⁴¹Ca in tooth enamel. part I: A biological signature of neutron exposure in atomic bomb survivors

The detection of ⁴¹Ca atoms in tooth enamel using accelerator mass spectrometry is suggested as a method capable of reconstructing thermal neutron exposures from atomic bomb survivors in Hiroshima and Nagasaki. In general, ⁴¹Ca atoms are produced via thermal neutron capture by stable ⁴⁰Ca. Thus any ⁴¹Ca atoms present in the tooth enamel of the survivors would be due to neutron exposure from both natural sources and radiation from the bomb. Tooth samples from five survivors in a control group with negligible neutron exposure were used to investigate the natural ⁴¹Ca content in tooth enamel, and 16 tooth samples from 13 survivors were used to estimate bomb-related neutron exposure. The results showed that the mean ⁴¹Ca/Ca isotope ratio was (0.17 ± 0.05) × 10^-14 in the control samples and increased to 2 × 10^-14 for survivors who were proximally exposed to the bomb. The ⁴¹Ca/Ca ratios showed an inverse correlation with distance from the hypocenter at the time of the bombing, similar to values that have been derived from theoretical free-in-air thermal-neutron transport calculations. Given that γ-ray doses were determined earlier for the same tooth samples by means of electron spin resonance (ESR, or electron paramagnetic resonance, EPR), these results can serve to validate neutron exposures that were calculated individually for the survivors but that had to incorporate a number of assumptions (e.g. shielding conditions for the survivors).