Abstract
We investigated the radiocesium (Cs-134 + Cs-137) concentration in trees (Quercus serrata) used in shiitake mushroom log cultivation in Tochigi Prefecture (where the concentration of radiocesium differs among three areas) by nondestructive inspection methods. Most of the logs in the Sakura city area exceeded the index value (50 Bq/kg or less at 12% log water content as specified by the Forestry Agency of Japan); therefore, trees of this area cannot be used as shiitake mushroom logs. On the other hand, all logs in the Ichikai town area were below the index value. Furthermore, 9.4% of logs in the Utsunomiya city area exceeded the index value. The surfaces of logs from Utsunomiya were rinsed, and then the radiocesium concentration of the rinsed logs was measured. An average 20 Bq/kg decrease was observed for all logs. Based on these test results, we calculated the reliable minimum sampling number (to determine whether logs meet a specified value allowing their use for shiitake cultivation) from a t-distribution (small sample theory method). With a confidence level of 95%, 202 logs need to be removed for sampling from the Sakura city set and 188 logs need to be removed for sampling from the Utsunomiya city set.
