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

Abstract

The long-lived radioisotope (36)Cl (half-life: 301,000 years) was measured in granite samples exposed to A-bomb neutrons at distances from 94 to 1,591 m from the hypocenter in Hiroshima, by means of accelerator mass spectrometry (AMS). Measured (36)Cl/Cl ratios decrease from 1.6 x 10(-10) close to the hypocenter to about 1-2 x 10(-13), at a distance of 1,300 m from the hypocenter. At this distance and beyond the measured (36)Cl/Cl ratios do not change significantly and scatter around values of 1-2 x 10(-13). These findings suggest that the (36)Cl had been predominantly produced by thermalized neutrons from the A-bomb via neutron capture on stable (35)Cl, at distances from the hypocenter smaller than about 1,200 m. 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 (36)Cl/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 (36)Cl for samples from large distances from the hypocenter, i.e., higher measured (36)Cl/Cl ratios than predicted by the previous dosimetry system DS86, was not confirmed.

Exclusion of natural radioisotopes for measurements of 152Eu in granites exposed to the Hiroshima atomic bomb

“…The DS86 was replaced by the dosimetry system 2002 (DS02) 5) . For the distant samples, the newly calculated 36 Cl/Cl ratios were also approximately coincident with those measured in Munich 6,7) . The height of the epicenter in the DS02 was 600 m while that in the DS86 was 580 m. The replacement of the dosimetry system made the calculated 36 Cl/Cl ratios lower at close ranges, but they were still larger than measured ratios 6,7) .…”

“…For the distant samples, the newly calculated 36 Cl/Cl ratios were also approximately coincident with those measured in Munich 6,7) . The height of the epicenter in the DS02 was 600 m while that in the DS86 was 580 m. The replacement of the dosimetry system made the calculated 36 Cl/Cl ratios lower at close ranges, but they were still larger than measured ratios 6,7) . Komura et al 8) measured 152 Eu/Eu ratios in several granite samples from ground distances of 146-1411 m. Most of the ratios were in agreement with those calculated by the DS02.…”

Prospects and status of the dosimetry system for atomic bomb survivor cohort study conducted at Research Institute for Radiation Biology and Medicine of Hiroshima University

“…Nevertheless, this problem in terms of the dosimetry for distant locations had been resolved within a few years from 2001. In the same year, 2001, Ruehm reported that the re-measurements of 36 Cl agreed with the preliminary results from new calculations for locations further than 1.2 km, which was later confirmed and published in 2005 [ 35 ]. The consistency in 36 Cl triggered the re-measurement of 152 Eu up to statistics of an order of magnitude.…”

Atomic spectrometry update. Industrial analysis: metals, chemicals and advanced materials

“…In the second paper, Huber and co-workers determined the long-lived radioisotope 36 Cl in granite samples exposed to the A-bomb neutrons at various distances from the Hiroshima hypocenter. 205 It was found that very close to the hypocenter, the 36 Cl/Cl ratio was 1.6 Â 10 À10 , but that this decreased to approximately 2 Â 10 À13 further away (1200 m) and then stayed approximately constant. It was concluded that the 36 Cl had been produced by the neutron capture by stable 35 Cl.…”