Investigation of Absorbed Dose Rate in Air by a Car-borne Survey in Namie Town, Fukushima Prefecture

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  • PORNNUMPA Chanis
    Department of Radiological life Sciences, Hirosaki University Graduate School of Health Sciences
  • IWAOKA Kazuki
    Department of Radiation Physics, Institute of Radiation Emergency Medicine, Hirosaki University
  • AKATA Naofumi
    National Institute for Fusion Science SOKENDAI [The Graduate University for Advanced Studies]
  • HOSODA Masahiro
    Department of Radiological life Sciences, Hirosaki University Graduate School of Health Sciences
  • SORIMACHI Atsuyuki
    Department of Radiation Physics, Institute of Radiation Emergency Medicine, Hirosaki University
  • TOKONAMI Shinji
    Department of Radiation Physics, Institute of Radiation Emergency Medicine, Hirosaki University

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

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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 29th to October 1st, 2011 to measure the absorbed dose rate by gamma-rays from 137Cs (T1/2: 30.05 y) and 134Cs (T1/2: 2.04 y) for the estimation of the radiation levels over long periods of time. The similar surveys were conducted again on August 22nd-24th, 2014 and September 14th-16th, 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.

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