Reevaluation of radioactive fallout in the black rain area by the Hiroshima atomic bomb

Abstract

It is well known from radiation surveys and sample measurements just after the bombing that the radioactive fallout from the Hiroshima atomic bomb contaminated the Koi-Takasu area located about 3 km west of the hypocenter. Meanwhile, "black rain" after the bombing was observed in the wider area, including villages 30 km away from the hypocenter, where radioactivity monitoring was not made just after the bombing. In 1970s measurements of ¹³⁷Cs and ⁹⁰Sr were carried out for soil samples taken from the black rain area. Unfortunately the contribution from the Hiroshima bomb could not be found because of large interference from the global fallout due to atmospheric nuclear tests. Recently, we succeeded to detect ²³⁶U in soil samples that was considered to be produced through the ²³⁵U(n,g)²³⁶U reaction in the Hiroshima bomb. We are now extending ²³⁶U measurements of soil samples from the black rain area and developing a method to evaluate the amount of fallout deposition based on the ²³⁶U data. The meteorological model is also investigated to evaluate the area of black rain as well as dosimetry models for external and internal irradiation.