Detection of the long-lived radionuclide 41Ca (t1/2 = 103 000 years) in tooth enamel is presented here as a means to reconstruct thermal neutron exposures of the A-bomb survivors of Hiroshima and Nagasaki. In general, 41Ca is produced via thermal neutron capture on stable 40Ca. Thus, any 41Ca present in enamel of the A-bomb survivors is both due to neutrons from natural sources and from the A-bomb explosion. Tooth samples from large-distant survivors with negligible exposure to a-bomb neutrons were used to investigate the natural 41Ca content in tooth enamel. Tooth samples from exposed survivors, all members of the so-called Life-Span-Study cohort, were measured for comparison. For those samples γ-ray doses had already been determined before by means of electron spin resonance measurements. All 41Ca measurements were performed by means of AMS using the Munich MP tandem accelerator. In terms of the 41Ca/40Ca isotope ratio, values significantly higher were found for survivors that had been located close to the hypocenter at the time of bombing, compared to those who survived at larger distances. For those who were not exposed significantly to A-bomb neutrons, the measured 41Ca/40Ca isotope ratios were found even lower. The 41Ca/Ca ratios for the exposed survivors show a significant correlation with distance from the hypocenter, as expected from calculations. Thus, for the first time an experimental signature of neutron exposure was found in tissue obtained from A-bomb survivors.
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