Photon ionizing radiation. What type of radiation refers to photon radiation Section iii. life safety management and economic mechanisms for its provision

Department of Oncology, Radiation Therapy and Radiation Diagnostics

Head department: prof., d.m.s. Redkin Alexander Nikolaevich

Lecturer: Ph.D. Cherkasova Irina Ivanovna

Abstract on the topic: "Physics of ionizing radiation"

Completed by: Vasilchenko Marina Gennadievna

Radiation used in medical radiology is divided into 2 groups:

1) Ionizing radiation

2) Non-ionizing radiation, which includes radio waves, infrared radiation, and ultrasonic waves in the range of 1-15 MHz.

ionizing radiation- these are electromagnetic radiations that are created during the ionization of atoms, their radioactive decay, forming ions of various signs when interacting with the environment.

Ionizing radiation is conditionally divided into 2 groups:

1) Corpuscular

2) Photonic (quantum)

Corpuscular ionizing radiation

This radiation is a stream of elementary particles:

α - particles, β - particles (electrons, positrons), protons, neutrons, mesons, etc. They have a charge, mass and energy, unlike photons.

alpha radiation represents a stream of nuclei of a helium atom, has a mass of 4 c.u. and charge +2. The energy of alpha particles is 4-7 MeV. The range of alpha particles in the air reaches 8-10 cm, in biological tissue 50-70 micrometers (mk). Since the range of alpha particles in matter is small, and the energy is very high, the ionization density per unit length of the range is very high (up to 10 thousand pairs of ions per 1 cm).

beta radiation - the flow of electrons or positrons during radioactive decay. Beta particles have a mass equal to 1/1838 of the mass of a hydrogen atom, a single negative (beta particle) or positive (positron) charge. The energy of beta radiation does not exceed a few MeV. The range in air is from 0.5 to 2 m, in tissues - 1-2 cm. Their ionizing ability is lower than alpha particles (several tens of pairs of ions per 1 cm of path).

Neutrons are neutral particles having the mass of a hydrogen atom. When interacting with matter, they lose their energy in elastic and inelastic collisions.

When corpuscular radiation interacts with matter, elementary particles transfer their energy to tissue atoms, causing their ionization and decay into opposite charged particles (ions).

Protons and α-particles, having a large mass, charge and energy, move in the tissues in a straight line and form dense clusters of ions.

An electron, having a small mass, travels a winding path in tissues and changes the direction of movement under the action of the electric fields of atoms.

Depending on the mass of nuclei and the energy of neutrons, the latter are divided into fast and slow. Fast neutrons lose energy as a result of collision with hydrogen nuclei or push out protons. Slow and thermal are captured by atoms of light elements such as sodium, phosphorus, chlorine, and they become radioactive (the so-called induced radioactivity).

Quantum ionizing radiation

It is an electromagnetic radiation consisting of photons, particles that do not have mass and charge, but have high energy and move at the speed of light.

Quantum ionizing radiation includes:

- γ-radiation

X-ray radiation (bremsstrahlung; characteristic)

γ radiation- photon radiation that occurs when the energy state of atomic nuclei changes, during nuclear transformations or during particle annihilation. It has energy from several thousand to several million electron volts. It propagates, like X-rays, in air at the speed of light. The ionizing ability of γ-radiation is much less than that of α- and β-particles. γ-radiation has a large penetrating power, which varies over a wide range.

x-ray radiation- photon radiation, consisting of bremsstrahlung and (or) characteristic radiation, occurs in X-ray tubes, electron accelerators, with a photon energy of not more than 1 MeV. It occupies the region of the electromagnetic spectrum between gamma and ultraviolet radiation and represents electromagnetic radiation with a wavelength from 10 -14 to 10 -7 m.

A) Bremsstrahlung - photon radiation with a continuous energy spectrum, which occurs when the kinetic energy of charged particles decreases.

B) Characteristic radiation is a photon radiation with a discrete energy spectrum that occurs when electrons change energy levels.

X-ray radiation, like gamma radiation, has a high penetrating power and a low ionization density of the medium.

Main properties x-rays

Invisibility- sensitive cells of the human retina do not react to x-rays, since their wavelength is thousands of times smaller than that of visible light;

Rectilinear propagation- rays are refracted, polarized (propagated in a certain plane) and diffracted, like visible light. The refractive index differs very little from unity;

photographic action- decompose silver halide compounds, including those found in photographic emulsions, which makes it possible to obtain x-rays;

penetrating power, on which X-ray diagnostics is based, depends on the density of tissues. Thus, the bone tissue has the highest density, and hence the absorbing capacity, therefore, during X-ray examination, it gives a darkening of high intensity. Parenchymal organs also look like darkening, but they delay X-rays 2 times less, and darkening has an average intensity. Air does not hold back the rays and creates enlightenment, like, for example, lung tissue, which is represented by alveoli filled with air.

Luminescent action- cause the luminescence of a number of chemical compounds (phosphors), this is the basis of the X-ray transmission technique. The intensity of the glow depends on the structure of the fluorescent substance, its amount and distance from the source of x-rays. Phosphors are used not only to obtain an image of the objects under study on a fluoroscopic screen, but also in radiography, where they make it possible to increase the radiation exposure to a radiographic film in a cassette due to the use of intensifying screens, the surface layer of which is made of fluorescent substances;

ionizing property is that under the action of X-rays in any medium through which they pass, ions are formed, by the number of which the radiation dose is judged. The method of dosimetry is based on this property - dose measurement using various types of special devices - dosimeters. Dosimetry is carried out by special departmental services.

Biological or damaging effect ionizing radiation on the human body makes it necessary to protect both the staff of X-ray rooms and patients from it when implementing methods of X-ray diagnostics. At the same time, this property is used in radiation therapy for the treatment of both tumor and non-tumor diseases.

Inverse square law- for a point source of X-ray radiation, the intensity decreases in proportion to the square of the distance to the source.

Types of interaction of photons with atoms of the medium:

A) Photoelectric effect (at low photon energies) - a photon pulls electrons out of an atom, giving it its energy.

B) Compton inelastic scattering of electrons on atoms (with energies up to 1 MeV) - both the photon itself and the recoil electrons formed by it also cause ionization of matter. The photon knocks out the electron, giving up some of the energy and changing its direction. Both of these effects cause the maximum absorption of energy in the surface layer of tissues (up to 0.5 cm), here the largest number of secondary electrons is formed.

C) The formation of pairs of elementary particles (at an energy greater than 1 MeV) - causes a maximum of ionization acts in the depths of tissues. Electron-positron pairs are formed. The photon itself disappears. The positron quickly loses energy and combines with the oncoming electron. After that, both particles disappear (annihilation) and instead of them, 2 photons appear, which diverge in opposite directions. Their energy is 2 times less than the initial photon.

Radiation dose units

Absorbed dose

Absorbed dose(D) - a value equal to the ratio of the energy ΔΕ transferred to the element of the irradiated substance to the mass Δm of this element:

The SI unit of absorbed dose is gray (Gy), in honor of the English physicist and radiobiologist Louis Harold Grey.

1 Gr - This is the absorbed dose of ionizing radiation of any kind, at which 1 kg of the mass of a substance absorbs the energy of 1 J of radiation energy.

In practical dosimetry, an off-system unit of absorbed dose is usually used - glad(1 glad= 10 -2 Gr).

Dose equivalent

Value absorbed dose takes into account only the energy transferred to the irradiated object, but does not take into account the "radiation quality". concept radiation quality characterizes the ability of a given type of radiation to produce various radiation effects. To assess the quality of radiation, a parameter is introduced - quality factor It is a regulated value, its values ​​are determined by special commissions and included in international standards designed to control radiation hazard.

Ionizing radiation can be conditionally divided into photon and corpuscular. Photon radiation is electromagnetic vibrations, to the corpuscular particle flow. The concepts of "electromagnetic", "quantum", "photon" radiation can be considered equivalent.

The type of interaction of photons with atoms of matter depends on the energy of 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 = 1.6 x 10 19 J. Multiple units: 1 keV = 10 3 eV; 1 MeV = 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.

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

Characteristic radiation occurs when the energy state of atoms changes. When an electron is knocked out of the inner shell

an atom by an electron or a photon, the atom goes into an excited state, and the vacated 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 energy difference at the corresponding levels. Characteristic radiation has a linear spectrum with wavelengths determined for a given substance, which, like the intensity of the lines of the characteristic X-ray spectrum, are determined by the atomic number of the element Z and the electronic structure of the atom.

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

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 ones. X-ray radiation of close-focus X-ray therapy devices is generated at an anode voltage of less than 100 kV, remote - up to 250 kV.

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

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 an antiparticle (for example, an electron and a positron).

Sources of gamma radiation are radionuclides. Each radionuclide emits γ-quanta of its specific energy. Radionuclides are produced at 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 per second occurs. The non-systemic unit of activity is the Curie (Ci). 1 Ki \u003d 3.7 x 10 10 Bq.

Sources of γ-radiation for remote and intracavitary radiation therapy are 60 Co and 137Cs. The drugs most commonly used 60Co with an average photon energy of 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 process of formation of electron-positron pairs occur.

photoelectric effect consists in the interaction of a gamma quantum with a bound electron of an atom (Fig. 10). In photoelectric absorption, all the energy of the incident photon is absorbed by the atom from which the electron is ejected. 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 transform into the energy of the characteristic X-ray radiation. Thus, during 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 an ejected electron - a photoelectron - at the end of its run loses energy, 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. photoelectric effect

Rice. eleven. Compton effect

Compton effect (incoherent scattering) occurs at a photon energy from 120 keV to 20 MeV, that is, with all types of ionizing radiation used in radiation therapy. With the Compton effect, the incident photon loses part of its energy as a result of elastic collision with electrons and changes the direction of the initial movement, 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 the primary photon into the kinetic energy of the electron and positron and into the energy of annihilation radiation. The quantum energy must be greater than 1.02 MeV (twice the electron rest energy). Such interaction of quanta with matter occurs when patients are irradiated at high-energy linear accelerators with a high-energy bremsstrahlung beam. The photon disappears in the Coulomb field of the nucleus (or electron).

Rice. 12. Formation of electron-positron pairs

In this case, the entire energy of the incident photon is transferred to the resulting pair minus the rest energy of the pair. 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) arises. The ionization ability of particles is much greater than that of photon radiation. When alternating 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 radiation avalanche, which decays with decreasing energy of each newly formed photons and particles.

The interaction of X-rays with matter is accompanied by its ionization and is determined by two main effects - photoelectric absorption and Compton scattering. When high-energy bremsstrahlung 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 varies in space inversely with the square of the distance.

Corpuscular radiation- streams of charged particles: electrons, protons, heavy ions (for example, carbon nuclei) with energies of several hundred MeV, as well as neutral particles - neutrons. Particle beam irradiation is now called hadron therapy. To hadrons (from the Greek word hadros- “heavy”) include nucleons, protons and neutrons included in them, as well as π -mesons, etc. The sources of particles are accelerators and nuclear reactors. Depending on the maximum energy of the accelerated protons, the accelerators are conditionally divided into 5 levels, and the accelerators of the 5th level with Ep > 200 MeV (meson factories)

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

high energy electron beam is generated by the same electron accelerators as in the production of bremsstrahlung. Electron beams with energy from 6 to 20 MeV are used. High-energy electrons have a high penetrating power. The mean free path of such electrons can reach 10-20 cm in the tissues of the human body. The electron beam, being absorbed in the tissues, creates a dose field at which the ionization maximum is formed near the surface of the body. Beyond the ionization maximum, the dose falls off quite rapidly. On modern linear accelerators, it is possible to regulate the energy of the electron beam, and, accordingly, create the required dose at the required depth.

Neutron is 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 thermal (slow) neutrons with an energy of 0.025 eV on biological tissue occurs under the action of protons formed in the (n, p) reaction and losing all their energy at the birthplace. 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 release of energy 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 exposure to neutrons is neutron capture therapy, which is a binary radiotherapy method that combines two components. The first component is a stable isotope of boron 10 B, which, when administered as part of the drug, can accumulate in the cells of certain types of brain tumors and melanomas. The second component is the flux of low-energy thermal neutrons. The heavy high-energy charged particles formed as a result of the capture of a thermal neutron by the 10 B nucleus (boron decays 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, preparations with gadolinium are promising in neutron capture therapy. For deep-seated tumors, it is promising to use epithermal neutrons in the energy range of 1 eV - 10 keV, which have a high penetrating power and, slowing down in tissues to thermal energies, allow neutron capture therapy of tumors located at a depth of up to 10 cm. Obtaining high fluxes of thermal and epithermal neutrons is carried out using a nuclear reactor.

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

my tumor. As a result, a large dose gradient is formed on the surface of the body and in the depth of the irradiated object, after which a sharp attenuation of the energy occurs. By changing the beam energy, it is possible to change the place of its complete stop in the tumor with great accuracy. Beams of protons with an energy of 70-200 MeV and the technique of multifield irradiation from different directions are used, in which the integral dose is distributed over a large area of ​​superficial tissues. During irradiation at the synchrocyclotron at PNPI (Petersburg Institute of Nuclear Physics), a fixed energy of the extracted proton beam is used - 1000 MeV and the continuous irradiation technique is used. Protons of such high energy easily pass through the irradiated object, producing uniform ionization along their path. In this case, a small scattering of protons in the substance occurs, therefore, a narrow proton beam with sharp boundaries 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 rotational irradiation technique, a very high dose ratio in the irradiation zone to the dose on the surface of the object is provided - about 200:1. A narrow proton beam with a half-intensity size of 5-6 mm is used to treat various brain diseases, such as arteriovenous malformations of the brain, pituitary adenomas, etc. carbon ions turns out to be several times higher in the Bragg peak than that of protons. Multiple double breaks of the DNA helix of the atoms of the irradiated volume occur, which after that can no longer be restored.

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

The advantages of using high-energy radiation for the treatment of malignant tumors located at depth are, with increasing energy, an increase in the deep dose and a decrease in the surface dose, a higher penetrating power with an increase in the relative deep dose, and a smaller difference between the absorbed dose in the bones and soft tissues. In the presence of a linear accelerator or a betatron, there is no need to dispose of a radioactive source, as with the use of 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 a high ionizing ability and low penetrating power, they 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 the 149 Tb radionuclide with a half-life of 4.1 hours for these purposes will be used.

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 the 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 a half-life of 375.59 days, 106 Rh, β - - emitter with an energy of 3540.0 keV and a half-life of 29.8 s. Both β-emitters 106 Ru + 106 Rh are included in the sets of ophthalmic applicators.

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 radionuclide 89 Sr is a practically pure β-emitter with a half-life of 50.6 days and an average energy of β-particles 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, a half-life of 46.2 h is part of the domestic drug samarium-oxabiphor, designed 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 labeled antibody radioimmunotherapy, treatment of liver tumors, and rheumatoid arthritis.

The radionuclide 59 Fe as part of a tableted radiopharmaceutical is used in the Russian Scientific Center for Roentgen Radiology (Moscow) for the treatment of patients with breast cancer. The principle of action of the drug, according to the authors, is the distribution of iron by blood flow, selective accumulation in tumor tissue cells and exposure to 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 to treat joint diseases, and balloon catheters with sodium perrhenate solution are used for endovascular brachytherapy. It is believed that there is a prospect for the use of β + -emitter 48 V with a half-life of 16.9 days for intracoronary brachytherapy using an arterial stent made of an alloy of titanium and nickel.

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 ​​the 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 - electronic capture with the transformation of iodine into tellurium and the release of a γ-quantum), is used as a closed microsource for brachytherapy. Half-life - 60.1 days.

mixedγ+ neutron radiation is inherent in 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 irradiation means that provide the optimal spatial and 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 already known physical laws of interaction of various types of radiation with matter. With the help of experimental methods, treatment situations are modeled with measurements in tissue-equivalent phantoms.

The tasks 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 on patients;

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

Carrying out absolute calibration of detectors for clinical dosimetry;

Conducting experimental studies of new therapeutic methods of irradiation.

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

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

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

Absorbed dose- this is 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 the substance in this volume:

where D is the absorbed dose,

E - average radiation energy,

m is the mass of the substance per unit volume.

Gray (Gy) is adopted as the unit of absorbed radiation dose in SI in honor of the English scientist L. H. Gray, known for his work in the field of radiation dosimetry. 1 Gy is equal to the absorbed dose of ionizing radiation, at which the energy of ionizing radiation equal to 1 J is transferred to a substance weighing 1 kg. In practice, an off-system unit of the absorbed dose, rad (radiation absorbed dose), is also common. 1 rad \u003d 10 2 J / kg \u003d 100 erg / g \u003d 10 2 Gy or 1 Gy = 100 rad.

The absorbed dose depends on the type, intensity of radiation, its energy and quality composition, exposure time, and also on the composition of the substance. The dose of ionizing radiation is the greater, the longer the radiation time. The dose increment per unit time is called dose rate, which characterizes the rate of ionizing radiation dose accumulation. It is allowed to use various special units (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, as a rule, higher than in the 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 nuclei of carbon, hydrogen, oxygen, and nitrogen. The absorbed dose in a biological substance depends on the energy of the neutrons, since neutrons of different energies selectively interact with the nuclei of the substance. In this case, charged particles, γ-radiation can appear, as well as radioactive nuclei, which themselves become sources of ionizing radiation.

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

Absorption of radiation energy causes processes leading to various radiobiological effects. For a specific type of radiation, the output of radiation-induced effects in a certain way

is related to the absorbed radiation energy, often a simple proportional relationship. This allows the radiation dose to be taken as a quantitative measure of the consequences of exposure, in particular to 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.

The RBE of radiation depends mainly on differences in the spatial distribution of ionization events 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 (LEP)< 10 кэВ/мкм) и плотноионизирующие (ЛПЭ >10 keV/μm) types of radiation.

Biological effects that occur with different types of ionizing radiation are usually compared with similar effects that occur in an X-ray field with a cutoff photon energy of 200 keV, which is taken as exemplary.

RBE coefficient determines the ratio of the absorbed dose of a standard radiation that causes a certain biological effect to the absorbed dose of a given radiation that gives the same effect.

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

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

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

Dose equivalent is defined as the average value of the absorbed dose in an organ or tissue, taking into account the average radiation weighting factor.

where H is the equivalent absorbed dose,

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

The SI unit of equivalent dose 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 radiation dose (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 a living tissue of a standard composition and the average radiation coefficient (W R) is equal to 1 J/kg.

In practice, an off-system unit of equivalent dose is also common - rem(1 Sv \u003d 100 rem), if in the same formula the absorbed radiation dose is expressed in rads.

Weighting coefficients for individual types of radiation when calculating the equivalent dose.

Effective equivalent dose- a concept used for 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 the organ or tissue and the corresponding weight factor (weighting factor) for the most important human organs:

where E is the effective equivalent dose,

H T - equivalent dose in the organ or tissue T,

W T - weighting factor for the organ or tissue T.

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

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

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

In practice, an off-system unit of exposure dose is often used. X-ray (R)- named after the German physicist 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 ​​the 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, one can 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 the specified mode of exposure.

Currently, more often as a dosimetric quantity characterizing the radiation field, kerma(KERMA is an abbreviation for 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 radiation interaction with this substance. Under certain conditions, kerma is equal to the absorbed radiation dose. For photon radiation in air, it is the energy equivalent of the exposure dose. The dimension of kerma is the same as that of absorbed dose, expressed as 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 field- this is the spatial distribution of the absorbed dose (or its power) in the irradiated part of the patient's body, tissue-equivalent medium or dosimetric phantom that models the patient's body according to the physical effects of the interaction of radiation with matter, 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 identical values ​​(absolute or relative) of the absorbed dose. Such curves are called isodoses, and their families - 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 should correspond to the target to be irradiated (that is, the area covering the clinically detected tumor and the expected area of ​​its spread).

The physical characteristic of the irradiation field is characterized by various parameters. The number of particles that penetrate the medium is called fluence. The sum of all penetrating particles and particles scattered in a given medium is flow ionizing 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 energy flow to the area of ​​an object. The radiation intensity depends on the particle flux density. Except linear power transmission (LET), characterizing the average energy losses of particles (photons), determine the linear ionization density (LPI), the number of pairs of ions per unit path length (track) of a particle or photon.

Formation of the dose field depends on the type and source of radiation. When forming the dose field with photon radiation, it is taken into account that the intensity of the photon radiation of a point source falls in the medium in inverse proportion 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 excitation energy of atoms, leading to secondary radiation, which also causes ionization. For photon radiation, the average ionization energy is equal to the average energy of ion formation of electrons released by photons.

The dose distribution of the γ-radiation beam is uneven. The 100% isodose section has a relatively small width, and then the relative dose value falls along the curve quite 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 drop in the dose at the field boundary, which is 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-ray beam, since the doses of irradiation of healthy organs and tissues near the pathological focus are reduced (Table 2.2).

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

Note. Distance source-surface for X-ray therapy apparatus - 50 cm; gamma therapeutic - 80 cm; linear accelerators - 100 cm.

From the data in Table. Figure 2.2 shows that megavolt radiation, in contrast to orthovoltage x-rays, has a maximum dose not on the skin surface, its depth increases with increasing radiation energy (Fig. 13). After the electrons reach the maximum, a steep dose gradient is noted, which makes it possible to reduce the dose load on the underlying healthy tissues.

Protons are distinguished by the absence of radiation scattering in the body, the possibility of beam deceleration at a given depth. At the same time, with the penetration depth, the linear energy density (LED) increases, the absorbed dose increases, reaching a maximum at the end of the particle path,

Rice. 13. Energy distribution 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 - at bremsstrahlung 25 MeV; 4 - when irradiated with fast electrons 17 MeV; 5 - when irradiated with 190 MeV protons; 6 - when irradiated with slow neutrons 100 keV

Fig.14. Bragg Peak

Rice. fifteen. Dose distribution of gamma radiation from two open parallel opposite fields

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

Often during irradiation, parallel opposite fields are used (Fig. 15, see Fig. 16 on the color inset). With a relatively central location of the focus, the dose from each field is usually the same; if the target location 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 those cases when the dose is delivered from two non-parallel fields, then the smaller the angle between their central axes, the more isodose equalization is carried out using the client.

novel filters that allow homogenizing the dose distribution (see Fig. 17 on the color insert). For the treatment of deep-seated tumors, three- and four-field irradiation techniques are usually used (Fig. 18).

On a linear electron accelerator, a rectangular radiation field of various sizes is formed using metal collets.

Rice. eighteen. Dose distribution of gamma radiation from three fields

limators built into the apparatus. Additional beam shaping is achieved by 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 LAE 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 allow you to control the positions and movement of the field-forming multileaf collimators. Typical multi-leaf collimators have 20 to 80 or more blades arranged in pairs. Computer control of the position of a large number of narrow petals tightly adjacent to each other makes it possible to generate a field of the required shape. By setting the petals to the desired position, a field is obtained that most closely matches the shape of the tumor. The adjustment of the field is carried out by means of changes in the computer file containing the settings for the petals.

When planning the 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 bremsstrahlung beam, that is, the supplied radiation dose is monitored. The dose monitor is often calibrated to dose at a reference point at the maximum ionization depth.

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

The presence of a computer-aided planning system for contact therapy makes it possible to carry out clinical and dosimetric analysis for each specific situation with the choice of dose distribution that most fully corresponds to the shape and extent of the primary focus, which makes it possible to reduce the intensity of radiation exposure to 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, analyzers

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

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

Radiation energy is absorbed in the detector of a dosimetric device, leading 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 dosimetric device are recorded by the output device (pointers, recorders, electromechanical counters, sound or light signaling devices, etc.).

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

Depending on the type of detector, there are ionization dosimeters, scintillation, luminescent, semiconductor, photodosimeters, etc.

Ionization chamber- a device for the study and registration of nuclear particles and radiation. Its action is based on the ability of fast charged particles to cause gas ionization. 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 recorded by the recording equipment. Distinguish current and impulse ionization chambers. In current ionization chambers, a galvanometer measures the current generated by electrons and ions. Current ionization chambers give information about the total number of ions formed during 1 s. They are commonly used for measuring radiation intensity and for dosimetric measurements.

In pulsed ionization chambers, voltage pulses are recorded and measured, which occur 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 due to secondary electrons knocked out of gas atoms or the 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 by 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 in such a way that, under identical irradiation conditions, the same absorption of energy (per unit mass) in the chamber and biological tissue is ensured. In dosimetric devices for measuring exposure doses, the chambers are filled with air. An example of an ionization dosimeter is the MRM-2 microroentgenmeter, which provides a measurement range from 0.01 to 30 μR/s for radiation with photon energies from 25 keV to 3 MeV. Reading of indications is made on the pointer device.

AT scintillation In dosimetric devices, light flashes that occur in the scintillator under the action of radiation are converted by a photomultiplier into electrical signals, which are then recorded by the measuring device. Scintillation dosimeters are most often used in radiation protection dosimetry.

AT luminescent Dosimetric 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 luminescence light flash, measured using special devices, is proportional to the radiation dose. Depending on the mechanism of luminescence and the method of additional excitation, there are thermoluminescent (TLD) and radiophotoluminescent dosimeters. A feature of luminescent dosimeters is the ability to store dose information.

A further stage in the development of luminescent dosimeters was dosimetric instruments based on thermionic emission. When heated, 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 for measuring dose on the patient, in the body cavity, and as personal dosimeters.

Semiconductor(crystal) dosimeters change conductivity with dose rate. Widely used along with ionization dosimeters.

Russia has a radiation metrological service that verifies clinical dosimeters and performs 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 engineer-physicist evaluates the dose distribution. The dose distribution obtained in the form of a set of isolines (isodose) is applied to 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 radiation beam axes 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 irradiation of a particular patient at the selected therapeutic unit.

When conducting brachytherapy, the device is used together with the appropriate ultrasound equipment, which makes it possible to evaluate the position of sources and isodose distribution in the organ in a real-time system thanks to a 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 certain dimensions is formed using an adjustable diaphragm, a collimating device, replaceable standard and individual protective blocks, wedge-shaped and compensating filters and boluses. They allow limiting the area and field of irradiation, increasing the dose gradient at its boundaries, leveling the distribution of the dose of ionizing radiation inside the field or, on the contrary, distributing it with the necessary unevenness, creating areas and fields, including curly and multiply connected (with internal shielded areas).

For correct reproduction and control of an individual patient irradiation program, beam visualization devices, mechanical, optical and laser centralizers, standard and individual fixators for patient immobilization during irradiation, as well as X-ray and other introscopy tools are used. Partially, they are built into the radiation head, the 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 locks for adjustment, in the required position of the patient.

All ionizing radiations are divided into photon and corpuscular.

Photon-ionizing radiation includes:

• a) Y-radiation emitted during the decay of radioactive isotopes or particle annihilation. Gamma radiation is, by its nature, short-wavelength electromagnetic radiation, i.e. a stream 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 the 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 produce ionization, but in the process of interaction with the medium they release charged particles (electrons, protons) that can ionize the atoms and molecules of the medium through which they pass:

a) neutrons - the only uncharged particles formed in some reactions of fission of the nuclei 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 dramatically increases the danger of neutron radiation. The penetrating power of neutrons is comparable to Y-radiation. Depending on the level of carried energy, fast neutrons (with energies from 0.2 to 20 MeV) and thermal neutrons (from 0.25 to 0.5 MeV) are conditionally distinguished. 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 (boron steel, boral, boron graphite, cadmium-lead alloy).

Alpha, beta and gamma particles have an energy of only a few megaelectronvolts, and cannot create induced radiation;

• b) beta particles - electrons emitted during the radioactive decay of nuclear elements with an intermediate ionizing and penetrating power (run 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 a low penetrating ability (run 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, as they are able to knock out electrons from 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 pairs of ions.

Rice. one

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

Each radionuclide has an invariable, unique half-life (the time required for the 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.

Photon IRs include radiation from radioactive substances, characteristic and bremsstrahlung generated by various accelerators. The LPI of photon radiation is the lowest (1-2 pairs of ions per 1 cm 3 of air), which determines its high penetrating ability (the path length in air is several hundred meters).

-radiation occurs during radioactive decay. The transition of the nucleus from the excited to the ground state is accompanied by the emission of a -quantum with energies from 10 keV to 5 MeV. The main therapeutic sources - radiation are - devices (guns).

Bremsstrahlung X-ray arises due to the acceleration and sharp deceleration of electrons in vacuum systems of various accelerators and differs from X-ray by a higher photon energy (from one to tens of MeV).

When a photon flux passes through a substance, it is weakened as a result of the following interaction processes (the type of interaction of photons with atoms of a substance depends on the photon energy):

Classical (coherent or Thompson scattering) - for photons with energy from 10 to 50-100 keV. The relative frequency of this effect is small. An interaction takes place, which does not play a significant role, since the incident quantum, colliding with an electron, is deflected, and its energy does not change.

Photoelectric absorption (photoelectric effect) - at relatively low energies - from 50 to 300 keV (plays a significant role in X-ray therapy). The incident quantum knocks out an orbital electron from the atom, is itself absorbed, and the electron, having slightly changed direction, flies away. This escaped electron is called a photoelectron. Thus, the energy of a photon is spent on the work function of the electron and on giving it kinetic energy.

Compton effect (incoherent scattering) - occurs at a photon energy from 120 keV to 20 MeV (i.e., almost the entire spectrum of radiation therapy). The incident quantum knocks out an electron from the outer shell of the atom, transferring part of the energy to it, and changes its direction. The electron flies out of the atom at a certain angle, and the new quantum differs from the original one not only in a different direction of motion, but also in lower energy. The resulting quantum will indirectly ionize the medium, and the electron - directly.

The process of formation of electron-positron pairs - the quantum energy must be greater than 1.02 MeV (twice the rest energy of the electron). This mechanism has to be taken into account when a patient is irradiated with a beam of high-energy bremsstrahlung, i.e., on high-energy linear accelerators. Near the nucleus of an atom, the incident quantum experiences acceleration and disappears, transforming into an electron and a positron. A positron quickly combines with an oncoming electron, and the process of annihilation (mutual annihilation) occurs, and instead two photons appear, the energy of each of which is half the energy of the original photon. Thus, the energy of the primary quantum is converted into the kinetic energy of the electron and into the energy of the annihilation radiation.

Photo nuclear takeover - the quantum energy must be more than 2.5 MeV. A photon is absorbed by the nucleus of an atom, as a result of which the nucleus passes into an excited state and can either give up an electron or fall apart. This is how neutrons are produced.

As a result of the above processes of interaction of photon radiation with matter, secondary photon and corpuscular radiation (electrons and positrons) arises. The ionization ability of particles is much greater than that of photon radiation.

The spatial attenuation of the photon beam occurs according to an exponential law (the inverse square law): the radiation intensity is inversely proportional to the square of the distance to the radiation source.

Radiation in the energy range from 200 keV to 15 MeV has found the widest application in the treatment of malignant neoplasms. Great penetrating power allows you to transfer energy to deeply located tumors. This sharply reduces the radiation exposure to the skin and subcutaneous tissue, which allows you to bring the required dose to the lesion without radiation damage to these areas of the body (unlike soft x-rays). With an increase in photon energy above 15 MeV, the risk of radiation damage to tissues at the exit from the beam increases.

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

Figure 5.10 shows a primitive diagram of an X-ray machine that demonstrates what has been said.

Fig.5.10. Primitive scheme of the 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 thickness of the target, and is in the range from 10 5 to 10 14 s -1. Approximately, the source power can be expressed by the formula:

J ~ i Z V 2 (5.34),

wherein i- current on the tube, Z is the atomic number of the target material, V- voltage on the tube.

The energy distribution of 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 Fig. 5.11.

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

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

The fundamental difference between a linear electron accelerator and an X-ray machine is only in the energy of accelerated electrons, which in X-ray machines usually does not exceed 400 keV, and on accelerators reaches tens MeV. This is also manifested 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 of electron accelerators, the often shown spectral distribution is replaced by a monoenergetic one with an 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 >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 emitted photons (Fig. 5.12).

Fig.5.12. Angular distribution of photons emitted from an 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 voltages on the tube, 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, according to the reaction (γ,n), powerful accompanying neutron radiation arises, which sometimes determines the radiation situation near the accelerator.

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

Instantaneous gamma radiation represents gamma quanta emitted during the fission of a heavy nucleus and the decay of short-lived fission products, i.e. photon radiation emitted over time t<5·10 -7 с after the fission reaction. The total energy of this gamma radiation is approximately 7 MeV/div, the spectrum of emitted photons decreases 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.

Fission product gamma radiation 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 formed fission products. In general, approx. 1000 radionuclides - fission products, each of which has a spectrum of discrete energy lines of gamma rays 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 operating time of the reactor and the time of its shutdowns. Fission product activities at any point in time can be calculated from data on independent or cumulative yields of fission products and 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 with medium energy 0.8 MeV and a total energy of about 7.5 MeV/div.

Capture gamma radiation occurs when neutrons are captured, 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 structures surrounding it, including the biological protection of the reactor. If, as a first approximation, we assume that in the process of division 235 U formed by thermal neutrons 2,43 neutron/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 ​​for 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 neutron binding energy 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 rays. 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 heavy materials. Taking into account the low values ​​of the cross sections of inelastic scattering and the low energy of the resulting gamma rays (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 capture gamma radiation.

Activation gamma radiation due to neutron capture reactions by stable nuclei of reactor materials with the formation of radioactive nuclides. This is mainly due to reactions (n,γ) or (n,p). When choosing the structural elements of the reactor, all measures are taken to reduce the concentrations 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 activation radiation radionuclides are well known, since they belong to the radionuclides described above.

It should be noted the features of the formation of gamma radiation fields of the reactor. If the instantaneous, capture, gamma radiation of inelastic neutron scattering and the short-lived activation activity of the coolant of the 1st circuit are formed only during the operation of the reactor, and it is these sources that determine its safe operation, then the gamma radiation of fission products accumulated during the operation of the reactor and long-lived radionuclides of activation radiation determine the gamma radiation of a shutdown reactor, and, consequently, determine the issues of handling spent nuclear fuel and radioactive waste accumulated in 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 source of neutrons . Nuclear fission can be carried out under the action of various elementary particles (neutrons, protons, alpha particles, etc.) or photons that carry significant energy. It is mainly heavy nuclei that are subject to fission. Of all the known fission reactions, reactions under the action of neutrons are of the greatest practical importance. One of the conditions for the fission of an excited nucleus, which is formed during the capture of a neutron, is the excess of the excitation energy of a certain threshold - the critical energy E cr, i.e. E + E St> E cr, where E is the kinetic energy of the incident neutron, and E St is the binding energy of the neutron in the nucleus. For isotopes 231 Pa, 232 Th, 237 Np and 238 U, etc. E cr> E St, so their fission requires neutrons with high kinetic energy ( E >1 MeV), or fast neutrons. At the same time for 233 U, 235 U, 239 Pu and 241 Pu E light> E cr. This ratio explains the ability of these isotopes to fission on thermal neutrons; such nuclides are called fissile.

In general, the reaction of neutron capture, the formation of a compound nucleus and the subsequent realization of its excited state, for example, 235 U can be written in the following form:

92 236 U + γ

(absorption without fission -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 produced. For example, during the fission of uranium, two new neutrons are more often produced (up to 30%), less often one, three or even four neutrons (up to 25%). In some fission events, secondary neutrons are not produced at all (up to 10%).

An important point determining the possibility of developing a fission chain reaction is the average number of secondary neutrons ν per 1 fission event. Table 5.4 shows the values ​​of ν for the main fissile nuclides during fission by thermal and 238 U fast neutrons.