Geographical distribution of plutonium derived from the atomic bomb in the eastern area of Nagasaki

The detonation of Nagasaki atomic bomb occurred on August 9, 1945. A large amount of unfissioned plutonium was dispersed into Nagasaki area. In our previous study, we measured 240Pu/239Pu ratios as well as 239+240Pu concentrations in sediment collected at Nishiyama reservoir in the center of Nishiyama area, where 'lack rain' was precipitated, and found that trace amount of the atomic-bomb plutonium has flown into the reservoir even now. A further study on soil samples collected within 10 km from the hypocenter including the basin of the reservoir elucidated the geological distribution of the atomic-bomb plutonium, which was locally deposited at Nishiyama area. In addition, the ratios of 0.13±0.01, which was lower than the value (0.176) of global fallout, were found in samples taken about 8 km east from the hypocenter, while the concentrations were comparable with back ground level. This means a possibility that the plutonium was deposited farther in eastern area. For this reason, we expanded the search area to the eastern area of Nagasaki for determining the deposition of the atomic-bomb plutonium from view point of 240Pu/239Pu ratio. Until now, we collected eight soil samples within 100 km north eastward of the hypocenter. As preliminary results of 2 samples collected at about 25 km east and 45 km from the hypocenter, the 240Pu/239Pu ratios were 0.154±0.017 and 0.111±0.004, which were lower than the value of global fallout indicating the existence of the atomic-bomb plutonium. In the presentation, we will report the details and other results in soils collected in farther area and discuss about deposit area of the atomic-bomb plutonium.