A method of deducing the I 131 concentration in a radioactive plume from the time history of peak count rates determined from pulse height spectra obtained from an NaI(Tl) scintillation detector employed as a detector of a monitoring post was presented. The contribution to the count rates from I 131 accumulated around the monitoring post was subtracted in accordance with the time history, taking into consideration the cumulative attachment and its decay. The concentrations of I 131 in the

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... mes were estimated from the count rates using the calculated response of the NaI(Tl) detector with egs5 for a model of a plume uniformly containing I 131. This method was applied to the data from the monitoring posts at Nuclear Science Research Institutes of Japan Atomic Energy Agency (JAEA). The estimated time history variation of I 131 concentrations in plumes was in fair agreement with those measured directly by an air sampling method. The diŠerence was less than a factor of 4 for plumes that arrived on March 15 and March 21, indicating relatively high I 131 concentrations among the plumes studied in this work.