Plutonium Isotopes, 241Am and 137Cs in soil samples collected from Nagasaki and Hiroshima were measured to evaluate the contribution of residual radioactivity derived from the atomic bombs dropped in 1945.In the soils from Nishiyama area of Nagasaki City, activity ratios different from those of global fallout were found for the 238Pu/239, 240Pu (0.05-0.06), 239, 240Pu/137Cs (0.2-0.3), 241Am/239, 240Pu (0.04-0.05) and 240Pu/239Pu (0.08-0.13), activity ratios indicating that a fair amount of plutonium derived from the atomic bomb had accumulated in this area. On the other hand, in the soils from the “Black-rain” area of Hiroshima, no significant difference from the global fallout was found for the activity ratios among the Pu isotopes, 241Am and 137Cs except for the soils collected at 10 km and 12 km in the NNW direction from the epicenter, where a factor 2 to 3 lower 241Am/239, 240Pu activity ratio was observed. However, no evidence of the contribution of the atomic bomb was found by the measurement of 240Pu/239Pu isotopic ratio.
Yamamoto et al. (Tue,) studied this question.