Effectiveness of tree rings for recording Pu history at Nagasaki, Japan

A 78-year-old tree was harvested in 1988 at 2.8 km east of the Nagasaki Pubomb hypocentre, where the local fallout of the 1945 blast was highest. The surface soil concentration of 239 + 240Pu was 64.5 mBq g −1 and that of 137Cs was 87.4 mBq g −1. The tree rings were analyzed for their concentrations of 239 + 240Pu, 137Cs and 40K. Interestingly, the concentration profiles over seven decades showed that the Pu was immobile, while Cs and K were mobile in the tree rings. In other words, the Pu concentration profile revealed a history of Pu in the surrounding environment of Nagasaki. However, the combined routes, via leaves from the atmospheric deposition and roots from surface soils to tree rings, made the record less clear. Surprisingly, the Pu from the Nagasaki Pu-bomb in the tree rings of 1946–44 played a minor role in the concentration profile compared to that from global fallout. This meant that the Pu in the local fallout was less bio-available compared to that of the global fallout.