Irradiation of laboratory animals by neutron activated dust: development and application of the method - first results of international multicenter study

The radionuclide 56Mn (T1/2=2.58 hours) is one of the dominant beta- and gamma-emitters within few hours after the neutron irradiation of soil dust following nuclear explosion in atmosphere. The effects of exposure to residual radioactivity from nuclear explosions are the subject of discussions and research of the consequences of nuclear tests and the atomic bombing. Special experimental device was constructed for irradiation of laboratory rats by 56Mn powder. Neutron activation of powdered MnO2 was performed by using nuclear reactor IVG.1M (experimental facility “Baikal-1”, Kurchatov, Kazakhstan) with neutron fluence of 4´1014 n/cm2. The obtained 56Mn-containing powder was sprayed over the animals (rats of Wistar line) with pneumatic system, the rats were placed in an isolated box. The initial activity of radioactive 56Mn powder was 2.74´108 Bq. Exposition of rats to radioactive powder was carried out by two scenarios: a) the experimental box was supplied with air filter only (targeted for animal's breathing); b) the experimental box was supplied with the system of forced ventilation. After exposure to radioactive powder of 56Mn radioactivity in organs and tissues of the rats was measured. The used equipment was calibrated in A. Tsyb MRRC (Obninsk, Russia) using standard source of 56Mn obtained by neutron activation in the research nuclear reactor. Absorbed fractions of energy in organs and tissues of rats was calculated in A. Tsyb MRRC (Obninsk, Russia) using the Monte-Carlo code (version MCNP-4C) with the corresponding library of nuclear constants for electrons and gammas and using mathematical phantom of an experimental rat. Assessment of internal radiation doses was performed on the basis of the results of measuring of 56Mn activity in organs and tissues of experimental animals and on the basis of calculated values of absorbed fractions of internal exposure to photons and electrons. The first results of this international multicenter study showed that the most irradiated organs and tissues of the experimental animals are: the large intestine, small intestine, stomach, lungs, and skin. In the first scenario of exposure (without forced ventilation) accumulated absorbed doses of internal radiation to the large intestine, small intestine, stomach, lungs and skin were 1.65 Gy, 1.33 Gy, 0.24 Gy, 0.10 Gy and 0.076 Gy respectively. In the second scenario of exposure (with forced ventilation) the accumulated absorbed doses of internal radiation to the large intestine, small intestine, stomach, lungs and skin were 0.45 Gy, 0.15 Gy, 0.11 Gy, 0.03 and 0.073 Gy respectively. The results of assessments of internal doses from neutron-activated 56Mn in powder, sprayed over experimental animals can be used for understanding of biological effects of this type of radiation, which is the subject of future publications.