Plutonium isotopes derived from Nagasaki atomic bomb in the sediment of Nishiyama reservoir at Nagasaki, Japan

The source of plutonium in sediments deposited at Nishiyama reservoir at Nagasaki was characterized by their 240Pu/239Pu atom ratio. The average ratio was approximately 0.03, except in two layers. The main source of the plutonium was the Nagasaki atomic bomb. The plutonium continues to flow into the reservoir even now. The 240Pu/239Pu atom ratios in two layers were higher than the average, which showed that plutonium in these layers were made of those of nuclear tests added to those of the atomic bomb.