Estimation of neutron dose of atomic bomb survivors by ⁴¹Ca/Ca ratio in tooth enamel using AMS

Abstract

⁴¹Ca is a radioisotope of Calcium, mainly produced by absorption of low-energy neutrons to ⁴⁰Ca. We measured ⁴¹Ca/⁴⁰Ca ratio for evaluation of neutron dose using accelerator mass spectrometry (AMS) method in tooth enamel of atomic-bomb survivors in Hiroshima. The background-subtracted ⁴¹Ca/Ca ratio was found to decrease from 19.5x10^-15 to 2.8x10^-15 with increasing distance from the hypocenter. Further, the measured ratios are in good correlation with gamma-ray doses assessed by the electron spin resonance (ESR) method in the same enamel samples, and agree well with calculated ratios based on the current DS02 or more customized dose estimates. It is noted that the calculated DS02 neutron dose to the survivors was about 10 to 80 mGy. The contribution of low energy neutrons (responsible for ⁴¹Ca activation) was smaller (10%) when shielding conditions were lighter while it was larger (26%) when they were heavier because of local moderation of neutrons by shielding materials. We conclude that AMS is useful for verifying calculated neutron doses under mixed exposure conditions with gamma rays.