First biological signature of A-bomb neutrons obtained from Hiroshima-survivors

Full text: The so-called life-span-study is an extensive study to evaluate late effects of radiation received by the survivors of the Hiroshima/Nagasaki A-bombs. It consists of about 120000 individuals and represents the main database for our understanding of late effects of ionizing radiation. To correlate radiation and its effects, γ-ray and neutron-doses have to be determined and verified. The measurement of radionuclides, produced by neutrons originating from the a-bomb explosions, offers the possibility to reconstruct neutron fluences to which survivors were exposed. The radionuclide 41Ca, is presented here as a means for a retrospective determination of neutron fluences, directly within the human body of a survivor using tooth enamel as the most suitable material. Quantifying such small signals implies an extremely sensitive technique, solely represented by Accelerator-Mass-Spectrometry (AMS). The Munich tandem-accelerator which provides high particle energies in combination with a dedicated particle-detection-setup (GAMS) allowed the measurement of 41Ca down to naturally occurring levels. In comparison to non-exposed survivors, those exposed to A-bomb radiation showed higher 41Ca/Ca isotopic-ratios with a significant correlation with distance from the hypocenter. Thus, for the first time an experimental signature of neutron exposure was found in tissue obtained from A-bomb survivors. Since γ-ray doses had been determined by means of electron-spin-resonance method in the same material, both components of the mixed field that were present in Hiroshima at the time of bombing could be, for the first time, quantified in the same sample. The results will be useful to reconstruct neutron exposures that were calculated for the survivors on an individual scale. (author)