DOSES OF IONIZING RADIATION — the physical quantities accepted in dosimetry of ionizing radiation for the quantitative characteristic of the field of radiation and impact of radiation on the irradiated object.
The main size applicable to any kind of ionizing radiation (alpha and beta particles, gamma radiation, protons, neutrons, mesons etc.), is the absorbed dose of radiation (D) — the relation of energy of dE transferred by ionizing radiation to substance in elementary volume to the mass of DM substance in this volume (D — dE/dm). Special unit of an absorbed dose — I am glad (pad). 1 I am glad there correspond to absorption of a radiation energy 100 erg in 1 g of substance (1 I am glad = 100 erg/g). In the International System of Units (SI) unit of an absorbed dose of radiation is heat (Gy) which is defined as 1 J/kg. Units I am glad and heat are connected by the following ratio: 1 I am glad = 10 - 2 Gy.
Derivative units of an absorbed dose — kilorad (krad), the microrad (mkrad) etc. milli-is glad (mrad).
The increase in an absorbed dose of radiation referred to a unit of time is called the power of an absorbed dose (P). P = dD/dt, where dD — an increment of an absorbed dose for a time slice of dt. A power unit of an absorbed dose is any private from division is glad (heating) or its derivative unit per a unit of time (hour is glad/, I am glad / mines, hour, mkrad / is glad sec. / sec., mrad/, to Gy / with etc.).
Physical. corrective action of radiation on all irradiated body or on its certain part is the integral absorbed dose of Dint. It is equal to an absorbed energy of radiation in body weight (or its parts). The integral dose of radiation is measured in units a hail, kg - is glad, etc.
As the absorbed dose of radiation ambiguously defines impact of photons and particles of different types and energy on a live organism, for comparisons at hron, radiation size an equivalent dose of radiation is entered (D ekv ), the cut is unit rem (rem). Such absorbed dose of any kind of ionizing radiation, edge is accepted to 1 rem at hron, radiation causes same biol, the effect, as 1 is glad x-ray or gamma radiations (see. Relative biological efficiency of radiations , Factor of quality ).
Along with the absorbed dose of radiation which is universal size widely use an air dose (D 0 ) radiations, applicable only for air and for photon (x-ray and gamma) radiations with energy to 3 MEV.
The air dose is based on the ionizing radiation effect.
For photon radiation unambiguous communication between absorbed (i.e. transferred to electrons as a result of elementary acts of interaction) energy of photons in this volume and the ionization made by these secondary electrons since a part of secondary electrons which run more linear sizes of this volume or which are formed at its borders will make ionization out of this volume is not always observed. Besides, in volume the secondary electrons formed by the photons absorbed out of this volume can make ionization.
Proceeding from features of interaction of photon radiation with substance, the air dose is defined as the relation of a boundary space charge of dQ of all ions of one sign created in air when all electrons and positrons released by photons in elementary air volume with a mass of DM completely stopped in air, to the mass of DM air in the specified volume: D0 — dQ/dm.
Special unit of an air dose of radiation — X-ray (see. Radiological sizes, units ). In the International System of Units (SI) unit of an air dose of radiation is the coulomb per kilogram (C/kg). Unit X-ray is connected with it the following ratio: 1 P == 2,58*10 - 4 C/kg. Derivative units of an air dose of radiation — milliroentgen (mr) and microroentgen (μR). The air dose of radiation carried to a unit of time is called the power of an air dose. It is measured in R / hour, mr / mines, μR/hour, μR/sec., etc.
At an air dose in 1 P electrons and positrons formed by photons in 1 cm 3 air (at 0 ° and 760 mm of mercury.), will create in air 2,08*10 9 steam of ions. If to consider that the mean energy spent for formation of one couple of ions in air is equal 34 ev, then at an air dose 1 P energy of photons transmitted to electrons and positrons to 1 cm 3 , it is equal 0,114 erg/cm 3 , and an absorbed dose — 88 erg/g, or 0,88*10 - 2 Gy.
Unambiguous communication between the exposition and absorbed doses can be established when the absorbed dose is measured in the air volume surrounded with a layer of air or airequivalent substance thickness to-rogo more or is equal to a run of secondary electrons i.e. when the condition of electronic balance is met.
In this case at an air dose 1 P absorbed dose is equal in air 88 erg/g. It is a power equivalent of X-ray.
Between an air dose of D0 and the absorbed dose of D measured in the conditions of electronic balance in any other environment there is the following ratio of D = kD0 where k has dimension is glad/Ruble.
The absorbed dose in water and muscular tissue differs for 4 — 10% of an absorbed dose in air because effective atomic number of Z eff water and muscular tissue it is close, but Zeff of air is not equal. Thereof in the range of energy of photon radiation 150 kev — 3 MEV k = 0,93 P for water and muscular tissue is glad / and 0,97 I am glad / R for a fatty tissue, i.e. at an air dose in 1 P, the absorbed dose in the conditions of electronic balance will be equal in water and muscular tissue 93 is glad. For a bone tissue, Zeff cover more, than at air and consequently, and photo-electric absorption in the field of small energy is more essential, value k will change from 4,74 to 0,88 P with increase in energy from 10 to 200 kev is glad/; since 200 kev value k remains approximately constant and equal 0,88 is glad/Ruble.
At a gamma therapy, and also at a row biol, experiments it is important to know distribution dozny field (see) in the irradiated object on the basis of what it is possible to judge an absorbed dose of radiation in various points. The air-vessel can make definition of a dose in any point in the irradiated object in the presence of inside it that allows to measure in it ionization. Such measurements are taken usually on models (phantoms). Phantoms are produced from tkaneekvivalentny substances, i.e. from substances at which easing and dispersion of radiation happen as well as in muscular tissue (see. Phantoms dosimetric ). Such substances are water, paraffin, a cardboard, plexiglass. Placing the ionization chamber with tkaneekvivalentny walls in various points of the phantom, define distribution of the dozny field, on Krom it is possible to judge distribution of an absorbed dose.
The dose created in the depth of the irradiated object is called a deep dose (D hl ). It consists of the dose created by the direct radiation of a source and scattered radiation. The dose created by scattered radiation depends on a radiation energy, geometry of radiation and the size of an object.
A superficial dose (Dp) — the dose created on a surface of the irradiated object. It is more, than the dose measured in air in the same point for lack of an object that is caused by back-scattering. E.g., for radiation with energy 200 kev back-scattering can reach 20 — 25% of a dose of primary radiation in the same point, for gamma radiation 60 With it 1 — 3% depending on the sizes of the field of radiation is equal.
The relation of a deep dose to a dose in air in the location of a surface of the irradiated object D' is called a relative deep dose (Dgl/D'). This size expressed as a percentage is called a percentage deep dose (Dgl/D '×100). Sometimes a relative deep dose call the relation of a deep dose to superficial (Dgl/Dp).
Doses of ionizing radiation in medicine and biology. Under natural conditions the organism of animals and the person is exposed to continuous influence of space beams and radiations of the natural radioelements which are present at air, the soil and at fabrics of the organism. Levels of natural radiation from all sources on average correspond 100 mrem a year, but in certain areas — to 1000 mrem a year.
In modern conditions in the course of life activity of people faces excesses of this average level of radiation. For the persons working in the field of action of ionizing radiation, setpoint values of the marginal dose (MD) for all body (see. Marginal doses, radiations) which at long influence do not cause in the person of disturbance of the general state, and also change of functions of a hemopoiesis and reproduction. For ionizing radiation it is established to traffic regulations of 5 rem a year. Calculation of dose loadings is made taking into account coefficient of quality of different types of ionizing radiation.
For assessment of the remote manifestations of a radiation effect in posterity consider a possibility of increase in a mutation rate. The dose of radiation most likely doubling the frequency of spontaneous mutations at the person does not exceed 100 rem on generation; there are, however, instructions and on smaller values of this dose (3 — 12 rem).
Genetically significant doses for the population are in limits of 7 — 55 mrem/year.
Use of radiation in medical practice leads to increase in dose loads of the population. Rentgenol. inspection is followed by beam impact on these or those body surfaces in doses of 0,04 P — 7,0 P by production of pictures and to 50 P at raying (tab. 1 — 4). These values of a dose tend to decrease.
Dose loadings at radio isotope diagnosis depending on the used radioactive nuclide at single use fluctuate from 0,01 to 600 rem / mkki on all body and from 0,003 to 6000 rem/mCi on separate bodies and fabrics (see. Critical body ).
The medical staff of X-ray departments, doctors-radiologists and medical staff of radio handling offices during the performance of various works are exposed to beam impact on certain areas of a body in doses of 0,03 — 0,18 rem/days (tab. 5).
At radiation therapy of malignant tumors depending on character patol, process local radiations in doses on average to 8000 rem in 3 — 4 weeks are carried out.
In radiobiology distinguish the following dose sizes characterizing death of animals during the fixed time (30 — 60 days): the minimum lethal dose (DLM), dose half (50%) survival or mortality (DL50) during a certain term, the minimum absolutely lethal dose (MALD) — the minimum dose causing death of all animals.
Values of these doses fluctuate depending on a look and the line of animals. So, e.g., DL50 at single uniform influence by gamma radiation lie ranging from 250 is glad (2,5 Gy) for dogs to 900 is glad (9 Gy) for separate lines of mice. For the person at total radiation is accepted by the gamma radiation of MALD equal 600 I am glad (6 Gy), a DL50 — 400 is glad (4 Gy).
Dependence of mortality on a dose is expressed to a S-shaped curve (fig. 1). Dependence of average life expectancy on a dose (fig. 2) is shown that in process of increase in a dose there is a gradual reduction of life expectancy while it does not reach 3 — 3,5 days (apprx. 1000 I am glad) — a piece of AB. At further increase of a dose to 6000 — 10 000 I am glad (60 — 100 Gy) average life expectancy does not change — a piece of BV. Increase in a dose St. 10 000 I am glad (100 Gy) leads to reduction of life expectancy up to one days, and then and several hours — a piece of VG. Since a dose 20 000 I am glad cases of «death under a beam» are noted. Depending on these data forms are defined radial illness (see): acute, the most acute and fulminant.
Table 1. An air dose on the body surfaces and an integral dose received inspected at roentgenoscopy without the electron-optical
Table 2 converter. The exposition and integral doses of radiation received inspected at a X-ray analysis (one picture)
Table 3. An air dose of radiation on a body surface and in the field of gonads inspected at pictures
Table 4. An air dose on the body surfaces and an integral dose of radiation received inspected at some special X-ray diagnostic testings
Table 5. The dose of radiation received by the radiologist at roentgenoscopy without electron-optical converter
Bibliography: Zolnikova N. I., Merkulova T. I. and Ishchenko 3. G. Beam loads of personnel during the work on various gamma and therapeutic installations, Medical radio-gramophones., t. 20, No. 5, page 46, 1975; Ivanov V. I. Course of dosimetry, M., 1970; Katsman A. Ya. Beam loadings and antiactinic protection at rents-geno-diagnostic procedures, JI., 1966, bibliogr.; Krongauz A. N., Lyapidevsky V. K. and Frolova A. V. Physical fundamentals of clinical dosimetry, M., 1969, bibliogr.; Standards of radiation safety (NRB-76), M., 1977; Standards of radiation safety for patients during the use of radioactive materials with the diagnostic purpose, Medical radio-gramophones., t. 18, No. 6, page 87. 1973; Radiation safety, Sizes, units, methods and devices, the lane with English, under the editorship of I. B. Keirim-Markus, M., 1974, bibliogr.
U. Ya. Margulies; N. G. Darenskaya (doses of ionizing radiation in medicine and biology), A. N. Krongauz (tab).