[H-CG22] Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Status, operations, and scientific application

[HCG22-P08] Extraction and Analysis of Molten Particles in Soil from the Remains of the Former Army Stockpile Warehouse, Hiroshima

*Satoru Endo¹, Takuya Sigimoto¹, Tsuyoshi Kajimoto¹, Yutaka Yamada², Masakazu Oikawa², Yasuhito Igarashi³ (1.Graduate School of Advanced Science and Engineering,Hiroshima University, 2.National Institutes for Quantum Science and Technology, 3.Institute for Integrated Radiation and Nuclear Science, Kyoto University)

Keywords:black rain, molten particle, element analysis

Wannier et al. in 2019 reported the presence of about 3,000 tons of molten particles of high-temperature origin (above about 1,800 degrees) in the sand of Moto-Ujina beach, Hiroshima City, and it was argued that the origin could reasonably be attributed to the Hiroshima atomic bomb on August 6, 1945. These molten particles are expected to be an indicator material for estimating black rainfall areas, and we have been investigating whether or not the molten particles originated from the atomic bomb. We have collected sea sand from the Moto-Ujina beach in Hiroshima Prefecture and extracted spherical molten particles. The extracted particles (5.86 g) were used to quantify ¹³⁷Cs concentration, uranium concentration, and thorium concentration by Ge detector measurement. The ¹³⁷Cs concentration was 1.3+/-0.5 mBq g^-1, which is too low to consider that the molten particles were attributable to the A-bomb. The uranium and thorium concentrations were 8.67 ppm and 20.1 ppm, respectively, which were similar to or slightly higher than those in the Ota Riverbed sediments in Hiroshima City. Trace element analysis by inductively coupled plasma mass spectrometry (ICP-MS) was also conducted to quantify Be, Co, Rb, Ag, Cs, Tl, Pb, Bi, Th, and U concentrations. These concentrations were close to those of the Hiroshima soil, which seemed to indicate that the molten particles were of soil origin. Furthermore, the ²³⁵U/²³⁸U ratio of the molten particles was evaluated by thermal ionization mass spectrometry (TIMS) measurements, and the ²³⁵U/²³⁸U ratio was consistent with the natural abundance ratio of 0.00726. These results indicated the possibility that the spherical molten particles could have originated in the city's soils, but the results did not suggest that they were produced by the atomic bomb fireball.
In order to resolve this contradictory information, soil cores were collected at the remains of a former army stockpile warehouse discovered in Hiroshima City to directly confirm whether molten particles were present in the A-bomb-exposed soil and what their properties were with respect to the soil. By measuring depth profile of ¹³⁷Cs concentration of the soil core, the exposed surface was identified, and molten particles were extracted from the soil close to the exposed surface.. These molten particles were analyzed using Micro-PIXE analysis by PASTA at the Quantitative Research Institute of Japan (QST). This presentation will introduce the results of the identification of the exposed surface, the extraction of molten particles, and results of the PIXE analysis of the molten particles.