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 238U, 232Th, and 40K 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 238U, 232Th, and 40K 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) μGy/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 coefficient 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.
It is necessary to consider the effect of photoneutrons produced by photonuclear reactions in the shielding calculation of a medical linac room with incident electron energies greater than 10 MeV. For photonuclear reaction files, the validation of photoneutron energy distributions has not been reported. We compared the evaluated data of photoneutron energy distributions in LA150 and JENDL/PD-2016.1 with the experimental data for 11 nuclides taken from the EXFOR database. For dominant shielding materials of linac, we validated the tendency of energy distributions by comparing the experimental data of neighborhood nuclides. Consequently, we found problems in the evaluations of the photoneutron energy distributions of the photonuclear reaction files.