Time Variations in Dose Rate and γ Spectrum Measured at Tsukuba City, Ibaraki, due to the Accident of Fukushima Daiichi Nuclear Power Station

The time variations in the dose rate and γ spectrum of radio nuclides originating from the accident of Fukushima Daiichi Nuclear Power Station were measured at Tsukuba City, Ibaraki, during the period from 15th March to 9th April 2011. The radiation dose peaked three times during the period from 15th to 16th March (1.27 μSv/h at maximum). The contribution of Xe-133 to the dose rate was observed from the γ spectrum obtained from the 5 h measurement during the peaks on 15th and 16th March, indicating that radioactive plume passed through Tsukuba City at that time. After the peaks, a dose rate increase with rainfall was observed on 21th March, dominating the integral dose rate measured at Tsukuba City. The dose after the rainfall comes from I-131, Cs-134, and Cs-137 that can be observed as peaks in the spectra.