Effective half-lives of Cs-137 from persimmon tree tissue parts in Japan after Fukushima Dai-ichi Nuclear Power Plant accident

摘要

To estimate the radiocesium decreasing rates from persimmon trees during a period of about 3 y following the accident at Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we conducted measurements of tree tissue parts collected in 2011-2013. The sampling was carried out in Chiba, 220 km south of FDNPP; radioactive fallouts discharged from FDNPP had mainly been observed in March-April 2011 on the sampling site. We measured Cs-137 concentrations in the tree tissue parts, i.e., fruits (flesh, skin and seeds), leaves and newly emerged branches, and then the effective half-lives (T-eff) of Cs-137 were calculated. Leaf samples were classified into two types by sampling months according to the growing stages, that is, immature (April-May) and mature (June-November) leaves. All these parts showed exponential declines in Cs-137 concentration with good adjusted contribution ratios of higher than ca. 0.7. The calculated T-eff values from all tissue parts were similar with the average of 229 d (range: 216-243 d). From these results, we concluded that each tree tissue was representative for the calculation of T-eff. For comparison to these observation results, open source food monitoring data from 2011 to 2013 including Cs-137 data for persimmon fruits collected in Fukushima Prefecture were used to calculate T-eff for persimmon trees. Values of T-eff were obtained for persimmon fruits grown in each local government area in Fukushima Prefecture and they ranged from 303 to 475 d. (C) 2014 Elsevier Ltd. All rights reserved.