Spectrometric Estimation of Dose Rate Induced from Radioactive Cesium in the Ground Using a Mobile Gamma-Ray Spectrometry Based on a LaBr3(Ce) Detector

The site characterization around the Fukushima Daiichi nuclear power plant (FDNPP) was conducted to measure the dose rate of radioactive cesium using mobile gamma-ray spectrometry through a backpack survey based on a LaBr3(Ce) detector. Four sites were selected in the Fukushima prefecture with diverse dose rate levels in residence and non-residence areas. One reference site in Sendai city was also designated with a low dose rate in comparison with sites in the Fukushima prefecture. The ambient dose rate was distributed from several tens of dose rate to above 1 mu Gy h(-1) due to the radioactive cesium distributed on the ground of the Fukushima prefecture. To assess the dose rates of Cs-134 and Cs-137 using the backpack survey with a short acquisition time, a good correlation was identified between the dose rate of radioactive cesium and the gross count rate in specific regions of interest (ROIs) with gamma rays from radioactive cesium. The dose rates of Cs-134 and Cs-137 accounted for more than 25% of the ambient dose rate during the survey period. The ratio of the Cs-134 dose rate to the Cs-137 dose rate was shown to be about 30% for all survey sites.