Abstract
A car-borne survey of air dose rate measurements was performed yearly from 2011 to 2013 to determine the levels of environmental radiation in Gunma prefecture after the Fukushima Daiichi Nuclear Power Plant accident in 2011. The results revealed that the average ambient doses in each year were 85.3 ± 34.1 nGy/h in 2011, 60.3 ± 19.9 nGy/h in 2012, and 43.5 ± 15.1 nGy/h in 2013. The ambient dose rate in 2011, which was about three times higher than the average of 27.0 ± 7.1 nGy/h in 1998, was still in safety level considering the public health, and the ambient dose rate subsequently decreased in 2013 to approximately half the 2011 level. A contour map of the ambient dose rate showed relatively higher levels in the northern and western parts of the prefecture, with relatively lower levels toward the eastern and southern parts.
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