Gamma survey in Fukushima prefecture right after the accident at Fukushima nuclear power plant

On March 15-17 and April 8, 2011, right after the accident at Fukushima nuclear power plant, γ-ray dose-rate distributions and spectra were measured on expressways in Fukushima and neighboring prefectures using an NaI(Tl) dosimeter and a LaBr₃ spectrometer. A radioactive plume was observed near Koriyama-city on 15. Although the distributions of ¹³²Te, ¹³²I, ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs resembled each other, the distribution of ¹³¹I was different from the others. The dominant radionuclides of dose rate were ¹³²Te and ¹³²I on March 15-17 and ¹³⁴Cs and ¹³⁷Cs on April 8. The initial distribution profile of the dose rate on March 15-17 remained on April 8, which showed the deposited radioactive material was not moved from the initial location and there was no additional effective deposition of radioactivity after March 15. (author)