The Hiroshima thermal-neutron discrepancy for ³⁶Cl at large distances. Part II: Natural in situ production as a source

For Hiroshima, a large discrepancy between calculated and measured thermal-neutron fluences had been reported in the past, for distances to the epicenter larger than about 1,000 m. To be more specific, measured ³⁶Cl concentrations in environmental samples from Hiroshima were too large at these distances, and the ratio of measured to calculated values reached about 70, at a distance of 1,800 m. In an attempt to identify other sources that might also produce ³⁶Cl in Hiroshima samples, the role of cosmic rays and of neutrons from natural terrestrial sources was investigated. Four reaction mechanisms were taken into account: spallation reactions of the nucleonic (hadronic) component of the cosmic rays on potassium (K) and calcium (Ca) in the sample material, particle emission after nuclear capture of negative muons by K and Ca, reactions of fast-muon induced electromagnetic, and hadronic showers with K and Ca, and neutron capture reactions with ³⁵Cl in the sample where the neutrons originate from the above three reaction mechanisms and from uranium and thorium decay. These mechanisms are physically described and mathematically quantified. It is shown that among those parameters important for the production of ³⁶Cl in granite, the chemical composition of the sample, the depth in the quarry where the sample had initially been taken, and the erosion rate at the site of the quarry are most important. Based on these physical, chemical, and geological parameters, ³⁶Cl concentrations were calculated for different types of granite that are typical for the Hiroshima area. In samples that were of these granite types and that had not been exposed to atomic bomb(A-bomb) neutrons, the ³⁶Cl concentration was also determined experimentally by means of accelerator mass spectrometry, and good agreement was found with the calculated values. The ³⁶Cl signal due to natural in situ production was also calculated in granite samples that had been exposed to A-bomb neutrons at distances up to 1,500 m from the hypocenter. It is demonstrated that, for granite samples from Hiroshima exposed to A-bomb neutrons beyond distances of about 1,300 m from the hypocenter, the ³⁶Cl signal is dominated by natural in situ production.