Inoue Kazumasa, Arai Moeko, Fujisawa Makoto, Saito Kyouko, Fukushi Masahiro
Department of Radiological Sciences, Graduate School of Human Health Sciences, Tokyo Metropolitan University, Arakawa-ku, Tokyo, Japan.
Department of Radiological Technology, Faculty of Health Sciences, Nihon Institute of Medical Science, Iruma-gun, Saitama, Japan.
PLoS One. 2017 Jan 27;12(1):e0171100. doi: 10.1371/journal.pone.0171100. eCollection 2017.
A car-borne survey was carried out in the northwestern, or Tokatsu, area of Chiba Prefecture, Japan, to make a detailed distribution map of absorbed dose rate in air four years after the Fukushima Daiichi Nuclear Power Plant accident. This area was chosen because it was the most heavily radionuclide contaminated part of Chiba Prefecture and it neighbors metropolitan Tokyo. Measurements were performed using a 3-in × 3-in NaI(Tl) scintillation spectrometer in June 2015. The survey route covered the whole Tokatsu area which includes six cities. A heterogeneous distribution of absorbed dose rate in air was observed on the dose distribution map. Especially, higher absorbed dose rates in air exceeding 80 nGy h-1 were observed along national roads constructed using high porosity asphalt, whereas lower absorbed dose rates in air were observed along local roads constructed using low porosity asphalt. The difference between these asphalt types resulted in a heterogeneous dose distribution in the Tokatsu area. The mean of the contribution ratio of artificial radionuclides to absorbed dose rate in air measured 4 years after the accident was 29% (9-50%) in the Tokatsu area. The maximum absorbed dose rate in air, 201 nGy h-1 was observed at Kashiwa City. Radiocesium was deposited in the upper 1 cm surface layer of the high porosity asphalt which was collected in Kashiwa City and the environmental half-life of the absorbed dose rate in air was estimated to be 1.7 years.
在日本千叶县的西北部（即常津地区）开展了一项车载调查，目的是绘制福岛第一核电站事故四年后空气中吸收剂量率的详细分布图。选择该地区是因为它是千叶县放射性核素污染最严重的地区，且与东京都相邻。2015年6月，使用一台3英寸×3英寸的碘化钠（铊）闪烁光谱仪进行了测量。调查路线覆盖了包括六个城市的整个常津地区。在剂量分布图上观察到空气中吸收剂量率的分布不均匀。特别是，在使用高孔隙率沥青铺设的国道沿线，观察到空气中较高的吸收剂量率超过80纳戈瑞/小时，而在使用低孔隙率沥青铺设的地方道路沿线，观察到空气中较低的吸收剂量率。这些沥青类型的差异导致了常津地区剂量分布不均匀。事故发生四年后，常津地区人工放射性核素对空气中吸收剂量率的贡献比例平均值为29%（9 - 50%）。在柏市观察到空气中的最大吸收剂量率为201纳戈瑞/小时。放射性铯沉积在柏市采集的高孔隙率沥青表层1厘米处，空气中吸收剂量率的环境半衰期估计为1.7年。
