Radioactivity in atomic-bomb samples from exposure to environmental neutrons

For about one decade, activation measurements performed on environmental samples from a distance larger than 1 km from the hypocenter of the atomic-bomb explosion over Hiroshima suggested much higher thermal neutron fluences to the survivors than predicted. This caused concern among the radiation protection community and prompted a complete re-evaluation of all aspects of survivor dosimetry. While it was shown recently that secondary neutrons from cosmic radiation and other sources have probably been the reason for the high measured concentrations of the long-lived radioisotope Cl-36 in these samples, the source for high measured concentrations of the short-lived radionuclides Eu-152 and Co-60 has not yet been investigated in detail. In order to quantify the production of Eu-152 and Co-60 in environmental samples by secondary neutrons from cosmic radiation, thermal neutron fluxes were measured by means of a He-3 gas proportional counter in various buildings where these samples had been and still are being stored. Because a Cf-252 neutron source has been operated occasionally close to one of the sample storage rooms, additional neutron flux measurements were carried out when the neutron source was in operation. The thermal neutron fluxes measured ranged from 0.00017 to 0.00093 n cm(-2) s(-1) and depended on the floor number of the investigated building. Based on the measured neutron fluxes, the specific activities from the reactions Eu-151(n,y)Eu-152 and Co-59(n,gamma)Co-60 in the atomic-bomb samples were estimated to be 7.9 mBq g(-1) Eu and 0.27 mBq g(-1) Co, respectively, in saturation. These activities are much lower than those recently measured in samples that had been exposed to atomic-bomb neutrons. It is therefore concluded that environmental and moderated Cf-252. neutrons are not the source for the high activities that had been measured in these samples.