GAMMA RADIATION

From Big Medical Encyclopedia

GAMMA RADIATION — the electromagnetic radiation emitted at radioactive decay and nuclear reactions, i.e. upon transition of an atomic nucleus from one power state in another.

- and. apply in medicine to treatment of tumors (see. Gamma therapy , Radiation therapy ), and also for sterilization of rooms, the equipment and medicines (see. Sterilization , cold). As G.'s sources - and. use gamma-emitters — natural and artificial radioisotopes (see. Isotopes , radioactive) in the course of

which disintegration gamma quanta are let out. Gamma-emitters apply to production of sources of G. - and. various intensity and a configuration (see. Gamma devices ).

By the nature gamma-rays are similar to x-ray, infrared and ultraviolet rays, and also to visible light and radio waves. These types electromagnetic radiation (see) differ only in conditions of education. E.g., as a result of braking of quickly flying charged particles (electrons, alpha particles or protons) arises bremsstrahlung (see); upon various transitions of atoms and molecules from excited state in unexcited there is an emission of visible light, infrared, ultra-violet or characteristic x-ray emission (see).

In the course of interaction with substance electromagnetic radiation shows as wave properties (interferes, refracts, diffracts), and corpuscular. Therefore it can be characterized on wavelength or to consider as a flow of uncharged particles — quanta (photons) which have a certain mass of Mk and energy (E = hv where h = 6,625×10 27 эрг×сек — quantum of action, or a constant Level, ν = c/λ — the frequency of electromagnetic radiation). Than the high frequency and consequently also energy of electromagnetic radiation, subjects its corpuscular properties are to a large extent shown.

Properties of different types of electromagnetic radiation do not depend on a way of their education and are defined by wavelength (λ) or energy of quanta (E). At the same time it must be kept in mind that power border between brake and G. - and. does not exist, unlike such types of electromagnetic radiation as radio waves, visible light, ultraviolet and infrared radiation, the certain range of energy (or lengths of waves) which is almost not blocked is characteristic of each of which. So, energy of the gamma quanta which are let out in the course of radioactive decay (see. Radioactivity ), the kilo-electron-volt to several megaelectron-volt lies ranging from several tens, and at some nuclear transformations can reach tens of mega-electron-volts. At the same time on modern accelerators the bremsstrahlung with energy from zero to hundreds and thousands of mega-electron-volts is generated. However brake and G. - and. significantly differ not only under the terms of education. A range of a bremsstrahlung — continuous, and G.'s range - and., as well as a range of characteristic radiation of atom — discrete (line). It is explained by the fact that kernels, just as atoms and molecules, can be only in certain power states, and transition to another comes from one state in steps.

In the course of passing through substance gamma quanta interact with electrons of atoms, electric field of a kernel, and also with a kernel. Easing of intensity of primary bunch of G. results - and. generally due to three effects: photo-electric absorption (photoeffect), incoherent dispersion (Compton effect) and formations of couples.

Photo-electric absorption — process of interaction with electrons of atoms, at Krom gamma quanta transfer them all the energy. As a result the gamma quantum disappears, and its energy is spent for a separation of an electron from atom and the message to it a motive energy. In this case energy of gamma quantum is transferred preferential to the electrons which are on the K-cover (i.e. on a cover, the closest to a kernel). With increase in atomic number of substance absorber (z) the probability of photoeffect grows approximately in proportion to the 4th degree of atomic number of substance (z 4 ), and with increase in energy of gamma quanta the probability of this process sharply decreases.

Incoherent dispersion — interaction with electrons of atoms, at Krom gamma quantum transfers to an electron only a part of the energy and number of the movement and after collision changes the direction of the movement (dissipates). In this case interaction happens generally to external (valent) electrons. With increase in energy of gamma quanta the probability of incoherent dispersion decreases, but more slowly, than probability of photoeffect. The probability of process increases in proportion to increase in atomic number of substance absorber, i.e. approximately in proportion to its density.

Formation of couples — process of interaction of G. - and. with electric field of a kernel, to-rogo transformation of gamma quantum into couple of particles results: electron and positron. This process is observed only at energy of gamma quantum more than 1,022 Mev (more than the sum of the energy interconnected with a mass of rest of an electron and positron); with increase in energy of gamma quantum the probability of this process increases in proportion to a square of atomic number of substance absorber (z 2 ).

Along with basic processes of interaction of G. - and. with substance coherent (classical) dispersion of G. is observed - and. It is such process of interaction with electrons of atom, as a result to-rogo gamma quantum changes only the direction of the movement (dissipates), and its energy does not change. Before process of dispersion and after it the electron remains connected with atom, i.e. its power state does not change. This process sushchestven only for G. - and. with energy to 100 kev. At a radiation energy higher than 100 kev the probability of coherent dispersion on 1 — 2 order is less, than incoherent. Gamma quanta can interact also with atomic nuclei, causing various nuclear reactions (see), called by photonuclear. The probability of photonuclear reactions is several orders less, than probability of other processes of interaction of G. - and. with substance.

Thus, at all basic processes of interaction of gamma quanta with substance a part of a radiation energy will be transformed to a motive energy of electrons which, passing through substance, make ionization (see). Ionization in complex chemical substances is resulted by change of their chemical properties, and these changes finally bring in living tissue to biol, to effects (see. Ionizing radiation , biological effect).

Specific weight of each of the specified processes of interaction of G. - and. with substance depends on energy of gamma quanta and atomic number of substance absorber. So, in air, water and biol, fabrics absorption due to photoeffect makes 50% at G.'s energy - and., equal about 60 kev. At energy 120 kev the share of photoeffect makes only 10%, and since 200 kev the basic process causing G.'s weakening - and. in substance, incoherent dispersion is. For substances with average atomic number (iron, copper) the share of photoeffect is insignificant at energy more than 0,5 Mev; for lead the photoeffect needs to be considered to G.'s energy - and. about 1,5 — 2 Mev. Process of formation of couples begins to play a nek-ry role for substances with small atomic number approximately from 10 Mev, and for substances with big atomic number (lead) — from 2,5 — 3 Mev. G.'s weakening - and. in substance occurs the stronger, than less energy of gamma quanta and than is more density and atomic number of substance. At the narrow direction of a bunch of G. - and. reduction of intensity of monoenergetic G. - and. (consisting of gamma quanta with identical energy) occurs under exponential law:

where I — a radiation intensity in this point after passing of a layer of an absorber thickness of d, I o — a radiation intensity in the same point in the absence of an absorber, e — number, the basis of natural logarithms (e = 2,718), μ (cm - 1 ) — the linear extinction coefficient characterizing relative easing of intensity of G. - and. layer of substance 1 cm thick; the linear extinction coefficient represents the total size consisting of linear extinction coefficients τ, σ and χ, processes of photoeffect, the incoherent dispersion and formation of couples caused respectively (μ = τ+σ+χ).

Thus, the extinction coefficient depends on properties of an absorber and on G.'s energy - and. Than substance and than less energy of G. is heavier - and., the extinction coefficient is more.

See also Ionizing radiation .


Bibliography: Aglintsev K. K. Dosimetry of ionizing radiation, page 48, etc., M. — L., 1950; Bibergalya. Century, Margulies U. Ya. and Vorobyov E. I. Protection from x-ray and gamma-rays, M., 1960; Gusev N. G. and d river. Physical bases of protection against radiations, page 82, M., 1969; Kimmel L. R. and Mashkovich V. P. Protection against ionizing radiation, page 74, M., 1972.

U. Ya. Margulies.

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