Fast neutrons measured in copper from the hiroshima atomic bomb dome

The first measurements of ⁶³Ni produced by A-bomb fast neutrons (above ~1 MeV) in copper samples from Hiroshima encompassed distances from ~380 to 5062 m from the hypocenter (the point on the ground directly under the bomb). They included the region of interest to survivor studies (~900 to 1500 m) and provided the first direct validation of fast neutrons in that range. However, a significant measurement gap remained between the hypocenter and 380 m. Measurements close to the hypocenter are important as a high-value anchor for the slope of the curve for neutron activation as a function of distance. Here we report measurements of ⁶³Ni in copper samples from the historic Hiroshima Atomic Bomb Dome, which is located ~150 m from the hypocenter. These measurements extend the range of our previously published data for ⁶³Ni providing a more comprehensive and consistent A-bomb activation curve. The results are also in good agreement with calculations based on the current dosimetry system (DS02) and give further experimental support to the accuracy of this system that forms the basis for radiation risk estimates Worldwide.