There were two sources of ionizing irradiation after the atomic bombings of Hiroshima and Nagasaki: (1) initial gamma-neutron irradiation at the moment of detonation and (2) residual radioactivity. Residual radioactivity consisted of two components: radioactive fallout containing fission products, including radioactive fissile materials from nuclear device, and neutron-activated radioisotopes from materials on the ground. The dosimetry systems DS86 and DS02 were mainly devoted to the assessment of initial radiation exposure to neutrons and gamma rays, while only brief considerations were given for the estimation of doses caused by residual radiation exposure. Currently, estimation of internal exposure of atomic bomb survivors due to dispersed radioactivity and neutron-activated radioisotopes from materials on the ground is a matter of some interest, in Japan. The main neutron-activated radionuclides in soil dust were Na,Al, Si,P, Cl,K, Ca,Sc, Mn,Fe, Co, andCs. The radionuclide Mn (T = 2.58 h) is known as one of the dominant beta- and gamma emitters during the first few hours after neutron irradiation of soil and other materials on ground, dispersed in the form of dust after a nuclear explosion in the atmosphere. To investigate the peculiarities of biological effects of internal exposure to Mn in comparison with external gamma irradiation, a dedicated experiment with Wistar rats exposed to neutron-activatedMn dioxide powder was performed recently by Shichijo and coworkers. The dosimetry required for this experiment is described here. Assessment of internal radiation doses was performed on the basis of measured Mn activity in the organs and tissues of the rats and of absorbed fractions of internal exposure to photons and electrons calculated with the MCNP-4C Monte Carlo using a mathematical rat phantom. The first results of this international multicenter study show that the internal irradiation due to incorporatedMn powder is highly inhomogeneous, and that the most irradiated organs of the experimental animals are: large intestine, small intestine, stomach, and lungs. Accumulated absorbed organ doses were 1.65, 1.33, 0.24, 0.10 Gy for large intestine, small intestine, stomach, and lungs, respectively. Other organs were irradiated at lower dose levels. These results will be useful for interpretation of the biological effects of internal exposure of experimental rats to powdered Mn as observed by Shichijo and coworkers.

The experiment was performed in support of a Japanese initiative to investigate the biological effects of irradiation from residual neutron-activated radioactivity that resulted from the A-bombing. Radionuclide 56Mn (T1/2 = 2.58 h) is one of the main neutron-activated emitters during the first hours after neutron activation of soil dust particles. In our previous studies (2016–2017) related to irradiation of male Wistar rats after dispersion of 56MnO2 powder, the internal doses in rats were found to be very inhomogeneous: distribution of doses among different organs ranged from 1.3 Gy in small intestine to less than 0.0015 Gy in some of the other organs. Internal doses in the lungs ranged from 0.03 to 0.1 Gy. The essential pathological changes were found in lung tissue of rats despite a low level of irradiation. In the present study, the dosimetry investigations were extended: internal doses in experimental mice and rats were estimated for various activity levels of dispersed neutron-activated 56MnO2 powder. The following findings were noted: (a) internal radiation doses in mice were several times higher in comparison with rats under similar conditions of exposure to 56MnO2 powder. (b) When 2.74 × 108 Bq of 56MnO2 powder was dispersed over mice, doses of internal irradiation ranged from 0.81 to 4.5 Gy in the gastrointestinal tract (small intestine, stomach, large intestine), from 0.096 to 0.14 Gy in lungs, and doses in skin and eyes ranged from 0.29 to 0.42 Gy and from 0.12 to 0.16 Gy, respectively. Internal radiation doses in other organs of mice were much lower. (c) Internal radiation doses were significantly lower in organs of rats with the same activity of exposure to 56MnO2 powder (2.74 × 108 Bq): 0.09, 0.17, 0.29, and 0.025 Gy in stomach, small intestine, large intestine, and lungs, respectively. (d) Doses of internal irradiation in organs of rats and mice were two to four times higher when they were exposed to 8.0 × 108 Bq of 56MnO2 (in comparison with exposure to 2.74 × 108 Bq of 56MnO2). (e) Internal radiation doses in organs of mice were 7–14 times lower with the lowest 56MnO2 amount (8.0 × 107 Bq) in comparison with the highest amount, 8.0 × 108 Bq, of dispersed 56MnO2 powder. The data obtained will be used for interpretation of biological effects in experimental mice and rats that result from dispersion of various levels of neutron-activated 56MnO2 powder, which is the subject of separate studies.

No abstract

При ядерных испытаниях в нижних слоях атмосферы, а также при атомной бомбардировке, в результате нейтронной активации химических элементов в составе почвы образуются бета-и гамма-излучающие радионуклиды. Радионуклид 56 Mn (T1/2=2,58 ч) -один из основных ней-тронно-активированных бета-излучателей в течение первых часов после нейтронной актива-ции частиц почвенной пыли, поднявшейся в момент ядерного взрыва. Эффекты облучения остаточной радиоактивностью, образовавшейся в результате ядерных взрывов, являются предметом обсуждения и исследований последствий ядерных испытаний и атомных бомбар-дировок. Экспериментальное моделирование облучения лабораторных животных остаточной радиоактивностью в пылевых частицах почвы можно осуществить с использованием ней-тронно-активированного 56 Mn в виде порошка. Нейтронная активация порошкообразного MnO2 проведена на ядерном реакторе ИВГ.1М (экспериментальная установка «Байкал-1», Курчатов, Казахстан) при флюенсе нейтронов 410 14 н/см 2 . Полученный активированный по-рошок с 56 Mn был распылён пневматической системой над экспериментальными животными (крысы линии Вистар), которые находились в специальном боксе. Начальная активность рас-пылённого порошка была равна 2,7410 8 Бк. Облучение проведено в двух вариантах -в пер-вом из них в боксе имелся только воздушный фильтр для обеспечения дыхания животных. Во втором варианте была осуществлена принудительная вентиляция бокса. После распыле-ния порошка было выполнено измерение активности