Measurement of Residual ¹⁵²Eu Activity Induced by Atomic Bomb Neutrons in Nagasaki and the Contribution of Environmental Neutrons to This Activity

抄録

Residual ¹⁵²Eu activities induced by neutrons from the Nagasaki atomic bomb were measured for nine mineral samples located up to 1,061 m in the slant range and one control sample at 2,850 m from the hypocenter. A chemical separation to prepare europium-enriched samples was performed for all samples, and gamma ray measurements were carried out with a low background well-type germanium detector. In this paper, the measured specific activities of ¹⁵²Eu are compared with activation calculations based on the DS86 neutron fluence and the 93Rev one. The calculated-to-measured ratios are also compared with those of ⁶⁰Co and ³⁶Cl. The present results indicate that the measurements agree to the calculation within a factor of three as observed in the nuclear tests at Nevada. The activation level of environmental neutrons and the detection limit for ¹⁵²Eu are also discussed.