Investigation of absorbed dose rate in air by a car-borne survey in Namie Town, Fukushima Prefecture

Namie Town, Fukushima Prefecture was instituted as an evacuation zone and a controlled area after the Fukushima Daiichi Nuclear Power Plant accident in April, 2011 which immediately resulted in a significant amount of radionuclides leakage to the environment. In this study, a car-borne survey utilizing a NaI(Tl) scintillation survey meter was carried out along ordinary roads and forest areas in Namie Town from September 29^th to October 1^st, 2011 to measure the absorbed dose rate by gamma-rays from ¹³⁷Cs (T_1/2: 30.05 y) and ¹³⁴Cs (T_1/2: 2.04 y) for the estimation of the radiation levels over long periods of time. The similar surveys were conducted again on August 22^nd-24^th, 2014 and September 14^th-16^th, 2015 using a NaI(Tl) scintillation spectrometer. The ratios of data obtained from urban areas to total surveyed areas in the same measurements were 27%, 34% and 48% in 2011, 2014 and 2015, respectively. The ratios of those measured from forest areas and mountain regions were 73%, 66% and 52% in 2011, 2014 and 2015, respectively. The absorbed dose rates in air obtained from these surveys were used to draw dose rate distribution maps to monitor the variation of the dose rates, and the distribution was found to be heterogeneous. The maximum value, minimum value and geometric mean of the absorbed dose rates in air for 2011 were estimated to be 47.6, 0.15 and 3.7 μGy h^-1, respectively. The values for 2014 were estimated to be 5.8, 0.09 and 1.2 μGy h^-1, respectively. In 2015, the values were estimated to be 5.6, 0.05 and 0.9 μGy h^-1, respectively. The annual effective doses in 2014 and 2015 dropped by 66% and 75% from 2011, respectively. The annual effective doses estimated in 2014 and 2015 were lower than the annual effective dose limit of 20 mSv recommended by the Japanese government for an evacuation directive lift in the prepared area. (author)