Basis of dosimetry
Dosimetry – is the section of applied nuclear physics considering the ionizing radiation, physical quantities, characterizing radiation field or interaction of radiation with substance, and also principles and methods of definition of these quantities. Dosimetry deals with such physical quantities of ionizing radiation, which define its chemical, physical and biological effect. The major characteristic of dosimetric quantities – is the established connection between the measured physical quantity and expected radiating effect.
THE HISTORY OF DEVELOPMENT OF DOSIMETRY
In the first years of scientists’ work with x-ray radiation and radioactive elements, attempts to limitation of the person’s irradiation were not undertaken, despite understanding of ionizing radiation danger. Only after almost 7 years from the moment of x-ray radiation invention, English scientist Rollinz in 1902 year suggested to limit irradiation of workers, by dose, which caused darkening of photoemulsion, applied during that period of time, which corresponded to exposure dose 10 R/day.
However the first clear representation of physically proved concept of dose which is close enough to modern was developed by the Swiss doctor and physicist Kristen in article «Measurement and a dosage of X-rays». Biological methods of dosimetry had applied before started to apply physically proved methods in dosimetry. So discovered and thereafter well studied early damages of skin integuments of persons, working with an ionizing radiation, formed the basis for offers of leading radiologists of the world about limitation of professional irradiation.
Thereafter specially created national committees on protection against ionizing radiation which were created in 1921 in many countries began to be engaged in these questions. "These years such unit of x-ray radiation as roentgen was introduced. In 1925 the American radiologist Matcheller recommended as a tolerant (transferable) dose for a month - a dose equal to 340 R (about 100 mR/day). However, only in 1934, the International commission on protection against x-ray radiation and radium, which was created in 1928 (now is the International commission on radiating protection (ICRP), recommended for the first time to accept as a tolerant dose 200 mR/day to the national governments. In 1936 this commission reduced the indicated dose to 100 mR/day.
Further accumulation of scientific data about ionising radiation effect, particularly about life duration reduction of experimental animals, a term tolerant dose was changed by the more cautious one - maximum permissible dose (MPD). In 1948 ICRP recommended to lower MPD irradiations of professionals to 50 mR/day (6 Sv for 40 years of work), having formulated concept MPD as «such dose which should not cause considerable damage of human body at any moment during his life».
In 1953 the International commission on radiating units and measurements (which was created in 1925), introduced generally applicable dose quantity into practice - the absorbed dose instead of X-ray, which began to be applied as unit of exposure dose. In 1958, on the basis of new scientific data, ICRP lowered MPD to 0,6 Sv at the age of till 30 years. In the former USSR, in 1987 MPD was limited by quantity of 50 mSv/year.
MPD equal to 20 mSv/year for professionals (the category A - professional workers who constantly or temporarily work with ionizing radiation sources), for the personnel (the category B - persons not working directly with ionizing radiation sources, but can be exposed to ionizing radiation influence according to working conditions or residing) - 2 mSv/year, and for the population - 1 mSv/year, was accepted by radiation safety standards of Ukraine (RSSU-97) in 1997.
FORMATION OF IRRADIATION DOSE IN THE BIOLOGICAL ENVIRONMENT
There are directly ionizing particles and indirectly ionizing particles in the biological environment under formation of irradiation dose. Directly ionizing particles are the charged particles: alpha rays (helium cores), beta particles (electrons, positrons), etc., and indirectly ionizing particles are not charged particles: neutrons, gamma-quantum.
There are strong (developed by early irradiation effects) and prolonged (long), unitary and repeated (fractionating) irradiation under biological individuals irradiation. Both strong and prolonged irradiation can be unitary or fractionating. Besides, chronic irradiation is possible, which can be considered as a kind of fractionating, but produced long on very low dose rate.
The dose formed by radiation in substance can be estimated, measuring, for example, the rise in temperature caused by it. However, even on dangerous to the human life doses, released energy is not enough for heating of irradiated organism on thousand shares of degree. Therefore during studying of radiation effect on biological objects, doses are estimated with application of more sensitive methods of dosimetry.
Dose distribution in time for various radiations on linear energy transfer (LET) can differ considerably and affect differently on radio biological irradiation effects. It is developed particularly on the remote consequences of biological effect of radiations differing by LET, in this connection, the serious attention is given to definition of temporal dose distribution in radio biology.
The ionizing radiation, reacting with substance, transfers to it energy in small, final portions. The energy transfer is a casual process. The energy transferred to substance in each action of reaction is also casual. Therefore absorbed in some volume of substance energy under its repeated irradiation in identical conditions and by the same dose of ionizing radiation of one kind, strictly speaking, is a little differing. It is necessary to remember about essentially always presented, but not always essential fluctuations (dispersion) of absorbed energy (and, accordingly, of the absorbed dose).
In case of small irradiated volumes, commensurable with volume of separate cells on size, or sub cellular structures, the situation, under which fluctuation of the absorbed doze is to be commensurable and even surpass dose size, is possible. In such conditions comparison of an output of the radiating-induced effects to the absorbed doze becomes ambiguous and there is a necessity to consider these fluctuations. The fluctuation becomes more considerably, the less volume is, in which the size of absorbed doze is estimated, and the more size of LET radiation forming this doze is.
In case of formation so-called "small dozes" of irradiations (in microdosimetric understanding of the given term which not always coincides with its biological understanding), quantity of penetrated by tracks of ionizing radiation of sensitive micro volumes in irradiated object is essentially less than its general number. In this case in average, the linear change of degree of developing process of this or that radiobiological effect from radiation doze is connected simply with growth of number of sensitive micro volumes penetrated by tracks of radiation, instead of actually linear character of output dose dependences of this effect.
The similar situation is realized more often in standard conditions of professional irradiation and under influence on the person of radiation of Earth’s radiating background, forming, as it is known, absorbed radiation dose on the level of hundred milligray in year (mGy/per year). It means, that through sensitive volumes of separate cells of an organism of the person during year very seldom passes more than one track, and through their other part for the same time tracks don’t pass at all.
The quantitative radiobiology, vice versa, studies radiation effect in such conditions when each sensitive micro volume in irradiated biological object is penetrated by greater number of tracks more often and the increase in irradiation doze corresponds to condition of increase in number of tracks through each of its sensitive micro volumes.
THE BASIC PHYSICAL QUANTITIES OF DOSIMETRY
Original cause of radiating effects are absorption of radiation energy by irradiated object, and doze as the measure of the absorbed energy, is the basic dosimetric quantity. Therefore, the basic physical dosimetric quantity used for estimation of measure radiation influence on environment, is the absorbed radiation doze.
The absorbed radiation doze (D) is the quantity defined by radiation energy (J) of absorbed mass unit (kg) of irradiated substance. For a doze unit in SI system gray (Gy) is accepted:
D = 1J/1kg=1 Gy.
Gray is such ionizing radiation doze, under which energy 1J is transferred to a site of substance of 1 kg. Off-system unit is "rad". 1 rad = 0,01 Gy.
The absorbed doze characterizes not the radiation, but the degree of its influence on environment. The same stream of radiation in various environments and even in various sites of one environment can generate various quantity of the absorbed doze in principle. Therefore, when speak about the absorbed doze, it is necessary to specify, in what environment is it generated: in air, water or a soft biological tissue.
In order to measure the distribution of radiation dose in time, capacity value of the absorbed dose, or intensity of irradiation are used. Hereunder quantity of the radiation energy, absorbed in unit of time by mass unit of irradiated substance (Gy/hour; Gy/year) is considered.
Under practical use of radiations the person, excepting special cases of medical influences and radiating failures, influences small dozes of irradiation. Working conditions of professionals now most often answer situation when sensitive targets of cells of their organism of individual tracks of the ionizing particles forming doze of irradiation, are essentially more than that time, during which operate reparative (regenerative) systems of cells eliminating infringements, caused the passed particle.
Induced biological effects don’t depend on such factors as doze capacity, its distribution, conditions and rhythm of irradiation in these conditions. The output of effects is defined only by the cumulative exposure (independent of irradiation time), i.e. consequences of irradiation will be identical under unitary irradiation of the given doze, or at its reception within several days of months and even year. The degree of expressiveness of effect will be influenced only with spatial distribution of ionization effects and the excitation created in tracks, i.e. linear energy transfer (LET) of ionizing radiation. Therefore, for such conditions the special doze quantity, considering both of these factors - equivalent doze is introduced. It is possible to connect unequivocally the output of radiating consequences of irradiation with irradiation doze by this quantity.
The equivalent doze (H) is defined as product of the absorbed doze (D) of the given radiation kind on average value of the weighing factor (quality factor) of ionizing radiation (WR) in the given element-volume of biological tissue. Values WR for various kinds of radiations are presented in table 1. This doze is the measure of expressiveness of stochastic effects of irradiation. It is applicable for estimation of radiating danger of chronic irradiation of radiation of any structure (and strong doze irradiation, less than 0,25 Sievert) and also is defined under the formula:
Í = D • WR
As a unit of equivalent doze in SI Sievert (Sv) is accepted. Sievert is equal to such equivalent doze under which, the size of product of the ionizing radiation doze absorbed in biological tissue on average value of the weighing factor for this radiation is equal to 1 J/kg. Off-system unit is "rem" (biological equivalent of Roentgen). 1 rem = 0,01 Sv.
From definition follows, that for radiation with WR = 1, the equivalent doze 1 Sv is realized on the absorbed doze 1 Gy, i.e. for this case 1 Sv = 1 Gy. If WR is distinct from 1, the equivalent doze 1 Sv will be generated in biological tissue under quantity of the absorbed doze in it equal to (1/WR) Gy. Summation of equivalent dozes for estimation of the general level of irradiation for a long time interval if each single doze which was on fractionating acute irradiation for this time did not exceed 0,25 Sv is supposed.
Table 1 - Value of radiation weighing factors (WR)
The equivalent doze is defined as the sum of products of the absorbed dozes of separate kinds of radiations on corresponding values of weighing factors of these radiations for the mixed radiation.
Under the set equivalent irradiation doze the probability of the output of stochastic consequences depends on tissue or organ irradiated by it. Therefore, one more coefficient considering specificity of various tissues from the point of view of probability of induction of stochastic consequences of irradiation in it - the tissue weighing factor (WT) is introduced. Values of WT accepted now are indicated in the table 2 and are used only for calculation of the effective doze. Tissue weighing factors are introduced, proceeding from the concept of no threshold radiations effect, and its quantities correspond to the output of stochastic consequences for various organs and tissues, received on the basis of linear extrapolation of data available from area of high irradiation dozes (as real output of stochastic consequences in the field of small dozes is unknown).
Table 2 - Values of tissue weighing factors (WT)
In contrast to stochastic effects, not stochastic (determined) are developed only on reception of the certain dozes (tab.3).
Table 3 - Value of dozes below of which occurrence of not stochastic (determined) effects is excluded
Absolute values of factors are matched so that their total quantity was equaled to one. It allows treating tissue weighing factors as a set of coefficients defining relative contributions of corresponding organs in the total output of stochastic consequences, arising under uniform irradiation of all organism. The most radiosensitive organ by criterion of output of these consequences is sexual glands, completely responsible for genetic effects and a part of somatic stochastic consequences of irradiation.
Physical sense of concept of effective doze is the following: value of effective doze (E) corresponds to such level of uniform irradiation of all organism under which the total output of stochastic consequences of irradiation in it will be the same, as well as in case of local irradiation of organ (Ò) by the equivalent doze of quantity (Í):
Å = Í • WT
As a unit of effective doze in SI system sievert (Sv) was accepted too. Under uniform irradiation - the effective doze is equal to equivalent doze. Under non-uniform irradiation - the effective doze is equal to product of equivalent doze on the tissue weighing factor, or equal to such equivalent doze (under uniform irradiation), which creates the same risk of adverse consequences.
It is impossible to measure the effective doze of irradiation of organism. It is calculated as the sum of products of equivalent dozes (Í) in separate organs and tissues on corresponding values of weighing factors (WT) specified in table 2.
The effective doze represents a measure of output of stochastic consequences of biological effect of small dozes of irradiation on the given individual, i.e. it is a measure of the individual danger caused by effect on organism of small dozes of ionizing radiation..
For photon radiation the specific quantity in dosimetry – the exposure dose is introduced. Numerically it is equal to the absolute value of a full ion charge of like-sign formed in air mass unit under full deceleration of electrons and positrons released by photons (by x-ray radiation). That is, it is air-equivalent doze unit which is not intended for dosimetry in substance.
Unit of measurement of exposure dose in system of SI is coulomb/kg (C/kg), off-system unit is Roentgen (R).
1 Ð = 2,58 • 10-4 C/kg (exactly).
The exposure dose can be used for the approached estimation of the absorbed and exposure dose in substance (tab. 4).
Table 4 - Doses converting for gamma-rays (muscles)