DETERMINATION OF NATURAL ALPHA-RADIONUCLIDES RELATED TO MILLISECOND ORDER LIVES IN ENVIRONMENTAL-SAMPLES USING THE TIME-INTERVAL ANALYSIS (TIA) METHOD

The time files due to radioactive decay events were analyzed for the s elective extraction of successive alpha-decay pulse events in millisec ond orders, such as Po-216 (145 ms thorium series), At-217 (32.3 ms, n eptunium series), and Po-215 (1.78 ms, actinium series) after respecti ve preceding parents decay, using a multiple time interval analysis (T IA-MTA) technique from both theoretical and experimental viewpoints. I n the theoretical treatments, the detection sensitivities of each corr elated event in natural decay series was compared with more generalize d conditions. It was noteworthy that the evaluated detection sensitivi ty had increased proportionally with shorter half-lives in the followi ng order: Po-214 > Po-215 > At-217 > Po-216. Finally, the present TIA was applied to the clarification of thorium series disequilibrium stat es in Tamagawa (Akita, Japan) Hot Spring waters. Activities of Ra-224 were evaluated to be 3.41 +/- 0.38 and 0.60 +/- 0.08 Bq/l at the upper and lower stream, respectively. The results from activity ratios of r adium to thorium isotopes showed a preferential precipitation of radiu m isotopes in comparison with thorium, reflecting the sensitivity to t he lowering acidity of strongly acidic hot spring waters in the lower end of the stream.