Assessment of radioactive cesium deposition using ground-based gamma-ray spectrometry with a LaBr₃(Ce) detector

Ground-based gamma-ray spectrometry using a LaBr₃(Ce) detector was conducted to assess radioactive cesium deposition in soil contaminated by the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Japan. Five sites, including a reference site with relatively low contamination, were selected as having different levels of ambient dose rate due to significant effects of radioactive fallout of ¹³⁴Cs and ¹³⁷Cs. According to ICRU Report 53, the radioactivity in the ground and dose rate at 1 m above the ground were determined from the measured net count rates of gamma-rays induced from radioactive cesium. Because the radioactivity and dose rate depended on the depth profile of radioactive cesium in the ground, a database of possible radioactivity and dose rate could be established according to several depth distributions. A new approach to estimate the depth profile was then developed by directly calculating dose rates of ¹³⁴Cs and ¹³⁷Cs at the same geometry through dose rate spectroscopy and comparing them with the database of possible dose rates of radioactive cesium. Once the depth profile was determined, radioactivity was estimated from the database depending on the depth profile in the ground. The activity ratio between two radioactive cesium was shown to average about 0.112, in December 2017. It was in good agreement with the originally same released amount of ¹³⁴Cs and ¹³⁷Cs at the time of the FDNPP accident, when physical decay correction was applied to the results of the radioactivity assessment.