dose(redirected from minimum lethal dose (MLD))
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the energy of ionizing radiation absorbed by a unit of mass of irradiated matter (the absorbed dose Da). The absorbed energy is expended in the heating and the chemical and physical transformation of matter. The magnitude of the dose depends on the type of radiation (for example, X-ray or neutron flux), the energy of its particles, the density of their flux, and the composition of the irradiated matter. All other things being equal, the longer the time of irradiation, the greater the dose. Thus, doses accumulate with time. The dose per unit time is called the dose rate.
The dependence of dose magnitude on the energy of the particles, their flux density, and the composition of the irradiated matter is different for different types of radiation. For example, the dose for X- and y-radiation depends on the atomic numbers (Z) of the elements that make up the matter; the character of this relationship is determined by the photon energy (hv; h being Planck’s constant and v, the electromagnetic oscillation frequency). For these types of radiation the dose is greater in heavy substances than in light ones (under identical conditions of irradiation). Neutrons interact with atomic nuclei; the character of that interaction depends substantially on the energy of the neutrons. If elastic collisions of neutrons and nuclei occur, the average quantity of energy transferred to the nucleus in one interaction is greater for nuclei of lower weight. In this case (under identical conditions of irradiation) the absorbed dose in a substance of lower weight will be higher than that in a substance of greater weight. Other types of ionizing radiation have their own characteristics of interaction with matter, which determine the dependence of dose on the energy of radiation and on the composition of the matter. The absorbed dose in the International System of Units (SI) is measured in joules (J) per kilogram. The rad, a subsidiary unit, is widely used: 1 rad = 102 J/kg = 100 ergs/g. Dose rate is measured in rads per sec, rads per hour, and so on.
In addition to the absorbed dose there are the concepts of the exposure dose and the dose equivalent. The exposure dose, a measure of the ionization of air under the action of Xand y-radiation, is measured by the number of charges formed. Coulombs per kg is the unit of exposure dose in the system. An exposure dose of 1 coulomb per kg means that the total charge of all ions of the same sign formed in one kilogram of air is equal to one coulomb. The roentgen is a subsidiary that is widely used for exposure dose; 1 roentgen = 2.57976 x 10-4 coulombs per kg, which corresponds to the formation of 2.08 x 109 pairs of ions in 1 cu cm of air (at 0°C and 760 mm mercury). In order to create that number of ions it is necessary to expend energy equal to 0.114 ergs per cu cm, or 88 ergs per g. Thus, 88 ergs per g is the energy equivalent of the roentgen. The absorbed dose of X-radiation or y radiation in any substance can be calculated from the magnitude of the exposure dose. In order to do this it is necessary to know the composition of the substance and the photon energy.
When living organisms (in particular, man) are irradiated, biological effects arise whose magnitude determines the degree of radiation danger. For any given form of radiation the observed radiation effects are in many cases proportional to the absorbed energy; however, for identical absorbed doses in body tissues the biological effect turns out to be different for different types of radiation. Consequently, knowledge of the magnitude of an absorbed dose is insufficient for evaluation of the degree of radiation danger. Conventionally, the biological effects produced by any ionizing radiation are compared with the biological effects produced by X-radiation and -Σ-radiation. The figure that indicates how many times higher the radiation danger is for a given form of radiation than the danger from X-radiation, given the identical absorbed dose in the body tissues, is called the quality factor (K). The concept of relative biological effectiveness is used to compare radiation effects in radiobiological research. K = 1 for X-radiation and γ-radiation. For all other ionizing radiation the quality factor is established on the basis of radiobiological data. The quality factor may be different for different energies of the same type of radiation. For example, for thermal neutrons, K=3’, for neutrons with an energy of 0.5 MeV, K=l0. The dose equivalent De is defined as the product of the absorbed dose Da and the quality factor K; De= DJK. The coefficient K is a dimensionless quantity, and the dose equivalent can be measured in the same units as the absorbed dose. There is, however, a special unit of dose equivalent called the rem (roentgen-equivalent-man). A dose equivalent of 1 rem is numerically equal to an absorbed dose of 1 rad multiplied by the quality factor K.
Thus, the radiation danger to which a human being is subjected under any form of radiation corresponds to the same magnitude of dose equivalent. Natural sources of ionizing radiation (cosmic rays, the natural radioactivity of soil, water, air, and the human body) produce an average equivalent dose rate of 125 millirems per year. A dose equivalent of 400-500 rems received in a short time during irradiation of the whole body may, without special treatment, lead to death. However, the same dose equivalent received evenly during the course of one’s life leads to no perceptible changes. A dose equivalent of 5 rems per year is considered the maximum permissible exposure (MPE) in occupational irradiation.
The minimum dose of γ-radiation that will suppress the reproductive capacity of certain cells after a single irradiation is 5 rems. Inceptive changes in the blood are observed with prolonged daily doses of 0.02-0.05 rems, tumor formation, with 0.11 rems. The long-term consequences of irradiation are judged according to the increase in the frequency of mutation in the offspring. The dosage that will double the frequency of spontaneous mutation in man probably does not exceed 100 rems per generation. With local irradiation—that is, for the purpose of treating malignant tumors—high doses (6,000-10,000 rems in three or four weeks) of X-radiation or γ-radiation are used (carefully shielding the rest of the body).
Radiobiology distinguishes the following doses that lead sooner or later to death in animals: a dose that causes the death of 50 percent of the animals in 30 days (a lethal dose— LD30/50) with single-exposure unilateral X-radiation or γ-radiation is 300 rems for guinea pigs and 1,000 rems for rabbits; the minimum absolute lethal dose (MALD) with general γ-radiation is approximately 600 rems for man. With increases in dosage, the survival time is reduced to 2.8-3.5 days; further increases in dosage do not change that period. Only doses higher than 10,000-20,000 rems shorten the survival time to one day, and with subsequent irradiation, to a few hours. With a dose of 15,000-25,000 rems, cases of “death under the ray” are noted. Particular forms of radiation injury correspond to each dosage range. A number of invertebrate animals, plants, and microorganisms have considerably lower sensitivity.
Dosimeters are used in measuring radiation doses in order to predict radiation effects.
REFERENCESGOST 8848-63: Edinitsy radioaktivnosti i ioniziruiushchikh izluchenii. Moscow, 1964.
GOST 12631-67. Koeffitsient kachestva ioniziruiushchikh izluchenii. Moscow, 1967.
Ivanov, V. I. Kurs dozimetrii, 2nd ed. Moscow, 1970.
Golubev, B. P. Dozimetriia i zashchita ot ioniziruiushchikh izluchenii, 2nd ed. Moscow, 1971.
V. I. IVANOV and N. G. DARENSKAIA