Precision spectroscopy of cesium-137 from the ground to 150 m above in Fukushima

After the Fukushima nuclear disaster in 2011, large amounts of radioisotopes (mainly 137Cs and 134Cs) were released into the environment. Various monitoring activities have revealed radiation on the ground both in local and wide areas; however, aerial dose variation in the vertical direction is poorly known. This paper presents the results of airborne gamma-ray spectroscopy of a contamination field in Namie, Fukushima, as measured from 0 m to 150 m above the ground by drone. We found that the gamma-ray dose rate measured at 100 m height is about seven times higher than that expected based on ground measurements, which is caused by two factors: (1) the integrated dose includes contamination of upward scattered 662-keV gamma rays and (2) radiation from 137Cs is vertically collimated because 137Cs is buried in the soil. We also propose a novel method to obtain the distribution of radioactive substances in the soil only through aerial mapping.