Measurements of Fukushima Daiichi nuclear power plant accident-related radiation and radioactivity at KEK Tsukuba campus

Described are results and findings of several measurements in the title conducted from Mar. 14, 2011, at the Campus of High Energy Accelerator Research Organization, Tsukuba City, Ibaraki Prefecture (KEK) localizing 165 km afar from the Power Plant and having the approximate area of 2 km x 1 km. Ambient dose rates close to the ground and at 1 m height were measured with Hitachi-Aloka's NaI survey meter TCS-171 at 15 spots in the Campus, and gamma ray energy spectrum with LaBr survey meter at one of the outdoor spots near the main office. The maximum rates including the back ground value at 1 m height were 0.25 (April 1) to 0.12 (Dec. 12) mcSv/h. Soil and withered grass were subjected to gamma ray measurement and subsequent conversion to radioactivity with CANBERRA's portable Ge detector (GC1019) and with its software ISOCS, respectively, at several spots above. Radioactivity of smears on 10 articles was measured with Hitachi-Aloka's GM survey meter TGS-146B. The Campus ambient dose rates were also monitored continuously with NaI detector and GM counter, which gave significant spike peaks in mcSv/h of 0.5 (Mar. 15), 0.3 (21) and lower thereafter, with 134Cs/137Cs ratio of 1.15 at early Apr. Atmospheric radioactivity was measured with the Ge detector above after adsorption on the charcoal fiber filter of gaseous body and on Si fiber filter of particulate, collected from 100-1,700 m3 air with a high-volume air sampler (collaboration with the National Institute for Environmental Studies, Tsukuba City). Findings on those measurements were: After the Accident on Mar. 11, air containing radioactive materials released from the Plant had passed as north wind on Mar. 15-16 over Tsukuba City, which had brought about a transient small increase of particulate radioactivity and dose rate; and on Mar. 20-22 radioactive aerosol in the fog had come to the City to drop as rain on the ground, resulting in the similar increase. (T.T.)