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 Nagasak i Pu-bomb hypocentre, where the local fallout of the 1945 blast was hi ghest. The surface soil concentration of Pu-239+240 was 64.5 mBq g-1 a nd that of Cs-137 was 87.4 mBq g-1. The tree rings were analyzed for t heir concentrations of Pu-239+240, Cs-137 and K-40. Interestingly, the concentration profiles over seven decades showed that the Pu was immo bile, while Cs and K were mobile in the free rings. In other words, th e Pu concentration profile revealed a history of Pu in the surrounding environment of Nagasaki. However, the combined routes, via leaves fro m the atmospheric deposition and roots from surface soils to tree ring s, 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 concen tration 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.