
simplest case, G = 1; if only part of the decays lead to excited
nuclear levels (and another part to the
non-radiative ground state of the daughter nucleus) , G <1. In the case of cascade deexcitation, G> 1;
similarly for positron b + radionuclide
preparations , G » 2 (simultaneous emission of two
annihilation photons g ) . To determine the energy
fluctuation and the radiation dose , the mean
energy of the emitted quanta would be multiplied
(more detailed is derived in §5.1., Passage " Radiation dose from radioactivity ") .
In practice, to determine the
intensity of radiation from radionuclide sources, it is necessary
to take into account the effects of radiation absorption
in the source itself or its envelope, as well as in the environment
between the source and the measured site - the resulting
intensity will be lower : I = G. A / 4p r 2 . e -m . r , where m is the linear
absorption coefficient of the medium.
Most often, the radiation intensity is
quantified for photons of gamma radiation, or characteristic
X-rays. For radiation a and b, it is problematic to use this relationship, because a
significant part of such radiation is already absorbed in the
source itself and another significant absorption occurs in the
air or other environment lying between the source and the
measured place.
Radiation beam definition - collimation
In the vast majority of processes of ionizing radiation, this
radiation is emitted almost isotropically in all
directions *).
*) The exceptions are the interactions of high-energy
particles , where due to the relativistic laws of conservation of
momentum, the emerging secondary particles and radiation are
kinematically directed (collimated) in the
direction of movement of primary high-energy particles.
However, we often need to
direct the radiation (collimate) to a certain angle, or
to concentrate it to a certain place; Radiation
in other directions can be undesirable - disruptive or even
harmful and dangerous.
¨ Electromagnetic collimation of
charged particles
In the case of corpuscular radiation of charged particles,
suitable direction - collimation - can be achieved by the action
of electric and magnetic fields, which exert a force on the
charged particles. This deflects the direction of the beam, which
can be directed to the desired location.
¨ Mechanical
absorption collimation of radiation
However, a simpler way, which works both for charged
particles and for g and X radiation , is to use collimators. A
collimator is a mechanical and geometric arrangement of
materials absorbing a given type of radiation,
which transmits only radiation from certain desired
directions (angles), while it absorbs and does not transmit
radiation from other directions **).
**) However, such absolutely sharp
collimations cannot always be achieved in practice. For
high-energy radiation penetrating grams occurring in peripheral
edges of the collimator partial radiography , in
which edge portions of the collimated beam produces a " penumbra
."
Collimators are used in virtually all
applications of ionizing radiation. Most of them are simple
collimators in the shape of various tubes or
orifices (as shown, for example, in Fig.2.8.1). Intricately
configured collimators then play a key role, for example, in scintigraphy
(imaging collimators with a large number of holes - §4.2
"Scintillation cameras", section " Collimators
"), in X-ray diagnostics (Bucky-Potter or Lysholm
screen - §3.2 " X-ray diagnostics ") and
in radiotherapy (eg multilamellar MLC
collimators - §3.6 "Radiotherapy", passage " Modulation of radiation beams "). Reflective mirror optics work for soft X-rays
under certain circumstances, but only for very small angles of
incidence-reflection - see the appendix " X-ray telescopes " at the
end of §3.2.
Some detection and imaging methods use so-called electronic
radiation collimation (see eg " PET cameras " or " Compton cameras.
However, this is not a matter of defining the beam of radiation -
all the radiation falls into the detector, from which a certain
part is selected only for the purposes of
detection and display on the basis of coincidence detection and
electronic directional reconstruction of particle paths.
Interaction
of ionizing radiation during passage through matter
Under normal natural and laboratory conditions, matter
(substance) is composed of atoms , which are
possibly. bound in molecules and can form crystalline or
amorphous structures of solid, liquid or gaseous state (we do not consider "exotic" forms of the
substance, such as ionized plasma ) . The interaction of radiation with matter therefore
takes place primarily at the atomic level , or
at higher energies at the nuclear and particle levels.
Collective and
individual interactions with atoms
Macroscopic bodies when moving in a material environment
(eg a stone thrown into water) interact simultaneously, collectively,
with many billions of atoms and molecules. Similarly, electromagnetic
radiation of longer wavelengths - radio waves, light:
collective electromagnetic interaction with a large number of
atoms leads to the known laws of optics -
reflection, refraction, bending. However, high-energy
quanta of ionizing radiation have such a short
(effective) wavelength that they interact individually
with individual atoms of matter, with electrons, or
atomic nuclei and elementary particles. Therefore, the
interaction of ionizing radiation with matter is fundamentally
different from conventional "soft" radiation such as
light.
Before we
start to describe the ways of interaction of specific types of
radiation with substances of different composition, we will
mention some general mechanisms.applicable to
the passage of radiation through matter. Above all, in all types
of radiation we encounter cases of radiation passing without
interaction , where the quantum of radiation can flow
freely between the atoms of matter; this case occurs more often
for hard radiation passing through a substance with a lower
density.
When different types of ionizing
radiation pass through a substance, quantum radiation generally
interacts with envelope electrons and atomic nuclei. In
principle, all three interactions that can be considered here can
be applied - strong, weak and electromagnetic interaction :
All these interactions and
processes lead to the loss of energy of these
particles during the passage of quantum ionizing radiation
through the substance , their braking and
finally to stopping (if the substance
environment is large enough) - the radiation has a limited range
or range in the substance *) . Along the path of
their flight, the quantum of radiation leaves an ionization
trace of free negative electrons and positive ions. Some
of these ions and electrons recombine with each
other again , but some of them can cause new chemical
bonds and reactions in the surrounding substance (unless
the substance is an element composed of the same type of atoms),
especially when it is a more complex organic substance. The use
of ionizing effects of radiation for its detection and
spectrometry is discussed in more detail in Chapter 2 " Detection and spectrometry of ionizing radiation ", the chemical effects of ionizing radiation on
substances and especially on living tissue in Chapter 5 " Biological effects of ionizing radiation ".
*) Range of radiation in matter
Because the individual processes of interaction and collisions of
radiation quanta with atoms of matter have a random character,
the range of radiation particles is not always the same - it is
around a certain mean value called the mean range R s .
Sometimes the value of the maximum range R is givenmax .
The range of radiation in a substance is often also described by
the effective range R 90 , which is the distance at which 90% of the original
emitted energy of the particles is absorbed (or the radius of the
spherical space of the substance around the point source at which
90% of the energy emitted by the source is absorbed) .
What happens
to a quantum or particle of radiation after they are braked and
absorbed in matter - what is their ultimate "fate"? It
depends on the type of radiation :
- In
the case of photon radiation (X,g), the photons
transfer all their energy to the particles of matter, mostly
electrons, and they themselves disappear during the photo effect.
-
Electron b - is
gradually inhibited by collisions with the electrons of the
atomic shells of the substance, then it almost stops (performs
only thermal oscillations) - the substance is
"enriched" by one electron, which remains either free
or binds in an atom.
- The positron b + is also inhibited by collisions with envelope
electrons, but after braking it does not remain in the substance
- it annihilates with the electron to form two opposing photons g (and those either
fly out of the substance or are absorbed).
- The proton
p +, after
its braking, "gains" an electron and the substance is
enriched by one hydrogen atom.
- Neutron
n 0 has
two possibilities: Either it is absorbed by some nucleus and
causes a nuclear reaction - in the substance the relevant atom is
then transmutated to another isotope, or the core may split into
other elements. If a slow neutron remains in matter for longer,
it spontaneously transforms into a proton, an electron, and an
(anti) neutrino.
- Particles a after braking
"puts on" two electrons and is enriched in one helium
atom.
More complex situations occur at high
energies of quantum and radiation particles, where nuclear
reactions and the formation of new particles
can occur . All the various processes are discussed below.
Thermal
and electrical effects of radiation
Another phenomenon little known in common applications
accompanies all interactions of radiation with matter: it is heat
. During the absorption of radiation, part of the energy is
transferred to the substance at the level of the kinetic
energy of the atoms . And the kinetic energy of the
motion of the atoms of matter is nothing but heat. With each
subsequent interaction, the atoms of the substance will oscillate
to greater and greater kinetic energy - the irradiated
substance will heat up . At low radiation fluxes, this
phenomenon is unobservably weak, but during intense irradiation
the substance "warms up" quite clearly - for example,
targets in accelerators must often be cooled. If the substance is
irradiated with quantum carrying electric charges (radiation a, b -, +, proton), or secondary charged particles fly out of the
substance, the substance will be electrically charged
. This phenomenon would be observable only in a vacuum; in the
air, ionizing radiation releases free charge carriers (electrons,
ions), which dissipate and neutralize the charge of the
irradiated body.
Effective
cross-section of the interaction of radiation with matter
In §1.6 "Elementary particles" the concept of the
so-called effective cross-section of the
interaction was introduced , which expresses the probability of
particle interactions in a clear geometric way. Even when
studying the interactions of radiation with a substance, it is
possible to apply the idea that each atom of the irradiated
substance behaves as an "absorbing body" of radius r
with respect to the incident particle , which either hits the
particle and does not hit it. around) and the interaction does
not occur. The larger the radius of this body, resp. its
effective area s = p .r 2 - effective cross section , the
greater the probability of interaction (probability that the
particle "hits").
Expression of the probability of interaction of radiation quanta
(firing particles) with a target particle (atom) using an
effective cross section
The cross section may, but need
not be directly related to the "geometric mean" atoms r
geom or
their "geometrical cross section" s geom = p .r 2 geom . For "effectively interacting" particles it
is s > sgeom , for weakly
interacting particles is s
< s geom
. In addition, the same firing particle can cause different
interactions on the same atom , the different probabilities of
which are described by different effective cross sections. These
effective cross sections no longer have anything to do with the
geometric dimensions of atoms - they are the result of the
internal mechanisms of specific types of interactions.
The unit of effective cross section in the SI
system would be m 2 , which is, however, inadequately large, and therefore
the unit barn (bn) is used in nuclear physics :
1 bn = 10 -28 m 2 , which has the order of magnitude of the geometric
cross section of atomic nuclei.
The effective cross section of the interaction
is very closely related to the absorption coefficient, the
so-called linear attenuation coefficient m , in the
exponential law of absorption of ionizing radiation in
substances. This connection will be clarified below in the
section " Absorption of radiation in substances ".
Multiple
interactions - cascades of interactions and sprays of particles
When the interaction of high-energy radiation in a sufficiently
voluminous medium environment, the effect of multiple
interactions occurs . The secondary particles released
during the first interaction of the incident primary particle
cause further interactions , producing
additional (tertiary) particles that do the same. From one
incident particle, a whole spray of secondary particles
is formed in a cascade of interactions. As the evolving spray
penetrates to the depth of the material, the number of secondary
particles increases and their average energy decreases. Once this
energy falls below a certain threshold, the multiplication
process will stop and the energy of the particles will be
dissipated by ionization and excitation; the number of particles
in the spray will decrease until the spray finally disappears. In
practice, we distinguish two types of cascade interactions :
¨ Electromagnetic
sprays
arising from the interaction of high-energy photons or electrons
with atoms of matter. Secondary electrons and photons emitted
during the primary interaction, due to paired e - e + production, Compton
scattering, photoeffect and bremsstrahlung, produce additional
electrons (+ positrons) and photons; etc.
¨ Hadron sprays
arising from inelastic interactions of high-energy hadrons with
atomic nuclei of the material. Nuclear fragments are formed and
new secondary particles are produced - p, n, p, K. The number of
these secondary particles is approximately proportional to the
logarithm of the energy n ~ lnE.
In many cases in practice, this spray is not
purely hadron or electromagnetic, but mixed .
The hadron spray includes pions that immediately
disintegrate: p +, - ®m +, - + n m , p o ®g + g ; this
leads to the formation of an electromagnetic electron-photon-muon
spraythat accompanies the hadron cascade. Thus, each
hadron shower also has an electromagnetic component. And with the
interaction of high-energy photon or electron radiation, photonuclear
reactions emit protons and neutrons, which can enrich the
electromagnetic spray with a hadron component .
Cascades of interactions and sprays of secondary particles are
observed in cosmic rays (Fig. 1.6.7) and in particle interactions on
accelerators (in bubble chambers, trackers and calorimeters).
Secondary radiation generated by
radiation-substance interactions
Any object that is irradiated with (primary) radiation generally
becomes a source of weaker secondary radiation. Even with the
interaction of ionizing radiation with matter occurs marches in
which is generated by secondary radiation of
various kinds:
¨ Braking
radiation (bremsstrahlung) generated primarily during
movement of electrons and positrons in the fabric
¨ Compton dispersed g -radiation
or X-rays
¨ Diffracted neutrons
¨ Photoelectrons released
from the atomic packaging due to the photoeffect of primary
radiation
¨ Characteristic X-rays
following the photoeffect of primary radiation
¨ Auger electrons
generated by internal conversion of characteristic X-rays
¨ Electron
and positron radiation arising from primary high-energy
radiation in the formation of electron-positron pairs
¨ Annihilation g -radiation
of 511keV energy, arising by annihilation of positrons formed by
electron-positron pairs
¨ Protons and neutrons
generated by nuclear by interactions of primary radiation
¨ Mesons p and K, muons,
resp. hyperons, caused by particle interactions of high-energy
quanta of primary radiation
¨ Light radiation
generated by deexcitation of electrons on the outer shells of the
atomic shell, when Cherenkov's radiation of
secondary electrons, or during deexcitation in the luminescent
centers of certain substances (scintillators).
Photoelectrons, Auger electrons and
electron-positron pairs are mostly absorbed in the substance,
only a very small part of them can fly out of the layers at the
surface of the irradiated substance. However, Compton-scattered g -radiation,
characteristic X-rays, bremsstrahlung, and annihilation g- rays can easily fly
out of the irradiated substance and thus enrich the original
radiation field. Similarly, neutrons produced by scattering or
nuclear reactions. The mechanisms of secondary radiation will be
discussed in more detail below.
Albedo
The name albedo is sometimes used to measure the
secondary radiation emitted by an irradiated body (lat. albus = white; albedo = whiteness ) . In science, the term albedo generally means
light reflectancediffusely reflecting matte areas and is
quantified as the ratio of the intensity of the reflected light
to the incident light; it is usually expressed in %. It depends
on the frequency, or wavelength or energy, ie on the spectrum of
the considered radiation and also on the angle of incidence of
the radiation. Albedo is often used in astronomy, where the light
reflectance of planets or asteroids irradiated by sunlight
suggests the composition of their surface, such as the proportion
of ice. The average albedo of the planet Earth is about 30%, for
the Moon it reaches only 12%. Here on Earth, fresh snow has an
albedo of about 90%, a grass area of 15-25%, a coniferous forest
of about 10%, the water surface of the sea only about 4% (light
easily penetrates into the water and is absorbed in depth). Of
the chemicals, high albedo has 96-98% magnesium oxide and barium
sulphate, and very low (less than 1%) amorphous carbon.
Albedo can be determined not only for light,
but for any electromagnetic radiation, and also for other types
of radiation (ionizing, corpuscular). However, ionizing radiation
is not reflected radiation, but secondary radiation caused by
scattering and other interactions of quantum primary radiation
with atoms of matter - with electrons in the envelope or with
atomic nuclei. For X and g radiation , the albedo of common substances (such as
water or living tissue) is very low, below 1%. It is caused
mainly by Compton scattering, partly also by X-ray fluorescence.
Higher albedo, up to 40%, may be in neutrons.
Interaction
of charged particles - directly ionizing radiation
First we will deal directly ionizing radiation, wherein the first
mention common features of the interaction of the radiation
passes through matter, then we analyze the specific features of
the interaction of radiation a
, b+, - and proton radiation.
Excitation
and ionization
The charged particle, as it passes through the substance, loses
its kinetic energy mainly by electrical Coulomb interaction with
electrons in the atoms of the substance. If the energy
transferred to an electron in the atomic shell is relatively
small and is only enough to "raise" the electron to a
higher energy level, it is a process of excitation of
atoms. The excited state of the atom is not constant - then the
electron jumps back to the original level - dexcitation
occurs , and the energy difference is radiated in the form of a photon
of electromagnetic radiation. During the excitation of
electrons on the outer shells, visible light is emitted, on
medium UV radiation, during excitation on the inner shells, then
photons of characteristic X-rays (with
spectral lines K a, b) .
If an electron receives enough
energy to be completely released from its bond to the parent
atom, it moves away from it permanently - the atom
is ionized , divided into a negative electron
and a positive ion. By primary ionization is
meant the number of ion pairs formed by the ejection of electrons
by the primary particle. Some electrons punched out during
ionization have so much energy that they can ionize further along
their path - secondary ionization (such electrons were formerly called delta
rays because their trace in a nuclear emulsion or nebula chamber
has a characteristic branched shape) .
Linear energy transfer LET
During ionization and excitation, a fast charged particle loses
its kinetic energy by imparting momentum to electrons by the
action of electric Coulomb forces. The magnitude of the momentum
transmitted to the electrons is proportional to the magnitude of
the Coulomb forces and the time for which these forces act
(interaction time). Coulomb forces are proportional to the charge
of the particle q and the electron density of the
substance. The interaction time is inversely proportional to the
velocity of the particle v , so that the energy
transferred to the electrons is proportional to 1 / v 2 . The amount of
energy loss per unit path of a particle - linear energy
transfer LET *) - is therefore directly proportional to
the electron density of the substance (this is given by the
density r and the proton number Z) and inversely
proportional to the square of the particle velocity: -dE / dx ~
q. r .Z
/ v 2 (the exact value is given
by the so-called Bethe-Bloch formula below , which also
includes the mean excitation potential of the atoms of the
substance, approximately proportional to the proton number Z) .
*) Linear Energy Transfer (LET) expresses the amount
of energy transferred to an ionizing particle per unit length of
its path in a given environment. In practice, it is usually
expressed in [keV / m m] or [MeV / cm]. For alpha particles with energies of
4-8MeV, LET in water is about 100keV / micrometer, at the end of
the path in the Bragg maximum it can increase locally up to
300keV / mm. For beta-particles with typical energies of hundreds
of keV, LET is only about 0.2 keV / micrometer.
Energy transfer
charged particles interacting with electrons
Dissipation (or "braking power") -de / dx charged
particles when passing through the fabric is called. Bethe-Blochovým
formula (whose simpler variant
derived N.Bohr (1913) - in the lower part of the formula in
frame, complete specified variant then H.Bethe and F.Bloch
(1930-33) - the upper part of the formula) :
This is a primary particle with charge Q and mass M
(M >> m e ), flying at instantaneous velocity in
[relativistic designation b = v / c, g = 1 / Ö (1-v 2 / c 2 )] a material medium
of density r , whose atoms have a proton number Z and a mass
number A , I is the mean excitation (ionization)
energy of the atoms of the substance [eV]. N A = 6,022.10 23 is the Avogadro's
constant, m is the rest mass of the electron, e is the elementary
charge of the electron. kin E max is the maximum value of energy that can be
kinematically transferred from the flying primary particle (mass
M) to the free electron (mass m e ) in one collision. Parameter d (b)is the density
correction caused by polarization (applied at high energies), C /
Z is the correction for slow particles with velocities comparable
to the velocity of bound electrons. The mean excitation
(ionization) energy I is approximately proportional to the
proton number: I = 10 [eV] . OF; for lighter atoms (Z <20) the
empirical dependence I = 10 [eV] was measured . Z 0.9 .
The energy of the secondary
electrons ejected from the atoms of matter during the
passage of the heavy charged particle was derived in an analogous
manner to the energy return of the primary charged particle
according to the Bethe-Bloch formula. The number of these
secondary electrons is the intermittent Q 2 / b 2and their energy
distribution (spectrum) is:
where d 2
N e is the
number of kinetic energy electrons E e (E e >> I) in the energy interval dE e released on the path
d x . The curves to the right of the formula show the
approximate energy spectrum of secondary
electrons released during the passage of protons with an energy
of the order of hundreds of MeV and MeV units through water (roughly corresponds to the situation around the site of
the Bragg maximum) . The average energy of
secondary electrons increases with the energy of primary charged
particles, but for proton radiation of units up to hundreds of
MeV it is generally very low (during their rapid flight through the atomic envelope,
the protons only need to transfer Coulombovsky a small amount of
energy to the electrons) - in the order of
tens of eV. However, the number of secondary electrons increases
significantly at low proton energies, leading to the existence of
a Bragg maximum , as discussed below.
Depth dependence of
ionization - Bragg curves
Specific or linear ionization is the number of
ion pairs formed per unit length of the path of the interacting
particle. In Fig. 1.6.1 at the bottom right are the so-called Bragg
curves of the dependence of specific ionization on the
depth of penetration of a charged particle into matter. As the
particle slows down and decreases in velocity,
the ionizing effects increase - slower motion
leads to a longer time of action of the Coulomb
interaction, which is enough to transfer more energy and pull out
more electrons; the transmitted energy is inversely proportional
to the square of the particle velocity. Just before the heavy
charged particle is braked, the greatest energy is transferred -
the curve of the depth dependence of the specific ionization has
a significant so-called Bragg maximum . After
braking, further ionization does not continue; if the particle
has been positively charged, it is neutralized by trapping
electrons to form neutral atoms. The
possibilities of using this depth dependence of ionization in the
so-called hadron radiotherapy are discussed in
§3.6 "Radiotherapy", part " Hadron radiotherapy ".
![]() |
Fig.1.6.1. Interaction
of fast charged particles with matter. Top left: Schematic representation of ionization mechanisms in the passage of beta - and alpha particles . Top middle: Three basic mechanisms of proton radiation interaction with matter. Bottom: Interaction of positron beta + radiation with a substance ending in annihilation of a positron with an electron. Right: Bragg curves of depth dependence of absorption and specific ionization along the path of gamma photons, accelerated electrons and protons. |
The ionizing and excitatory effects of high-energy radiation described above are the most important effects we encounter when ionizing radiation passes through matter. We will also mention here some accompanying phenomena (which, however, can play an important role in certain situations) , in which secondary radiation is usually emitted :
Scattering
When particles interact with atoms and atomic nuclei, they are
subjected to electric and nuclear forces, which can change
the direction of movement of particles - causing them to
scatter . Scattering is mainly applied to light
particles (electrons) and lower kinetic energies. During the
passage of charged particles through the material environment,
the interaction with the electric Coulomb field of atoms and
their nuclei manifests itself. In terms of energy balance, the
variance is divided into two categories :
When charged particles pass through a substance medium that contains a large number of atoms, the particle after one scattering is usually subject to further collisions and scattering on other atoms - there is multiple scattering (as shown in Fig. 1.6.1 at the top left) ; individual scatterings can be elastic or inelastic.
Interaction of positron ( beta+
) radiation
It interacts with the substance b+ i.e. positron e+ radiation in a very specific way - lower part
fig.1.6.1. As long as the positron has a high velocity, it pulls
electrons out of the shell with its electric forces as it passes
around the atoms, and thus ionizes , similarly
to the beta- electron . However, after sufficient braking (in water
or tissue after about 1-4 mm), the posron e + meets the electron e - , and since they are
"antagonistic" antiparticles, they are mutually
eliminated ("eaten"): they are annihilated by
e + + e -
® 2g - is converted to two
quantums of hard radiation g with
energies 511keV *).
*) It is interesting that according to the laws of quantum
electrodynamics, there should be an opposite process
to annihilation of a positron with an electron e + + e - ® 2 g , the so-called Breit-Wheeler
production of e + e - pairs. In this process, on the other
hand, electron-positron pairs could theoretically be formed by
the collision of two photons g 1 + g 2 ® e + + e -
. However, this two-photon process has a
very low probability (slight effective cross section), to
demonstrate it would require an extremely intense collimated beam
of gamma photons with energy higher than 511keV - so far failed
... Some possibilities of realization of multiphoton pair
production are briefly discussed in §1.5, passage " Electrons and positrons ".
Both photons
of annihilation radiation fly out of the annihilation site
exactly in opposite directions - at an angle of
180 o (in the center of gravity
system) . This fact is used in the
scintigraphic method of positron emission tomography PET (as described in detail in §4.3, section " Positron emission tomography PET ") .
Thus, if we have a sample of the radioactive substance b + , positrons with electrons annihilate already inside
this sample, so that we do not register practically any positrons
in its vicinity, but such a sample will be a source of intense
hard radiation g with an energy of 511keV. And just as when we apply a
radiolabel labeled with b +
radionuclide to the body - each positron at a distance of about 1-3mm from the
place of its origin annihilates with an electron in the tissue
and we can detect in coincidence two quanta of g radiation with
energy 511keV flying in opposite directions - established PET
scintigraphy (see also " PET
cameras " in chapter 4
"Radioisotope scintigraphy") .
In terms of the distribution of different types of radiation,
positron radiation interactions are discussed once again in the
section " Beta + radiation interactions ".
Positrons
e + are
actually a kind of " visitors from the anti-world
" - particles of antimatter . The
properties of antiparticles, antimatter, antiworlds or
"antiuniverses" are discussed in §1.5 " Elementary
particles and accelerators ", passage " Antiparticles
- antiatoms - antimatter - antiworlds " (including the possibility
of "production" and use of antimatter, with a little
light sci-fi story about the meeting of a " hmman
" with a "anti-human " ...
Braking radiation (bremsstrahlung)
During the passage of fast charged particles through matter, due
to the Coulomb interaction with electron shells and nuclei of
atoms, the velocities and direction of motion of the particles
change - their scattering. The scattering of a charged particle
on atoms at a large angle causes a large and rapid change
of the velocity vector with time , ie a large
"acceleration" of the particle, which according to
Maxwell's electrodynamics leads to the emission of
electromagnetic radiation - photons called braking
radiation X or g with continuous spectrum. This type of scattering occurs
on the one hand in the field of electrons, but especially during
the passage of a charged particle near the nucleus with charge Z
(Fig. 1.6.2 in the middle), during which they will be on a
particle with mass mand the charge q exerts
electric Coulomb forces proportional to the product qZ, so that
they will impart to the particle an acceleration proportional to
q.Z / m. According to the laws of electrodynamics, each
accelerated charge emits electromagnetic radiation, the intensity
of which is proportional to the square of acceleration *), ie Z 2 .q 2 / m 2 . It follows that
energy losses by braking radiation will be significantly greater
in heavy substances with a large proton number Z and that braking
radiation will be applied mainly to light charged particles, ie
electrons (protons lose a million times
less energy to braking radiation than electrons) . The effective cross section for excitation of
bremsstrahlung is larger in the field of the atomic nucleus than
in the field of envelope electrons.
*) The physical-mathematical
derivation is given in §1.5 " Electromagnetic
field. Maxwell's equations.
", Larmor's formula (1.61 '), monograph " Gravity,
black holes and space-time physics
". The effective cross section for the
production of bremsstrahlung in matter is generally given by the
highly complicated Bethe-Heitler formula (derived from quantum radiation theory, corrected by the
Sauter and Elwert factors of the Coulomb
shielding of the electron shell) . For a
not very wide range of energies of incident electrometers E e (tens to hundreds of
keV) and proton numbers of target material (medium to
heavy materials), the total approximate the efficiency of
braking radiation production h by a simplified formula:
h = E e [kev] .Z . 10 -6 [photons /
electron].
Only a relatively small part (only about 1%) of
the original kinetic energy of the incident particle changes to
braking radiation when braked in the fabric. Most of the energy
is eventually transferred to the kinetic energy of the atoms of
matter by multiple Coulomb scattering - it is converted into heat
.
The
graphical shape of the energy spectrum I (E) of
continuous braking X-rays is described in the first approximation
by the so-called Kramers formula :
I
(E) = K. I. Z . (E max - E),
where I (E) is the relative intensity of energy photons E
, K is a constant, Z is the proton (atomic) number
of the target material, E max is the maximum energy of X-ray photons, given by the
kinetic energy of the incident electrons. For E = E max , I (E max ) = 0 and the
formula applies only to E <E max .
It is logical that the efficiency of brake radiation
production is higher for high Z - large electric Coulomb forces
act around such nuclei, causing abrupt changes in the velocity
vector of the incident electrons that get close to the nucleus.
The efficiency of bremsstrahlung [number of photons / electron]
increases with energy E eincident electrons. However, the overall energy
efficiency - the ratio of the total energy of the emitted photons
to the energy of the incident electrons - is lower for higher
energies (due to the higher percentage of low-energy photons).
And the heat production in the target fabric is higher.
Braking radiation
has a continuous spectrumfrom energies close to
zero to the maximum energy given almost by the value of the
kinetic energy of the incident particles. The energy of the brake
radiation photons depends on the speed (acceleration) at which
the electrons are braked when interacting with the substance. The
individual electrons penetrate differently close to the nuclei of
the material, thus emitting different wavelengths or energies of
photons. Those electrons, which "softly" slow down with
repeated multiple scattering on the outer electron shells of
atoms, emit low-energy X-rays. The deeper the electrons penetrate
into the interior of the atoms of matter, the closer to the
nucleus, the faster the intense Coulomb forces change their
velocity vector and the harder the bremsstrahlung radiation is
produced. The shortest wavelengths arise for electrons that have
penetrated close to the nucleus to the level of the shell K or
closer and were braked once. Depending on the impact factor of
the individual electrons relative to the atoms of the substance,
all possibilities are continuously realized. Such a different
degree of braking of electrons causes a mixture of radiation of
different wavelengths or energies of photons - the result iscontinuous
spectrum of braking radiation (see
eg Fig.3.2.5 at the top right in §3.2 " X-rays - X-ray diagnostics ") .
The angular distribution of
the emitted photons of bremsstrahlung depends on the
energy of the primary charged particles. The mean angle J of the emission of
the quantities of bremsstrahlung excited by the electrons with
kinetic energy E e is approximately given by the relation J = m 0e .c 2 / E e (= 0,511 / E e for the energy in MeV) . At low
energies, the bremsstrahlung is emitted practically isotropically
in all directions from the point of interaction. With increasing
energy E e
of the electron exciting the braking radiation, the mean angle J of the emitted
quanta g is ever smaller - at high energies of the incident
charged particles, the braking radiation is preferably
transmitted in a narrow cone "forward"
in the direction of incidence of the primary particles. The
directional radiation pattern of high-energy bremsstrahlung has
the shape of a sharp "lobe" in the direction of the
primary beam (see, for example, the figure
" Radiotherapy-HomogFiltr.gif
" at the top left in §3.6 " Radiotherapy ") .
Braking radiation finds significant
use in the excitation of X-rays by the impact of
electrically accelerated electrons on the anode in X
- rays - see §3.2 "X-rays - X-ray
diagnostics "), or when exciting hard g- radiation
by the impact of high-energy electrons from a betatron
or a linear accelerator (see §1.5
"Elementary particles", part " Charged
particle accelerators ") on a suitable target; it is often used in radiotherapy
(§3.6 " Radiotherapy ") .
Terminological
note: For braking radiation - English braking
radiation or deceleration radiation, for historical
reasons the German name " bremsstrahlung "
often occurs .
The very designation " braking
" is somewhat misleading, deceleration (deceleration, braking) of charged
particles. The same radiation but also arises when accelerating
(acceleration) of
charged particles. However, in nature and in the laboratory, the
particles do not accelerate sufficiently fast, so that the
"acceleration radiation" is negligible and is not
observed. On the other hand, the braking of fast charged
particles, especially light electrons, in substances is quite
rapid, so that the braking radiation can be very significant.
Braking radiation arises not only during the actual
"braking" of a charged particle (perhaps in a strong
electric field of a uniformly charged particle), but also during
a curved motion in an electric or magnetic field (see the
following paragraph - synchrotron radiation), which does
not primarily involve braking. However, braking nevertheless
occurs here secondarily, because the electromagnetic radiation
carries away the kinetic energy and thus effectively inhibits
the movement of the charged particle.
Cyclotron and
synchrotron radiation
A special type of braking radiation is the so-called cyclotron
and synchrotron radiation *). It arises during
the movement of charged particles in a magnetic field
, where these particles are acted upon by a Lorentz force
curving their paths - forcing them to move in a circle or spiral
(helix) with an axis parallel to the magnetic induction vector.
Due to the uneven movement of electrically charged particles
during the circular circulation under the influence of the
magnetic field, bremsstrahlung radiation is generated. According
to the well-known Larmor formula of electrodynamics, the
intensity of this radiation is proportional to the electric
charge and the square of the acceleration of the particle motion,
here it is a centripetal acceleration circular motion.
Thus, for a given kinetic energy of a particle, the intensity of
synchrotron radiation is inversely proportional to the square of
the mass of the particle. This phenomenon therefore applies
almost exclusively to the motion of light particles, electrons
, with high kinetic energies (and thus high velocities), which
move in the magnetic field with high radial accelerations. Due to
their high mass, protons emit cyclotron or synchrotron radiation
a million times smaller.
*) The names derive from the
fact that these radiations occur in the respective circular
accelerators (§1.5, section " Circular accelerators "). Synchrotron radiation was first observed in
1947 on a large accelerator - 70MeV GE synchrotron.
Cyclotron
radiation emitted by slower ones
(non-relativistic) electrons is monochromatic with a
frequency corresponding to the Larmor cyclotron frequency
f = e.B / (2p m e ), where e is the charge am e the rest mass of the
electron, B is the magnetic induction; falls into the
field of radio or microwave radiation.
Synchrotron
radiation is emitted by high-energy (ultra)relativistic
ones electrons. The radiation is emitted in a narrow cone in
the direction of electron movement. Multiples of the Larmor
frequency also appear in its spectrum. For relativistic
electrons, there is a blurring of the cyclotron frequency due to
different relativistic time dilation at different points in the
circular orbit relative to the observed location. The emitted
synchrotron radiation therefore has a continuous spectrum
. Synchrotron radiation takes place in the visible region of
the spectrum , in strong magnetic fields also in the field
of X-rays . In circular high-energy accelerators,
synchrotron radiation can be disruptive - causing
unwanted energy losses of the accelerated particles. On the other
hand, it can be used to intentionally create intense sources
of radiationwith advantageous properties for some laboratory
applications (see §1.5, section " Synchrotron radiation generators ") .
Cyclotron and especially synchrotron
radiation is also used in a number of processes in space
- in hot corona stars, in nebulae, around neutron stars, in
massive quasar jets - where fast electrons move in magnetic
fields (see eg §4.2 " Final
stellar phases Gravitational collapse of the " book " Gravity, Black Holes and the Physics
of Spacetime ") .
Photoeffect
and characteristic X-rays
In addition to braking X-rays with a continuous spectrum, a
certain smaller amount of characteristic X-rays
with a line spectrum is emitted (characteristic pair of peaks K a , K b , or weaker and
lower peaks of the L series), whose energy does not depend on the
energy of incident particles, but it is given by the material
- the type of atoms of which the irradiated substance is
composed. This characteristic radiation manifests itself as
"peaks" on a continuous curve of the bremsstrahlung
spectrum. The characteristic X-ray is caused by two processes :
¨ The direct process of
the impact photo effect at the internal energy
levels of the envelope in the atoms of the irradiated substance
(left part of Fig.1.6.2) - fast charged particles penetrate into
the interior of the atoms and eject bound electrons from the K
and L shells. ) or characteristic X-radiation is then emitted
from the shell M to L (L-series) (cf. also with Fig.1.1.3 in
§1.1).
¨ Indirect
process of photoelectric absorption of bremsstrahlung
- braking X-rays, generated by the above-mentioned mechanism
during the passage of a charged particle, interacts with other
atoms inside the substance, including a photon photoeffect (described below " Interaction
of gamma and X-rays ", Fig.1.6.3
left), emitting electrons from the inner
shells, followed by an electron jump and the emission of
characteristic X-rays.
The impact photoeffect of charged
particles and the emission of photons also occurs when electrons
jump in the outer shells, but the energy of these photons is low
and this radiation is covered by continuous bremsstrahlung at the
beginning of the spectrum.
Fig.1.6.2. Mechanisms of characteristic X-rays, brakind
radiation, Cherenkov radiation and transient radiation
Cherenkov
radiation
When an electrically charged particle passes through the medium,
the electric field of the particle causes local
polarization of atoms and molecules of the medium along
the path - small electric dipoles are formed . After the
passage of the particle, the atoms of the environment quickly depolarize
again , while the obtained energy is radiated in the form of
electromagnetic waves - light. This electromagnetic wave emitted
along the path of the particle is subject to interference,
the effect of which depends on the velocity of the particle.
During the slow movement of the charged particle, the
polarization energy is elastically transferred back to the
particle. As the particle moves rapidly, a limited rate of
depolarization occurs, the particle "escapes" from the
location, and "delayed" depolarization occurs by the
emission of an electromagnetic wave. If the velocity of the
charged particle in the environment is greater than the
phase velocity of light , light waves emitted during
depolarization at various points in the path may enter the phase
and "constructive" interference and observable
radiation may occur at a suitable angle J. In other words, a coherent
emission of dipoles formed by polarization occurs as a charged
particle passes.
Geometric
analysis particle motion, propagation of emitted light
and interference properties is shown in Fig.1.6.2, the second
picture from the right. Due to the depolarization of the medium,
each part of the particle path becomes a source of a weak
electromagnetic signal, which propagates in the material
environment at a speed of c / n. During the elementary time t
, this signal propagates into a spherical wavefront of radius (c
/ n) .t, during which time the particles travel the distance v.t.
they reach smaller radii. The common envelope of these wavefronts
forms the mantle of the cone, in the section in Fig.1.6.2 on the
right the hinge of a right triangle. Individual partial signals
arrive at the same phase and positive interference may occur.
Such an analysis can be done for each point of the particle path
and time interval t . It follows that the
"constructive" (positive, amplifying) interference will
occur at the angle J given by the mentioned right triangle, whose cosine cos J = (v / c) .n.
The resulting radiation thus conically
diverges from the path of the particle flying at
velocity v at an angle J
given by the relation cos J = 1 / b .n, where b = v / c, n = c / c 'is the refractive index of
the optical medium ( c is the speed of light in a
vacuum, c' the speed of light in a given optical medium).
The refractive index of optical media depends somewhat on the
wavelength of light, n = n ( l
) - light dispersion .
Thus, if a charged particle passes
through a substance medium with a velocity exceeding the
speed of light c ' in this medium (this
is given by the electrical permittivity e and magnetic permeability m of the substance: c' = Ö[e.m], otherwise also
the refractive index n of the substance: c '= c / n) , occurs electromagnetic shock waves (similar to the formation of acoustic shock waves when a
body passes through air at supersonic speed) , at which visible light called Cherenkov
radiation is emitted .
This radiation
was first observed in 1934 by the Soviet physicist P.A.Čerenkov
in water exposed to ionizing radiation. Together with
S.I.Vavilov, they performed a number of experiments to elucidate
the properties of this radiation, reaching a partial explanation
that the observed radiation is caused by fast electrons
. The definitive explanation of the mechanism of this phenomenon
on the basis of the laws of electrodynamics in the
material environment was given in 1937 by their other
colleagues I.M.Frank and I.J.Tamm.
The condition for the formation of Cherenkov radiation
is therefore the movement of a charged particle at a speed at
least equal to the threshold speed vmin = c ' = c / n,
exceeding the speed of light c' in a given environment. From the
relativistic relation for kinetic energy (E
kin = m o c 2 / Ö (1 - v 2 / c 2 ) - m o c 2 - see formula (1.79)
in §1.6 "Four- dimensional spacetime and
special theory of relativity
" of the book Gravity , black holes and space - time
physics ") then
it follows that the (kinetic) threshold energy
corresponding to this velocity in min charged particles for the formation of Cherenkov
radiation when passing through a medium with a refractive index n
is: E min
= m o c 2 [1 / Ö ( 1- 1/n 2 ) - 1]. For this case
of the threshold velocity, cos J min = 1, ie J min = 0 - the radiation
goes in the direction of particle motion. At lower speeds or
energy, no radiation occurs .
For ultrarelativistic particles moving at the maximum possible
velocity v max = c, the maximum radiation angle cos J max = 1 / n. In the water with a refractive index n = 1.33,
the threshold velocity for the formation of Cherenkov radiation vmin = 0.75c, which for
the electron corresponds to the threshold kinetic energy E min = 0.26MeV; the
ultrarelativistic electron flying through water (cos J max = 0.75) will then emit Cherenkovsky at an angle J max = 41.5 °. The threshold energies of some particles for
the formation of Cherenkov radiation in plexiglass, water and air
(at normal atmospheric pressure) are given in the following table:
Substance | electron e - | mion m -, + | pion p -, + | proton p + |
plexiglass (n = 1.5) | 0.173 MeV | 36 MeV | 49 MeV | 320 MeV |
water (n = 1.33) | 0.26 MeV | 50 MeV | 68 MeV | 460 MeV |
air (n = 1,0003) | 20.35 MeV |
The energy dW emitted along the path dl
by a particle with charge q, flying at velocity v ( b = v / c),
radiation with angular frequency w = 2 p f and in frequency interval
d w ,
is given by Frank-Tamm equation
d
2 W = (q 2 e / 4 p ). w . [1 - 1/b 2 n 2 ( l )] dl d w .
The total amount of energy *) dW emitted by the particle
per unit dl of the path is then given by the relation
after integration
dW
/ dL = (Q 2
/4 p ) ň
[1 - 1/b 2 n 2 ( l )] e.w dw ,
which integrates over a circular radiation frequency w = 2p f = 2p / l (the boundary
condition v> c / e ). It follows from this expression that the number of
Cherenkov photons dN with energy hf = h .
w emitted
per unit of path dl is
dN
2 / dl =
(dW / dl). ( l / hc) = (4p2 q 2 / hc). ň
[1 - 1 / b 2 n 2 ( l )] / l 2 d l.
The spectrum of Cherenkov radiation, ie the
number of photons dN emitted along the path of a charged
particle per unit path dl and per unit energy interval, or
equivalent wavelength d l , is thus given by
the formula:
dN
2 / dl d l = (4 p 2 q 2 / hc). [1 - 1 / b 2 n 2 ( l )] / l 2 .
Or, the number of photons N l
2 ¸ l 1 in the spectral region between the wavelengths l 1 ¸ l 2 emitted along the path l is:
N
l 2 ¸ l 1 = (4 p 2 q 2 .l / hc). (1 / l 2 - 1 / l 1 ). [1 - 1 / b 2 n 2 ( l )] .
It follows that the intensity of Cherenkov radiation
increases with the refractive index n of the material
environment, its spectrum is continuous and is
the same for all particles with the same charge q, the number of
photons decreases with the square of the wavelength l . The relative
intensity of Cherenkov radiation increases with frequency, so
higher frequencies (shorter wavelengths) are more intensely
represented. That is why in the optical field Cherenkov radiation
appears to us as bright blue ; we do not see
most of it lying in the ultraviolet region with our eyes. The
maximum in the continuous spectrum is usually around 330 nm.
*) Cherenkov radiation in principle
contributes to energy losses and braking of the particle during
flight through the environment, but in comparison with other
mechanisms (ionization, excitation, bremsstrahlung) this effect
is negligible.
![]() |
Cherenkov radiation generated in an
aqueous phantom during irradiation with electron and
photon radiation beams. Left: A cylindrical phantom (diameter 20 cm and height 18 cm) filled with water was irradiated with a wide (magnetically scattered) beam of 9MeV energy electrons from a linear accelerator. Middle: . During passing through the upper part of the phantom, fast electrons generated Cherenkov radiation to a depth of about 4.5 cm, when the energy of the electrons fell below the threshold level of 260 kV. Right: When irradiating the same phantom with a beam of photon radiation (max. Energy 6MeV, beam with a diameter of 4cm)form secondary electrons along the g beam Cherenkov radiation - with a deep decrease in intensity as the primary photon beam weakens as it passes through water (just below the surface, a slight increase in intensity is initially seen - build-up effect to a depth of about 1 cm, discussed below) " Secondary radiation generated by X and g interactions ") . Note: In the upper and lower part, optical reflections of light from the cover and from the bottom of the phantom are visible. Due to the relatively weaker intensity of the images, the images contain a higher amount of disturbing noise. .. Colleagues ing.L.Knybel, ing.L.Molenda and Ing.B.Otáhal took pictures of Čerenkov's radiation on Varian TrueBeam and Accuray CyberKnife devices. |
If the material environment for light radiation
is transparent, ie it is an optical environment
, Cherenkov radiation can be visible - bluish
fluorescence observed at stronger particle flux - see phantom
measurement on the accelerator from above in Fig. " Cherenkov radiation of
irradiation beams " (known is blue fluorescence around highly radioactive
nuclear reactor fuel elements) . This
radiation can also be used with the help of photomultipliers to detect
fast charged particles , either primary or secondary
interactions of primary radiation with matter - see §2.4 " Scintillation
detectors " , section " Cherenkov
detectors ". These detection
methods find their application in accelerators, in the detection
of neutrinos and cosmic radiation (see also
the passage " Neutrinos " in §1.2 " Radioactivity " or below
" Cosmic radiation ") .
Minor interest:
Cherenkov radiation can also occur in our eye
during the interaction of high-energy particles. Indeed, faint
bluish flashes of light caused by cosmic rays are occasionally
observed with astronauts with their eyes closed.
Askaryan radiation
In addition to the above-described classical light Cherenkov
radiation, which arises during the passage of electrically
charged particles, a dielectric environment and the passage of
high-energy uncharged particles create a spray of fast
secondary charged particles, which emit a cone of electromagnetic
waves at radio or microwave frequency 5 GHz),
so-called Askaryan radiation . If this
phenomenon occurs in an environment permeable to radio waves -
such as ice, salt, quartz, sand, the atmosphere - this radiation,
generated in the form of short pulses, can be detected by radio antennas
.
Askaryan radiation is tested for neutrino
detection, especially in Antarctica, where high - energy
neutrinos pass through a layer of ice. The ANITA (....) antenna,
located on a balloon above Antarctica, detects these radio
pulses. It works in collaboration with photomultipliers detecting
Cherenkov radiation in Antarctic ice in the IceCube system. ...
.......
Transition radiation
is another radiation-optical effect when fast
charged particles passing through inhomogeneous medium fabric is
called emission. Transition radiation ( transition
radiation ). This radiation is generated when a charged
particle passes through the optical interface of
material media with different refractive indices, especially if
the electrical permittivities e
1 and e 2 of the two media differ . During the transition
(rapid passage) of a charged particle through such an interface,
according to Coulomb's law E = (1/4 pe ) .q / r2 the intensity of the
electric field around the particle changes very quickly
from the value of E1 (r) to E 2 (r), which according to Maxwell's equations of
electrodynamics arouses electromagnetic waves ,
called according to the mechanism of their formation transient
radiation *) - Fig.1.6.2 on the right. The
intensity of this radiation is approximately proportional to the
energy of the charged particle and is generally very small. ........ add: Frank-Ginzburg equation ....? ..
*) The mechanism of transient radiation was elucidated on the
basis of the laws of electrodynamics in the material
environment in 1945 I. Frank and V. Ginzburg.
For fast charged particles, both Cherenkov and transient
radiation are generated in the material environment. However,
transient radiation differs from Cherenkov
radiation in two aspects :
¨ Transient radiation is in principle also generated for
charged particles moving at a speed less than the speed
of light in a given environment, it is sufficient that
the environment is optically (electrically) inhomogeneous.
However, at lower velocities (energies) of charged particles, or
a gradual change of the dielectric constant e of the medium, transient radiation
is very weak and long-wave - in the optical field
(an optically transparent environment is a prerequisite here), or
in the field of infrared radiation or radio waves. Such radiation
is usually not detectable.
¨ The passage of
relativistic high-energy particles (especially
electrons) through the interface with a step change in the
refractive index creates short-wave transient X-ray
radiation- soft X-rays, photons with an energy of several keV,
which can be detected by methods for ionizing radiation (TRD - Transition
Radiation Detector ), eg proportional ionization chambers.
In general, transient radiation is
the least significant of all types of secondary
radiation generated by the interaction of charged particles with
matter. Since it is very weak (often only less than one photon per particle to pass
through the interface), is usually
irradiated by much more intense bremsstrahlung radiation and
radiation from atom deexcitation. X-ray transient radiation is
sometimes used in high-energy radiation analysis to detect
electrons (TRDs) and separate them from heavier particles (pions
or protons), which emit X-ray transient radiation at many times
higher energies than electrons.
Impact transition
radiation
Transition radiation arises also when the impact of
fast charged particles on the surface elements. If they are
bodies made of non-conductive material (dielectric), the
formation of transient radiation can be explained by the
above-mentioned mechanism of sudden change of electric field of a
particle when passing from vacuum with permittivity e o to environment with permittivity e > eo . However, transient radiation also arises when a
charged particle hits an metal surface , eg when
electrons hit an anode at X-ray. It arises from the fact that
when a fast charged particle approaches a metal surface, the dipole
moment d of the pair [incoming
charged particle q « electron or cation on the
metal surface q´] changes rapidly over time ,
which effectively forms an electric dipole (which disappears on impact) .
And according to the laws of electrodynamics, the time change of
the dipole moment of the charges leads to the emission of
electromagnetic waves *) - Fig.1.6.2 at the bottom
right, in this case the impact transient radiation.
This radiation can be observed as a faint bluish fluorescence (it is polarized) at the anodes of
high-voltage vacuum tubes (first observed
in 1919 by J.E.Lilienfeld at the anode of the cathode ray tube-
but not proven...) . It also occurs at the anode
of the X-ray tube (where it is not
possible to observe them, as it is completely irradiated with
light from a hot cathode) .
*) See §1.5 " Electromagnetic
field. Maxwell's equations.
", Formula (1.61), in the book " Gravity, black
holes and space-time physics ".
Electric
charging
An obvious, but mostly completely neglected phenomenon in the
interaction of electrically charged particles with matter, is the
electric charging of the originally neutral
substance environment. According to the law of
conservation of electric charge , the electric charge of
each place where the electrically charged particle is absorbed
and braked increases by the value of the charge of the particle.
At low radiation fluxes, or if the irradiated body is at least
partially conductively connected to earth, this phenomenon is
negligible. However, if we irradiate an electrically insulated
body with an intense flux of radiation a or b, it will gradually be
positively or negatively charged even to a high electrical
potential of up to hundreds of kV (depending on its electrical
capacity). This phenomenon is fully manifested only in a vacuum,
because in the air the radiation causes ionization, the
environment becomes partially electrically conductive and the
charge is continuously removed from the irradiated body.
The actual radioactive emitter a or b (if electrically
isolated) is also electrically charged, as
these particles carry an electric charge and the charges of the
emitter then have the opposite charges than the sign of the
charge of the emitted particles.
We can now turn to the specific properties of the interactions of specific particles of directly ionizing radiation :
The interaction of
heavy charged particles - alpha, proton and deuteron radiation,
heavier ions
a - radiation ,
which is a current of fast-flying helium nuclei 4He 2 (2p + , 2n o ) , is characterized by the fact
that of all common quantum radiation they have a - particles the largest
mass and especially the largest electric charge
- it is the positive charge of two protons p + . Permeating a particles capable
to substances acting in its flyby atoms substantial electric
(Coulomb) force on the electrons that very efficiently
rips from atomic shells - fig.1.6.1 top left. Due to
these strong ionizing effects , the particle a , although usually
having a high kinetic energy, brakes very quickly
in the substance, so that its range is very small
- at an energy of the order of MeV, the range is about 0.1 mm in
water density substances. The strongest ionizing effects arise at
the end of the particle's range - the Bragg maximum
.
If the particles have a sufficiently high
energy (several MeV), they can overcome the repulsive
electrical forces of the nuclei as they pass through the
substance and enter into nuclear reactions with
the atoms of the irradiated substance. Most often it is the
reactions ( a , n) in which they radiateneutrons .
This process is used in radioisotope neutron sources
: a -radioactive
substance (most often americium 241 Am) is mixed with a suitable material containing light
atoms (most often beryllium), from the nuclei of which energetic and -particles emit
neutrons.
Proton and deuteron
rays
Largely similar properties of interaction with matter has proton
radiation - stream of fast protons p + (hydrogen nuclei) and
deuteron rays (which is the current instant
deuterium nuclei D = 2 H 1 consisting of proton p+ and neutron n0) - Fig.1.6.1 top center. Fast-flying protons interact
mainly with atomic shells, from which secondary electrons
emerge . To a lesser extent, there are interactions with atomic
nuclei - mostly elastic scattering , but also nuclear
reactions in which secondary protons, neutrons, deuterons,
gamma photons are formed. All these interactions are discussed in
more detail in §1.6, passage " Proton radiotherapy ".
We do not encounter this radiation
in terrestrial nature, but in the upper atmosphere and in space,
high-energy proton radiation is a major component of primary cosmic
radiation (see " Cosmic radiation " below).. Proton and
deuteron radiation of artificial origin are generated in
accelerators (§1.5, section " Charged
particle accelerators ") . It is used for the production of radionuclides (especially positron - §1.4, part " Production
of artificial radionuclides
") , proton radiation in the so-called
hadron radiotherapy (§3.6, part
" Hadron radiotherapy ") .
Radiation of heavier ions
Fast-flying nuclei of heavier elements than
helium, also called heavier ions , produce analogous
ionization effects as radiation a or D, but
proportionally higher ionization densities due
to its larger charge. Even heavier nuclei with high kinetic
energies are contained in a smaller percentage in cosmic rays.
They are produced on accelerators for research purposes (study of
nucleus structure and strong interactions, formation of
quark-gluon plasma - §1.5, part " Quark
structure of hadrons "),
formation of heavy transuranic nuclei (§1.4, part " Transurans ") and for hadron radiotherapy (
accelerated carbon nuclei are mainly used here, see §3.6,
section " Hadron radiotherapy ").
Muon radiation
Muon radiation , which is a
current of fast muons m + or m - , can also be found on the earth's surface. Is part
secondary cosmic rays , caused by interactions of
high-energy particles of primary cosmic rays (see " Cosmic rays
" below). The muon radiation is highly penetrating
, in addition to its own ionization, the muons m ± eventually decay into electrons e ± (which is
actually beta ± ionizing radiation ) and neutrinos.
Pion radiation ,
which is the current of fast pi-mesons p + or p -, occurs
on Earth in the upper atmosphere, where it is formed by
interactions of high - energy primary cosmic rays. At large
accelerators, pions are formed during nuclear interactions of
protons accelerated to high energies (> 300MeV). In addition
to basic research in nuclear physics, p - experimental testing is being performed
as an effective tool in hadron
radiotherapy (§1.6, section " Hadron radiotherapy ", but it is quite problematic .!..) .
Antiproton radiation ,
formed by a stream of fast antiprotons (antiparticles to
protons), arises during nuclear interactions of protons
accelerated to very high energies (> min.6GeV, for higher
yields > 20GeV). It is being tested experimentally for antiproton
radiotherapy (radiation efficiency is
about 3 times greater than that of protons, but debatable, apart
from the extreme complexity and cost, is the problem of
production of secondary pion radiation - see also §1.6, passage
" Hadron
radiotherapy ") .
For better illustration, we will again present the important figure 1.6.1 of the interaction of charged particles with matter :
![]() |
Fig.1.6.1. Interaction
of fast charged particles with matter. Top left: Schematic representation of ionization mechanisms in the passage of beta - and alpha particles . Top middle: Three basic mechanisms of proton radiation interaction with matter. Bottom: Interaction of positron beta + radiation with a substance ending in annihilation of a positron with an electron. Right: Bragg curves of depth dependence of absorption and specific ionization along the path of gamma photons, accelerated electrons and protons. |
Interaction of electrons - beta- radiation and
high-energy electrons
Electrons interact with matter primarily through the
electron-electron interaction : incoming primary
electron electric forces acting on the electrons in the atomic
packing - causing excitation and ionization of bound electrons.
The primary electrons thus transfer part of their energy to the secondary
electrons , originally bound in the atoms of the
irradiated matter environment. In addition to this " collision
" electron-electron interaction, primary electrons can lose
their energy by radiation , emitting bremsstrahlung
. Quantum electrodynamics (QED) describes the general
mechanism of electron interaction. Two basic properties apply
here (which result from the general
analysis of the interaction of charged particles outlined above) :
¨ The
more forceful electric action between electrons is the
"tighter" their collision. The transmitted energy
(energy loss) and the scattering angle (electron deflection after
interaction) are therefore inversely proportional to the
collision parameter b .
¨ The
time for which electrons are in electrical interaction with each
other (for which they "pass" each other) is shorter the
higher the velocity of the incident electron (its energy) *).
With a shorter force action, the arriving electron
"manages" to transfer less energy to the target
electron and also deviates less from the original direction.
*) At energies of the order of MeV, the speed of electrons is
already very close to the speed of light, so a further increase
in energy should no longer seem to cause a shortening of the
interaction time. However, in the center of gravity system of
interacting relativistic electrons, the interaction time
continues to decrease with respect to the energy increase
(relative to the laboratory system) due to the effect of relativistic
time dilation . Therefore, even in the field of relativistic
electron energies, energy loss and scattering angle decrease with
increasing energy.
These two mechanisms lead to the regularity that the energy
DE
transmitted during the electron-electron interaction and the
scattering angle J are inversely proportional to the energy E
of the flying electron and the valuecollision
parameter b : DE ~ 1 / b.E, J ~ 1 / b.E
First we will approach what happens in matter during the
interaction of medium and lower energy electrons (radiation b - from
radioactivity), then we will look at the interaction of
high-energy electrons.
If the particle b- , which is a negatively charged electron e- from
the radioactivity of beta kinetic energy of the order of hundreds
of keV, penetrates into the substance , then during its passage
around atoms it acts by electric repulsive forces on electrons
which eject from the atomic shell and thus ionize
atoms.. Since electrons are very light particles, with
each such ionization of the atom, the electron b abruptly
changes the direction of its motion - it is reflected
by the repulsive electric forces from the atom. And then from the
next and next atom - the electron b will "zigzag"
move and reflect between the atoms, which it ionizes and at the
same time loses energy - Fig.1.6.1 top left. Depending on its
energy, it brakes to a depth of up to 1-4 mm in the water density
substance, and in heavy metals it does not fly deeper than about
0.1 mm. The mean range R s
of radiation b in a substance *) depends
on the energy of the radiation and on the density and proton
number of the substance (.......... -empirical
formula? ). For energies in the range of
approx. 0.6-3 MeV depends on the range of energy approximately
linearly, for lower energies the dependence is somewhat
flattened. The range of 3 mm, given as an example in the scale in
Fig. 1.6.1 on the left, corresponds to approximately harder
radiation b with an energy of about 1.5 MeV (as it has, for example,
32 P) in
water. For medium energies around 500keV the average range in
water is less than 1 mm, for soft b (as has 3 H) the range is very
small, of the order of hundredths of a mm.
*) Due to precipitation and scattering, the
trace of the particle is bin the matter it is very curvilinear, so that even two
electrons of the same initial energy, emitted from the same place
and in the same direction, can brake at considerably different
depths. Those electrons that moved in a more direct direction and
suffered fewer collisions with less energy loss penetrate
further, while electrons that have changed direction many times
and lost more energy in collisions slow down. In addition,
particles b from radioactive sources have a continuous spectrum of
energies, which further "blurs" the actual range length
around the value of the mean range R s . For radiation b , therefore, the value of the maximum range R max .
The range of radiation in a substance is often also described by
the effective range R90 , which is the distance at which 90% of the original
emitted energy of the particles is absorbed (or the radius of the
spherical space of the substance around the point source at which
90% of the energy emitted by the source is absorbed).
Towards the end of the path, when the energy of
the electron is no longer sufficient for ionization, the electronb willlose energy by
exciting the electrons in the atoms. If this is not the
electron trapped in some of the atoms, then its kinetic energy to
reduce the thermal value » 3/2 kT ( to the Boltzmann constant), which at room
temperature is only about 0.04eV.
High energy electrons from an
accelerator (betatron or linear accelerator) with unit energies
of up to tens of MeV, after entering the material environment,
they initially move almost in the original straight direction and
only brake slowly. The higher the electron energy
, the smaller the effective interaction cross
section, energy loss and scattering angle . Towards the depth,
along the path of the braked electron, as the energy decreases,
the mean free path shortens, the number of collisions increases,
the proportion of transmitted energy increases and the number of
significant changes in the direction of electron movement
increases (scattering to larger angles). Originally a straight
path of a high-energy electron, it gradually becomes a curvature,
ionization thickens and in the end section, before the complete
stop of the electron, a very dense chaotic cluster of
ionization is formed in the matter.and excitation of
atoms (as described above for the beta
radiation interaction) .
The curve of the depth dependence of the radiation dose
of a high-energy electron beam shows an initial slight
rise (corresponding to a lower effective cross-section
of high-energy interactions), after reaching a maximum in a few
centimeters followed by a steep drop to zero
(corresponds to complete braking of electrons in matter) - red
curve in Fig.1.6.1 bottom right.
During the passage of electron
radiation through a substance, as already mentioned above, secondary
electromagnetic radiation is generated : braking
X-radiation with a continuous spectrum, characteristic
X-radiationwith the line spectrum of the given type of
substance; when high-energy electron radiation passes through an
optically transparent substance (eg water), visible Cherenkov
radiation is also generated (
bluish fluorescence is visible around strong b- emitters) , in inhomogeneous optical environments event. weak transient
radiation .
These optical effects
were discussed above in the section " Cherenkov radiation ", including the image from the measurement itself
with electron and photon irradiation beams.
Interaction of beta+ radiation
Permeating agent particles b+ , which is positively charged positron e+ will initially - until it has a high energy and moving
high a velocity of - like b - during
its flyby carbon Coulomb electric forces pull electrons
from atoms , and due to its as small mass as the electron, it
will again "zigzag" move and bounce between the atoms,
which it will ionize while losing energy. It
brakes , depending on its energy, similar to the
electrons from radioactivity b
-, also at a depth of about 0.5-4 mm in the substance of
water density, the process of braking and thermalization is
similar to that of b - . Only
after almost complete braking of the positron (at "thermal" energy) does its "close" interaction with the electron
occur.
After braking, however, the fate of the positron is
completely different from that of the electron b - (Fig.1.6.1 below): upon encounter with the electron,
the electron and positron e + + e - ® 2 g mutual annihilate
, during which the positron and electron disappear
and transform to two photons of hard radiation g with energies of
511keV, which fly from the point of annihilation in exactly in
opposite directions - at an angle of 180 ° *). This
perfect angular correlation is widely used in positron
emission tomography imaging in nuclear medicine (§4.3, section " Positron emission tomography PET ") .
*) These regularities apply exactly in the center
of gravity frame of the positron and electron. The
energy of photons 2 ´ 511keV is a consequence of the law of
conservation of energy (rest energy of electron and
positron is m 0e .c 2 = 511keV), the opposite direction 180 ° is a
consequence of the law of conservation of momentum.
In the case of collisions of positrons and electrons of higher
energies, the angle of inclination of annihilation photons would
differ from 180 °. However, in the matter, the positron and the
electron already have relatively low velocities at the moment of
annihilation, so that the emitted quanta actually fly in almost
opposite directions.
Positronium
Just before the actual annihilation, the electron e - and the positron e + can orbit for a
while (they orbit the common center of gravity) - they form a
special bound system (similar to a hydrogen
atom) called positronium (Ps). The dimension of
the "atom" of the positron is twice the hydrogen atom,
the binding energy of the positron is 6.8 eV. Depending on the
mutual orientation of the electron and positron spins, the
positron can be either in the singlet state1S 0 with oppositely oriented spins -
so-called parapositon p-Ps (1/4 cases), or in triplet
state 3 S 1
with concordantly oriented spins - so-called orthopozitronium
o-Ps (3/4 cases).
However, this
system of positronium is unstable , the two
particles approaching each other in a spiral under the emission
of electromagnetic waves; in p-Ps in about 120ps they
"fall" on each other and there is a self-annihilation
on two photons g. In the case of o-Ps,
annihilation into two photons is prohibited by quantum selection
rules (related to the law of conservation of the spin momentum -
each of the photons has spin 1), so o-Ps would decay in a vacuum
by emitting 3 photons with a relatively long lifetime of about
140ns. ; in the substance, however, the positron bound in o-Ps
much earlier is enough to annihilate with some
"foreign" electron from the surrounding environment,
which has the opposite spin orientation - again two photons g are formed .
The
annihilation of a positron with an electron produces 2 gamma
photons in the vast majority of cases, as mentioned above.
Sometimes, however, more of them can occur , but with a
very small probability (the probability that 2 + n photons will
be formed during e -
e + -annihilation is proportional and n,
where a = 1/137 is the fine
structure constant). If a positron interacts with an electron
bound in an atomic shell, the extinction of such a pair may be
accompanied by the emission of only a single photon, and
some of the energy and momentum may be transferred to either the
atomic nucleus or one of the other electrons; however, the
probability of this process is very small and does not apply in
practice.
The
lifetime of positrons in substances is in the order of
hundreds of picoseconds. However, the exact value depends on
local electron densities and configurations, which is used in the
PLS (Positron Lifetime Spectroscopy)
spectroscopic method . The investigated material is locally
irradiated with a b +
- g emitter (most often 22
Na), wherein the positron lifetime is determined by measuring the
delayed coincidence between the detection of photon radiation g of irradiating radionuclides (from 22 Na it is g
1274 keV) annihilation photons, and detecting g
511 keV.
When
b+
-radiation passes through the substance, braking and
characteristic X-radiation and Cherenkov radiation are generated
in a manner analogous to b- radiation .
In terms of
the general properties of the interactions of different types of
radiation, positron radiation was also discussed above in the
section " Interactions of positron radiation ".
Interactions
of indirectly ionizing radiation
Interactions
of gamma and X radiation
Photons of g
radiation and X-rays
*) do not have an electric charge, so they cannot ion atoms with
direct electric forces. However, a photon is a quantum of a
rapidly oscillating electric and magnetic field, so when an
electron comes in "close proximity" to this oscillating
field, it can receive electromagnetic energy and
be accelerated by the photon . If this happens
to the envelope electron in the atom, the atom may be excited
or ionized . A photon can also electromagnetically
interact with nucleons in the nucleus - the atomic nucleus
can be excited .
*) Both these X and gamma rays have the
same physical nature (electromagnetic photon radiation) and
largely similar properties, they may differ in the way they
originate. In §1.2 "Radioactivity", part " Radioactivity gamma
", we introduced a terminological agreement that photon
radiation emitted from atomic nuclei is called radiation g
(even if it has a low energy of a few keV), while radiation
arising from electron jumps in the atomic shell and the
bremsstrahlung of electrons is called X - rays
(X-rays - even if it has a higher energy of tens and hundreds of
keV). However, photon radiation with very high energies (of the
order of MeV) is usually called gamma radiation
, regardless of the way it is generated. Quantum X a g -radiation moves exactly at the speed of light
in vacuum c (for
interest, however, see below the theoretical note " Is high-energy g
-radiation moving slower than
light? ") .
Ordinary photons of gamma and X of lower energies (tens or hundreds of keV) interact
almost exclusively with the electron shell of
atoms. Their interaction with the nucleus is very unlikely (with some exception being the Mösbauer resonant
nuclear absorption described below) . Only
at energies above 5MeV do interactions with nuclei - photonuclear
reactions - begin to take effect (see
below) .
The way photon radiation interacts
with matter is determined primarily by its kinetic energy
(wavelength). "Soft" electromagnetic radiation of
longer wavelengths (low energies) behaves mainly like a wave
that interacts collectively with a larger number
of electrons or atoms (which oscillates) in the material
environment , which in the case of light leads to known optical
phenomena of light reflection and refraction. During the
interaction with the atom, the oscillating electromagnetic field
hits either the whole electron shell or most of it, while due to
the slower course of the interaction of the binding force of the
electrons, it is enough to "divert" the excitation to
the whole atom.
As the frequency-energy increases,
the radiation acquires a photon character, wherein the
photons have the properties of free point particles of relatively
high energy. Such photons then interact - collide - individually
with individual electrons, whether free or bound in atoms. At
higher energies, therefore, direct collisions of
photons with electrons are applied ("point-located"
photons can strike individual electrons in atoms directly,
without affecting the surrounding orbitals in the atomic shell) . Due to the very short time that the photon is "in
contact" with the electron, it is not enough to
"convert" the binding forces of the electrons to the
rest of the atom.
On the electron side, the mode of
interaction depends on whether the electron is free or how
strongly it is bound in the atomic shell. For free or
weakly bound electrons, the interaction usually has the character
of a direct "collision" of the photon with the electron
and the result is scattering - the photon
"bounces" in another direction and its energy is
reduced by the value passed to the electron. However, with
electrons strongly bound in the atomic shell, the photon can
interact to some extent "collectively": the primary
photon is first absorbed by the electron shell, creating a
transient excitation state that then decays spontaneously and
emits the received energy - either again as a secondary photon or
in in the form of kinetic energy of the released electron.
Non-ionization
processes
Without further analysis, we will only mention the non-ionization
processes that occur during the interaction of
electromagnetic radiation with matter :
From the point of view of the physics of ionizing radiation itself, these processes have almost no significance - for gamma radiation they occur only with a very small effective cross section and do not lead to ionizing effects. However, they are important from the point of view of atomic physics, the interaction of softer radiation with atoms. X-ray interference in coherent scattering on atoms in crystal lattices is used in X-ray diffraction analysis of the structure of solids (see §3.3 " Radiation measurement of mechanical properties of materials "). Thomson scattering on electrons is important in plasma physics and in astrophysics, the excitation and subsequent deexcitation of atoms is the source of much of the visible, infrared and UV electromagnetic radiation that we observe in nature.
Ionization
processes
The interaction of g and
X radiation (for short we will
write only g , for X-radiation the situation is analogous) with the substance, leading to ionization effects, can
take place in four different ways marked in Fig.1.6.3 (fifth method, resonant nuclear absorption - the
Mösbauer effect, not shown here, but described in detail below) :
Fig.1.6.3. Four ways of gamma radiation interaction with matter.
Both basic phenomena - the photo effect and Compton scattering - are often combined in the interaction of photon radiation in matter . In most cases, Compton scattering occurs first, or multiple, and scattered photons with lower energy then interact with the photo effect.
Effective
cross-section of gamma radiation absorption in matter
The total effective cross-section of
the interaction of photon (X, gamma) radiation with matter is given by the sum for
photoeffect, Compton scattering and electron-positron pair
formation (nuclear photoeffect and
formation of heavier particles (pions,
hyperons) has an effective cross section
significantly lower) . A typical dependence
of the effective cross section of the interaction on the energy
of gamma radiation E g is graphically shown in Fig.1.6.4 for light material (water) , medium-heavy (iron) and heavy material (lead) . The overall trend in the
field of low and medium energy (units up to
hundreds of keV) isa significant
decrease in the total ("Total")
effective cross section with an increase in
energy E g (except for smaller "teeth" - the edges of the
local increase in the vicinity of the binding energy of electrons
on the shells K, L, M) . For high energies (MeV units) , this decrease
gradually stops and is replaced by a slight increase in the
effective cross section due to the formation of electron-positron
pairs.
Fig.1.6.4. The energy dependence of the effective cross section
of the interaction of photon radiation (gamma, X) with the
substance for the photoeffect, Compton scattering and
electron-positron pair formation - is expressed by appropriate
contributions to the linear absorption coefficient m (normalized to
density r ). In the area of lower energies, resonantly increased
"teeth" of the photoeffect efficiency can be seen in
the vicinity of the binding energy of E K electrons on the K shell - K-edge , analogously
L and M edge.
Due to the large range of energy values and
effective cross sections, it is necessary to use a logarithmic
scale - graph [log-log] for a clear display.
Secondary radiation generated by
interactions of X and g with matter
In the interactions of primary radiation g and X with matter discussed
above , there are processes in which secondary radiation
is generated :
Photoelectrons, Auger electrons and electron-positron pairs are mostly absorbed in the substance, only a very small part of them can fly out of the layers at the surface of the irradiated substance. However, Compton-scattered g -radiation, characteristic X-radiation, bremsstrahlung, and annihilation g -radiation can easily fly out of the irradiated substance and thus enrich the original field of g- radiation . Secondary light radiation can be used in the case of optically transparent substances; has an important use in scintillation detection and spectrometry of g - rays - see §2.4 " Scintillation detection and spectrometry of gamma rays ", or in the detection of Cherenkov radiation using photomultipliers. As stated above in the passage " Secondary radiation generated by radiation-substance interactions ", for the amount of secondary radiation emitted by an irradiated body, is sometimes called albedo. For X and g radiation, the albedo of common substances (such as water or living tissue) is very low, below 1%. caused mainly by Compton scattering, partly also by X-ray fluorescence.
Theoretical
curiosity:
Is high-energy g-radiation moving slower than light?
All electromagnetic radiation
propagates in a vacuum at exactly the speed of light c ,
independent of the movement of the source and the observer. This
is a basic finding, firmly rooted in the special theory of
relativity. Regardless of the wavelength - speed c
propagates radio waves, visible light *), X and gamma radiation.
*) The classical dispersion
observed in light in the matter's optical environment originates
in the (collective) interactions of the electromagnetic wave with
the atoms of matter; does not occur in a vacuum. This is
something else ..! ..
Influence of fluctuations in space-time geometry on the speed of
motion of high-energy photons of gamma radiation.
However, in connection with
quantum-gravitational effects leading to fluctuations in
space-time geometry (see §B.4
" Quantum Geometrodynamics " of the book "Gravity, Black Holes and
Space- Time Physics) , there may be
phenomena that may somewhat call this basic statement into
question in certain circumstances. The figure shows a situation
where two photons are emitted from a certain source at the same
time: one photon with lower energy, ie longer wavelength, the
other photon of high-energy gamma radiation with a very short
wavelength. are averaged and completely smoothed out in the
corresponding longer scale of quantum fluctuations of the metric,
so that this radiation will move in the classical vacuum exactly
at the speed of light v = c. Photons of high-energy radiation ghowever, with a
very short wavelength, they will be "more sensitive" to
fine-time fluctuations in space-time metrics than low-energy
photons. Such waves will move along a slightly undulating
geodetic orbit, photons will in a sense "penetrate" the
unevenness of the orbit, caused by subtle metric perturbations,
and their effective velocity v ef will be slightly less than c .
We can compare this to the movement of a car with small wheels
and large wheels on a bumpy road: when driving the wheels at the
same circumferential speed, a car with small wheels will travel a
little slower than a car with a large wheel diameter.
*) This phenomenon cannot be
considered as a violation or failure of a special theory of
relativity, which is exactly valid in flat spacetime without
metric defects.
These differences are manifested
only at very high energy g , in the area of GeV and TeV. Here, too, the differences
in speed are very small (of the order of 10 -20 ), without the possibility of laboratory measurements.
In the future, they could only be demonstrated by a time
comparison of the detection of light and flashes of hard g- radiation from
catastrophic processes in outer space. At cosmological distances
of billions of light-years, even these slight differences in
speed could "accumulate" and have measurable effects
(the problem, however, is to distinguish these differences from
the differences in emission times in the sources themselves ...).
Interactions with
quantum-gravitational fluctuations of space can lead to
dissipative phenomena and slight modification of
kinematics not only for hard photon radiation, but also
for high-energy particles in space.
Neutron radiation and its interactions
Neutron radiation is a stream of moving neutrons
. Neutrons are normally bound in nuclei by a strong interaction,
along with protons. They are released from the nuclei by nuclear
reactions , which arise during irradiation with
high-energy particles from accelerators and during the fission of
heavy nuclei. Intensive sources of neutron radiation
are nuclear reactors , whether fissile or experimental
fusion thermonuclear (§1.3, part " Fission
of atomic nuclei " and " Fusion
of atomic nuclei "). Specific small charged particle accelerators (mostly
deuterons, with a tritium target) called neutron generators
(§1.5, section "Charged particle
accelerators", section " Neutron generators
") are constructed as laboratory
sources of neutrons . However, the most common are radioisotope
neutron sources consisting of a mixture of a - emitter with a
light element such as beryllium (a mixture of 241 Am + Be, 239 Pu + Be, 226 Ra + Be, 210 Po + Be). Energetic
alpha radiation emits neutrons from beryllium nuclei by the
reaction 9
Be ( a ,
n) 12 C.
Heavy transuranic radionuclides are rarely used, most
often californium-252, during the spontaneous fission of which
neutrons are released (§1.3, " Transurans ") .
In a vacuum, neutrons move freely
and without resistance, but their "range as neutrons"
is not unlimited as might be expected: free neutrons
spontaneously decay by radioactivity b - with a half-life of about 13 minutes into protons,
electrons and (anti) neutrinos. Since neutrons do not have an
electric charge, they do not ionize themselves as they pass
through the substance (this is indirectly ionizing radiation).
The ionization of the environment is caused only by secondary
particles , which are formed during the interaction of
neutrons with the nuclei of atoms (reflected light nuclei, g radiation ,
protons, alpha particles, etc.).
After entering the substance,
neutrons interact almost exclusively with atomic nuclei (not with
electrons in the shell), in four ways :
Basic ways of neutron interaction with
matter
In practice, the individual mechanisms of interaction of neutron radiation with matter are often combined. E.g. fast neutrons easily penetrate the substance, they quickly lose their energy during elastic or inelastic collisions, especially with light nuclei; these reflected nuclei then ionize and excite the surrounding atoms. After slowing down, neutrons enter the nuclei and cause nuclear reactions there with the formation of radioisotopes - neutron activation , which can become a long-term source of ionizing radiation. We do not mention here the interactions of neutrons with very heavy nuclei in the area of uranium and transuranium, leading to nuclear fission , which is discussed in more detail in §1.3 "Nuclear reactions", part " Fission of atomic nuclei ".
Neutrinous
radiation
Although in terms of particle fluency, neutrino radiation is one
of the most abundant and most intense radiation in nature
, its radiation significance is negligible
(practically zero) and usually does not even belong to ionizing
radiation. This is due to the extremely small effective
cross-section of the interaction of neutrinos with the substance.
The origin and properties of neutrinos are discussed in detail in
§1.2, section " Neutrinos ".
" Visibility
" of invisible ionizing radiation?
Ionizing radiation is invisible to our eyes , we
can only register it using special methods of detection
and spectrometry (Chapter
2 " Detection and spectrometry of ionizing radiation ") . For better clarity,
however, it would be appropriate to somehow directly " make
visible " this radiation, respectively. its
interaction with the substance. One of the methods is described
in §2.2 - 3-D gel dosimeters ; however, it is a relatively complicated and demanding
method, it is used very rarely. There are two other ways to
directly and easily "make visible" the passage of
ionizing radiation through a substance: Cherenkov
radiation in an optically transparent environment (even in water ) and
scintillation radiation (preferably
in a liquid scintillator ) .
We used these methods experimentally for electron and photon
beams at our workplace and for proton beams at PTC :
The results of these measurements are discussed in more detail in
§3.6, passage " Make the invisible
visible " - display of
radiation beams ").
Radiation
absorption in substances. Shielding.
All of the above-described
mechanisms of the interaction of radiation with matter cause a
certain part of the quantum of ionizing radiation to be absorbed
as it passes through the substance . With low-penetrating
radiation, practically everything is absorbed, with penetrating
radiation, part of the quanta is absorbed and part passes. We
will deal mainly with the absorption of g- radiation , which is
penetrating.
Different absorption of ionizing
radiation depending on the type and energy of radiation,
thickness and density of irradiated material is used in a number
of radiation analytical methods, described in more detail in
Chapter 3. It is mainly X-ray diagnostics (§3.2
" X-rays - X-ray diagnostics ") , defectoscopy,
measurement of thicknesses and densities of materials or height
of levels(§3.3 " Radiation measurement of mechanical properties
of materials ") . The absorbed radiation energy is used in radiation
technologies, especially in radiotherapy (§3.6
" Radiotherapy ") . And the phenomenon
abrorpce appropriate material is based shielding and collimation
of ionizing radiation (see below " ionizing
radiation schielding ') .
Fig.1.6.5. Basic laws of absorption of ionizing radiation in
matter of density r , proton number Z and thickness d
.
In the exponential graph on the right,
"low-medium-strong" absorption is meant only
ideologically and unspecified: "strong" absorption
occurs either for low-energy gamma radiation and / or for heavy
shielding material; "low" absorption can be caused by
either high radiation energy and / or light material.
Fig. 1.6.5 on the left shows the basic situation where we place a layer of absorbing substance (with density r and proton number Z) of thickness d in the path of a parallel beam of radiation g of initial intensity I o . Part of the radiation is absorbed, the intensity of the transmitted radiation denoted I . What will be the amount of absorbed and passed radiation? Of course, primarily on the thickness of the material d , the dependence will be exponential (it is derived below):
I = I o . e - m . d , |
where the absorption coefficient m is called the linear
attenuation factor . Its value depends on the density
and proton number of the absorbent material and
significantly also on the radiation energy E g : m = m ( r , Z, E g ). The
linear attenuation factor is higher the higher the density r and the proton
number Z of a given substance and the lower the higher the
radiation energy E g .
The total linear attenuation coefficient m is the sum of the
individual partial absorption coefficients for
photon radiation (gamma, X)for the photo effect m f , the Compton scattering m C and the formation of electron-positron pairs m e : m = m f + m C + m e . The relative
proportion of these subcomponents depends on the material and
very significantly on the energy of the radiation g (see Fig. 1.6.4 of
the section " Effective cross section of the absorption of
gamma radiation in substances " ).
In addition to
the linear attenuation factor m
, a mass
attenuation coefficient
m / r is sometimes introduced which is independent of
density. Furthermore, especially for technical purposes (such as
shielding design - see below), instead of the linear attenuation
coefficient m , the values ??of the so-called
absorption half - layer (half-thickness) d 1/2 = ln2 / m @ 0.693 / m are often
given in the tables , which is such the thickness of a layer of a
given material, which halves the intensity of that radiation . And sometimes the mass half-layer of
absorption r / d 1/2 [g / cm 2 ] is given, which depends mainly on the energy and the
type of radiation.
Note: The basic property of an
exponential function with a negative exponent is that it
approaches zero up to the limit at infinity. After passing the
layer d = d 1/2 , exactly half of the particle-quantum is absorbed: I
(d 1/2 ) =
I o / 2.
At the end of the next half-layer, half of the half of the
particles remain, ie a quarter: I (2.d 1/2 ) = I o / 4. And so on to infinity, so only in the limit d ® Ą will the limit be I (d ® Ą ) = 0 and all particles will really be absorbed.
This would mean that the radiation can practically never be
completely shielded. However, this is only theoretically the
case. In fact, each emitter emits only a finite number
quantum particles. After passing through a sufficient thickness
(tens or hundreds of d 1/2 ) in practice, the last quantum is
always finally absorbed ...
Attenuation of a wide
beam of radiation
The exponential law applies exactly to a parallel narrowly
collimated beam of radiation, where only photons passed through
the absorber without scattering. In the case of a wide
beam of radiation without collimation, the detector can
also be affected by scattered radiation in the space behind the
absorbing material, so that the intensity I will be
somewhat higher. This circumstance is the exponential law
expressed by so-called absorption. growth factor
B: I = I a
.B.e - m .d. The magnitude of the growth factor (B ł 1) depends on the
thickness and type of substance, the energy of the radiation, as
well as on the geometric arrangement of the radiation source, the
irradiated layer and the detector.
Effective
cross-section of interaction and linear attenuation coefficient
In §1.3 "Nuclear reactions" and §1.5 "Elementary
particles" the concept of effective cross-section of
interaction with was introduced , which geometrically expresses the
probability of a given type of interaction. This is the effective
cross-section of the interaction of radiation, eg g , with the atoms
of the substance (either the total effective cross-section of the
interaction, or individual partial effective cross-sections for
photoeffect, Compton scattering and electron-positron pair
formation). In the interaction of a parallel beam of radiation of
intensity I , 1 cm 2 of the substance passes every second I particles, which
interact with the atoms of the substance with an effective cross
section s . The number of atoms in 1cm 3 is L» R .m p / N, where r is the density of the material and N is the nucleon
number of the atoms of the substance. The layer dx of thickness
dx contains 1 cm 2 of L.dx atoms, each of which represents an
radiation-effective shielding surface of the interaction of size s , so that the
intensity I of the beam is attenuated by -dI = I. s .L.dx. By
integrating this differential relation, we obtain for the
intensity I (x) of the beam at depth x the relation I (x)
= I o .e - s .Lx = I o .e - m .x , where I o is the original
intensity of the beam on the surface and the coefficient m = s .L = s . r .m p / N represents the linear
attenuation factor . The linear attenuation factor is
proportional to the density r
of the absorbing material and, thanks to
the effective cross section s,
significantly depends on the radiation
energy and also on the proton number of the atoms of the
substance (because the proton number also determines the electron
density of atoms).
In addition to the linear attenuation factor m , a mass
attenuation coefficient m
/ r , which is independent of density, is sometimes
introduced .
Absorption of
polyenergetic beams of radiation - "spectrum hardening"
The beams used in X-ray diagnostics and radiotherapy usually have
a continuous spectrum covering a relatively wide
range of photon energies. Here we observe deviations
from the exponential course of attenuation. The initial layer of
material here weakens mainly low energies, while leaving high
energies almost without absorption. In the depth of the material,
this increases the proportion of higher energies - the beam
becomes more penetrating, its spectrum " hardens
". Due to this spectral shift, the course of attenuation of
polyenergetic beams at depth is no longer exactly (mono)
exponential, but the rate of absorption gradually decreases with
depth.
The "spectrum hardening" effect is used to filter
the X-ray beam spectrum (§3.2. "X-rays - X-ray diagnostics ", passage" Sources of X-rays - X-rays ").
Shielding
of ionizing radiation
In many applications of ionizing radiation, it is necessary to
prevent ionizing radiation from penetrating to certain
places or from certain directions - it is therefore necessary to
shield a certain part of the radiation . This need
arises, for example, for protection against
ionizing radiation (§5.3, section " Factors of radiation protection ") , for detection of
ionizing radiation (where the detector
needs to be shielded from the background, or to detect only
radiation from certain directions - §2.1, passage " Shielding, collimation and
filtration of detected radiation
") , in imaging methods
such as scintigraphy (where by collimation we detect only radiation from
precisely defined directions - §4.2, part " Scintigraphic collimators ") , in radiotherapy
where by collimation we define a narrow beam of radiation affecting only the
target tumor tissue (§3.6, part " Isocentric radiotherapy ") , etc. With reference to
The above-mentioned mechanisms of the interaction of radiation
with matter (" Interaction of ionizing
radiation in the passage of matter ") will briefly mention here
some general principles for achieving optimal shielding
for individual types of radiation.
Gamma
radiation shielding
The shielding properties of light, medium and heavy materials
depending on the energy of gamma radiation are plotted above in
Fig.1.6.4 in the section " Effective cross section of gamma radiation
absorption in substances ". For gamma and
X radiation , the most effective shielding materials are substances
with a high
specific gravity (density) and a proton number, ie with a high electron density - especially lead , tungsten, or uranium *). Lead
containers for the transport and storage of radiators, lead sheet
metal screens, shaped lead bricks, etc. are used.
A layer of 2 mm thick lead is sufficient for effective shielding
of gamma radiation with an energy of approx. 100 keV; the higher
the energy of the gamma radiation photons, the thicker the
shielding layer must be used. If is the need to maintain the optical
visibility , used lead glass
with a high content of lead oxide in the melt.
*) Due to its high density (19g /cm 3 ) and proton number
(Z = 92), uranium is a very good shielding
material for hard radiation g
. In addition to the high price, its main
disadvantage is that uranium itself is radioactive
(see §1.4, passage " Radioactive
decay series "). Its
specific activity can be reduced by removing the isotope 235U, which (despite
its low proportion of 0.7%) forms a significant component of the
radioactivity of natural uranium. The so-called " depleted
uranium " - 238 U , thus formed , has a
specific activity of approx. 12 kBq /g and is suitable for
shielding preparations with high activity and small dimensions.
Uranium is not suitable for shielding and collimation of low
activities and weak radiation fluxes, where the actual
radioactivity of the shielding material interferes.
For economic reasons, it is sometimes more
advantageous to use larger material thicknesses with lower
specific shielding capabilities, if the configuration of the
radiator, irradiated substances and detector allows it. This is
usually the case with the construction solution of workplaces
with ionizing radiation, where, in addition to brick masonry, denser building materials are used - concrete with event. barite admixture, barite plasters, etc.
Absorption
half- thickness
The thickness of the shield
required depends on the density (and nucleon number) of the
shielding material, the radiation energy g and the
attenuation required. In addition to the
linear attenuation factor m , the tables often give the values of the
so-called absorption half - layer (half-thickness) d 1/2 = ln2 / m @ 0.693 / m , which is the thickness of the layer of
shielding material that attenuates the intensity of the radiation
by half. to 1/4
, 3 half-thickness to 1/8
etc. - the shielding effect
increases exponentially with the shielding thickness according to
the above formula). For some common materials and radiation
energies g (resp. X), the half-layers are as follows
:
|
||||||
E g [keV] | water | concrete | iron | lead | ||
100 | 42 | 17 | 4.8 | 0.15 | ||
200 | 51 | 21 | 6.6 | 1.4 | ||
500 | 74 | 32 | 11.1 | 4.2 | ||
1000 | 102 | 45 | 15.6 | 9.2 | ||
2000 | 144 | 59 | 21 | 13.5 | ||
5000 | 231 | 905 | 28.8 | 14.7 |
The attenuation of the radiation intensity by the absorption layer of thickness d can be expressed by means of a half -thickness d 1/2 by a simple relation I / I o = 2-d/d1/2 . A shield with a thickness corresponding to 7 half-thicknesses attenuates the radiation to approximately 1%, and 10 half-thicknesses below 0.1%.
Beta
radiation shield
To shield radiation b - just lightweight
materials
(such as plexiglas or aluminum) with a thickness of
about 5-10 mm. For harder beta radiation, it is best combined
with a subsequent thin layer of
lead to
shield the braking electromagnetic radiation generated by the braking of electrons b in the shielding
material . Lead itself is not the optimal shielding material for
energetic radiation b , because it produces hard and intense bremsstrahlung
radiation, for the shielding of which it is necessary to use an
need thick layer of lead.
For shielding positron
radiation b +
in addition to the layer of light material, it is necessary to
use relatively thick layers of lead (approx. 3 cm), or tungsten
to shield hard gamma radiation with an energy of 511keV, arising
from the annihilation of positrons b
+ with electrons e - .
Alpha radiation shielding
Radiation a,
due to its low penetration , can be shielded
very easily. A thin (millimeter) layer of light material, such as
plastic, is enough. It is often not necessary to shield against
alpha radiation at all, because even in the air there is a range
of particles a only a few
centimeters, at higher energies max. Tens of centimeters. If the
emitter is mixed with a + g , the gamma shield automatically
completely shields the alpha radiation as well.
Neutron
shielding
Neutron shielding is generally a more complex problem than beta
or gamma radiation. Neutrons are the only common types of
particles that do not interact with the electron shell of atoms
of matter, but only with the nuclei of atoms, through a strong
interaction (the interaction of neutrons
with matter has been discussed in more detail in the section
" Neutron radiation and its
interactions ") . In the case of fast neutrons , they
must first be slowed down so that they can be
effectively absorbed by a suitable absorber..
Neutrons are most effectively slowed down by the passage of
substances from lighter atoms, such as those rich in hydrogen,
where they lose energy during elastic scattering on hydrogen
nuclei (protons). To reduce the number of fast neutrons about 10
times, a layer of about 20 cm of paraffin or plastic is needed.
For the absorption of such slowed neutrons, their capture
by suitable atomic nuclei is then used . The most
effective absorption takes place in cadmium, boron or indium. The
absorption of neutrons in the nuclei of cadmium or boron is
accompanied by the emission of gamma radiation (these are reactions (n, g ) of neutron radiation
capture) , which also need to be shielded,
by a heavy material - lead. Thus, neutron shielding generally
must consist of three layers: a layer of light
hydrogen-rich material (eg polyethylene), a layer of cadmium or
boron, and finally a layer of lead.
Note: When shielding neutrons, it is
necessary to keep in mind that radionuclides are
formed during the capture of neutrons in some nuclei , when
originally inactive materials can become emitters b and g . These radionuclides then
"internally" contaminate the shielding
and construction materials. E.g. if cobalt-alloyed steel 59 Co is exposed to
neutrons, the known radionuclide 60 Co with a half-life of over 5 years is formed by
neutron capture !
Cosmic
radiation
Light of cosmic origin from the
Sun and stars has been observed by humans since time immemorial.
The first indication that invisible ionizing radiation
came to us from space was the observation of the Austrian
researcher Viktor Hess in 1912 during a bold ascent on a balloon
that the level of radiation indicated on the electroscope
increases with altitude *). Further measurements during altitude
flights into the stratosphere, terrestrial measurements with more
advanced detectors and later measurements on space probes, not
only reliably confirmed the existence of this cosmic
radiation , but also measured its properties in detail.
*) This finding was quite surprising
at the time, as experts at the time believed that all natural
radiation, causing ionizing discharge of electroscopes,
has its origin in the emission of radioactive substances
contained in the earth's crust. The radiation level should
therefore decrease with height above the ground
. This was partly indicated by the observation of T. Wulf, who
brought the electrometer to the top of the Eiffel Tower, where at
a height of 330 m he measured about half the ionization than at
ground. However, even this decrease was much smaller than if it
were only radiation ( g ) coming from the earth's surface. During the balloon
ascents of V.Hesse (the flight took place
in Ústí nad Labem, the balloon was filled with hydrogen from
the local chemical factory)and his
followers, airtight electrometers were used so that the rate of
discharge could not be affected by changes in air pressure. Upon
ascending to the first 800m, the ionization actually decreased,
but more slowly than expected; on the other hand, the ionization increased
with a further increase , which at a height of
about 3 km was already quite steep; at a height of 5 km, the
ionization was 3 times greater than at the Earth's surface. The
only explanation was: "from above", from outer space,
comes penetrating radiation of extraterrestrial origin
, which partially passes through the atmosphere and contributes
to other natural radiation and ionization even at the Earth's
surface. These
experiments were then confirmed in 1925 by R. Milikan using an
electrometer with automatic recording of measurements on film,
which could ascend to far greater heights on a balloon without a
human crew; he called this radiation " cosmic
radiation ".
The electrometers were only able to show if the
radiation was present and what its approximate intensity was. The
use of particle detectors , especially the
Geiger-Müller detector (see §2.3) and also particle
detectors - fog chambers and nuclear photographic
emulsions (see §2.2), has brought great progress in
understanding the nature of cosmic rays . The first trace of a
cosmic ray particle in the nebula chamber was recorded by D.
Skobelcyn in 1922. With the help of coincidence
measurements G.-M. Detectors have been found to contain
cosmic rays charged particles with high
energies exceeding 1GeV. In 1938, Pierre Auger detected
the coincidence of impulses coming from sprays of
particles of (secondary) cosmic radiation generated in
the atmosphere. Using traces of cosmic radiation in fog chambers
and photographic emulsions, not only was the composition of
cosmic radiation revealed, but a number of new particles
, hitherto unknown to physics - were discovered - positron e + , muon m , mesons p and K, and finally
some heavy hadrons (hyperons). , see §1.5 "Elementary
particles". The study of cosmic rays thus played an
extremely important role in understanding the laws of the
microworld(After all, we cannot yet achieve such high
energies as occur in cosmic rays in terrestrial accelerators, see
below).
Cosmic
radiation that comes from space is called primary
; we will deal with this first. During the passage of primary
cosmic radiation through the Earth's atmosphere, secondary
cosmic radiation is created (we will mention it in the
second part of this passage on cosmic radiation).
Composition
and energy of cosmic radiation
By cosmic radiation (primary) we mean
high-energy radiation of cosmic origin, which consists mostly of protons
(88%), helium nuclei (10%) and other elements (1%); the content
of the various nuclei in cosmic rays roughly corresponds to the
representation of elements in the universe, as established as a
result of primordial and stellar nucleosynthesis. From light
particles then fast electrons and neutrinos. High-energy photons
of gamma radiation are also part of cosmic radiation. The energy
of particles of (primary) cosmic radiation varies in a wide
range. The lower limit is about 10 9eV - charged particles with lower energies have
difficulty penetrating the Earth due to the magnetic field
created by charged particles moving from the Sun ("solar
wind"). Due to its local significance, radiation from the
Sun itself (the so-called solar component consisting
mainly of protons and 5-10% of helium ions) is usually not
classified as cosmic radiation. The upper energy limit of the
cosmic rays registered so far is about 10 20 eV; If we compare this with the highest particle
energies of about 10 12 eV achieved at Earth accelerators so far, cosmic rays
contain by far the highest particle energies we
know *) - they exceed by 8 orders of magnitude (ie one hundred
million times!) the highest energies achieved so far in
terrestrial accelerators . accelerators.
*) In 1991, a particle of cosmic radiation
with an energy of 3.10 20 eV was recorded , which in common units corresponds to
about 50 Joules. So the microparticle - the proton - has
"macroscopic" energy!
With increasing energy E the
number of cosmic ray particles decreases rapidly (is proportional
to about E -3 ), so while the flow of particles with energies around
1GeV is relatively intense (about 10 4 /sec./m 2 ), there are very few high-energy particles - for
energy of 10 16 eV we observe only a few particles per 1m 2 in 1 year, for the
highest energy around 10 19 eV it is only about 1 particle / 1km 2 per year. Particles
with the highest energies 1020 eV are detected only rarely in a few years.
The energy
representation of particles, ie the spectrum of primary
cosmic radiation , is schematically shown in the left
half of Fig. 1.6.6 on a logarithmic scale. The shape of this
spectrum is sometimes compared to the shape of an outstretched
human foot: after a more or less uniform decrease in the number
of particles to energies of about 10 15-16 eV, a bend-like bend appears on the curve, behind which
the decrease in the number of particles with energy begins to be
slightly faster to to very high energies approx. 10 18-19eV, where the loss
of particles starts to be a bit slower again - a kind of
"ankle" and "instep" appear on the shape of
the curve. This slowdown is somewhat surprising, as from an
astrophysical point of view, an even faster decrease in frequency
could be expected in the highest energy region, partly due to
pionic interactions of energy protons with cosmological relic
radiation (GZK-limit, see below). However, the frequency of these
highest energy particles is very small and the quantification of
energy is difficult here, so that due to statistical fluctuations
the frequency and energy in this region may be overestimated. The
problem remains open for the time being, with decisive results
expected from measurements of a large number of cosmic ray
showers at the AUGER observatory (see below).
![]() |
|
Fig.1.6.6. Left: Energy spectrum of primary cosmic rays. |
Right:
Reduction of the energy of high-energy proton radiation by interactions with relic photon radiation depending on the distances traveled in space. |
Cosmic ray
propagation; GZK limit
The directional distribution of cosmic rays is almost isotropic
, which is related to the complex curved orbits of charged
particles in magnetic fields within the galaxy and in
intergalactic space. The curvature of the path is directly
proportional to the charge of the particle and the intensity of
the magnetic field and indirectly proportional to the mass of the
particle and thus its energy - the so-called Larmor
radius of the circle along which the motion takes place.
The particles we capture have undergone very complex
curved orbits on their way to Earth , which
unfortunately loses directional information
about the source in which they formed *). Only the most energetic
particles (above 10 19eV) have a sufficiently large Larmor radius of curvature
(of the order of kiloparsecs) and largely retain their direction
to allow their approximate location ; since no
increased number of such particles are observed coming from
around the plane of our Galaxy, these high-energy particles are
probably of extragalactic origin .
*) We can compare it to an arrow
fired at a greater distance in a strong wind. During the flight,
the wind "plays" with the arrow and changes the
direction of its flight. So at the point of impact, it can be
difficult to determine the direction from which it was originally
fired. If the charged particles of cosmic radiation have low
energy, they can arrive in the opposite direction, even in the
opposite direction than they were sent, in a short time of flight
in the cosmic magnetic field. At very high energies, the
curvature of the trajectory is small, the particle keeps its
direction - just like a bullet fired from a rifle, compared to an
arrow, it is only slightly affected by the wind.
In addition to
the curved orbit, there is also a gradual loss of energy
during the propagation of charged particles of cosmic radiation
through space.interactions with photons of relic microwave
radiation (Fig.1.6.6 on the right), in which these particles lose
energy by inverted Compton scattering . At
sufficiently high energies - higher than the so-called GZK
energy limit *), which for protons is about 5.10 19 eV, a collision with
a photon of relic radiation even leads to the production
of a pion by reactions p + g 2.7 ° K ® p + p o , p +
g 2.7
° K ® n + p + (the neutron then changes again to a proton
+ electron + by b- conversion neutrino) **). These processes, associated
with a significant loss of kinetic energy (approximately 2.10 8 eV per interaction),
are more intense the higher the energy of the particle, which
means that no matter how high the energy of the particle was at
the beginning (for example 10 20 -10 22 eV), after overcoming a distance of about 100-200 Mpc
with gradual collisions with relic photons to form p- mesons will
reduce the energy to the value of GZK-energy ( » 5.10 19 eV); below this
limit, the effective cross section for pion formation is already
very small and the braking of charged particles by relic
radiation is considerably slower - it takes place only by
inverted Compton scattering.
*) This energy limit is so named after K.Greisen,
G.T.Zacepin and V.A. Kuzmin,
who studied the interactions of high-energy protons of cosmic
radiation with photons and determined the energy above which p- mesons
are efficiently produced in this interaction by
the reaction p + g 2.7 ° K
® p + p
o , resp.
by an analogous reaction to the formation of a neutron (Fig.1.6.6
on the right). **) It may seem strange that the photon of relic
radiation, which is a relatively long-wave microwave radiation
corresponding to a temperature of 2.7 ° K, has been termed
"gamma radiation" ( g 2.7 ° K)! However,
this is justified by the effects of the special theory of
relativity. The cosmic ray particle moves at a relativistic
speed, so that the photons of relic radiation from the point of
view of its rest frame have such a large blue Doppler shift that
they become gamma-photons , which interact with
the "photonuclear" reaction to form a pion. The
reaction occurs via D + : p + g 2.7 ° K ® D + ® p + p o (analogous to neutron), where the GZK limit is given by
the threshold energy for the formation of D + and subsequently pions: it is the energy of the primary
proton at which in the resting system of the proton (or the
center of gravity system) the photons of relic radiation reach
this threshold energy.
Thus, a large
portion of high-energy particles gradually " slow
down the relic radiation " - when such a particle
has an initial energy higher than about 5.10 19 eV, it loses this
high energy very quickly.
Note: On the one
hand, it is a great pity for the " astronomy of
cosmic rays ", which thus loses an interesting
observation "window" into turbulent processes in outer
space. On the other hand, relic radiation may protect us
from high-energy particles from outer space (see also section
below).').
This analysis also shows that the
cosmic ray particles that have energy higher than » 5.10 19 eV must come from an
area more than » 50 ¸ 100 Mpc, and explain this
is difficult, suitable nearby sources capable of producing
particles about such a large energy, we do not know
(apart from the unproven hypothetical
possibilities mentioned in point 3. " Energy
interactions of exotic particles " of the following
paragraph on the formation of cosmic rays) ...
How does
cosmic radiation arise ?
Due to the above-mentioned facts about the energy spectrum and
the nature of propagation, the explanation of the mechanism of
cosmic ray formation is very difficult and encounters
considerable difficulties. Potential sources of cosmic
radiation and the mechanisms of its formation can be
divided into three categories :
1. Continuous acceleration
Since normal particle interactions do not produce as high energy
particles as observed, the appropriate " cosmic
accelerator " must be detected . E. Fermi proposed
the mechanism of a certain continuous or diffuse
acceleration during repeated interaction of particles
with moving large clouds of ionized gas (either within the galaxy
or intergalactic gas, or in galaxy collisions), with the
interaction of magnetic and electric fields. The magnetic field
must be either very strong (for neutron stars) or very large
(radio lobes of active galaxies).
2. Catastrophic astrophysical processes
The high energies of cosmic ray-forming particles suggest that
this radiation probably does not arise during the normal
equilibrium processes of star and galaxy evolution, but rather
during the cataclysmic processes associated with the release of
extreme amounts of energy. During these processes, an electric
field with a high potential of the order of up to 10 19 V can be generated
. The particles are usually accelerated here once
. Two types of such "catastrophic" processes could be
the source of cosmic ray energy :
3. Energetic interactions of exotic
particles
There has also been speculation about the possible formation of
high-energy cosmic radiation during the decay of hitherto unknown
very heavy particles with a long lifetime. Occasionally, such a
particle disintegrates (spontaneously or by interaction with
another particle), emitting high-energy particles. The possible
existence of these hypothetical superheavy particles
with rest masses up to 10 24 eV is predicted by some so-called supersymmetric
theories (magnetic monopoles, domain walls, cosmic strings
...). According to some hypotheses, there could be extremely
energetic neutrinos in the universe(perhaps also of
relict origin after stormy processes during the Big Bang), which
could form Z bosons during collisions with other (slow) neutrinos
o weak
interactions, the decay of which could also form protons and
electrons with high energies up to 10 21 eV.
Processes of this kind could take
place everywhere in space, including near Earth. These
hypothetical mechanisms could then explain the observed cosmic
ray particles with the highest energies , which
due to interactions with relic radiation (see the above-mentioned
GZK limit ) could not retain this energy during a
journey from outer space.
Particles with very high energies
could also form in the final phase of quantum evaporation of a
black hole (Hawking effect), with a hypothetical quantum
exposure of a black minidire - see §4.7 " Quantum
radiation and thermodynamics of black holes " of the book "Gravity, black holes and
space-time physics".
This whole third category of
possible sources of cosmic radiation has no support in
the results of observation or experiment.
Is sadly have to admit that the question of the origin of cosmic rays , particularly its components with the highest energies yet not definitively clarified (some light into this issue could bring new complex method of observation of cosmic rays, mainly built extensive facilities AUGER - see below) .
Cosmic X
and gamma radiation
In addition to corpuscular ionizing radiation, ionizing radiation
of a wave nature also comes from space - electromagnetic X-rays
and gamma rays.
Using X-ray satellite detection (" X-ray telescopes ") , a large number of X-ray
sources have been observed in space. X-rays are created
in space during various processes. It can be synchrotron
radiation emitted by relativistic electrons moving in a strong
magnetic field, bremsstrahlung, radiant recombination of atoms in
an ionized gas. Flashes of X-rays can occur when thermonuclear
ignites hydrogen accumulated by accretion from a red giant to a
white dwarf in a tight binary system. A large amount of X-rays is
created during the accretion of matter to neutron stars and black
holes, when in the inner parts of the accretion disk the gas is
heated to such a high temperature that it also emits X-rays. Due
to turbulence and shock waves in the accretion disks, this X-ray
radiation has an irregular, rapidly changing intensity. A certain
amount of X-rays comes from all directions in space and is
referred to as the X-ray cosmic background. Previously,
it was thought to be scattered, diffuse, continuous radiation of
a similar type to microwave relic radiation. However,
improvements in the resolution of X-ray telescopes have shown
that it is not a continuous cosmic background, but a set of
millions of separate individual sources spread across
the sky (which earlier instruments could not distinguish from
each other). According to astronomers, these sources are probably
the active nuclei of galaxies , in the center of
which is a supermassive black hole with a massive accretion disk,
from which radiation is emitted in the X-ray region of the
spectrum.
Gamma radiation comes from space
(each time from a different place) in the form of relatively
short flashes of g radiation
, abbreviated GRB (Gamma Ray Burts ),
whose duration ranges from tenths of a second, through units and
tens of seconds, sometimes to minutes. Short and long flashes
differ spectroscopically. The radiation energy g is observed in the
range of about 100keV to several MeV; it is interesting that
short flashes of radiation g with a duration of less than about 2sec. they contain
relatively more high-energy radiation than long flashes. Flashes
of g radiation
are usually accompanied by " afterglow
", in which the energy is reduced to X-rays, then to visible
light and finally to radio waves.
Origin of flashes of radiation ghas not yet been
clarified with complete certainty. The most likely sources of
GRBs could be "catastrophic" processes - supernova or
hypernova explosions, black hole accretion, or collisions and
fusions of compact structures such as neutron stars
(these would probably be short flashes of g ) . The mechanisms of these high-energy processes in space
are briefly discussed in §4.8 " Astrophysical
significance of black holes ",
passage " Binary
systems of gravitationally bound black holes. Collisions and
fusion of black holes . " Of the book Gravity, Black Holes and the Physics
of Spacetime . Self radiation git does not arise
directly in the region of a black hole or a neutron star, but in
a surrounding disk of remaining (or ejected) material in which
jets with a velocity close to the speed of light cause shock
waves . Gradual deceleration of the jet wave wave upon
interaction with the surrounding material can then lead to the
emission of "afterglow" with gradual degradation of
energy from g- radiation to X-ray, visible light and finally to radio
waves. It is likely that every time an intense gamma-ray burst in
space, a stormy "catastrophic" event occurred somewhere
in the depths of outer space - a supernova
exploded , a black hole was born , or " neutron
stars " collided in a wild circle and merged
into two neutron stars .
Next to g radiation must occur in such processes also issue
massive amounts of high-energy particles, i.e. cosmic radiation
in the truest sense. However, no flashes of corpuscular radiation
following the g- flash with an appropriate time delay were observed. This
is because the orbits of charged particles are deflected and
scattered in all possible directions by galactic and
intergalactic magnetic fields, so that they either do not
penetrate us at all or their "diluted" flux merges with
the overall cosmic background.
The possible threat
to life on Earth by an intense flash of g- radiation and a
subsequent spray of corpuscular radiation from a nearby cosmic
source is discussed in the passage " Cosmic radiation and life" at the end of this §1.6.
Cosmic X and g radiation do not
penetrate the Earth's atmosphere and must therefore be detected
in space, using instruments placed on satellites (see below" Cosmic radiation detection ") *). Although this radiation is weaker than other
components of the primary cosmic radiation, especially than the
proton component, provides important information about turbulent
processes in outer space (see also the
above-mentioned mention of the possibilities of testing
quantum-gravitational effects " Is high-energy g-
radiation moving slower than
light? ") .
*) In hard g interactions-radiation with
the upper layers of the atmosphere, however, produces sprays of
electrons which propagate in the atmosphere at a speed higher
than the speed of light in this environment. This creates bluish
flashes of Cherenkov radiation, which can be detected by
sensitive photomultipliers placed in the focus of large mirror
telescopes. If there is a whole matrix of photomultipliers in the
focus of the parabolic mirror, it is possible to display the
place in the atmosphere where the Cherenkov radiation originated,
to reconstruct the electron spray and possibly determine the
source of primary radiation from space; cosmic g- radiation
propagates straight through space (as opposed to charged
particles), so by reconstructing the direction of the electron
spray on the basis of Cherenkov radiation, it is possible to
determine the place (direction) from which the cosmic g-The radiation
comes from. Since 2004, the MAGIC ( Major Atmospheric Gamma
Imaging Cherenkov ) telescope with a segment mirror diameter
of 17 m has been in operation on the Canary Island of La Palma at
an altitude of 2200 m , with an imaging system of 576
photomultipliers.
Secondary cosmic rays
As primary cosmic rays pass through the Earth's
atmosphere, there are a number of interactions
with air particles, creating secondary cosmic
rays. Photons of bremsstrahlung are formed, and fragmentary
reactions of atomic nuclei occur . The
interaction of high-energy primary protons p with nucleons N (in
the nuclei of nitrogen, oxygen, carbon) creates energy protons,
neutrons, p- mesons: p + N ® p + N + p + + p -
+ p
o + ....
The resulting p ±-mezons
are unstable, they immediately decay (with a half-life of » 2.5.10 -8 s) into muons m ± and neutrino: p - ® m - + n ' m , p + ® m + + n m
(neutral p o -mezons with a very
short half - life » 10 -16 s they decay into two quantum gamma: p o ® g + g) .
Muons are also unstable, but their
half-lives are » 2.10 -6 s is 100 times longer than pions, so many muons fall to
the earth's surface (this allows the effect of relativistic time
dilation - see passage "Muons" §1.5 "Elementary
particles"). Muons m ± decay
into electrons e ± and neutrinos *): m
- ® e - + n ' e + n m , m + ® e + + n e + n ' m , while the resulting electrons and positrons have a
kinetic energy up to 50MeV .
*) During the decay of pions and
muons, muon and electron neutrinos are formed; they are sometimes
referred to as " atmospheric neutrinos
". The total balance of neutrinos arising
from the decay of pions p ± and
subsequently mions m ± leads
to the ratio of the number of muon and electron neutrinos n ( n m ): n ( n e ) = 2: 1.
Confrontation of this expected ratio of "atmospheric"
neutrinos with the actually measured proportion of neutrinos in
experiments Super KamiokaNDE made it
possible to experimentally prove the so-called neutrino
oscillation - see §1.2, part "Radioactivity
beta", passage " neutrino ".
Interactions
of primary cosmic radiation with the atmosphere most often occur
at a height of about 30 km. The released particles often still
have high energies, so they are able to further fragment the
nuclei. With the precipitation, more and more particles are
formed in cascades , the reaction branches off
until the energy of the secondary particles falls below about 80
MeV, when the interactions no longer lead to the formation of new
particles, but only to their absorption. The whole spray
of cosmic secondary radiation , most often containing
electrons e ± , photons g , muons m ±and a
smaller number of high-energy protons and neutrons (Fig.1.6.7).
Tens of thousands to millions of secondary radiation particles
can be formed from a single high-energy proton of primary cosmic
radiation. A significant part of electrons, positrons and gamma
photons at low altitudes is formed by the decay of muons
m .
Secondary cosmic radiation is sometimes divided into a soft
component (e ± , g with energy up to 100MeV - electron-photon spray
) and a hard component ( m ± , smaller amount p ± , p + , with
energy higher than 500MeV - muon and hadron spray). At
the Earth's surface, a spray of cosmic rays often covers a large
area of many square kilometers.
Note: The high
permeability of muons is due to the fact that they have
about 200 times higher rest mass than electrons and show only
electromagnetic and weak interaction (unlike protons or pions,
which can interact strongly and atomic nuclei).
These large sprays of secondary
cosmic radiation in the atmosphere were first detected by Pierre
Auger in 1938 in the Alps at an altitude of around 3 000 m.
Fig.1.6.7. The interaction of high-energy particles of primary
cosmic radiation with the Earth's atmosphere creates sprays of
secondary cosmic radiation.
Cosmogenic
radionuclides
One of the side effects of cosmic radiation is the activation
of some nuclei with the formation of natural
cosmogenic radionuclides (eg 14 C, 3 H) - Fig.1.6.7 on the right. Most important is radiocarbon
14 C to produce the effect of neutrons
ejected cosmic rays from nuclei of atoms on the nitrogen in the
higher-layer earth's atmosphere n a + 14 N 7 ® 14 C 6 + P + . This creates about 2 atoms of 14 C per second per 1 cm 2 of atmosphere. Carbon 14 C, as a long-term radionuclide (T
1/2 = 5730 years,
pure b - , energy 158keV)
constantly contaminates the
biosphere , oxidizes to 14CO 2 in the atmosphere , enters the biocycle (photosynthesis
enters plants from the atmosphere, then food into animal bodies)
and is therefore contained in all living organisms. The
concentration of 1 atom of 14 C is set at about 8.10 13 atoms of ordinary 12 C; one gram of natural carbon in all living organisms
contains an activity of about 0.25 Bq 14C. After the death of the organism, its metabolic
contact with the atmosphere and the supply of 14C interrupts, so that
the concentration of radiocarbon begins to decrease by its
radioactive b- decay with a half-life of 5730 years. This also changes
the relative proportion between the carbon isotopes 14 C, 13 C and 12 C. The carbon-carbon
dating method (also called carbon
chronometry ) is based on this :
From the ratio between the relative proportion of 14 C radioactive
isotope and stable carbon isotopes in the studied historical
subject origin (for example wood, remains of organisms, etc.) we
can approximately determine the age of this
object - the time that has elapsed since the death of the
organisms from which the object originated. The radiocarbon
dating method and other dating methods in geology (using other
long-term natural radionuclides) are described in more detail in
§1.4 " Radionuclides ",
section " Natural radionuclides , passage" Radioisotope (radiometric) dating ".
Less important is cosmogenic
tritium 3 H (T 1 / 2 = 12.3 years, pure b - ,
energy only 18keV, it is formed in the amount of about 0.25 atom
/ cm 2 /
s), which oxidizes to "heavy" water in the atmosphere 1H3HO, which reaches the earth's surface with rainfall.
Some other cosmogenic radionuclides are also formed in very small
amounts - eg 7,10 Be, 32 P, 35S, 36 Cl.
Cosmic
radiation detection and spectrometry
The motivation for detecting and analyzing cosmic rays comes from
three different areas :
¨ Astrophysics :
Cosmic rays provide us with useful information about processes
in outer space , often the most tumultuous processes in
star death by gravitational collapse (supernova explosions) or
accretion of matter to a black hole (in quasars).
¨ Nuclear and particle physics:
In cosmic rays we encounter particles of the highest
energies, by many orders of magnitude higher than the
energies that we will be able to create on Earth 's accelerators
in the foreseeable future. The interactions of these high-energy
particles can provide important insights into the internal
structure of particles and the properties of their interactions.
During proton interactions at very high energies, for example, new
states of quark-gluon plasma may appear (which do not materialize at unit energies and tens of
TeVs in terrestrial accelerators) ..? ..
¨ Influence of cosmic radiation on
nature and life:
Cosmic radiation is the most important a natural source
of permanent ionizing radiation from humans, animals and
other living creatures. It also probably played an important role
in the processes of chemical development of the universe, the origin
and evolution of life (See "Cosmic Radiation and Life " below) .
The detection and spectrometry of
individual types of ionizing radiation is systematically
discussed from a physical point of view in Chapter 2 " Detection
and spectrometry of ionizing radiation
". However, the detection of cosmic radiation has some
significant specifics, which are more appropriate to discuss at
this point in the discussion of cosmic radiation. From the global
point of view, the issue of cosmic ray detection can be divided
into two areas:
1. Direct detection of
primary cosmic radiation; 2. Detection of secondary cosmic rays .
Depending on where we perform the
detection, we have three options:
a) Ground detection; b) Atmospheric detection; c)
Detection in space .
The individual methods of cosmic ray
detection will be briefly discussed below.
Detection
of primary cosmic radiation
The possibilities of direct detection of primary cosmic
radiation, especially particles with the highest energies, are
very limited for us for three reasons: 1. Cosmic
radiation already interacts with atoms in the upper atmosphere; 2.
Low flux density of high energy particles; 3.
Low effective cross section of the interaction of high-energy
particles with the detector material.
From the standpoint of
methodology of detection are primary cosmic ray
particles suitable two types of detectors :
× Photographic
emulsions, mist and bubble chamber
,
stored in the magnetic field recorded tracks of particles (described in §2.2, the " track detectors particles') . In particular, photographic
emulsions were handed out balloons to great heights, then caused
a trace particle analysis to obtain a series of important
information about the composition of the primary cosmic radiation
and partly also energy particles.
× Comprehensive electronic
detection systems of particles ,
including semiconductor or ionization trackers ,
spectrometers and calorimeters (see §2.1,
section " Arrangement and
configuration of radiation detectors ") The spectrometric system
is located in a magnetic field, the charge and momentum of the
particles can be determined from the curvature of the charged
particle tracks. smaller and significantly lighter
so that they can be brought into orbit. On the other hand, this
requirement significantly reduces the detection efficiency and
spectrometry possibilities, especially for the highest energy
particles. This limits the detected energy range to a maximum of
hundreds of GeV; there are still relatively many such particles
and they are able to lose the necessary part of the energy in a
not very massive detector. If such an electronic detection system
is located on a space satellite (Fig. 1.6.8 on the left), it can
transmit data on the type, energy and interactions of cosmic ray
particles for a long time. X- ray telescopes are
used to image X-ray sources (section "
X-ray telescopes " at the end of §3.2) .
Special ones are used to identify sources of hard gamma
radiation from spaceCompton telescopes (see §4.2, section " High energy gamma cameras ") .
From the point of view of the
detection site, primary cosmic radiation can be detected in two
ways :
l Detectors placed
on balloons ,
able to ascend to a height of tens of kilometers, perform
measurements there and then descend to the ground again
(Fig.1.6.8 left). Mostly special film emulsions were installed in
the balloons, later also electronic detectors.
l Detectors in
space on space probes (satellites)
Compared to balloons, detectors on space probes have two main
advantages :
1. They actually detect primary particles, without being
affected by interactions with the atmosphere; 2.
They can work for a long time. The most suitable are the
above-mentioned electronic multidetector systems
(Fig.1.6.8 left), with automatic radio sending of
measured signals to the terrestrial coordination center.
The Russian-Italian
project is currently preparing a PAMELA
spacecraft (Payload for Antimatter-Matter Exploration and
Light-nuclei Astrophysics) for the detection of particles and
antiparticles in cosmic rays and the measurement of their
energies. It contains a magnetic particle spectrometer
(magnetic induction 0,4T) with a silicon pixel tracker
, an absorption spectrometer
("calorimeter") consisting of absorption layers of
tungsten interspersed with silicon secondary radiation detectors
and alsohadron detector consisting of helium
ionization tubes for the detection of neutrons and protons (there
is a 3 He
isotope in the tubes , which has a high effective cross section
for neutron capture, slowed down in a polyethylene moderator
surrounding the tubes).
The issue of direct detection of
primary cosmic radiation outside the Earth is complex, but future
detection systems will certainly yield interesting results.
However, our terrestrial nature provides us with an effective
"means" for detecting cosmic radiation: such an
"detector" is the Earth's atmosphere.
By interacting with atmospheric atoms, the high and
difficult-to-detect energy of a primary particle is
"comminuted" into a large number of secondary particles
with energies that are easier to detect, even by ground-based
detectors. Furthermore, high-energy radiation (primary particles,
but mainly secondary particles in the spray) causes light effects
(Cherenkov radiation, fluorescent radiation of excited atoms) as
it passes through the atmosphere - the atmosphere can serve as a
huge " scintillation detector " of
primary cosmic radiation. Thanks to these two mechanisms, the
detection of secondary cosmic radiation, discussed in the
following paragraph, can also say a lot about the properties of
primary cosmic radiation - it can serve as an indirect
detection of primary cosmic radiation .
Fig.1.6.8. Possibilities of cosmic ray detection.
Left: Detection by space probes and balloons. Right:
Ground detection of secondary cosmic rays.
Detection
of secondary cosmic radiation
Individual quantities of secondary cosmic radiation are commonly
detected by ionization, scintillation and semiconductor
detectors, they form part of the natural radiation
background (often undesirable). However, one simple
detector is not enough for a more complex analysis of entire
secondary cosmic ray showers , more complex detection
systems are needed . There are basically two ways to
proceed (according to Fig. 1.6.8 on the right) :
Fluorescent radiation, which arises during the passage of cosmic radiation through the atmosphere, can in principle be detected from the "opposite side" - from space , using space probes, whose sensitive light flash detector, photodetector, is directed into the Earth's atmosphere (schematically shown in Fig.1.6.8 left).
AUGER
detection system
For efficient and comprehensive detection of (secondary) cosmic
ray showers was now built in the Argentine steppe in the province
of Mendoza an extensive detector system called Pierre
AUGER (according to French
physicist Pierre Auger, who first detected cosmic ray showers in
1938 and who also discovered electrons emitted during the
internal conversion of characteristic X-rays in excited atoms). In international cooperation (led by J. Chronin and A.
Watson), a large number of cosmic ray spray detectors it was
deployed here on an area of about 3,000 km2. The northern branch is in the preparation
stage AUGER project in Colorado, USA. The northern and southern
branches of the observatory will allow the observation of almost
the entire sky, especially the core of the Galaxy from the
southern hemisphere and the extragalactic structure observable
rather from the northern hemisphere.
The AUGER Observatory is a substantial improvement and extension
of the basic scheme according to Fig.1.6.8 on the right. It works
as a hybrid detection system : cosmic ray
showers record systems of two different types of
detectors (all of these detectors are electronically
interconnected):
Cherenkov's fast
charged particle detectors
incident on the earth's surface, formed by tanks with water,
where flashes from the passage of particles are captured by
photomultipliers. A ground network of 1600 of these detectors
will be created at distances of 1.5 km. Each contains 1200 liters
of high purity water and 3 photomultipliers.
Atmospheric
fluorescent telescopes
detect flashes of fluorescent radiation that are generated when
secondary cosmic ray spray particles pass through the Earth's
atmosphere. Charged particles of cosmic radiation ionize
and excite molecules in the atmosphere (especially nitrogen) as
they pass, and they emit visible light and UV radiation as they
return to their ground state. These flashes of fluorescent
radiation, lasting on the order of microseconds, are detectedoptical telescopes with a high luminosity and a viewing
angle of 180-360 ° (they have the shape of a "fly's
eye" from many mirror segments), equipped with
photomultipliers or CCD detectors. 24 of these detectors is
deployed, each with a collection area of 3.6 x 3.6 m 2 and 440
photomultipliers.
The Pierre Auger
Observatory detects secondary
cosmic rays, but with the main goal of analyzing primary
cosmic rays , especially those with the highest
energies. Analysis of data from a number of telescopes located at
different locations in the system, in correlation with data from
ground Cherenkov detectors, could provide geometric
(stereoscopic) and energetic reconstruction of the spray,
which could also contribute to the kinematic reconstruction of
the direction of the primary high - energy
quanta of cosmic rays - and thus find out where
they come from space.
Is cosmic radiation coming randomly
from all directions, or are there any significant directions
corresponding to certain specific sources in space? As mentioned
above, the cosmic ray paths of low and medium energies are very
curvilinear (curved by magnetic fields) and it is impossible to
determine the direction of their source. Only particles with very
high energies above 10 19 eV are able to maintain their direction ; the AUGER
project focuses on these.
It is hoped that a significant
number of high-energy particles will be able to find their
direction of arrivaland identify it towards one of the
observed supernovae or a galaxy with an active nucleus (quasar -
a massive black hole) inside it. Such observations could
significantly move us to finding sources and
explaining the mechanisms of cosmic ray. At the
same time, high-energy cosmic rays could become a new
"observation window" into turbulent processes in space.
In addition to optical, radio, infrared, X-ray and gamma
astronomy, a new branch of particle astronomy is
gradually emerging - cosmic ray astronomy.
Detection and spectrometry of
secondary cosmic rays, derived from the interaction of the
highest energy primary protons, can also be useful for nuclear
and particle physics. It can reveal new mechanisms of
high-energy interactions of protons (with the participation of
quarks and gluons), unattainable in terrestrial accelerators in
the foreseeable future.
Cosmic
radiation and life
In addition to the important information that cosmic radiation
carries about the properties of elementary particles and
phenomena in outer space, cosmic radiation is also interesting
for its relationship to the phenomenon of life - its biological
significance . It probably played an important role in
the origin and evolution of life , in at least
two ways :
Cosmic radiation, together
with the then high level of radiation from natural radionuclides,
also took care of the development of effective repair
mechanisms of cells against radiation damage (see §5.2.
Biological effects of ionizing radiation ).
Cosmic radiation forms an important
part of the natural radiation background to
which life on Earth is exposed *) from its inception to the
present day. The flow of cosmic ray particles is about 200
particles / m 2per second (at low altitudes), the average annual
effective cosmic radiation dose for humans is about 0.4 mSv.
However, the dose rate from cosmic radiation depends on the
altitude - at sea level it is about 0.3 mSv / year, at 1000
meters above sea level 0.45 mSv / year, at a height of 5km about
2mSv / year, in 8km it is already about 10mSv /year. It also
depends on latitude - due to the magnetic field of the Earth, it
is larger in the region of the poles and smaller at the equator.
*) Protection
of the biosphere from external ionizing radiation
Some mechanisms that protect life from the
adverse effects of ionizing radiation are important for life on
Earth . Ozone in the upper atmosphere protects
us from ultraviolet radiation from the Sun. A massive layer of
the atmosphere does not transmit X-rays and
gamma rays; strongly slows down and absorbs even high-energy
quantum. The Earth's magnetic field
protects us from particles of the "solar wind" (these
have energies significantly less than "true" cosmic
rays, but high intensity - they could, among other things,
destroy the ozone layer). Microwave relic radiation
in space may protect life from high-energy particles of primary
cosmic radiation coming from outer space (this protection by
relic radiation would only be more important if cosmic rays with
the highest energies above 10 19 eV were sufficiently frequent - which is not expected.
.).
Deadly
cosmic rays ?
So far, we have focused on the "peaceful" cosmic rays
that come from space with an almost constant average intensity
for millions of years. However, in the passage on cosmic X and
gamma rays, we have already mentioned the intense flashes
of g- rays that we observe coming from space
(fortunately distant). These flashes, as well as other
astronomical observations and theoretical analyzes (see Chapter 4 " Black Holes
" in the book " Gravity, Black Holes and the Physics
of Spacetime ") , show that very turbulent and catastrophic
processes are taking place in space , during which a
huge amount of radiant energy. These are supernova
explosions, gravitational collapse and formation of
black holes, collisions or fusions of neutron stars and black
holes. These processes are through their massive glowing speeches
astronomically observed not only in the distant universe
, which on one hand makes it difficult to analyze and correct
understanding, on the other hand, a huge attenuation of radiation
intensity long distances and scatter by intergalactic and
galactic magnetic fields on charged particles protects
us here on Earth from the dangerous effects of hard radiation.
But what would happen if a similar
energy process took place in our Galaxy in one of the relatively close
ones stars (several tens or hundreds of light-years
away)? The immense amount of radiant energy that would strike the
Earth would probably be a huge natural disaster
that could seriously jeopardize the very existence of life here
on Earth! At first we would be struck by a short-lasting but
powerful flash of g- radiation , which would decompose a large part of the
molecules in the upper layers of the Earth's atmosphere with its
ionizing effects; Subsequent chemical reactions would produce
large amounts of nitrogen oxides, which, by their light-absorbing
properties, would darken the sky on the side facing the flash. In
addition, the ozone layer would be destroyed. Already these
atmospheric effects would have very serious ecological
consequences for plants, animals and the overall climate on
Earth.
Behind the flash of gamma radiation,
however, an even more destructive massive stream of corpuscular
cosmic rays would reach Earth in a few days , from
which, due to its relatively short trajectory through space, a
weak galactic magnetic field would not be enough to protect us;
it would take tens of days. Each such particle with an energy of
the order of GeV would cascade by interacting with atoms in the
atmosphere to cause a spray of energetic secondary radiation,
including muons. This radiation would penetrate the earth's
surface and even deep below the water surface and below the
earth's surface. The radiation dose would be many times higher
than the lethal dose for humans and other higher
organisms; only highly radiation-resistant species could survive.
High energy particles would further cause nuclear
reactionsin the atmosphere and on the earth's surface,
with radioactive nuclei forming , many of which
would have half-lives of many years (eg tritium), some even
millions of years. The earth's surface and surface and
groundwater would remain radioactively contaminated
for a long time and uninhabitable for a long time .
It is possible that such a " cosmic
radiation catastrophe " has already affected the
Earth in the distant past and may have caused the sudden extinction
of species *), or, conversely, the accelerated
development of new species by increased mutations. Some estimates
of the number of massive stars at the end of their evolution
(which then explode as supernovae) and the occurrence of close
pairs of neutron stars (which will gradually approach each other
due to losses of orbital energy by gravitational waves until they
finally fuse to emit a massive flash) show that Such catastrophic
events in the vicinity of about hundreds of light-years around
the Solar System could occur about once every 100 million years.
This estimate remarkably corresponds to the mean time (but again
only estimated) among the largest paleontological changes in the
early prehistory of the Earth.
*) However, there could have
been several reasons for the mass extinction of animal species
during the evolution of life on Earth, and some of them are
considered even more likely or more frequent - Earth collisions
with asteroids, tectonic crustal movements, volcanic eruptions.
The same applies to the possible threat to life
on Earth in the future - even here, a
destructive flash of cosmic radiation is a potential risk in the
distant future.
A relatively
early supernova explosion, over a time horizon of millions of
years, can be expected in very massive stars observed in the
so-called red giant phase.. Such "old" stars,
at the end of their lives, have already burned all the hydrogen
inside, their envelope has heavily "inflated" and
cooled (the "red giant"), and thermonuclear
"burning" of helium and other heavier elements occurs
in the shrinking nucleus. (For the
evolution of stars, see " Black Holes ") . This less
energy-intensive "fuel" is only enough for a few
million years. Once everything is burned, the star collapses
rapidly into a neutron star or black hole, with a huge type II
supernova explosion (one such
"endangered" relatively close to very massive stars is
the red giant Betelgeuse in the constellation Orion,
about 20 Suns away). Earth about 1000 light-years;. The largest amount of radiation is emitted in the
direction of the supernova's axis of rotation; potentially the
most dangerous are those nearby supernovae that face us at one of
their "poles."
How to protect
yourself from the deadly cosmic rays?
However, the question arises, how
can we protect ourselves from this danger ?
Unlike the impact of an asteroid, where the development of
observation methods and rocket technology in the near future will
hopefully avert such "local events", it will probably
never be in our power to prevent such massive processes as a
supernova explosion or neutron star collisions, tens or hundreds
of light. years away. The only way to save life on Earth would be
shielding cosmic ray beam. Shielding by terrain
barriers on Earth (mountains, canyons, opposite hemisphere in the
event of an explosion in the direction of the North or South
Pole) would not be very effective in the long run due to the
spread of the already mentioned radioactivity by atmospheric
flow. A suitable solution would be to shield the entire
Earth with a sufficiently thick shield
(thickness of at least hundreds of meters) placed in a suitable
orbit in outer space above the ground, in the direction of the
intended radiation source. Such a shield could perhaps be
assembled from asteroids that otherwise threaten us locally. At
present, such a project belongs more to the field of science
fiction. However, unlike asteroids, which can appear suddenly in
Earth's orbit, a supernova explosion or neutron star collision is
a "prepared" process that is predictable several
million years ahead. For the advanced civilization of the
future, which will certainly have in detail a
"mapped" of all potentially dangerous objects in the
surrounding parts of our galaxy, this could be a technically and
temporally feasible task. Another option would be to relocate
humanity to another part of the universe, hundreds or thousands
of light years away - but we are already entirely in science
fiction ....
A brief
reflection on the global perspectives of life in
space is in the passage " Astrophysics and Cosmology: - Human hopelessness? " §5.6 "The future of the universe. The arrow
of time." monograph "Gravity,
black holes and the physics of spacetime".
Back: Nuclear physics and physics of ionizing radiation | |||
Nuclear and radiation physics | Radiation detection and spectrometry | Radiation applications | |
With cintigraphy | Computer evaluation of scintigraphy | Radiation protection | |
Gravity, black holes and space - time physics Anthropic principle or cosmic God | |||
AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy |