|AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy||Physics and nuclear medicine|
Nuclear and radiation physics
1.0. Physics - fundamental natural science
1.1. Atoms and atomic nuclei
1.3. Nuclear reactions
1.5. Elementary particles
1.6. Ionizing radiation
1.6. Ionizing radiation
Radiation - an important natural phenomenon
|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
macroscopic 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
This radiative energy transfer can be performed by two types of mechanisms :
Radiation can spread either :
of information by radiation
The transmission of energy by radiation, owing to its structuring, 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 material 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 " and 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 are not able to eject electrons from atomic shells. They mainly show mechanical and thermal effects on the substance, 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 ionization of the substance occurs. 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 tumorous (§3.6 " Radiotherapy ").
- 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 also in the detection of ionizing radiation, in electronic sources of ionizing radiation (X-rays tubes, accelerators), as well as in the relevant measurement and control technology.
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, gamma, 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 Z2 -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 (for example in 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 of 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 "radiation" generally includes all types of radiation, not just ionizing radiation.
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 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
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 circumstance 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 substance (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 ".
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 mo of 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). All this types of radiation are
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 12C 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 p0 ) 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 - "ghosts" between particles ".
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 radiator or emitter . This radiators 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 "kilovoltage" . These include X-rays and most radiosotopes.
¨ High-energy , producing quantum radiation with energies of unit, tens or hundreds of MeV , GeV, up to TeV . In radiological jargon, they are also called "megavoltages" . These are mainly high-energy accelerators, occasionally some radioisotopes (such as 60Co).
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 sources - their size is much smaller than the distance at which we examine the emitted radiation or where the irradiated object is located.
¨ Line sources - 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 sources - the radioactive substance is applied in a thin layer on a substrate, usually in the shape of a rectangle or circle .
¨ Volumetric (spatial) sources - 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 a 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 ".
quantities characterizing ionizing radiation
The basic physical quantity characterizing ionizing radiation is the kinetic energy of its quanta (particles, photons). The properties of radiation during the interaction with substances - ionization, range, possible nuclear reactions, formation of secondary radiation, significantly depend on the energy of the emitted particles. The basic physical unit of energy joule [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 tube (§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 ", passage "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 - 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 drawn 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 fundamentally 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 quantity emission 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.m2 .s-1 ], which is the dose or kerm rate 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, event. 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 radiation 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).
*) Most light sources radiate almost isotropically, i.e. they have a spherical radiation diagram, eg small light bulbs (except for the direction of the base), as well as many stars in universe (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). At ionizing radiation sources, radionuclide sources emit isotropically (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
A quantum of radiation propagating from a radiation source, 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 flow - 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 2p , ie over the full
spatial angle 4p ) we obtain a spherical distribution I (E) = 0 ò
2p 0ò 2p I (E,
r, J , j ) d J d j , expressing the
total flow of particles (or energy flow) passing
per 1 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 also 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 connections of points with the same dose. Usually, isodose curves are plotted for certain percentages from the site with the 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 emission and propagation of radiation quanta in a vacuum or in the 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 consistently 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 (convergent, focused) shape of the radiation beam is required . 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 can (to a limited extent) only ba achieved by 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 source 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 / m2 ) emitted from a radioactive source such, is therefore directly proportional to the activity A in preparation, and inversely proportional to the square of the distance r from the source to the measuring point (this is exactly true for a point emitter, approximately in situations where the distance is significantly greater than the dimensions of the source) :
I = G. A / 4p 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 fluence 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 material 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 delimitation - 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 radiation only 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 penetrating radiation g, partial shine trough occurs at the peripheral edges of the collimator, which creates a kind of "half-shadow" in the edge parts of the collimated beam - 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 apertures (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 filter - §3.2 " X-ray diagnostics ") and in radiotherapy (eg multileaf 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 collimation of radiation (see eg " PET cameras " or "Compton cameras"). However, this is not a matter of delimitation 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.
of ionizing radiation during passage through matter
Under normal natural and laboratory conditions, matter (substance) is composed of atoms, which are eventually 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 separately 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 begin to describe the ways of interaction of specific types of radiation with substances of different composition, we will mention some general mechanisms applies 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 freely pass 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 outreach 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 Rmax is given. 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 themselves disappear during the photo effect.
- Electron b- is gradually braked 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 braked by collisions with envelope electrons, but after stoping it does not remain in the substance - it annihilates with the electron to form two opposing photons g (and those from the substance either fly out, or are absorbed).
- The proton p+, after its braking, "captures" the 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 cleave 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 "captures" two electrons and the substance 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.
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-, +, protons), or secondary charged particles fly out of the substance, the substance will be to charged electrically. 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.
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 in the study of the interactions of radiation with a substance, it is possible to apply the illustrative idea, that each atom of the irradiated substance behaves as an "absorbing body" of radius r, which the incident particle either hits and the given interaction occurs, or does not hit them (passes them, flies around) and the interaction does not occur. The larger the radius of this body, resp. its effective area s = p .r2 - 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
rgeom , or
their "geometrical cross section" s geom = p .r 2geom . For "effectively interacting" particles it
is s > sgeom , for weakly
interacting particles is s
. 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 m2 , which is, however, inadequately large, and therefore the unit barn (bn) is used in nuclear physics : 1 bn = 10-28 m2 , 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 ".
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 ~ ln E .
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 spray that 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
Any object that is irradiated with (primary) radiation generally becomes a source of weaker secondary radiation. During the interaction of ionizing radiation with matter, processes occurs in which secondary radiation of various kinds is generated :
¨ Braking radiation (bremsstrahlung) generated during the movement of mainly electrons and positrons in matter
¨ Compton-scattered g-radiation or X-radiation
¨ Diffracted neutrons
¨ Photoelectrons released from the atomic packaging due to the photoeffect of primary radiation
¨ Characteristic X-rays following after 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 interactions of primary radiation
¨ Mesons p and K, muons, or hyperons, formed by particle interactions of high-energy quanta of primary radiation
¨ Light radiation generated by deexcitation of electrons on the outer shells of the atomic envelope, during 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 creation will be discussed in more detail below.
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 reflectance diffusely 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). In the case of ionizing radiation, however, it 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%. Is caused mainly by Compton scattering, partly also by X-ray fluorescence radiation. Higher albedo, up to 40%, may be in neutrons.
of charged particles - directly ionizing radiation
First we will deal directly ionizing radiation, wherein the first mention common features of the interaction of this radiation at it passes through the substance, then we analyze the specific features of the interaction of radiation a, b+, - and proton radiation.
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 shells UV radiation, during excitation on the inner shells, then photons of characteristic X-rays (with spectral lines Ka,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 - this is a 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 / v2 . 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/v2 (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/mm] 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 by interaction with electrons
Dissipation (or "braking power") of the -dE /dx charged particles when passing through the substance is given by the co-called Bethe-Bloch formula (whose simpler variant derived N.Bohr (1913) - in the lower part of the formula in frame, complete more accurated variant then H.Bethe and F.Bloch (1930-33) - the upper part of the formula) :
This is here a primary particle with charge Q and mass M (M >> m e ), flying at instantaneous velocity v [relativistic designation b=v/c, g=1/Ö(1-v2/c2)] trough 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, me is the rest mass of the electron, e is the elementary charge of the electron. kinEmax 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].Z; for lighter atoms (Z <20) the empirical dependence I = 10[eV].Z0,9 was measured .
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 proportional Q2 / b2 and their energy distribution (spectrum) is :
where d2 Ne is the number of kinetic energy electrons Ee (Ee >> 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 units of MeV 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 Coulomb transfer 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 ".
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 :
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+
Very specific way interacts with the substance the b+ i.e. positron e+ radiation - in 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 positron e+ meets the electron e- , and since they are "antagonistic" antiparticles, they destroy each other ("eaten"): their annihilattion e + + e - ® 2g occurs - they are converted into two quantums of hard radiation g with energies 511keV *).
*) It is interesting that, according to the laws of quantum electrodynamics, there should be also the 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 from the annihilation site exactly in opposite directions - at an angle of 180o (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 the same, when we apply a radioidicator 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 - this is the basis of 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" below.
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 bit of humorous sci-fi story about the meeting of a "woman" with a "anti-man "...).
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 occur. 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 m and the charge q exerts electric Coulomb forces proportional to the product q.Z, 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 Z2 .q2 /m2 . 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 braking radiation 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 braking radiation 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 electrons Ee (tens to hundreds of keV) and proton numbers Z 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 / 1 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 substance. 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, Emax is the maximum energy of X-ray photons, given by the kinetic energy of the incident electrons. For E = E max , I (Emax ) = 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 e incident 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 material 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 at 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 braking radiation depends on the energy of the primary charged particles. The mean angle J of the emission of the quantities of braking radiation excited by the electrons with kinetic energy E e is approximately given by the relation J = m 0e .c2 / E e (= 0,511 / E e for the energy in MeV) . At low energies, the braking radiation 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 braking radiation 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 - ray tubes- 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 ") .
For 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 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, braking 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 strong 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 and m 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 (Larmor) 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 radiation with advantageous properties for some laboratory applications (see §1.5, section " Synchrotron radiation generators ") .
Cyclotron and especially synchrotron radiation also occurs in a number of processes in universe - 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... " in book "Gravity, Black Holes and the Physics of Spacetime ") .
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 Ka , Kb , 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 braking radiation spectrum. The characteristic X-ray is caused by two processes :
¨ 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. When electrons jump from the L shell to the emptied K shell (K-series), or from the M shell to the L (L-series), a characteristic X-radiation is then emitted (cf. also with Fig.1.1.3 in §1.1). ¨ Indirect process of photoelectric absorption of braking radiation - braking X-rays, produced by the above-mentioned mechanism during the passage of a charged particle, interacts with other atoms inside the substance, e.g 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 braking radiation at the beginning of the spectrum.
Fig.1.6.2. Mechanisms of characteristic X-rays, braking radiation, Cherenkov radiation and transient 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 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 will manifest, the particle "runs away" from a given place, 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 rapidly 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. During this time interval, spherical wavefronts gradually diverge from all other points of the path, which during this time 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. Into these sites the individual partial signals arrive in 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.Èerenkov 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 (Ekin= moc2/Ö(1 - v2/c2) - moc2 - 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 vmin charged particles for the formation of Cherenkov radiation when passing through a medium with a refractive index n is: Emin = mo.c2[1/Ö(1-1/n2) - 1]. For this case of the threshold velocity, cos Jmin = 1, ie Jmin = 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 vmax = c, the maximum radiation angle cos Jmax = 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 Emin = 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), by
radiation with angular frequency w = 2pf and in frequency interval
dw , is
given by Frank-Tamm equation
d2W = (q2.e/4p).w.[1 - 1/b2n2(l)] dl dw .
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 = (q2/4p) n [1 - 1/b2n2(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 =
per unit of path dl is
dN2/dl = (dW/dl).(l/h.c) = (4p2q2/h.c).n [1 - 1/b2n2(l)]/l2 dl .
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 dl , is thus given by the formula:
dN2/dl dl = (4p2q2/h.c). [1 - 1/b2n2(l)]/l2 .
Or equally, the number of photons Nl2¸l1 in the spectral region between the wavelengths l1¸l2 emitted along the path l makes :
Nl2¸l1 = (4p2q2.l/h.c). (1/l2 - 1/l1).[1 - 1/b2n2(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 with our eyes most of it, lies in the ultraviolet region . 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, braking radiation) 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 electrons with energy 9MeV 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), the secondary electrons along the g beam form the Cherenkov radiation - with a deep decrease in intensity, as the primary photon beam attenuates 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. ..
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
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, whether primary or secondary, caused
by the interactions of primary radiation with a substance - 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. Faint bluish flashes of light caused by cosmic rays, indeed are occasionally observed with astronauts with their eyes closed.
In addition to the above-described classical light Cherenkov radiation, which arises during the passage of electrically charged particles, in dielectric environment even at the passage of high-energy uncharged particles create a spray of fast secondary charged particles, which by the Cherenkov effect 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. ... .......
Another radiation-optical effect during the passage of fast charged particles through an inhomogeneous medium, is the emission of so-called 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 e1 and e2 of the two media differ . During the transition (rapid passage) of a charged particle through such an interface, according to Coulomb's law E = (1/4pe ) .q / r2 the intensity of the electric field around the particle changes very quickly from the value of E1(r) to E2(r), which according to Maxwell's equations of electrodynamics evokes 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 drouwn out by much more intense braking 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 only at many times higher energies than electrons.
Impact transition radiation
Transition radiation arises also when the impact of fast charged particles on the surface of bodies. 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 eo 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 tube. 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 at the 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 already in 1919 by J.E.Lilienfeld at the anode of the cathode ray tube - but it was 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 drown out 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 ".
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 led off from the irradiated body.
Self radioactive emitter a or b (if is electrically isolated) is also electrically charging, because these particles carry an electric charge and in the substance of the emitter then dominates 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,
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+. If the a-particle enters into substance, it exerts a considerable electric (Coulomb) force on the electrons as it passes around the atoms, which it very effectively pulls out of the atomic shells - Fig. 1.6.1 at the 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 a have sufficiently high energy (several MeV), as they pass through the substance they can overcome the repulsive electrical forces of the nuclei and enter into nuclear reactions with the atoms of the irradiated substance. Most often it is the reactions (a , n) in which they radiate neutrons . This process is used in radioisotope neutron sources : a -radioactive substance (most often americium 241Am) is mixed with a suitable material containing light atoms (most often beryllium), from the nuclei of which energetic a-particles set off the 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 of fast deuterium nuclei D = 2H1 composed of proton p+ and neutron n0) - Fig.1.6.1 top center. Fast-flying protons interact mainly with atomic shells, from which secondary electrons knock-out . 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 universe, 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 , 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 :
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
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 braking radiation. 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 stronger electrical action between the electrons is in the case of a "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 value of the collision 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 with 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 immediately 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 (outreach) Rs 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 in energy approximately linearly, for lower energies the dependence is somewhat flattened. The mean 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, 32P) in water. For medium energies around 500keV the average range in water is less than 1 mm, for soft b (as has 3H) the range is very small, of the order of hundredths of a mm.
*) Due to collisions and scattering, the trace of the particle b in the matter is 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 Rmax is more appropriate. 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 electron b will lose energy by exciting the electrons in the atoms. If this electron is not the trapped in some of the atoms, then its kinetic energy decreases to a thermal value of » 3/2 kT ( k is the Boltzmann constant), which at room temperature is only about 0.04eV.
High energy electrons from an accelerator (betatron or linear accelerator) with energies of unit 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 devious, ionization thickens and in the final section, before the complete stop of the electron, a very dense chaotic cluster of ionization is formed in the matter by ionization 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-radiation with a line spectrum determined by the 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
If a particles b+, which is positively charged positron e+, enter the substance, it will initially - as long as it has a high energy and moves at high speed - like b- during its flyby around atoms by Coulomb electric forces, pulling electrons from atoms, and due to its as small mass like an electron, it will again "zigzag" move and bounce between the atoms, which it will ionize while losing energy. It brakes and stops, 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 electrons occur.
After complete 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 m0e .c 2 = 511keV), the opposite direction 180° is a consequence of the law of conservation of momentum. In the case of collisions of positrons of higher energies with electrons, the angle of inclination of annihilation photons would differ from 180°. However, in the matter, the positron already have relatively low velocities at the moment of annihilation with the electron, so that the emitted quanta actually fly in almost opposite directions.
Just before the actual annihilation, the electron e- and the positron e+ can circulate each other (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 positronium 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 state 1S 0 with oppositely oriented spins - so-called parapositon p-Ps (1/4 cases), or in triplet state 3S 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 120 ps 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 140 ns. ; 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 a 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 22Na it is g 1274 keV) and detecting the annihilation photons 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 ".
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 ionize atoms by 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 core - 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 braking radiation of electrons is called 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 and more) is usually called gamma radiation, regardless of the way it is generated. Quantum of X and 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 the energies above 5MeV begin to take noticeably manifests the interactions with nuclei - photonuclear reactions (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 "redirected" 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, the binding forces of the electrons it is not enough "convert" the excitation 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 (Compton scatering). 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 the form of kinetic energy of the released electron.
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 substances (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.
The interaction of g and X radiation (for short we will in the next 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.
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 Eg 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) is a significant decrease in the resultant ("Total") effective cross section with an increase in energy Eg (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) , this decrease gradually stops and is replaced by a slight increase in the effective cross section due to the formation of electron-positron pairs e-e+.
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 EK 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, braking radiation, 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.
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. For radiation with a longer wavelength, the
quantum fluctuations of the metrics 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 universe.
Neutron radiation and its interactions
Under neutron radiation means 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 fission 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 241Am + Be, 239Pu + Be, 226Ra + Be, 210Po + Be). Energetic alpha radiation emits neutrons from beryllium nuclei by the reaction 9Be (a , n) 12C. 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 ".
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-"ghosts" between particles ".
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 ").
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 on the phenomenon of absorption in suitable materials is based shielding and collimation of ionizing radiation (see below "Ionizing radiation schielding ') .
Fig.1.6.5. Basic regularities 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 Io . Part of the radiation is absorbed, the intensity of the transmitted radiation denoted I . On what will depend the amount of absorbed and passed radiation? Of course, primarily on the thickness of the material d , where 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) d1/2 = 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 / d1/2 [g / cm2 ] 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 = d1/2 , exactly half of the particle-quantum is absorbed: I (d1/2 ) = I o / 2. At the end of the next half-layer, half of the half of the particles remain, ie a quarter: I (2.d1/2 ) = Io /4. And so on to infinity, so only in the limit d ® ¥ will the limit be I(d ® ¥ ) = 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 d1/2 ) 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 is takes into account only the photons that have 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 expressed in the exponential law of absorption by introduction of the so-called growth factor B: I = Io.B.e-m .d. The magnitude of the growth factor (B ³ 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.
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 s 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 1cm3 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 is sometime introduced, which is independent of density .
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 ").
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.
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 /cm3 ) 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" - 238U , 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) d1/2 = ln2 /m @ 0.693 / m , which is the thickness of the layer of shielding material that attenuates the intensity of the radiation by half (2 half-layers then 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|
The attenuation of the radiation intensity by the absorption layer of thickness d can be expressed by means of a half -thickness d1/2 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 about 1%, and 10 half-thicknesses below 0.1%.
To shield radiation b- are sufficient 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 braking 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 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 59Co is exposed to neutrons, the known radionuclide 60Co is formed by neutron capture, with a half-life of over 5 years !
Light of cosmic origin from the Sun and stars has been observed by humans since time immemorial. The first indication that invisible ionizing radiation was coming to us from universe, 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.Hess (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. When ascending to the first 800m, the ionization actually decreased, but more slowly than expected; on the contrary, the ionization then increased with a further ascending , 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 trace 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 by 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).
Stelar - Solar - "wind"
Inside the stars, the hot gases are held firmly by gravity. Nevertheless, by thermoemission and the pressure of the radiation release a smaller amount of gas particles (plasma) from the hot surface of the star, which are carried to the surrounding space. This stream of charged particles, especially protons, electrons and alpha-particles (helium nuclei), flowing from the star's surface into interstellar space, is called the stellar "wind".
This phenomenon is well known in our Sun - the solar "wind". Solar wind particles (with kinetic energy of about 0.5-10 eV) also hit our Earth. If the clouds of the solar wind hit the earth's surface fully, it would be dangerous for life. The long-term impact of solar wind particles on an unprotected planet would gradually "blow off" the atmosphere, with the subsequent evaporation of water from the surface - the planet would become inhospitable, without life (cf part "Stars-Planets-Life" in treatise "Antropic principle and/or cosmic Got").
Fortunately, our planet Earth has an atmosphere and a magnetic field. The orbits of charged particles curve in the Earth's magnetic field, and most particles deflect or bounce further into space - clouds of charged particles seem to "wrap" us along the curves of magnetic field lines. Only a small part of them enter the atmosphere, especially in the polar regions, where magnetic field lines approach the Earth's surface. In the upper layers of the atmosphere, a stream of solar wind particles interacts with nitrogen and oxygen atoms, causing their excitation and ionization. During deexcitation, light is then emitted, observed as aurora borealis.
Radiation from the Sun, the so-called solar component (consisting mainly of protons and 5-10% of helium ions), is usually not classified as cosmic radiation, due to its local importance and low energy.
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 109 eV - charged particles with low energies have difficulty penetrating the Earth due to the magnetic field of Earth. The upper energy limit of the cosmic rays registered so far is about 1020 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 large terrestrial accelerators .
*) In 1991, a particle of cosmic radiation with an energy of 3.1020 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 104 /sec./m2 ), there are very few high-energy particles - for energies of 1016 eV we observe only a few particles per 1 m2 in 1 year, for the highest energy around 1019 eV it is only about 1 particle /1km2 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 1015-16 eV, there is a bend in the curve apears as if the shape of "knee", after which the decrease in the number of particles with energy begins to be slightly faster up to very high energies of about1018-19 eV, 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 of occurence 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).
Left: Energy spectrum of primary
Reduction of the energy of high-energy
proton radiation by interactions with relic photon
radiation depending on the distances traveled in space.
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 detect 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 1019 eV) 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, in a short time of flight in the cosmic magnetic field, they can arrive even in the opposite direction than they were sent. 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 by interactions of photons with 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 5.1019 eV, a collision with a photon of relic radiation even leads to the production of a pion by reactions p + g2.7 °K ® p + po , p + g2.7 °K ® n + p+ (the neutron then changes again to a proton + electron + neutrino by b- conversion ) **). These processes, associated with a significant loss of kinetic energy (approximately 2.108 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 1020 -1022 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 ( » 5.1019 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 + g2.7 °K ® p + po , resp. by an analogous reaction to the formation of a neutron (Fig.1.6.6 on the right).
**) Microwave relic radiation gamma? 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" ( g2.7 °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 + g2.7 °K ® D+ ® p + po (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 "brakes againt a relic radiation" - when such a particle has an initial energy higher than about 5.1019 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 the section "Cosmic radiation and life" below).
This analysis also shows, that the cosmic ray particles that have energy higher than » 5.1019 eV must come from an area closer 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) ...
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 relevant "cosmic accelerator" must be uncover. 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 1019 V can be generated . The particles are usually accelerated here one-time. Two types of such "catastrophic" processes could be the source of cosmic ray energy :
3. Energetic interactions of exotic
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 1024 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 Zo bosons during collisions with other (slow) neutrinos weak interactions, the decay of which could also form protons and electrons with high energies up to 1021 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 distant 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 explosion of a black mini-hole - 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.
Unfortunatelly, it must to admit that the question of the origin of cosmic radiation, particularly its components with the highest energies, has not yet been definitively clarified (some light into this issue could bring new complex method of observation of cosmic rays, mainly extensive facilities AUGER - see below) .
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, braking radiation, 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 100 keV 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 g it does not arise directly in the inner 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 two neutron stars in a wild circulation collided and merged .
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 parhs 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 trough 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 that 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- radiation originate. 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 , in the fosus of with is an imaging system of 576 photomultipliers.
As primary cosmic rays pass through the Earth's atmosphere, there are a number of interactions with air particles, creating secondary cosmic radiation. Photons of braking radiation 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 ± - mesons 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 50 MeV .
*) During the decay of pions and muons, the 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(nm) : n(ne) = 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 collisions, 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, hits the Earth's sutface (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.
One of the side effects of cosmic radiation is the activation of some nuclei with the formation of natural cosmogenic radionuclides (eg 14C, 3H) - Fig.1.6.7 on the right. The most important here is radiocarbon 14C, whis is formed by the effect of neutrons, ejected by cosmic radiation from the nuclei of atoms of nitrogen in the higher-layer Earth's atmosphere: no + 14N7 ® 14C6 + p+. This creates about 2 atoms of 14C per second per 1 cm2 of atmosphere. Carbon 14C, as a long-term radionuclide (T1/2 = 5730 years, pure b-, energy 158keV) constantly contaminates the biosphere, oxidizes to 14CO2 in the atmosphere, enters into biocycle (by photosynthesis enters plants from the atmosphere, then by food into animal bodies) and is therefore contained in all living organisms. The concentration of 1 atom of 14C is established at about 8.1013 atoms of ordinary 12C; 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 14C, 13C and 12C. The radio-carbon dating method (also called carbon chronometry ) is based on this :
From the ratio between the relative proportion of 14C 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 3H (T1/2 = 12.3 years, pure b-, energy only 18keV, it is formed in the amount of about 0.25 atom /cm2/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 7,10Be, 32P, 35S, 36Cl.
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 to 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 treatise on 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) Terestrial detection; b) Atmospheric detection; c) Detection in space .
The individual methods of cosmic ray detection will be briefly discussed below.
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, for primary cosmic ray particles are 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. They are basically similar to particle detectors on large accelerators, but must be 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). To identify sources of hard gamma radiation from outer space are used special Compton 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 universe on space probes (satellites)
Compared to balloons, detectors on space probes have two main advantages :
1. Detect the real primary particles, without affecting 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 also hadron detector consisting of helium ionization tubes for the detection of neutrons and protons (there is a 3He 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 terestrial detectors. Furthermore, high-energy radiation (primary particles, but mainly secondary particles in the spray) as it passes through the atmosphere, causes light effects (Cherenkov radiation, fluorescent radiation of excited atoms) that can be detected - 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.
of secondary cosmic radiation
Individual quanta 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 above) - 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).
For efficient and comprehensive detection of (secondary) cosmic radiation 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 detectors of fast charged particle
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 is created with 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 detected optical 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 m2 and 440 photomultipliers.
The Pierre Auger Observatory detects secondary cosmic radiation, but with the main goal of analyzing primary cosmic radiation, 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.
Does cosmic radiation come randomly from all directions, or are there any significant directions corresponding to certain specific sources in space? As mentioned above, low and medium energy cosmic ray paths 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 1019 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 arrival and 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 formation. 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 radiation, 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.
radiation and life
Cosmic radiation, as high-energy ionizing radiation, can exhibit the biological effects on living organisms (the effect of ionizing radiation on life is generally discussed in detail in §5.2., the treatise "Biological effects of ionizing radiation"). So, in addition to the important information that cosmic radiation carries about the properties of elementary particles and phenomena in outer universe, 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 previously high levels of radiation from natural
radionuclides, has also contributed to the development of
effective cells repair mechanisms against
radiation damage (see §5.2. "Repair
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 /m2 per 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 5 km about 2mSv /year, in 8 km 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 1019 eV were sufficiently frequent - which is not expected...).
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 universe , during which a huge amount of radiant energy suddenly released. These are supernova explosions, gravitational collapse and formation of black holes, collisions or fusions of neutron stars and black holes. These processes, through their massive radiant manifestations, are astronomically observed only in the distant universe , which on one hand makes it difficult to analyze and correct understanding, but on the other hand, the enomous attenuation of radiation intensity by long distances and scattering effect of 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 reactions in 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 land, surface and groundwater would remain radioactively contaminated for a long time and uninhabitable .
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 and they finally merge 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 only 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 explosion supernova of type II (one such "endangered" relatively close very massive stars is the red giant Betelgeuse in the constellation Orion, weighing about 20 Suns masses, about 600 light-years distant from Earth; a supernova explosion can be expected here within about 1 million 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 incidents", it will probably never be in our power to prevent such massive processes as a supernova explosion or neutron star collisions, moreover of 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 here we are already entirely in field of sci-fi....
A brief reflection on the global perspectives of life in universe is in the passage "Astrophysics and Cosmology: - Human hopelessness?" §5.6 "The future of the universe. The arrow of time." in monograph "Gravity, black holes and the physics of spacetime".
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