The dosimetry system DS86 and the neutron discrepancy in Hiroshima – historical review, present status, and future options

Abstract

The historical development of the dosimetry systems for Hiroshima and Nagasaki is outlined from the time immediately after the A-bomb explosions to the publication of the dosimetry system DS86 in 1987, and the present status of the so-called Hiroshima neutron discrepancy is summarized. Several long-lived radionuclides are discussed with regard to their production by neutrons from the A-bomb explosions. With the exception of ⁶³Ni, these radionuclides have not, up to now, been measured in samples from Hiroshima and Nagasaki. Two of them, ⁶³Ni in copper samples and ³⁹Ar in granite samples, were predominantly produced by fast neutrons. ⁶³Ni can be determined by accelerator mass spectrometry with a gas-filled analyzing magnet. It should be measurable, in the near future, in copper samples up to 1500 m from the hypocenter in Hiroshima. ³⁹Ar can be measured in terms of low-level beta-counting. This should be feasible up to a distance of about 1000 m from the hypocenter. Three radionuclides, ¹⁰Be, ¹⁴C , and ⁵⁹Ni, were produced predominantly by thermal neutrons with smaller fractions due to the epithermal and fast neutrons, which contribute increasingly more at larger distances from the hypocenter. State-of-the-art accelerator mass spectrometry is likely to permit the determination of ¹⁰Be close to the hypocenter and of ¹⁴C up to a distance of about 1000 m. ⁵⁹Ni should be detectable up to a distance of about 1000 m in terms of accelerator mass spectrometry with a gas-filled magnet. The measurements of ¹⁰Be, ¹⁴C, ³⁹Ar, ⁵⁹Ni – and potentially of ¹³¹Xe – can be performed in the same granitic sample that was already analyzed for ³⁶Cl, ⁴¹Ca, ⁶⁰Co, ¹⁵²Eu, and ¹⁵⁴Eu. This will provide extensive information on the neutron spectrum at the specified location, and similarly complete analyses can conceivably be performed on granite samples at other locations.