⁴¹Ca in tooth enamel. part II: A means for retrospective biological neutron dosimetry in atomic bomb survivors

⁴¹Ca is produced mainly by absorption of low-energy neutrons on stable ⁴⁰Ca. We used accelerator mass spectrometry (AMS) to measure ⁴¹Ca in enamel of 16 teeth from 13 atomic bomb survivors who were exposed to the bomb within 1.2 km from the hypocenter in Hiroshima. In our accompanying paper (Wallner et al., Radiat. Res. 174, 000000, 2010), we reported that the background-corrected ⁴¹Ca/Ca ratio decreased from 19.5 × 10^-15 to 2.8 × 10^-15 with increasing distance from the hypocenter. Here we show that the measured ratios are in good correlation with γ-ray doses assessed by electron paramagnetic resonance (EPR) in the same enamel samples, and agree well with calculated ratios based on either the current Dosimetry System 2002 (DS02) or more customized dose estimates where the regression slope as obtained from an errors-in-variables linear model was about 0.85. The calculated DS02 neutron dose to the survivors was about 10 to 80 mGy. The low-energy neutrons responsible for ⁴¹Ca activation contributed variably to the total neutron dose depending on the shielding conditions. Namely, the contribution was smaller (10) when shielding conditions were lighter (e.g., outside far away from a single house) and was larger (26) when they were heavier (e.g., in or close to several houses) 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 γ rays.