Environmental radiation monitoring and external dose estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant accident

Many nuclear facilities are located within Aomori Prefecture, Japan. However, no detailed dose rate distribution map of Aomori Prefecture, including its mountain regions has been reported since the Fukushima Daiichi Nuclear Power Plant accident. A car-borne survey which used a 3-in × 3-in NaI(Tl) scintillation spectrometer was carried out throughout the prefecture for the purposes of making a dose distribution map and estimating the annual external dose. The average absorbed dose rate in air and the annual effective dose were found to be 22 ± 5 nGy h^-1 and 0.20 ± 0.08 mSv, respectively. These average values for all of Aomori Prefecture were respectively 44% and 59% of the nationwide average values. The average values with standard deviations of activity concentrations in soil of ⁴⁰K, ²³⁸U and ²³²Th were 234 ± 148, 15 ± 6, 12 ± 6 Bq kg^-1, respectively. The average values of contributions of ⁴⁰K, ²³⁸U and ²³²Th to absorbed dose rates in air were 39%, 29% and 32%, respectively. The contributions of ¹³⁴Cs and ¹³⁷Cs to the absorbed dose rates in air were judged to be negligible. (author)