Feasibility of using 236U to reconstruct close-in fallout deposition from the Hiroshima atomic bomb

The first results on the feasibility of using U-236 to reconstruct the level and spatial distribution of close-in fallout deposition from the Hiroshima A-bomb are reported, coupled with the use of global fallout Cs-137 and Pu239 + 240. The results for global fallout U-236 in soil samples (0-30 cm) from Ishikawa prefecture showed that the deposition density of U-236 from the global fallout can be accurately evaluated using AMS. All deposited U-236, Cs-137 and Pu239 + 240 appeared to have been recovered using 30-cm cores. It was also noted from the depth profiles for U-236/Pu239 + 240 and U-236/Cs-137 ratios that the downward behavior of U-236 in the soil was apparently similar to that of Pu239 + 240, while the Cs-137 was liable to be retained in upper layers compared with U-236 and Pu239 + 240. The accumulated levels were 1.78 x 10(13) atoms m(-2) for U-236, 4340 Bq m(-2) for Cs-137 and tin Bq m(-2) for Pu239 + 240. The ratios of U-236/Cs-137 and U-236/Pu239 + 240 were (4.10 +/- 0.12) x 10(9) and (1.26 +/- 0.04) x 10(11) atoms Bq(-1), respectively. Results of U-236, (CS)-C-137 and Pu239 + 240 measurements for the seven soil cores (0-30 cm) from Hiroshima were discussed on the basis of ratios of U-236/(CS)-C-137 and U-236/Pu239 + 240 by comparing with those from the background area in Ishikawa, indicating that the global fallout dominates the current level of U-236 accumulation in soil in the Black-rain area around Hiroshima after the Hiroshima bomb, and the contribution of the close-in fallout U-236 produced by the Hiroshima A-bomb seems difficult to observe. (C) 2010 Elsevier B.V. All rights reserved.