||Radioactive cesium concentrations in purchased tea leaves and the other plant materials for brewing tea
35 , 2017-3-31 , 県立広島大学
Frequently drinking tea is an ordinary custom for a large number of Japanese people. Since the radioactive cesium (Cs) in tealeaves may harm human health, the concentration should be examined as precisely as possible. In order to determine 137Cs and 134Cs concentrations, a method to isolate Cs from soil, dead leaves and leaf mold was applied for Cs isolations from green tea leaves and the other kinds of plant materials for brewing tea. Sample was carbonized in a covered stainless steel beaker (300 cm3) heated for 3 hours in an electric furnace at 450 ºC, and the Cs atoms extracted by nitric acid were collected into 1 g of ammonium phosphomolybdate trihydrate. Gamma rays from Cs sample were measured by a pure Ge detector with volume of 160 cm3 shielded by lead blocks. Results in a supplemental experiment suggested that mass reduction rate by carbonization scarcely depended on the amount of dried sample in the beaker. The Japanese products used in this study were manufactured by companies located in or western from Ibaraki prefecture. The products were obtained on or after 4 May 2013. Among 21 tea leaves products obtained for this preliminary study, the highest concentration of 137Cs was 17.8 ±0.8 Bq/kg in a product purchased in Okayama Prefecture on 28 September 2013. (The concentrations and activity ratios shown in this summary are those decay-corrected to the end of March 2011.) In this product, the 134Cs/137Cs activity ratio was 1 within uncertainty. The 137Cs concentration was as low as 0.1-0.2 Bq/kg in two Sen-tea leaves products. One was manufactured in Hyogo Prefecture and the other was manufactured in Yamaguchi Prefecture. The 137Cs concentrations in the other tea leaves products were higher than or equal to 0.3 Bq/kg. In four tea leaves products of which raw material tea leaves were produced in Shizuoka, Yamaguchi and Kagoshima Prefectures, and in two oolong tea products, the 137Cs concentrations were higher than 0.5 Bq/kg and the differences from each 134Cs concentration were 0.5 – 1.3 Bq/kg. The differences were probably caused by the 137Cs atoms emitted by the Chernobyl disaster or atmospheric nuclear weapon tests. Except tealeaves, four plant materials for brewing tea were examined in this study. The products of dried mugwort leaves for brewing tea manufactured in Oita prefecture was obtained on 26 August 2013. Both 137Cs and 134Cs concentrations in the mugwort leaves were approximately 0.4 Bq/kg. In the other three plant materials for brewing tea, both 137Cs and 134Cs concentrations were less than 0.3 Bq/kg.