The Hiroshima thermal-neutron discrepancy for Cl-36 at large distances. Part I: New Cl-36 measurements in granite samples exposed to A-bomb neutrons

摘要

The long-lived radioisotope Cl-36 (half-life: 301,000 years) was measured in granite samples exposed to A-bomb neutrons at distances from 94 to 1,591 in from the hypocenter in Hiroshima, by means of accelerator mass spectrometry (AMS). Measured Cl-36/Cl ratios decrease from 1.6x10(-10) close to the hypocenter to about 1-2x10(-13), at a distance of 1,300 in from the hypocenter. At this distance and beyond the measured Cl-36/Cl ratios do not change significantly and scatter around values of 1-2x10(-13). These findings suggest that the Cl-36 had been predominantly produced by thermalized neutrons from the A-bomb via neutron capture on stable Cl-35, at distances from the hypocenter smaller than about 1,200 in. At larger distances, however, confounding processes induced by cosmic rays or neutrons from the decay of uranium and thorium become important. This hypothesis is theoretically and experimentally supported in a consecutive paper. The results are compared to calculations that are based on the most recent dosimetry system DS02. Close to the hypocenter, measured Cl-36/Cl ratios are lower than those calculated, while they are significantly higher at large distances from the hypocenter. If the contribution of the cosmic rays and of the neutrons from the decay of uranium and thorium in the sample was subtracted, however, no significant deviation from the DS02 calculations was observed, at those distances. Thus, the Hiroshima neutron discrepancy reported in the literature for Cl-36 for samples from large distances from the hypocenter, i.e., higher measued Cl-36/Cl ratios than predicted by the previous dosimetry system DS86, was not confirmed.