Kiyoshi Shizuma
Graduate School of Engineering, Hiroshima University

Abstract

137Cs was one of fission product of the atomic bombing of Hiroshima in 1945. Because of global fallout produced by later nuclear testing, it is difficult to distinguish the fission products caused by the atomic bomb itself from present-day surface soil samples. The only samples unaffected by global fallout are those collected just after the bombing. In previous studies, we used three types of samples to detect fallout from the Hiroshima bombing: (1) 22 soil samples collected by Nishina within 5 km of the hypocenter 3 days after the bombing, (2) 44 samples of roof tiles, concrete, granite, and other materials collected within 2 km of the hypocenter a few months after the bombing, and (3) black rain streaks on a plaster wall originally located in the Takasu area (4 km west of the hypocenter), where the fallout was the greatest. Here, we review our previous work on the relation between 137Cs deposition and rainfall area maps within 5 km of the hypocenter and 235U/238U ratios in black rain streaks. Cumulative dose estimates in the Hiroshima and Koi-Takasu areas are also presented. The deduced cumulative exposure by the fallout was 0.031 mC/kg within a 5-km radius of the hypocenter, except for a much heavier area of fallout near the Koi-Takasu district (1.0 mC/kg).