Cs-137 concentration in soil samples from an early survey of Hiroshima atomic bomb and cumulative dose estimation from the fallout

Low background gamma-ray measurement has been performed to determine the Cs-137 content in soil samples collected in a very early survey of the Hiroshima atomic bomb. These soil samples were collected just 3 d after the explosion within 5 km from the hypocenter and were not exposed to the global fallout from nuclear weapon tests. Out of 22 samples, Cs-137 was detected for 11 samples, and their radioactivities ranged from 0.16-10.6 mBq g(-1) at the time of the measurement. A comparison of the Cs-137 deposition with the rainfall area within Hiroshima city indicates that the rainfall area was wider than the previously proposed one. Cumulative exposure by the fallout has been estimated to be 0.31 mC kg(-1) (0.12 R) in Hiroshima city except for the heavy fallout area and at most 1.0 mC kg(-1) (4 R) in the heavy fallout area.