In situ spectrometry of terrestrial gamma rays using portable germanium detectors in area of 80 km radius around the Fukushima Daiichi nuclear power plant

To determine the background radiation level in an area affected by the Fukushima Daiichi nuclear power plant accident in 2011, natural terrestrial gamma rays had been measured using portable germanium detectors repeatedly from 2013 through 2019, at 370 locations within an area of 80 km radius around the Fukushima Daiichi nuclear power plant. At these locations, radioactive 238 U, 232 Th, and 40 K concentrations in soil and air kerma rates at a height of 1 m above the ground due to terrestrial gamma rays were obtained by the method indicated in ICRU report 53. The average 238 U, 232 Th, and 40 K concentrations were 18.8 （ standard deviation, SD: 6.37 ） , 22.7 （ SD: 11.7 ） , and 428 （ SD: 163 ） Bq / kg, respectively, and the average air kerma rate over the area was found to be 0.0402 （ SD: 0.0146 ） μ Gy / h. The obtained air kerma rates were compared with those reported in the literature. It was confirmed that the measured data positively correlated with each other with a regression coeffi cient close to one and agreed within a factor of two. A trend similar to that in previous findings was observed, that is, the air kerma rates at locations geologically classified as granite and rhyolite zones were statistically significantly higher than those at other locations.