Study of radial distribution of fallout nuclides and trace elements in tree rings as a method of determining environmental contamination over time.

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The ratios of ^<90>Sr to stable strontium and ^<137>Cs to potassium in the tree rings of two Japanese cedars (Cryptomeria japonica D.Don), one from Nagasaki and the other from Wakayama, were determined. The Nishiyama district of Nagasaki city was exposed to heavy "black rain" caused by the atomic bomb which was dropped there in 1945. Wakayama was not affected by the fallout from atomic bombs. The ratio of ^<90>Sr/Sr along a radial direction in each cedar decreased from the cambium to the inside. A clear peak in the ratio presumed due to the black rain from the Nagasaki atomic bomb was observed in the 1924-1925 rings of the cedar from Nagasaki.The ^<137>Cs/K ratios in the cedar from Nagasaki have almost the same value in every ring. No effect from the Nagasaki atomic bomb could be detected. The ratios in the cedar from Wakayama are similar but with a rather wider variation.The ratios of ^<90>Sr/Sr along a radial direction in the tree rings of a kaki (Diospros kaki Thunb.) from Hiroshima were determined. The ratios gradually increased from the cambium to the pith. No effect from the fallout from atomic bomb could be detected.The concentration profiles of 19 trace elements along radial direction in the stem of a kaki from Kyoto (which was not affected by the fallout from atomic bombs) were also determined by neutron activation analysis and/or the PIXE method.