Keywords:135Cs/137Cs isotope ratio, identification of origin, decommissioning, accident at the Fukushima Daiichi Nuclear Power Station
Radiocesium from the Fukushima Daiichi Nuclear Power Station (1F) accident can be treated as background radioactivity at completion of decommissioning, so determining the origin of radiocesium is essential. Therefore, we studied identification of radiocesium origin using 135Cs/137Cs isotope ratio. The ratio originated from 1F was 0.3628±0.0005. When the ratio originated from decommissioning facility was 0.05-0.2 larger than that originated from 1F, it is challenging to decrease the stand deviation less than 0.01 for 1 Bq/g of 137Cs because precision was decreased as decreasing the 137Cs concentration. If the ratio is able to be determined within the standard deviation of 0.0005 for 0.5 Bq/g of soil sample, the identification would be available.