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The Fukushima Daiichi nuclear disaster, which was triggered by the 2011 Tohoku earthquake, released large amounts of radio-nuclides. Soils in wide areas in eastern Japan were polluted by radioactive contamination. Polluted grazing lands are voluntarily unused even five years after the disaster because of the possibility of the consumption of polluted grass by cattle. Information on the spatial distribution of the radiation dose rate in soils and grasses is important for the management of the grazing land. In this study, we proposed a method to estimate the distribution of air radiation dose rate at ground level using paramotor observation data. The grazing lands in Kawatabi Field Science Center, Tohoku University are used as the study site. It is located in a mountainous area 150 km from the nuclear power plant and is a designated radiation hot spot. Air radiation dose rate measured by a radiation detection module containing a CsI (TI) scintillator and MPPC flying at a height of 500 m was 0.003-0.008 microSv/H. The data were obtained every 10 s with 50-300 m intervals over the entire target area. Air radiation dose rate at ground level was measured at eight paramotor observation points and it was 0.05-0.08 microSv/H except for one point, which showed 0.1 microSv/H in a valley. Distance decay rates were calculated from observed data, and the spatial air radiation dose rate at ground level was estimated. The observed air dose rate from the paramotor is possible to use for generating a distribution map of air dose rate at ground level. However, careful consideration is necessary for irregular observation value. Vegetation and soil (root mat) sample were collected at the eight ground measurement points and radioactive cesium content was measured. Radioactive cesium content in soil (root mat) was more than four times than that in vegetation on each sampling points. Radioactive cesium content in soil and vegetation increased with increasing air radiation dose rate and its correlation coefficient was 0.82. The air radiation dose rate observed from the paramotor is useful to assess the spatial distribution of radioactive soil.
