Detection of Uranium236 in Hiroshima Atom bomb "Black Rain" Soil

Abstract

There was "black rain" after the Hiroshima aton bombing in the wide area that extended outside Hiroshima city more than 30 km to the north-west direction from the hypocentre. Previous attempts failed to detect fission products such as ¹³⁷Cs and ⁹⁰Sr from the Hiroshima bomb in the black rain area because of large contribution from global fallout of nuclear test in the atmosphere. According to unofficial reports, about 51 kg of ²³⁵U was loaded in Hiroshiba bomb, of which 912 g was consumed by the 16-kt explosion and some fraction was converted into ²³⁶U through the ²³⁵U(n,g)²³⁶U reaction. Although most of ²³⁶U were assumed to have been dispersed upto the high altitude in the sky some part could be deposited on the ground with the black rain. Thermal ionization mass spectrometry (TIMS) was applied to detect ²³⁶U from soil samples collected in 1976 from the black rain area around the Hiroshima city. So far, the ²³⁶U/²³⁸U atom ratios of (1.2-8.6)x10^-8 were obtained for 7 soil samples, while ²³⁶U was not detected in the sample taken from the control area. The ²³⁶U measurement using TIMS can provide valuable information about the local fallout contamination by the Hiroshima atom bomb.