IPSHU Research Report Series 28号
2012-03 発行

Activation analysis for soils of Hiroshima city and estimation of gamma-ray dose rate due to neutron induced activated soil by Hiroshima atom bomb

Taguchi, Yuta
Imanaka, Tetsuji
Fukutani, Satoshi
Hoshi, Masaharu
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For the early entrance survivors in Hiroshima and Nagasaki atomic bomb (A-bomb), radiation doses from activated materials induced by the A-bomb neutrons are dominant. For estimation of such doses, element compositions of surrounded materials such as soil and nibbles are necessary. Especially Sc density in soil is important for estimating radiation doses at the time of a few 10 days after explosion. Because 46Sc which has the half-life of 84 days, is induced the A-bomb neutrons. However, few data of Sc density in soil are available in both of Hiroshima and Nagasaki cities. Purpose of this study is evaluation of Sc density in soil and the uncertainty using activation analysis.

Soil samples were taken from 11 locations within 4 km from A-bomb hypocenter at Hiroshima city. The soil samples and reference rock sample of JA-11) were activated in Kyoto University Reactor (KUR). Element compositions are relatively obtained from each identified radionuclide counting rates in soil and reference rock by Ge-detectors.

Twenty three element compositions including Al, Mn, Na and Sc are obtained by the activation analysis. The obtained element compositions are compared with values in Dosimetric System 1986 (DS86) and those are roughly the same as the reported values in DS86. Sc density in Hiroshima soil was estimated to be 5.12±0.59 (ppm). It was found the unevenness of Sc density in soils for 11 location of Hiroshima city is about 12%.

Using element compositions by the activation analysis, time variation of the exposure rate by activated soil are estimated. It was found that exposure rate in the few minute time range is dominated by 28A1, in the several days by 24Na, and in the a few 10 days by 46Sc. This estimated result is compared with measured dose rate measured after a few months after explosion. It was found that the estimated dose rate is quite similar to the measured one.
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Copyright (c) 2012 Institute for Peace Science, Hiroshima University