IONIZING RADIATION

From Big Medical Encyclopedia

IONIZING RADIATION — any radiations which interaction with Wednesday leads to ionization with formation of electric charges of different signs.

Ionization (see) and the excitement of atoms and molecules arising together with it are a releaser of the processes leading to development of radiation injury biol, structures — cells, fabrics, bodies, systems and all organism in general (see. Radial illness , Radiation therapy , Beam damages ).

And. and. can be directly or indirectly ionizing. Directly ionizing radiation are charged particles with the motive energy sufficient for ionization at collision with atoms of substance. To directly And. and. corpuscular radiations — cathode rayss, positrons, heavy charged particles — protons belong. deuterons, alpha particles, kernels of other atoms, the loaded mesons and hyperons (see. Alpha radiation , Space radiation , Proton radiation , Electronic radiation , atomic nucleus ).

Indirectly And. and. consists of photons and uncharged particles which interaction with Wednesday leads to release of secondary charged particles, i.e. secondary directly And. and. To indirectly And. and. electromagnetic {photon) radiation, in particular x-ray and the gamma radiation, neutron fluxes and not loaded mesons and hyperons belongs (see. Gamma radiation , Neutron emission , X-ray emission ). Though short-wave ultraviolet radiation is capable to ionize Wednesday, it cannot be carried to And. and. (in accordance with GOST 15484 — 74).

Sources And. and. natural and artificial radioactive materials are (see. Radioactivity , Nuclear weapon ), space (see. Space radiation ), nuclear reactors (see. nuclear reactors ), X-ray tubes (see. X-ray apparatus ) and various particle accelerators — betatrons, cyclotrons, linear accelerators, synchrotrons, microtrons, etc. (see. Particle accelerators ).

Charged particles at interaction with atoms of the environment lose the energy not only on excitement and ionization of atoms (ionization losses), but also on radiation during the braking (radiation losses). Power loss can happen also at elastic collisions since as a result of such collisions a part of a motive energy is transferred.

The average amount of energy spent by charged particle for excitement and ionization of atoms on a unit of length of a way in this substance is called linear power transmission (see) or linear ionization brake ability of substance. The number of couples of ions which are formed on average on a unit of length of a way of charged particle is called the linear density of ionization. Charged particle with an initial motive energy (E) creates on the way to dead stop p = E/W of couples of ions where W = so-called mean energy of an ionoobrazovaniye. Ionization brake ability is described by the equation to the Beta. With increase in atomic number (Z) of substance, about the Crimea there is an interaction of charged particles, the brake ability divided into density of substance slowly decreases. With increase in energy of charged particles from zero ionization brake ability at first increases, and then decreases since time of interaction of the flying particle with the atoms located on its way is reduced. The amount of the energy lost by charged particle on ionization and on a bremsstrahlung depends on the mass, a charge and energy of a particle. Energy, at a cut ionization power losss of particles are equal to radiation, is called critical energy. For electrons dependence of critical energy on atomic number (Z) of the environment can be expressed:

Ekr = 800 / (Z + 1,2) MEV

In water and biol, fabrics critical energy for electrons makes 9,2 MEV, and for protons 10 7 — 10 8 MEV. Thus, at usually found values of energy electrons can have considerable radiation losses at protons and at other heavy charged particles they are practically absent, only ionization power losss are observed. For alpha particles the minimum power transmission is 4 times more, than for protons of the same speed. Dependence of linear power transmission on energy of various charged particles has the similar form, but with increase in mass of charged particle the curve representing this dependence moves towards higher energy. In the field of small values of energy linear power transmission increases, reaches a maximum, and then decreases, passes through a minimum and at further increase in energy again slowly increases. For electrons the smallest value of linear power transmission is observed at energy apprx. 1 MEV, for protons apprx. 2000 MEV.

Losing the energy on ionization and radiation, charged particle passes the certain way in this substance depending on energy of a particle. Since the trajectory of the particle experiencing repeated collisions is not rectilinear also at each particle the, the concept «length of a run» is conditional. Passing a flow of the monodirected particles through various layers of this substance, investigate number of the particles which passed through layers of different thickness i.e. build a curve of easing. For particles with identical initial energy the number of the passing particles sharply decreases, beginning from a nek-swarm of thickness of a layer. Extrapolating the falling-down part of a curve to an axis of thickness, define the average or extrapolated run of particles of this energy in substance. At protons and other heavy charged particles the real run of separate particles differs from an average a little. At electrons the dispersion is high. For beta radiations (see) with a continuous power spectrum from zero to boundary energy the run (in cm) in substance can be determined by a formula:

R = (0,546 • Emax-0,16) ρ,

where Emax — boundary energy of electrons (beta particles) in MEV and ρ — density of substance in g/cm3. Use this formula during the calculations of thickness of the materials which are applied to protection against beta radiation (see. Antiactinic protection ). Beta radiation with boundary energy 50 kev has a run in air of 4,4 cm, and in water or biol, fabrics of 47 microns. At energy in 5 MEV the run in air makes 20 m, in water of 2,6 cm. The charged particle with this energy, the less length of its run in this substance is heavier. So, for protons with energy of 5 MEV length of a run in air about 40 cm, in water of 0,5 mm. At an alpha particle of the same energy a run in air about 3,5 cm, in water of 45 microns, Run of heavy-nuclei and their splinters make in air several millimeters, and in water several micrometers.

Heavy charged particles, having the small length of a run, create a large number of couples of ions on unit of a way. So, the alpha particle with energy of 5 MEV creates in air tens of thousands of plank beds of ions on 1 cm of a way, in biol, fabrics several thousands of couples of ions on the way to 1 micron. Protons with the same energy create on 1 cm of a way in air thousands of plank beds of ions and in biol, fabrics hundreds of couples of ions on the way to 1 micron. From charged particles electrons have the smallest linear density of ionization and at the same energy in 5 MEV create on 1 cm of a way in air less than one hundred couples of ions and in biol, fabrics several couples of ions on 1 micron.

Ionization on the way of a range of particle is made unevenly, density of ionization increases, forming so-called peak of Bragg on the end of a run. Density of ionization in Bragg's dive for heavy charged particles (alpha particles, protons, deuterons) can exceed in hundreds of times density of ionization at the beginning of a way. Existence of uneven number-density distribution of ionization in biol, creates to fabric for heavy charged particles favorable conditions for their use in radiation therapy (see) since allows to bring the necessary amount of radiant energy to the irradiated center almost locally. Changing energy of charged particles, it is possible to displace position of peak of Bragg on depth of fabric and to provide the maximum absorption of a radiation energy at the necessary depth. This property is used, in particular, at radiation therapy by the protons accelerated on powerful accelerators to energy in hundreds megaelectron-volt (see. Proton therapy ).

At interaction with substance x-ray and gamma radiations form secondary electrons — photoelectrons (at photoeffect), electrons of return (at a Compton effect) or electronic and positron couples. The electrons formed by photon radiation in substance make ionization as well as primary electrons of the same energy.

Neutrons (see. Neutron emission ) as a result of interaction with atomic nuclei can form both charged particles, and photons of gamma radiation. Bystry neutrons lose the energy as a result of elastic and inelastic collisions with kernels, telling them a part of the energy. At elastic dispersion the sum of motive energies of a scattered neutron and kernel of return is equal to an initial motive energy of a neutron. If the kernel of return gets a big motive energy, then it is capable to make ionization.

At elastic collision energy is distributed depending on the mass of the facing particles. The greatest number of energy is got by particles which mass is less or is equal to the mass of a neutron, napr, kernels of return of hydrogen, i.e. protons. Therefore the most effective delay of bystry neutrons happens on hydrogenous substances (water, paraffin, etc.) which use at protection against neutron emission (see. Antiactinic protection ), and also on biol, the fabric containing apprx. 10% of hydrogen. At each elastic collision with kernels of hydrogen the neutron loses on average a half of the energy and, having experienced several collisions, turns into a slow neutron. The main part of the energy transferred by bystry neutrons biol to objects, is caused by elastic dispersion on kernels of hydrogen.

Slow neutrons lose the energy as a result of capture by their kernels. At such capture new isotopes which part is artificially radioactive are formed and breaks up, letting out energy in the form of charged particles and photons of radiation. The main reactions in which slow neutrons transfer the energy irradiated biol to objects, are the reactions with hydrogen which are followed by emission of photons of gamma radiation with energy of 2,2 MEV and formation of a deuterium, and with nitrogen, 0,59 MEV which are followed by emission of protons with energy and formation of beta and active isotope of carbon 14 With with big half-life (see. Isotopes ). Neutrons widely use in medicine and biology (see. Neutron therapy ).

Depending on the linear density of ionization or linear power transmission initially or for the second time charged particles different types And. and. at an identical dose of radiation cause various in size biol, effect, i.e. possess various biol, efficiency (see. Relative biological efficiency of radiations ). The Relative Biol, Efficiency (RB,E) of subjects is more, than more linear power transmission And. and. or by what more energy is transferred by a particle to substance on a unit of length of the run. At very big linear power transmission of OBE ceases to increase and even decreases. During the ensuring radiation safety distinction biol, efficiency of different types of radiation is defined by the so-called coefficient of quality which is coefficient of proportionality of the absorbed and equivalent doses of radiation (see. Doses of ionizing radiation ).

Biological effect of ionizing radiation. Primary physical. the processes resulting from influence And. and. on biol, an object, cause formation of substances with high chemical activity. Biol, a radiation effect generally connect with products of a radiolysis of water, to the Crimea free atoms and radicals of H, OH, HO belong 2 and hydrogen peroxide H 2 O 2 . However And. and. can have also direct effect on biol, molecules and supermolecular structures. Radiation causes various denaturatsionny changes — a rupture of the least strong bonds, a separation of radicals, a depolymerization and other changes. In development biol, effect processes of migration of energy and formation of the stable metastable compounds arising owing to long preservation of a condition of excitement in some macromolecular substrates can have a certain value. Use in radiobiol. experiments radio spectroscopic, mass and spectrometer, luminescent and other physical. methods of a research allowed to show that the radiation energy received by macromolecules as a result of a direct or indirect radiation effect, as a rule, is not implemented in that place where there was an act of interaction. It migrates in the ways, specific to this structure, and strikes in the «weakest» place. It was shown that at radiation in macromolecules there are hidden long-living damages which can be revealed by means of influence of not radiation factors.

H. M. Emmanuel stated a hypothesis of an important role in development of beam damage of free radical states (see. Radicals ), arising under the influence of radiation in the major biochemical, components. Pointed B. to participation of chain oxidizing reactions in development of primary processes of radiation injury by H. T to are of owls. In addition to lipidic radio toxins (see), A. M. Kuzin identified toxic products of the quinoid nature. At impact of ionizing radiation in a cell there are physical. - chemical disturbances — are surprised biologically important macromolecules, activity of various fermental systems is suppressed, the structure of molecular surfaces of the multiphase environment of a cell changes. It leads to increase in permeability of membranes and change of diffusion processes that is followed by the phenomena of a denaturation of proteins, dehydrations of system, disturbances of internal environment of a cell.

Consider that radio sensitivity of a kernel and cytoplasm is identical. However the role of damages of a cellular kernel is defined biol, the importance of this structure for a cell. Structural changes of the chromosomal device, as a rule, lead to death of a cell in the course of a mitosis or to emergence of impractical posterity of cells. Oppression of mitotic activity of fabrics is considered as one of specific manifestations biol, actions of ionizing radiation. Cytogenetic effects of defeats of sex cells are connected with change of nuclear chromosomes.

The Porazhayemost of cells substantially depends on intensity of mitotic division and a metabolism, degree of a differentiation and development of cells. With increase in level of exchange processes the radioporazhayemost of fabrics increases. In a so-called scale of radio sensitivity of fabric hold (on the decreasing order) the following position: hemopoietic fabric, an intestinal epithelium, gonads, an epithelium of skin and a bag of a crystalline lens, fibrous fabric, a cartilage, a bone, muscles, nervous tissue (see. Radiochuvstvitelnost ).

Biological effect And. and. on an organism depends on a look and a dose of radiation (see. Doses of ionizing radiation ), linear power transmission, conditions of radiation and distribution of an absorbed dose in an organism (see. Radiation ), time factor of radiation (see), selective defeat of critical bodies (see. Critical body ), and also from funkts, conditions of an organism before radiation. At local external radiation survival at the doses much surpassing doses, admissible at general irradiation is possible. The fractional (fractioned) radiation is transferred easier, and the total dose at reirradiations can exceed single deadly considerably. Long hron, radiation in rather high doses leads to development hron, radial illness (see).

At external impact of x-ray, gamma and proton radiations on all body of the person in a dose apprx. 100 I am glad temporary change of a hemopoiesis is observed (see. the Hemopoiesis, in the irradiated organism ).

Radiation in small doses can lead to temporary stimulation of a number of functions at simultaneous oppression of activity of other, more radio sensitive systems.

The most brightly promoting effect of radiation is expressed at radiation of vegetable objects.

At the general external irradiation of the person a dose in 150 — 400 I am glad the radial illness of easy and average severity develops, at a dose in 400 — 600 I am glad — a serious radial illness; radiation in a dose higher than 600 I am glad is absolutely deadly if methods of prevention and therapy are not used.

At radiation by doses in the range of 100 — 1000 I am glad defeats the so-called marrowy mechanism of development of a radial illness is the cornerstone. At the general or local irradiation of a stomach in doses 1000 — 5000 I am glad the intestinal mechanism of radiation injury in combination with the phenomena of a toxaemia prevails. At acute exposition in doses more than 5000 I am glad the fulminant form of a radial illness develops, for a cut the cerebral mechanism of development of radiation injury (disturbance of a statics, a spasm, the general excitement, a nystagmus, vomiting) is characteristic. Radiation in doses St. 20 000 I am glad leads to death of an organism «under a beam».

Along with the processes causing a direct injury of fabrics and cells it is necessary to consider a radiation effect on the systems defining integral functions of an organism. The Soviet scientists M. N. Livanov and A. V. Lebedinsky showed high funkts, sensitivity of a nervous system. Shifts in bioelectric activity of a brain, a retinogramma are noted, changes in uslovnoreflektorny activity at radiation of experimental animals and people already in doses about 10 I am glad.

Influence And. and. on a nervous system it is carried out both through receptor systems, and as a result of a direct injury of nervous tissue. Disturbance of nervous control of functions can influence development of a row patol, changes in an organism.

As a result of external impact of neutron emission (see. Neutron emission ) in an organism various radioactive materials, napr, radionuclides of sodium, phosphorus, etc. are formed. At the same time an organism it becomes temporary the carrier of radioactive materials. Formation of this so-called induced activity allows to estimate a dose at the expense of a certain part of a range of neutrons, and its influence on the general dose is insignificant a little.

Local external radiation causes damage to varying severity depending on a dose (see. Cataract , Burns ). The oral syndrome arises in the conditions of preferential radiation of the head, the sialosis, epithelites are characteristic of it (see. Radioepithelitis ), development of ulcers in an oral cavity.

At radiation by heavy (multi-shooter) ions arise local damages which do not disappear independently and come under influence of the modifying factors a little.

At hit in an organism of radionuclides (see. Isotopes ), which can be sources alpha, beta or gamma radiations, there is a so-called internal radiation of an organism (see. Incorporation of radioactive materials ). Its danger is defined by features of metabolism, specific activity, ways of intake of radionuclides to an organism. The radionuclides having big half-life and which are badly brought out of an organism, e.g., radium-226 are most dangerous ( 226 Ra), plutonium-239 ( 239 Pl). The striking effect is influenced by the place of deposition of radionuclides: strontium-89 ( 89 Sr) and strontium-90 ( 90 Sr) — bones; strontium-137 ( 137 Sr) — muscles. The special importance and early the coming to light danger of defeat have quickly rezorbiruyushchiyesya radionuclides with hypodispersion in an organism, napr, hyzone ( 3 T) and polonium-210 ( 210 Po).



Bibliography: Aglintsev K. K. Dosimetry of ionizing radiation, M., 1957, bibliogr.; Brooms V. Ya. and Korenkov I. P. Radiation protection during the use of ionizing radiation, M., 1975; Grigoriev Yu. G. Radiation injuries and compensation of the broken functions, M., 1963, bibliogr.; Jones G. E. Physics of radiology, the lane with English, M., 1965; And with and e in B. M. and B r e-@ and d z e Yu. I. Neutrons in a radio biological experiment, M., 1967, bibliogr.; The reference book on a radiology and radiology, under the editorship of G. A. Zedgenidze, M., 1972; Tyubiana M., etc. Physical bases of radiation therapy and radiobiology, the lane with fr., M., 1969; Ade at with L. X. Physical and chemical bases of radio biological processes and protection against radiations, M., 1972.

A. N. Krongauz; Yu. G. Grigoriev (biological effect).

Яндекс.Метрика