Sources of photon radiation. What kind of radiation belongs to photon radiation Photon radiation is divided into X-ray and

The first studies of ionizing radiation were carried out at the end of the 19th century. In 1895, the German physicist V.K. Roentgen discovered "X-rays", later called X-rays. In 1896, the French physicist A. Becquerel discovered traces of the natural radioactivity of uranium salts on photographic plates. In 1898, the spouses Maria and Pierre Curie found that uranium, after radiation, turns into other chemical elements. They named one of these elements "radium" (Ra) (from the Latin "emitting rays").

Ionizing radiation is radiation, the interaction of which with the environment leads to the formation of ions of different signs. Ionizing radiation is subdivided into corpuscular and photonic.

Corpuscular radiation includes: a, b-, proton and neutron radiation.

a-radiation- This is a flux of helium nuclei formed during radioactive decay. They have a mass of 4 and a charge of +2. The a-emitter includes about 160 natural and man-made radionuclides, most of which are located at the end of the periodic table of elements (nuclear charge\u003e 82). a-particles propagate in media in a straight line, have an insignificant range (the distance at which particles lose their energy, interacting with matter): in air - less than 10 cm; in biological tissues 30-150 microns. a - particles have high ionizing and low penetrating power.

b-radiationIs a flow of electrons and positrons. Their mass is tens of thousands of times less than the mass of a-particles. The b-emitters include about 690 natural and man-made emitters. The range of b-particles is several meters in air, and in biological tissues - about 1 cm. They have a higher penetrating ability than a-particles, but less ionizing.

Proton radiation - stream of hydrogen nuclei.

Neutron radiation- the flow of nuclear particles that have no charge with a mass close to that of a proton. Free neutrons are captured by nuclei. In this case, the nuclei pass into an excited state and fission with the release of g-quanta, neutrons and delayed neutrons. Thanks to delayed neutrons, the fission reaction in nuclear reactors is controlled. Neutron radiation has a higher ionizing ability in comparison with other types of corpuscular radiation.

Photon Is a quantum of energy of electromagnetic radiation of high frequency. Photonic radiation is divided into X-ray and g-radiation. They have a high penetrating and low ionizing capacity.

X-ray radiation- This is artificial electromagnetic radiation that occurs in X-ray tubes ("X-rays").

g-radiation – this is electromagnetic radiation of natural origin. g-rays propagate in a straight line, do not deflect in electric and magnetic fields, and have a large range in air.

Directly ionizing radiation - This is radiation consisting of charged particles, for example, a, b-particles. Indirectly, ionizing radiation is radiation that consists of uncharged particles such as neutrons or photons. They create secondary radiation in the environments through which they pass.

Ionizing radiation is described by the following physical quantities

Activity of substance A determined by the rate of radioactive decay:

where: dN is the number of spontaneous nuclear transformations during the time dt.

Activity units:

in the SI system - Becquerel: 1 Bq \u003d 1 dec / s

off-system unit - Curie: 1 Ci \u003d 3.7. 10 10 dec / s, which corresponds to the activity of 1 g of pure Ra.

Half-life T 1/2 - the time required to reduce the activity of radionuclides by 2 times. For U-238, T 1/2 \u003d 4.56. 10 9 years, for Ra-226 T 1/2 \u003d 1622 years.

Exposure dose X - the energy of ionizing radiation, causing the formation of a charge dQ of the same sign in the air in an elementary volume, with a mass of dm.

Exposure dose units:

in the SI system 1 C / kg \u003d 3880 R.

off-system unit - X-ray: 1 R

The absorbed dose D is determined by the amount of absorbed energy dE per unit mass of the irradiated substance dm.

Absorbed dose units:

sI Gray: 1 Gy

off-system unit 1 rad \u003d 0.01 Gy

1 P \u003d 0.87 rad

1 rad \u003d 1.14 R

The name "rad" is from the first letters of the term "radiation absorbed dose".

Equivalent dose H R shows the danger of various types of radiation exposure of biological tissues and is equal to:

where: W R - weight factor reflecting the danger of one or another type of ionizing radiation for the body.

x-ray, g-radiation, b-radiation W R \u003d 1;

neutrons W R \u003d 5-20;

a-particles W R \u003d 20.

Equivalent dose units:

in SI 1 Sv in honor of the Swedish scientist Sievert

off-system unit - 1 rem \u003d 0.01 Sv

rem is the biological equivalent of glad.

Effective equivalent dose H E Is the magnitude of the risk of the long-term effects of irradiation of the entire human body and its individual organs, taking into account their radiosensitivity. Different organs and tissues have different sensitivity to radiation. For example, for the same equivalent dose of H R, lung cancer is more likely than thyroid cancer. Therefore, the concept of an effective equivalent dose has been introduced.

where: W T - weight coefficient for biological tissue.

All ionizing radiation is divided into photonic and corpuscular.

Photonic ionizing radiation includes:

• a) Y-radiation emitted by decay of radioactive isotopes or annihilation of particles. Gamma radiation is inherently shortwave electromagnetic radiation, i.e. flux of high-energy quanta of electromagnetic energy, the wavelength of which is much less than the interatomic distances, i.e. y
• b) X-ray radiation that occurs when the kinetic energy of charged particles decreases and / or when the energy state of the electrons of an atom changes.

Corpuscular ionizing radiation consists of a stream of charged particles (alpha, beta particles, protons, electrons), the kinetic energy of which is sufficient to ionize atoms in a collision. Neutrons and other elementary particles do not directly ionize, but in the process of interacting with the environment, they release charged particles (electrons, protons) that can ionize the atoms and molecules of the medium through which they pass:

a) neutrons are the only uncharged particles formed in some fission reactions of uranium or plutonium atoms. Since these particles are electrically neutral, they penetrate deeply into any substance, including living tissues. A distinctive feature of neutron radiation is its ability to convert atoms of stable elements into their radioactive isotopes, i.e. create induced radiation, which sharply increases the danger of neutron radiation. The penetrating power of neutrons is comparable to Y-radiation. Depending on the level of the carried energy, one can conditionally distinguish between fast neutrons (with energies from 0.2 to 20 MeV) and thermal (from 0.25 to 0.5 MeV). This difference is taken into account when carrying out protective measures. Fast neutrons are slowed down, losing ionization energy, by substances with a low atomic weight (the so-called hydrogen-containing ones: paraffin, water, plastics, etc.). Thermal neutrons are absorbed by materials containing boron and cadmium (boric steel, boral, boric graphite, cadmium-lead alloy).

Alpha-, beta-particles and gamma-quanta have energies of only a few megaelectronvolts, and cannot create induced radiation;

• b) beta particles - electrons emitted during the radioactive decay of nuclear elements with intermediate ionizing and penetrating ability (range in air up to 10-20 m).
• c) alpha particles - positively charged nuclei of helium atoms, and in outer space and atoms of other elements, emitted during the radioactive decay of isotopes of heavy elements - uranium or radium. They have low penetrating power (range in the air - no more than 10 cm), even human skin is an insurmountable obstacle for them. They are dangerous only when they enter the body, since they are able to knock electrons out of the shell of a neutral atom of any substance, including the human body, and turn it into a positively charged ion with all the ensuing consequences, which will be discussed later. Thus, an alpha particle with an energy of 5 MeV forms 150,000 ion pairs.

Figure: 1

The quantitative content of radioactive material in a human body or substance is defined by the term "activity of a radioactive source" (radioactivity). The unit of radioactivity in the SI system is a becquerel (Bq), corresponding to one decay in 1 s. Sometimes in practice the old unit of activity is used - curie (Ki). This is the activity of such an amount of substance, in which 37 billion atoms decay in 1 s. For translation, use the dependence: 1 Bq \u003d 2.7 x 10 Ci or 1 Ci \u003d 3.7 x 10 Bq.

Each radionuclide has a constant, inherent only half-life (the time required for a substance to lose half of its activity). For example, for uranium-235 it is 4,470 years, while for iodine-131 it is only 8 days.

Electron accelerators and X-ray installationsand . When charged particles pass in an electromagnetic field with acceleration or deceleration, the particle energy is lost in the form of bremsstrahlung photon radiation. This principle is based on obtaining beams of photon radiation during deceleration of electrons emitted by the cathode of an X-ray tube and accelerated by an electric field between the cathode and anode on the target.

Figure 5.10 shows a primitive diagram of an X-ray apparatus that demonstrates this.

Figure 5.10. A primitive diagram of an X-ray apparatus.

The power of such a photon source is determined by the electron current, the voltage between the cathode and the anode, the material and the thickness of the target, and ranges from 10 5 to 10 14 s -1... The power of the source can be approximated by the formula:

J ~ i Z V 2 (5.34),

wherein i- tube current, Zis the atomic number of the target material, V - tube voltage.

The energy distribution of the photons emitted by the target is continuous in the range from 0 to the energy of accelerated electrons and has a form similar to that shown in Figure 5.11.

Figure 5.11. Energy spectra of X-ray radiation from a tungsten target at various voltages across the tube.

Against the background of the continuous spectrum of bremsstrahlung radiation, characterized by the maximum photon energy equal to the energy of accelerated electrons, monoenergetic quanta of characteristic radiation of the target material are clearly distinguished, which in amplitude exceed the amplitude of bremsstrahlung radiation, and their position in energy depends on the target material.

The fundamental difference between a linear electron accelerator and an X-ray facility is only in the energy of accelerated electrons, which in X-ray devices usually does not exceed 400 keV, and on accelerators it reaches tens MeV... This also manifests itself in the bremsstrahlung spectrum, an approximate form of which for electrons is shown in Fig. 5.7. For the practice of calculating the protection against bremsstrahlung radiation of electron accelerators, the shown spectral distribution is often replaced by a monoenergetic one with effective energy equal to 2 / 3E e at the energy of accelerated electrons Her<1,7 МэВ ; 1/2 E e at Her in the range 1.7 - 10 MeV, 5 MeV at E e \u003d 10-15 MeV and 1/3 E e at E e\u003e 15 MeV.

In addition to the difference in the photon emission spectra of these installations, there is also a difference in the angular distribution of the emitted photons (Figure 5.12).

Figure 5.12. Angular distribution of photons emitted from the accelerator target at different accelerating voltages

On accelerators, photons, as a rule, fly in the direction of the primary electron beam; on an X-ray machine, at low tube voltages in the direction perpendicular to the primary beam.

One more feature of high-energy electron accelerators should be noted. If the energy of bremsstrahlung photon radiation exceeds the binding energy of neutrons in the core of the target material or structural elements, then a powerful accompanying neutron radiation arises by the reaction (γ, n), which sometimes determines the radiation situation near the accelerator.

Reactor as a source of photons. Sources of photon radiation in a nuclear reactor differ both in the nature of their formation and in the characteristics of the radiation emitted. The following main groups of photons from the reactor can be distinguished: instantaneous gamma radiation, gamma radiation from fission products, capture gamma radiation, gamma radiation from inelastic neutron scattering and activation gamma radiation.

Instant gamma radiationrepresents the gamma quanta emitted during the fission of a heavy nucleus and the decay of short-lived fission products, i.e., photon radiation emitted during the time t<5·10 -7 с after the fission reaction. The total energy of this gamma radiation is approximately 7 MeV / fission, the spectrum of emitted quanta decays with increasing energy and has a continuous energy distribution up to an energy of approximately 7.5 MeV with average photon energy 2.5 MeV... This radiation is generated in the reactor core directly during its operation.

Gamma radiation from fission products nuclear fuel is caused by gamma radiation of radionuclides accumulated in the fuel during the operation of the reactor, both directly in the fission process and due to the radioactive decay of these products and the capture of neutrons by the resulting fission products. In general, about 1000 radionuclides - fission products, each of which has a spectrum of discrete energy lines of gamma quanta and its own half-life. The abundance of radionuclides with different decay periods and the presence of many gamma transitions in their decay schemes form an almost continuous spectrum of gamma radiation from fission products, which varies depending on the reactor operation time and the time of its shutdowns. The activities of fission products at any time can be calculated based on data on the independent or cumulative yields of fission products and the cross sections of reactions leading to their formation. After about a year of exposure, the main contribution to the total spectrum is made by photons in the energy range from 0.5 to 0.9 MeV medium energy 0.8 MeV and the total energy is approximately 7.5 MeV / fission.

Captured gamma radiation arises during neutron capture, both in the fuel material and in the structural elements of the reactor, which leads to the fact that it is formed not only in the reactor core, but also in the surrounding structures, including the biological protection of the reactor. If we assume in the first approximation that in the process of fission 235 U thermal neutrons is formed 2,43 neutr. / fission, one of which is used for a self-sustaining fission reaction, then approximately 1,43 neutrons are captured with the formation of capture gamma radiation. Taking into account the fact that the cross-sections of neutron capture by the structural elements of the reactor have maximum values \u200b\u200bfor neutrons of thermal energies, and the binding energy of neutrons for the nuclei of these materials is in the range 7-11 MeV, then the energy of capture gamma quanta is determined mainly by the binding energy of the neutron in the nucleus and is equal to 7-11 MeV... This highly penetrating photon radiation in many cases determines the dimensions of the biological protection of the reactor.

Inelastic scattering gamma radiation accompanies the capture of a fast neutron by a nucleus, followed by the emission of a neutron with a lower energy. The difference between the energies of the captured and emitted neutrons is realized by the emission of gamma quanta. The dependences of the cross sections of inelastic scattering on the neutron energy have a threshold character; therefore, this process is possible only at neutron energies above approximately 0.8 MeV and on heavy materials. Taking into account the low values \u200b\u200bof the cross sections for inelastic scattering and the low energy of the generated gamma quanta (below 4 MeV), the contribution of this radiation to the characteristics of the gamma radiation field of the reactor is much lower than the contribution of captured gamma radiation.

Activation gamma radiation due to reactions of neutron capture by stable nuclei of reactor materials with the formation of radioactive nuclides. This mainly occurs as a result of reactions (n, γ) or (n, p)... When choosing structural elements of the reactor, all measures are taken to reduce the concentration of materials leading to the formation of activation radiation, however, it always occurs as a result of corrosion of materials and the ingress of corrosion products with the primary coolant into the reactor core. The characteristics of the resulting radionuclides of activation radiation are well known, since they belong to the radionuclides described above.

It should be noted the features of the formation of the gamma radiation fields of the reactor. If the instant, capture, gamma radiation of inelastic neutron scattering and the short-lived activation activity of the primary coolant are formed only during the operation of the reactor and it is these sources that determine its safe operation, then the gamma radiation of the fission products and long-lived radionuclides of activation radiation accumulated during the operation of the reactor determine the gamma radiation of the shutdown reactor, and, consequently, determine the issues of handling spent nuclear fuel and radioactive waste accumulated at the reactor. They also play a decisive role in the radiation environment created in the event of an emergency.

5.4.3. Sources of neutron radiation .

Nuclear reactor as a neutron source . Fission of nuclei can be carried out under the influence of various elementary particles (neutrons, protons, alpha particles, etc.) or photons carrying significant energy. Mostly heavy nuclei are subject to fission. Of all the known fission reactions, of the greatest practical importance are reactions under the action of neutrons. One of the conditions for the fission of an excited nucleus formed during neutron capture is the excess of the excitation energy of a certain threshold - the critical energy E cr, i.e. E + E st> E crwhere Eis the kinetic energy of the incident neutron, and E st is the binding energy of a neutron in the nucleus. For isotopes 231 Pa, 232 Th, 237 Np and 238 U, etc. E cr> E st, therefore, for their fission, neutrons with high kinetic energy ( E\u003e 1 MeV), or fast neutrons. At the same time for 233 U, 235 U, 239 Pu and 241 Pu E b> E cr... This ratio explains the ability of these isotopes to fission into thermal neutrons; such nuclides are called fissile.

In general terms, the reaction of neutron capture, the formation of a compound nucleus and the subsequent realization of its excited state, for example, 235 Ucan be written as follows:

92 236 U + γ

(absorption without division -10 - 15%)

92 235 U + 0 1 n 92 236 U

z1 A1 X + z2 A2 Y + γ + β +2.43 0 1 n + ν

(division - 85-90%)

In the fission of heavy nuclei, along with fission fragments z 1 A 1 X, z 2 A 2 Y several secondary neutrons are formed. For example, fission of uranium often produces two new neutrons (up to 30%), less often one, three or even four neutrons (up to 25%). In individual fission events, secondary neutrons are not formed at all (up to 10%).

An important point that determines the possibility of the development of a chain fission reaction is the average number of secondary neutrons ν per 1 fission act. Table 5.4 shows the values \u200b\u200bof ν for the main fissile nuclides upon fission by thermal and 238 U fast neutrons.

Ionizing radiation (AI) -this is radiation, the interaction of which with the medium leads to the formation of ions of different signs in this medium. Radiation is considered ionizing if it is capable of breaking chemical bonds between molecules. Ionizing radiation is divided into corpuscular and photonic.

Radio waves, light waves, thermal energy of the Sun do not belong to ionizing radiation, since they do not cause damage to the body by ionization.

Corpuscular - it is a stream of particles with a mass other than zero (electrons, protons, neutrons, alpha particles).

Photonic - this is electromagnetic radiation, indirectly ionizing radiation (gamma radiation, characteristic radiation, bremsstrahlung radiation, X-ray radiation, annihilation radiation).

Alpha radiation Is a stream of alpha particles (nuclei of helium atoms) emitted during radioactive decay, as well as in nuclear reactions and transformations. Alpha particles have strong ionizing properties and little penetrating power. In the air, they penetrate to a depth of several centimeters, in biological tissue - to a depth of a fraction of a millimeter, and are retained by a sheet of paper or cloth. Alpha radiation is especially dangerous if its source enters the body with food or inhaled air.

Beta radiation Is a stream of electrons or positrons emitted by the nuclei of radioactive elements during beta decay. Their ionizing power is less than that of alpha particles, but their penetrating power is many times greater, and amounts to tens of centimeters. In biological tissue, they penetrate to a depth of 2 cm, clothing is only partially retained. Beta radiation is dangerous to human health, both with external and internal exposure.

Proton radiation - this is the flux of protons that form the basis of cosmic radiation, as well as observed in nuclear explosions. Their range in air and their penetrating power are intermediate between alpha and beta radiation.

Neutron radiation - the flux of neutrons observed during nuclear explosions, especially neutron munitions and the operation of a nuclear reactor. The consequences of its impact on the environment depend on the initial neutron energy, which can vary within the range of 0.025–300 MeV.

Gamma radiation - electromagnetic radiation (wavelength 10 –10 –10 –14 m), which occurs in some cases during alpha and beta decay, annihilation of particles and during the excitation of atoms and their nuclei, deceleration of particles in an electric field. The penetrating power of gamma radiation is much higher than that of the above types of radiation. The depth of propagation of gamma quanta in the air can reach hundreds and thousands of meters. Ionizing ability (indirect) is much less than that of the above types of radiation. Most of the gamma rays pass through biological tissue, and only a small amount is absorbed by the human body.

Braking radiation - photon radiation with a continuous energy spectrum, emitted when the kinetic energy of charged particles decreases. The environmental impact is the same as gamma radiation.

Characteristic radiation - photon radiation with a discrete energy spectrum, which occurs when the energy state of the electrons of an atom changes. The effect on biological tissue is similar to gamma radiation.

Annihilation radiation - photon radiation resulting from the annihilation of a particle and antiparticle (for example, a positron and an electron). The effect on biological tissue is similar to gamma radiation.

Ionizing radiation can be conventionally divided into photonic and corpuscular. Photon radiation includes electromagnetic vibrations, to corpuscular - particle flow. The concepts of "electromagnetic", "quantum", "photon" radiation can be considered equivalent.

The type of interaction of photons with atoms of a substance depends on the energy of the photons. To measure the energy and mass of microparticles, an off-system unit of energy is used - electron-volt. 1 eV is the kinetic energy acquired by a particle carrying one elementary charge under the action of a potential difference of 1V. 1 eV \u003d 1.6 x 10 19 J. Multiple units: 1 keV \u003d 10 3 eV; 1 MeV \u003d 10 6 eV.

According to modern concepts, charged particles (α-, β-particles, protons, etc.) ionize matter directly, while neutral particles (neutrons) and electromagnetic waves (photons) are indirectly ionizing. The flow of neutral particles and electromagnetic waves, interacting with matter, cause the formation of charged particles, which ionize the medium.

2.1. PHOTON AND CORPUSCULAR RADIATION

Electromagnetic radiation.Radiation therapy uses X-ray radiation from X-ray therapy devices, gamma radiation from radionuclides, and high-energy bremsstrahlung (X-ray) radiation.

X-ray radiation- photon radiation, consisting of bremsstrahlung and (or) characteristic radiation.

Braking radiation- short-wave electromagnetic radiation arising from a change in the speed (deceleration) of charged particles when interacting with the atoms of the decelerating substance (anode). The wavelengths of bremsstrahlung X-ray radiation do not depend on the atomic number of the braking substance, but are determined only by the energy of the accelerated electrons. The spectrum of bremsstrahlung is continuous, with the maximum photon energy equal to the kinetic energy of the decelerating particles.

Characteristic radiationoccurs when the energy state of atoms changes. When knocking out an electron from the inner shell

of an atom by an electron or a photon, the atom passes into an excited state, and the vacant place is occupied by an electron from the outer shell. In this case, the atom returns to its normal state and emits a quantum of characteristic X-ray radiation with an energy equal to the difference in energies at the corresponding levels. The characteristic radiation has a linear spectrum with specific wavelengths for a given substance, which, like the intensity of the lines of the characteristic spectrum of X-ray radiation, are determined by the atomic number of the element Z and the electronic structure of the atom.

The intensity of the bremsstrahlung is inversely proportional to the square of the mass of the charged particle and is directly proportional to the square of the atomic number of the substance in the field of which the charged particles are decelerated. Therefore, to increase the yield of photons, relatively light charged particles - electrons and substances with a large atomic number (molybdenum, tungsten, platinum) are used.

The source of X-ray radiation for the purposes of radiation therapy is the X-ray tube of X-ray therapy devices, which, depending on the level of generated energy, are divided into close-focus and remote. X-ray radiation from close-focus X-ray therapy devices is generated at anode voltage less than 100 kV, remote ones - up to 250 kV.

High energy braking radiation,like bremsstrahlung X-ray radiation, it is short-wave electromagnetic radiation that occurs when the velocity (deceleration) of charged particles changes during interaction with target atoms. This type of radiation differs from high energy X-rays. Sources of high-energy bremsstrahlung are linear electron accelerators - LUE with bremsstrahlung energies from 6 to 20 MeV, as well as cyclic accelerators - betatrons. To obtain high-energy bremsstrahlung radiation, deceleration of sharply accelerated electrons is used in vacuum systems of accelerators.

Gamma radiation- short-wave electromagnetic radiation emitted by excited atomic nuclei during radioactive transformations or nuclear reactions, as well as during the annihilation of a particle and antiparticle (for example, an electron and a positron).

Radionuclides are sources of gamma radiation. Each radionuclide emits γ-quanta of its specific energy. Radionuclides are produced in accelerators and in nuclear reactors.

The activity of a radionuclide source is understood as the number of decays of atoms per unit time. Measurements are made in Becquerels (Bq). 1 Bq is the activity of the source in which 1 decay occurs per second. Non-systemic unit of activity - Curie (Ki). 1 Ci \u003d 3.7 x 10 10 Bq.

Sources of γ-radiation for external and intracavitary radiation therapy are 60 Coand 137 Cs.The most widely used drugs 60 Cowith photon energies on average 1.25 MeV (1.17 and 1.33 MeV).

For intracavitary radiation therapy, 60 Co is used,

137 Cs, 192 Ir.

When photon radiation interacts with matter, the phenomena of the photoelectric effect, the Compton effect, and the formation of electron-positron pairs occur.

Photo effectconsists in the interaction of a gamma quantum with a bound electron of the atom (Fig. 10). In photoelectric absorption, all the energy of the incident photon is absorbed by the atom, from which the electron is knocked out. After the emission of a photoelectron, a vacancy is formed in the atomic shell. The transition of less bound electrons to vacant levels is accompanied by the release of energy, which can be transferred to one of the electrons of the upper shells of the atom, which leads to its escape from the atom (Auger effect), or be transformed into the energy of characteristic X-ray radiation. Thus, with the photoelectric effect, part of the energy of the primary gamma quantum is converted into the energy of electrons (photoelectrons and Auger electrons), and part is released in the form of characteristic radiation. An atom that has lost an electron turns into a positive ion, and the knocked out electron - a photoelectron - loses energy at the end of its path, joins a neutral atom and turns it into a negatively charged ion. The photoelectric effect occurs at relatively low energies - from 50 to 300 keV, which are used in X-ray therapy.

Fig. 10.Photo effect

Figure: eleven.Compton effect

Compton effect (incoherent scattering)arises at a photon energy from 120 keV to 20 MeV, that is, with all types of ionizing radiation used in radiation therapy. In the Compton effect, the incident photon, as a result of elastic collision with electrons, loses part of its energy and changes the direction of its initial motion, and a recoil electron (Compton electron) is knocked out of the atom, which further ionizes the substance (Fig. 11).

The process of converting the energy of a primary photon into the kinetic energy of an electron and a positron and into the energy of annihilation radiation. The energy of a quantum must be greater than 1.02 MeV (double the electron rest energy). Such interaction of quanta with matter occurs when patients are irradiated at high-energy linear accelerators with a beam of high-energy bremsstrahlung radiation. The photon disappears in the Coulomb field of the nucleus (or electron).

Figure: 12.Formation of electron-positron pairs

In this case, the resulting pair is transferred all the energy of the incident photon minus the rest energy of the pair. The electrons and positrons arising in the process of absorption of gamma quanta lose their kinetic energy as a result of ionization of the molecules of the medium, and when they meet, they annihilate with the emission of two photons with an energy of 0.511 MeV each (Fig. 12).

As a result of the above processes of interaction of photon radiation with matter, secondary photon and corpuscular radiation (electrons and positrons) appears. The ionizing ability of particles is much greater than that of photon radiation. With the alternation of the processes of formation of electron-positron pairs, bremsstrahlung, a huge number of photons and charged particles are created in the medium, the so-called an avalanche of radiation,which decays with decreasing energy of each newly formed photons and particles.

The interaction of X-ray radiation with matter is accompanied by its ionization and is determined by two main effects - photoelectric absorption and Compton scattering. When high-energy bremsstrahlung radiation interacts with matter, Compton scattering occurs, as well as the formation of ion pairs, since the photon energy is greater than 1.02 MeV.

The intensity of photon radiation from a point source changes in space in inverse proportion to the square of the distance.

Corpuscular radiation- flows of charged particles: electrons, protons, heavy ions (for example, carbon nuclei) with energies of several hundred MeV, as well as neutral particles - neutrons. Irradiation with a particle stream is now called hadron therapy. To hadrons (from the Greek word hadros- "heavy") includes nucleons, protons and neutrons entering them, as well as π -mesons, etc. Sources of particles are accelerators and nuclear reactors. Depending on the maximum energy of the accelerated protons, the accelerators are conventionally divided into 5 levels, and the accelerators of the 5th level with Ep\u003e 200 MeV (meson factories)

are used to produce individual radionuclides. As a rule, the production of these radionuclides on cyclotrons of a different level is impossible or inefficient.

High energy electron beamgenerated by the same electron accelerators as when bremsstrahlung. Electron beams with energies from 6 to 20 MeV are used. High energy electrons have great penetrating power. The average free path of such electrons can reach 10-20 cm in the tissues of the human body. The electron beam, absorbed in the tissues, creates a dose field at which the maximum ionization is formed near the surface of the body. Beyond the ionization maximum, a fairly rapid dose decay occurs. On modern linear accelerators, it is possible to regulate the energy of the electron beam, and, accordingly, to create the required dose at the required depth.

Neutron - a particle that has no charge. The processes of interaction of neutrons (neutral particles) with matter depend on the energy of neutrons and the atomic composition of matter. The main effect of the action of thermal (slow) neutrons with an energy of 0.025 eV on biological tissue occurs under the action of protons formed in the reaction (n, p) and losing all their energy at the place of birth. Most of the energy of slow neutrons is spent on the excitation and splitting of tissue molecules. Almost all the energy of fast neutrons with energies from 200 keV to 20 MeV is lost in the tissue during elastic interaction. Further energy release occurs as a result of ionization of the medium by recoil protons. The high linear energy density of neutrons prevents the repair of irradiated tumor cells.

Another type of neutron exposure is neutron capture therapy, which is a binary method of radiotherapy that combines two components. The first component is a stable isotope of boron 10 B, which, when administered as part of a preparation, can accumulate in the cells of certain types of brain tumors and melanomas. The second component is a flux of low-energy thermal neutrons. Formed as a result of the capture of a thermal neutron by the 10 B nucleus, heavy high-energy charged particles (boron breaks down into lithium atoms and α-particles) destroy only cells that are in close proximity to boron atoms, almost without affecting adjacent normal cells. In addition to boron, the use of drugs with gadolinium is promising in neutron capture therapy. For deep-seated tumors, it is promising to use epithermal neutrons in the energy range 1 eV - 10 keV, which have a high penetrating ability and, slowing down in tissue to thermal energies, make it possible to carry out neutron capture therapy of tumors located at a depth of 10 cm. Obtaining high fluxes of thermal and epithermal neutrons are carried out using a nuclear reactor.

Proton is a positively charged particle. The method of irradiation is used at the "Bragg peak", when the maximum energy of charged particles is released at the end of the path and is localized in a limited volume of irradiation.

swollen tumor. As a result, a large dose gradient is formed on the surface of the body and in the depths of the irradiated object, after which a sharp decay of energy occurs. By changing the energy of the beam, it is possible to change the place of its complete stop in the tumor with great accuracy. Proton beams with an energy of 70-200 MeV and the technique of multi-field irradiation from different directions are used, in which the integral dose is distributed over a large area of \u200b\u200bsurface tissues. When irradiating at the synchrocyclotron at PNPI (Petersburg Institute of Nuclear Physics), a fixed energy of the extracted proton beam is used - 1000 MeV and the method of irradiation is applied continuously. Protons of such high energy easily pass through the irradiated object, producing uniform ionization along their path. In this case, there is a small scattering of protons in the substance; therefore, a narrow, sharp-bound beam of protons formed at the entrance remains practically the same narrow in the irradiation zone inside the object. As a result of continuous irradiation in combination with the rotary irradiation technique, a very high dose ratio in the irradiation zone to the dose on the object surface is provided - about 200: 1. A narrow proton beam with dimensions at half the intensity of 5-6 mm is used to treat various diseases of the brain, such as arteriovenous malformations of the brain, pituitary adenomas, etc. Striking effect carbon ionsturns out to be at the Bragg peak several times higher than that of protons. Multiple double ruptures of the DNA helix of the atoms of the irradiated volume occur, which after that cannot be restored.

π -Mesons- spinless elementary particles with a mass, the value of which occupies an intermediate place between the masses of an electron and a proton. π-mesons with energies of 25-100 MeV travel all the way through the tissue with practically no nuclear interactions, and at the end of the path they are captured by the nuclei of the tissue atoms. The act of absorption of the π-meson is accompanied by the emission of neutrons, protons, α-particles, Li, Be ions, etc. from the destroyed nucleus. The high cost of technological support of the process still prevents the active introduction of hadron therapy into clinical practice.

The advantages of using high-energy radiation for the treatment of malignant tumors at a depth are, with an increase in energy, an increase in the depth dose and a decrease in the surface dose, a higher penetrating power with an increase in the relative depth dose, and a smaller difference between the absorbed dose in bones and soft tissues. In the presence of a linear accelerator or betatron, there is no need to dispose of a radioactive source, as when using radionuclides.

When conducting brachytherapy, systemic radionuclide therapy, α-, β-, γ-emitting radionuclides are used, as well as sources with mixed, for example, γ- and neutron (n) radiation.

α -Radiation- corpuscular radiation consisting of 4 He nuclei (two protons and two neutrons), emitted during radioactive decay of nuclei or during nuclear reactions, transformations. α-Particles are emitted during the radioactive decay of elements heavier than lead or are formed in nuclear

reactions. α-Particles have high ionizing ability and low penetrating ability, carry two positive charges.

Radionuclide 225 Ac with a half-life of 10.0 days in combination with monoclonal antibodies is used for radioimmunotherapy of tumors. In the future, the use of 149 Tb radionuclide with a half-life of 4.1 hours for this purpose. Α-Emitters began to be used for irradiation of endothelial cells in coronary arteries after operations - coronary artery bypass grafting.

β -Radiation- corpuscular radiation with a continuous energy spectrum, consisting of negatively or positively charged electrons or positrons (β - or β + particles) and arising from radioactive β-decay of nuclei or unstable particles. β-Emitters are used in the treatment of malignant tumors, the localization of which allows direct contact with these drugs.

The sources of β-radiation are 106 Ru, β - -emitter with an energy of 39.4 keV and half-life of 375, 59 days, 106 Rh, β - -radiator with an energy of 3540.0 keV and half-life of 29.8 s. Both β-emitters 106 Ru + 106 Rh are included in the ophthalmic applicator sets.

β - - Emitter 32 P with an energy of 1.71 MeV and a half-life of 14.2 days is used in skin applicators for the treatment of superficial diseases. The 89 Sr radionuclide is an almost pure β-emitter with a half-life of 50.6 days and an average β-particle energy of 1.46 MeV. A solution of 89 Sr - chloride is used for the palliative treatment of bone metastases.

153 Sm with β-radiation energies of 203.229 and 268 keV and with γ-radiation energies of 69.7 and 103 keV, half-life of 46.2 h is a part of the domestic drug samarium oxabiphor, intended to affect bone metastases, as well as used in patients with severe pain in the joints with rheumatism.

90 Y with a half-life of 64.2 hours and a maximum energy of 2.27 MeV is used for a variety of therapeutic purposes, including radioimmunotherapy with labeled antibodies, treatment of liver tumors and rheumatoid arthritis.

The 59 Fe radionuclide as part of a tabletted radiopharmaceutical is used at the Russian Scientific Center for Roentgenoradiology (Moscow) to treat breast cancer patients. The principle of action of the drug, according to the authors, is the spread of iron by the blood stream, selective accumulation in the cells of tumor tissue and the effect on them of β-radiation. 67 Cu with a half-life of 2.6 days is combined with monoclonal antibodies for radioimmune therapy of tumors.

186 Re in the preparation (rhenium sulfide) with a half-life of 3.8 days is used for the treatment of joint diseases, and balloon catheters with sodium perrhenate solution are used for endovascular brachytherapy. It is believed that there is a prospect for using a 48 V β + emitter with a half-life of 16.9 days for intracoronary brachytherapy using an arterial stent made of titanium and nickel alloy.

131 I is used in the form of solutions for the treatment of diseases of the thyroid gland. 131 I decays with the emission of a complex spectrum of β- and γ-radiation. Has a half-life of 8.06 days.

X-ray and Auger-electron emitters include 103 Pd with a half-life of 16.96 days and 111 In with a half-life of 2.8 days. 103 Pd in \u200b\u200bthe form of a sealed source in a titanium capsule is used in tumor brachytherapy. 111 In is used in radioimmunotherapy using monoclonal antibodies.

125 I, which is a γ-emitter (a type of nuclear transformation - electron capture with the conversion of iodine into tellurium and the release of a γ-quantum), is used as a closed microsource for brachytherapy. The half-life is 60.1 days.

Mixedγ + neutron radiation is characteristic of 252 Cf with a half-life of 2.64 years. It is used for contact irradiation, taking into account the neutron component, in the treatment of highly resistant tumors.

2.2. CLINICAL DOSIMETRY

Clinical dosimetry- section of dosimetry of ionizing radiation, which is an integral part of radiation therapy. The main task of clinical dosimetry is to select and substantiate the means of irradiation that provide the optimal spatial-temporal distribution of the absorbed radiation energy in the body of the irradiated patient and a quantitative description of this distribution.

Clinical dosimetry uses computational and experimental techniques. Calculation methods are based on the already known physical laws of interaction of various types of radiation with matter. With the help of experimental methods, treatment situations are simulated with measurements in tissue-equivalent phantoms.

The objectives of clinical dosimetry are:

Measurement of radiation characteristics of therapeutic radiation beams;

Measurement of radiation fields and absorbed doses in phantoms;

Direct measurements of radiation fields and absorbed doses to patients;

Measurement of radiation fields of scattered radiation in canyons with therapeutic installations (for the purpose of radiation safety of patients and personnel);

Carrying out absolute calibration of detectors for clinical dosimetry;

Experimental studies of new therapeutic methods of radiation.

The main concepts and quantities of clinical dosimetry are absorbed dose, dose field, dosimetric phantom, and target.

Dose of ionizing radiation:1) a measure of the radiation received by the irradiated object, the absorbed dose of ionizing radiation;

2) a quantitative characteristic of the radiation field - exposure dose and kerma.

Absorbed doseis the main dosimetric quantity, which is equal to the ratio of the average energy transferred by ionizing radiation to a substance in an elementary volume to the mass of a substance in this volume:

where D is the absorbed dose,

E - average radiation energy,

m is the mass of a substance per unit volume.

As a unit of the absorbed radiation dose in SI, Gray (Gy) is adopted in honor of the English scientist Gray (L. N. Gray), known for his works in the field of radiation dosimetry. 1 Gy is equal to the absorbed dose of ionizing radiation, at which a substance with a mass of 1 kg is transferred the energy of ionizing radiation equal to 1 J. In practice, a non-systemic unit of the absorbed dose is also common - rad (radiation absorbed dose). 1 rad \u003d 10 2 J / kg \u003d 100 erg / g \u003d 10 2 Gyor 1 Gr \u003d 100 glad.

The absorbed dose depends on the type, intensity of radiation, its energetic and qualitative composition, time of exposure, and also on the composition of the substance. The longer the radiation time, the greater the dose of ionizing radiation. The dose increment per unit time is called dose rate,which characterizes the rate of accumulation of the dose of ionizing radiation. The use of various special units is allowed (for example, Gy / h, Gy / min, Gy / s, etc.).

The dose of photon radiation (X-ray and gamma radiation) depends on the atomic number of the elements that make up the substance. Under the same irradiation conditions in heavy substances, it is usually higher than in lungs. For example, in the same X-ray field, the absorbed dose in bones is greater than in soft tissues.

In the field of neutron radiation, the main factor determining the formation of the absorbed dose is the nuclear composition of the substance, and not the atomic number of the elements that make up the biological tissue. For soft tissues, the absorbed dose of neutron radiation is largely determined by the interaction of neutrons with the nuclei of carbon, hydrogen, oxygen and nitrogen. The absorbed dose in a biological substance depends on the neutron energy, since neutrons of different energies selectively interact with the nuclei of the substance. In this case, charged particles, γ-radiation can arise, as well as radioactive nuclei, which themselves become sources of ionizing radiation, can be formed.

Thus, the absorbed dose during neutron irradiation is formed due to the energy of secondary ionizing particles of various natures arising from the interaction of neutrons with matter.

The absorption of radiation energy causes processes that lead to various radiobiological effects. For a specific type of radiation, the yield of radiation-induced effects in a certain way

associated with the absorbed radiation energy, often a simple proportional relationship. This allows the radiation dose to be taken as a quantitative measure of the effects of radiation, in particular of a living organism.

Different types of ionizing radiation at the same absorbed dose have a different biological effect on the tissues of a living organism, which is determined by their relative biological effectiveness - RBE.

RBE of radiation depends mainly on differences in the spatial distribution of ionization acts caused by corpuscular and electromagnetic radiation in the irradiated substance. The energy transferred by a charged particle per unit length of its path in matter is called linear power transmission (LET).There are rare ionizing (LET< 10 кэВ/мкм) и плотноионизирующие (ЛПЭ > 10 keV / μm) types of radiation.

Biological effects arising from different types of ionizing radiation are usually compared with similar effects arising in an X-ray field with a boundary photon energy of 200 keV, which is taken as a model.

RBE coefficientdefines the ratio of the absorbed dose of standard radiation, which causes a certain biological effect, to the absorbed dose of this radiation, which gives the same effect.

where D x is the dose of a given type of radiation for which RBE is determined, D R is the dose of exemplary X-ray radiation.

On the basis of RBE data, different types of ionizing radiation are characterized by their radiative emissivity.

Weighting radiation coefficient (radiation coefficient of radiation)is a dimensionless coefficient by which the absorbed dose of radiation in an organ or tissue must be multiplied to calculate equivalent doseradiation to take into account the effectiveness of different types of radiation. The concept of an equivalent dose is used to assess the biological effect of radiation regardless of the type of radiation, which is necessary for the purposes of antiradiation protection of personnel working with sources of ionizing radiation, as well as patients during radiological research and treatment.

Equivalent doseis defined as the average value of the absorbed dose in an organ or tissue, taking into account the average weighting radiation coefficient.

where H is the equivalent absorbed dose,

W R is the weighting radiation factor currently established by the radiation safety standards.

The unit of equivalent dose in SI is Sievert (Sv)- named after the Swedish scientist R. M. Sievert, the first chairman of the International Commission on Radiological Protection (ICRP). If in the last formula the absorbed dose of radiation (D) is expressed in Grays, then the equivalent dose will be expressed in Sieverts. 1 Sv is equal to the equivalent dose at which the product of the absorbed dose (D) in living tissue of the standard composition by the average radiation coefficient (W R) is 1 J / kg.

In practice, a non-systemic unit of equivalent dose is also common - rem(1 Sv \u003d 100 rem), if the absorbed radiation dose is expressed in rad in the same formula.

Weighting factors for certain types of radiation when calculating the equivalent dose.

Effective equivalent dose- the concept used for the dosimetric assessment of exposure to healthy organs and tissues and the likelihood of long-term effects. This dose is equal to the sum of the products of the equivalent dose in an organ or tissue by the corresponding weighting factor (weighting factor) for the most important human organs:

where E is the effective equivalent dose,

Н Т - equivalent dose in organ or tissue Т,

W T - weighting coefficient for organ or tissue T.

The SI unit of effective equivalent dose is Sievert (Sv).

For the dosimetric characteristics of the photon-ionizing radiation field, exposure dose.It is a measure of the ionizing power of photon radiation in air. The unit of exposure dose in SI - Pendant per kilogram (C / kg).An exposure dose of 1 C / kg means that charged particles released in 1 kg of atmospheric air during the primary acts of absorption and scattering of photons

at full use of their range in air, form ions with a total charge of the same sign equal to 1 Coulomb.

In practice, a non-systemic unit of an exposure dose is often used X-ray (R)- by the name of the German physicist Roentgen (W. K. Rontgen): 1 P \u003d 2.58 x10 -4 C / kg.

The exposure dose is used to characterize the field of only photon-ionizing radiation in air. It gives an idea of \u200b\u200bthe potential level of human exposure to ionizing radiation. At an exposure dose of 1 R, the absorbed dose in soft tissue in the same radiation field is approximately 1 rad.

Knowing the exposure dose, it is possible to calculate the absorbed dose and its distribution in any complex object placed in a given radiation field, in particular in the human body. This allows you to plan and control a given irradiation regime.

Currently, more often as a dosimetric quantity characterizing the radiation field, they use kerm(KERMA is an abbreviation of the expression: Kinetic Energy Released in Material). Kerma is the kinetic energy of all charged particles released by ionizing radiation of any kind, per unit mass of the irradiated substance during the primary acts of interaction of radiation with this substance. Under certain conditions, the kerma is equal to the absorbed dose of radiation. For photon radiation in air, it is the energy equivalent of the exposure dose. The dimension of kerma is the same as that of the absorbed dose, expressed in J / kg.

Thus, the concept of "exposure dose" is necessary to assess the dose level generated by the radiation source, as well as to control the exposure regime. The concept of "absorbed dose" is used when planning radiation therapy in order to achieve the desired effect (Table 2.1).

Dose fieldis the spatial distribution of the absorbed dose (or its power) in the irradiated part of the patient's body, tissue-equivalent medium, or a dosimetric phantom that simulates the patient's body by the physical effects of the interaction of radiation with the substance, the shape and size of organs and tissues, and their anatomical relationships. Information about the dose field is presented in the form of curves connecting points of the same values \u200b\u200b(absolute or relative) of the absorbed dose. Such curves are called isodoses,and their families are isodose maps. The absorbed dose at any point of the dose field can be taken as a conventional unit (or 100%), in particular, the maximum absorbed dose, which must correspond to the target to be irradiated (that is, the area covering the clinically detected tumor and the assumed area of \u200b\u200bits spread).

The physical characteristics of the irradiation field are characterized by various parameters. The number of particles entering the medium is called fluence.The sum of all penetrated particles and particles scattered in a given environment is flowionizing particles, and the ratio of flux to area is flux density.Under radiation intensity,or flux density

Table 2.1. Basic radiation quantities and their units

energy, understand the ratio of the flow of energy to the area of \u200b\u200bthe object. The radiation intensity depends on the particle flux density. Besides linear power transmission (LET),characterizing the average energy losses of particles (photons), determine the linear ionization density (LPI),the number of ion pairs per unit length of path (track) of a particle or photon.

The formation of the dose field depends on the type and source of radiation. When forming a dose field with photon radiation, it is taken into account that the intensity of photon radiation from a point source decreases in the medium inversely proportional to the square of the distance to the source. In dosimetric planning, the concept of average ionization energy is used, which includes the energy of direct ionization and the energy of excitation of atoms, leading to secondary radiation, which also causes ionization. For photon radiation, the average ionization energy is equal to the average ionization energy of electrons released by photons.

The dose distribution of the γ-ray beam is uneven. The section of 100% isodose has a relatively small width, and then the relative value of the dose falls along the curve rather steeply. The size of the irradiation field is determined by the width of 50% of the dose. When the bremsstrahlung dose field is formed, there is a steep dose drop at the field boundary, determined by the small size of the focal spot. This leads to the fact that the width of 100% isodose is close to the width of 50% isodose, which determines the dosimetric value of the size of the irradiation field. Thus, in the formation of the dose distribution during irradiation with a bremsstrahlung beam, there are advantages over a γ-radiation beam, since the radiation doses to healthy organs and tissues near the pathological focus are reduced (Table 2.2).

Table 2.2. Depth of 100%, 80% and 50% isodose at the most commonly used radiation energies

Note. Source-surface distance for X-ray therapy apparatus - 50 cm; gamma therapy - 80 cm; linear accelerators - 100 cm.

From the data table. 2.2 it can be seen that megavoltage radiation, in contrast to orthovoltage X-ray, has a maximum dose not on the skin surface, its depth increases with increasing radiation energy (Fig. 13). After the electrons reach their maximum, a steep dose gradient is observed, which makes it possible to reduce the dose load on the underlying healthy tissues.

Protons are distinguished by the absence of scattering of radiation in the body, the possibility of decelerating the beam at a given depth. In this case, with the depth of penetration, the linear energy density (LET) increases, the magnitude of the absorbed dose increases, reaching a maximum at the end of the particle path,

Figure: 13.Distribution of energy of different types of radiation in a tissue-equivalent phantom: 1 - with close-focus X-ray therapy 40 kV and deep X-ray therapy 200 kV; 2 - with gamma therapy 1.25 MeV; 3 - with bremsstrahlung of 25 MeV; 4 - under irradiation with fast electrons 17 MeV; 5 - when irradiated with 190 MeV protons; 6 - under irradiation with slow neutrons 100 keV

Fig. 14.Bragg's Peak

Figure: 15.Dose distribution of gamma radiation from two open parallel opposite fields

the so-called Bragg peak, where the dose can be much greater than at the beam entrance, with a steep dose gradient behind the Bragg peak wave to almost 0 (Fig. 14).

Often, during irradiation, parallel opposing fields are used (Fig. 15, see Fig. 16 in the color insert). With a relatively central location of the focus, the dose from each field is usually the same; if the target area is eccentric, the dose ratio is changed in favor of the field closest to the tumor, for example, 2: 1, 3: 1, etc.

In cases where the dose is delivered from two non-parallel fields, then the smaller the angle between their central axes, the more the alignment of isodoses is carried out using a cli

novidny filters allowing to homogenize the dose distribution (see fig. 17 on the color insert). For the treatment of deep-seated tumors, three- and four-field radiation techniques are usually used (Fig. 18).

On a linear electron accelerator, a rectangular radiation field of one size or another is formed using metal pins

Figure: 18.Distribution of gamma radiation dose from three fields

limators built into the device. Additional beam shaping is achieved using a combination of these collimators and special blocks (a set of lead blocks or Wood's alloy blocks of various shapes and sizes) attached to the LEA after the collimators. The blocks cover parts of the rectangular field outside the target volume and protect tissues outside the target, thus forming fields of complex configuration.

The latest linear accelerators provide control over the position and movement of the field-forming multi-leaf collimators. Typical multi-lobe collimators have 20 to 80 lobes or more arranged in pairs. Computer control of the position of a large number of narrow, tightly fitting petals makes it possible to generate a field of the required shape. By setting the petals in the required position, a field is obtained that most closely matches the shape of the tumor. The field adjustment is done by changes in the computer file containing the settings for the petals.

When planning a dose, it is taken into account that the maximum dose (95-107%) should be delivered to the planned target volume, while ≥ 95% of this volume receives ≥ 95% of the planned dose. Another necessary condition is that only 5% of the volume of organs at risk can receive ≥ 60% of the planned dose.

Usually, linear accelerators have a dosimeter, the detector of which is built into the device for forming the primary beam of bremsstrahlung radiation, that is, monitoring of the supplied radiation dose is carried out. A dose monitor is often calibrated to a dose reference point at the depth of the ionization maximum.

Dosimetric provision of intracavitary γ-therapy with sources high activitydesigned for the individual formation of dose distributions taking into account the localization, length of the primary tumor, linear dimensions of the cavity. When planning, the calculated data in the form of an atlas of multiplanar isodose distributions applied to the intracavitary γ-therapeutic devices, as well as the data of planning systems for intracavitary devices based on personal computers, can be used.

The presence of a computer planning system for contact therapy allows for clinical and dosimetric analysis for each specific situation with the choice of dose distribution that most fully corresponds to the shape and length of the primary focus, which makes it possible to reduce the intensity of radiation exposure to the surrounding organs.

Before using radiation sources for contact radiation therapy, their preliminary dosimetric certification is carried out, for which clinical dosimeters and sets of tissue-equivalent phantoms are used.

For phantom measurements of dose fields, clinical dosimeters with small-sized ionization chambers or other (semiconductor, thermoluminescent) detectors are used, analyzers

dose field or isodosographs. Thermoluminescent detectors (TLD) are also used to monitor absorbed doses in patients.

Dosimetric devices.Dosimetric devices can be used to measure doses of one type of radiation or mixed radiation. Radiometers measure the activity or concentration of radioactive substances.

The radiation energy is absorbed in the detector of the dosimetry device, which leads to the appearance of radiation effects, the magnitude of which is measured using measuring devices. In relation to the measuring equipment, the detector is a signal sensor. The readings of the dosimetry device are recorded by the output device (dial gauges, recorders, electromechanical counters, sound or light signaling devices, etc.).

According to the method of operation, dosimetry devices are distinguished between stationary, portable (can be carried only when turned off) and wearable. A dosimetric device for measuring the dose of radiation received by each person in the irradiated area is called an individual dosimeter.

Depending on the type of detector, a distinction is made between ionization dosimeters, scintillation, luminescent, semiconductor, photodosimeters, etc.

Ionization chamberis a device for research and registration of nuclear particles and radiation. Its action is based on the ability of fast charged particles to cause ionization of the gas. The ionization chamber is an air or gas electric capacitor, to the electrodes of which a potential difference is applied. When ionizing particles enter the space between the electrodes, electrons and gas ions are formed there, which, moving in an electric field, are collected on the electrodes and are recorded by recording equipment. Distinguish currentand impulseionization chambers. In current ionization chambers, a galvanometer measures the current generated by electrons and ions. Current ionization chambers provide information on the total number of ions formed within 1 s. They are commonly used for measuring radiation intensity and for dosimetry measurements.

In pulsed ionization chambers, voltage pulses are recorded and measured, which arise on the resistance when the ionization current flows through it, caused by the passage of each particle.

In ionization chambers for the study of γ-radiation, ionization is caused by secondary electrons knocked out of gas atoms or walls of ionization chambers. The larger the volume of the ionization chambers, the more ions are formed by secondary electrons; therefore, large-volume ionization chambers are used to measure low-intensity γ-radiation.

The ionization chamber can also be used to measure neutrons. In this case, ionization is caused by recoil nuclei (usually proto-

us), created by fast neutrons, or α-particles, protons or γ-quanta arising from the capture of slow neutrons by nuclei 10 B, 3 He, 113 Cd. These substances are introduced into the gas or the walls of the ionization chambers.

In ionization chambers, the composition of the gas and the substance of the walls is chosen so that under identical irradiation conditions, the same energy absorption (per unit mass) in the chamber and in the biological tissue is provided. In dosimetry devices for measuring exposure doses, the chambers are filled with air. An example of an ionization dosimeter is the MRM-2 micro-roentgenmeter, which provides a measurement range from 0.01 to 30 μR / s for radiation with photon energies from 25 keV to 3 MeV. The readings are counted using a dial gauge.

IN scintillationin dosimetry devices, light flashes arising in the scintillator under the action of radiation are converted by a photomultiplier tube into electrical signals, which are then recorded by a measuring device. Scintillation dosimeters are most often used in radiation protection dosimetry.

IN luminescentdosimetry devices use the fact that phosphors are able to accumulate the absorbed radiation energy and then release it by luminescence under the action of additional excitation, which is carried out either by heating the phosphor or by irradiating it. The intensity of a light flash of luminescence, measured using special devices, is proportional to the radiation dose. Depending on the luminescence mechanism and the method of additional excitation, a distinction is made between thermoluminescent (TLD)and radiophotoluminescent dosimeters.A feature of fluorescent dosimeters is the ability to store information about the dose.

A further stage in the development of luminescent dosimeters was the dosimetric devices based on thermoexoelectronic emission. When heating some phosphors, previously irradiated with ionizing radiation, electrons (exoelectrons) fly out from their surface. Their number is proportional to the radiation dose in the phosphor substance. Thermoluminescent dosimeters are most widely used in clinical dosimetry to measure the dose to the patient, in the body cavity, and also as personal dosimeters.

Semiconductor(crystal) dosimeters change conductivity depending on the dose rate. They are widely used along with ionization dosimeters.

In Russia, there is a radiation metrological service that conducts verification of clinical dosimeters and dosimetric certification of radiation devices.

At the stage of dosimetric planning, taking into account the data of the topometric map and the clinical task, the physical engineer evaluates the dose distribution. The dose distribution obtained in the form of a set of isolines (isodoses) is plotted on a topometric map, and it serves to determine such irradiation parameters as the size of the irradiation field, the location of the centering point of the axes of the radiation beams and their directions.

The single absorbed dose, the total absorbed dose are determined, and the exposure time is calculated. The document is a protocol containing all the parameters of the irradiation of a particular patient at the selected therapeutic unit.

When carrying out brachytherapy, the apparatus is used in conjunction with the corresponding ultrasound equipment, which makes it possible to evaluate the position of the sources and the isodose distribution in the organ in real time thanks to the planning system. Another option is the introduction of sources into the tumor under the control of computed tomography.

A radiation beam of the required shape and specific dimensions is formed using an adjustable diaphragm, a collimating device, replaceable standard and individual protective blocks, wedge-shaped and compensating filters and boluses. They make it possible to limit the area and field of irradiation, to increase the dose gradient at its boundaries, to equalize the distribution of the ionizing radiation dose within the field or, on the contrary, to distribute it with the necessary non-uniformity, to create regions and fields, including curly and multiply connected (with internal shielded areas).

For correct reproduction and control of the patient's individual irradiation program, beam imaging devices, mechanical, optical and laser centralizers, standard and individual fixators for immobilizing the patient during irradiation, as well as X-ray and other means of introscopy are used. They are partially built into the radiation head, patient table and other parts of the apparatus. Laser centralizers are mounted on the walls of the treatment room. X-ray introscopes are placed near the therapeutic beam on a floor or ceiling stand with fixings for adjustment in the required position of the patient.