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 μGy h-1 due to the radioactive cesium distributed on the ground of the Fukushima prefecture. To assess the dose rates of 134Cs and 137Cs 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 134Cs and 137Cs accounted for more than 25% of the ambient dose rate during the survey period. The ratio of the 134Cs dose rate to the 137Cs dose rate was shown to be about 30% for all survey sites.
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