GEOGRAPHICAL-DISTRIBUTION OF FRACTIONATED LOCAL FALLOUT FROM THE NAGASAKI A-BOMB

Unfissioned plutonium (Pu-239 + Pu-240) along with fission products (including Cs-137) were released from the Nagasaki A-bomb on 9 August 1945 and deposited in the eastern side (Nishiyama) of the city of Nagasaki. The concentrations of these radioactive materials were determined in 47 undisturbed surface soils collected from both eastside and westside of the hypocenter. The maximum concentrations were 64.5 mBq g-1 (dry) (or 181 mBq cm-2) for Pu-239 + Pu-240 and 188 mBq g-1 (dry) (or 526 mBq cm-2) for Cs-137 in the Nishiyama district where 'black rain' precipitated 20 min after the detonation. The ratio Pu-239 + Pu-240/Pu-137 Cs was not a constant in the soil samples collected from the Nishiyama. The maximum ratio (73.8%) was observed 2.8 km east of the hypocenter, where the maximum Pu-239 + Pu-240 concentration was observed. This finding suggests that there was fractionation between Pu-239 + Pu-240 and Cs-137 during 'black rain' formation. Previously reported data support this fractionation hypothesis for the Nishiyama local fallout.