[H-TT35_1AM2] 地球人間圏科学研究のための加速器質量分析技術の革新と応用

[HTT35-14] 福島事故以前における表層土壌中の放射性核種塩素‐36とヨウ素‐129の分布

*末木 啓介¹、北川 潤一²、笹 公和¹、高橋 努¹、松村 万寿美¹、木下 哲一³、戸崎 裕貴⁴、松四 雄騎⁵、松崎 浩之⁶ (1.筑波大、2.高エネルギー加速器研究機構、3.清水建設、4.産業技術総合研究所、5.京都大学、6.東京大学)

キーワード：加速器質量分析, 塩素‐３６, ヨウ素‐１２９, 土壌

The long-lived radionuclides ³⁶Cl and ¹²⁹I are generated by the nuclear tests or interaction with cosmic rays. They have descended to ground or sea level surface, and they have remained ground surface afterward. We have measured amount of ³⁶Cl and ¹²⁹I by accelerator mass spectrometry (AMS) before nuclear accident at the Fukushima No. 1 nuclear power plant.We have collected surface soil samples from the Sea of Japan to the Pacific Ocean at the equal-latitude cross-sectional areas (37⁰20' N - 37⁰30' N) in the south Tohoku, Japan. Inorganic chlorine in soil developed an improved leaching process that uses diluted HNO3 as an extractant, activated carbon to remove organic matters without decomposition, and H₂O₂ to remove residual organic matters. After leaching from soils, the AgCl samples for AMS-target made from the obtained solutions at ordinary treatment. Isotopic ratios of ³⁶Cl/Cl were determined by AMS at Tandem Accelerator Complex, University of Tsukuba. Preparation of Iodine-129 target was following ordinary method. Isotopic ratios of ¹²⁹I/I were determined by accelerator mass spectrometry (AMS) at MALT, the University of Tokyo. Moreover, we determined ¹³⁷Cs concentrations by gamma spectroscopy and LOI (loss on ignition used by an electric furnace) which related to the amount of the organic matter in soil.We obtained the distributions of radionuclides ³⁶Cl and ¹²⁹I in surface soils. The measured ³⁶Cl/Cl ratios of 34 surface soil samples which were about 0-10 cm in depth from 6 sites at the equal-latitude cross-sectional areas were between 0.1 x 10^-13 and 4.1 x 10^-13. It was shown that the ³⁶Cl/Cl ratios are lower at both sea sides. The concentrations of ¹²⁹I and ¹²⁹I/I ratios in surface soil (0-10 cm) at 28 points were determined to be 0.18 - 1.13 mBq/kg and 4.3 x 10^-9 - 11.7 x 10^-9, respectively.The depth profiles of ³⁶Cl/Cl, ¹²⁹I and ¹³⁷Cs were examined that the difference of distribution. The concentrations are higher at close surface in each nuclide. The results of ³⁶Cl/Cl profiles in soil cores up to 1 m long suggested that bomb-produced ³⁶Cl remains in uppermost sections, typically for ~20 cm deep, in undisturbed soil layers. The observed close correlation between organic matter content and ³⁶Cl/Cl ratio implies that presence of biological activity contributes the retention of fallout ³⁶Cl in the surface zone. The concentration of ¹²⁹I is shown the highest in uppermost surface. It is thought that the influence of global fallout has been received until now. In both nuclides, a constant amount exists in deeper than 30 cm. The concentrations of ¹³⁷Cs are not detection in deeper than 40 cm. It is showed that ¹³⁷Cs was lower mobility in soils. Therefore, the sampling soils were not a disturbance.