[M-AG33] Basic study on the estimation of the "Black Rain" area caused by the atomic bomb

[MAG33-02] Evaluation of alteration of soil samples using depth distribution and inventory of radionuclides in soil

*Hiroshi Yashima¹, Yasuhito Igarashi¹, Keisuke Sueki², Naofumi Akata³, Shinji Sugihara⁴, Takashi Saito⁵, Yuichi Kurihara⁶, Sumi Yokoyama⁷, Naoko Fukuda⁷ (1.Institute for Integrated Radiation and Nuclear Science, Kyoto University, 2.University of Tsukuba, 3.Hirosaki University, 4.Kyushu University, 5.SHOKEI GAKUIN UNIVERSITY, 6.Nagaoka University of Technology, 7.Nagasaki University)

Keywords:cesium, excess lead, alteration of soil samples

[Introduction] Soil samples are being collected to obtain validation data for the "black rain" deposition in the study of the estimated "black rain" rainfall area that occurred after the atomic bombings of Hiroshima and Nagasaki in 1945. In order to confirm whether the validation data obtained are from the 1945 deposition, it is necessary to confirm that the soil samples have not been altered and that the stratigraphy has been preserved. In this presentation, we report on the evaluation of alteration of soil samples using radioactivity distribution profiles in soil samples and fallout inventories.

[Methods] The scraper plate method was employed as the soil sampling method in this study. After sterilization and drying, gamma-ray spectrometry using a high-purity Ge detector was performed to measure ¹³⁷Cs and excess ²¹⁰Pb radioactivity concentrations in each layer, and depth profiles were obtained. The ¹³⁷Cs and excess ²¹⁰Pb fallout inventories were calculated for each layer, and the total inventory amount was obtained by adding up the inventories.

Samples for which the obtained depth profiles and inventories matched the following conditions were judged to have been unaltered.

(1) There is a peak in the ¹³⁷Cs depth profile due to global fallout.

(2) The ¹³⁷Cs inventory is greater than 750 Bq m^-2. (Consider to previous survey, Yamamoto et al. (2000))

(3) Excess ²¹⁰Pb depth profile tends to decrease from the uppermost layer.

(4) The ¹³⁷Cs/excess²¹⁰Pb inventory ratio is in the range of 0.2-0.4. (Consider to previous survey, Yamamoto et al. (2000))

Although there were some cases in which the trend of the shallow layer profiles differed depending on the sampling conditions, as a basic rule, this criterion could be used to evaluate the alteration.

Acknowledgments: This study was commissioned by the Ministry of Health, Labour and Welfare of Japan. We wish to thank the Ministry of Health, Labour and Welfare of Japan.