Performance of in situ gamma-ray spectrometry in the assessment of radioactive cesium deposition around the Fukushima Daiichi nuclear power plant

Abstract In situ gamma-ray spectrometry using diverse survey platforms has been conducted in contaminated areas with various dose rate levels around the Fukushima Daiichi nuclear power plant (FDNPP). Six survey sites, including two evacuation zones around the FDNPP, were selected for ground-based gamma-ray spectrometry using HPGe (high purity Ge) and LaBr3(Ce) detectors to assess the radioactive cesium deposition in the ground. Laboratory analysis of soil samples taken by depth from the sites was performed to evaluate the depth profile of radioactive cesium. The diverse levels of radioactivity of 137Cs were then distributed to six survey sites from 30 to 3000 kBq·m−2 in the measurement period of October 2018. Finally, a method to directly calculate the depth profile using in situ measurement was introduced so as to have representation over a wide area, and the results were successfully compared with those of sample analysis at one point in the site. Except for the sites with relatively high lateral inhomogeneity of radioactive cesium, about 30% difference between two depth profiling methods using samples and in situ measurement was shown in the calculated radioactivity of 134Cs and 137Cs in the ground.