Activation Measurements for Thermal Neutrons

The neutron energy spectrum from the atomic bomb consisted of a range of energies from thermal to fast neutrons. Fast neutrons contribute a larger dose to the human body than lowenergy neutrons per unit flux. The evaluation of the fast neutron component has been performed based on analyses of P (>3 MeV) activity measured immediately after the Hiroshima bomb and Ni (>2 MeV), which has a much longer half-life (100 y) and which was measured by the AMS method. An indirect way to investigate the neutron spectrum is to measure the depth profile of Eu in massive rocks. Energetic neutrons incident on the rock surface lose their energy by scattering among the granite elements until they become slow or thermal neutrons. Then the neutrons induce Eu activity through neutron capture reactions along their paths in the rock. The resulting activity as a function of depth reflects the energy spectrum of incidence neutrons. The Eu depth profiles were measured for granite and concrete cores within 500 m of the hypocenter in Hiroshima. They were Motoyasu Bridge granite pillar (Hasai et al. 1987), other granite pillars of Motoyasu Bridge and Shirakami Shrine, and concrete cores of Hiroshima Bank and Gokoku Shrine (Shizuma et al. 1997). The results were compared with calculation.