Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation monitoring using manned and unmanned helicopters, the we have developed and established an analysis method concurrently with the development of this monitoring method. In particular, because the background radiation level differs greatly between East and West regions of Japan, we have developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation monitoring data was validated through comparison with large amounts of ground measurement data. The ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were 0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the radiation exposure of residents.

Ecological half-life (T_eco) of ¹³⁷Cs concentration over wide pond areas of surface sediments was evaluated from in situ monitoring with plastic scintillation fibers (PSF) between 2013 and 2019.

After the Fukushima Daiichi Nuclear Power Plant accident, the vertical distribution of radiocesium in soil has been investigated to better understand the behavior of radiocesium in the environment. The typical method used for measuring the vertical distribution of radiocesium is troublesome because it requires collection and measurement of the activity of soil samples. In this study, we established a method of estimating the vertical distribution of radiocesium by focusing on the characteristics of gamma-ray spectra obtained via aerial radiation monitoring using an unmanned helicopter. The estimates are based on actual measurement data collected at an extended farm. In this method, the change in the ratio of direct gamma rays to scattered gamma rays at various depths in the soil was utilized to quantify the vertical distribution of radiocesium. The results show a positive correlation between the abovementioned and the actual vertical distributions of radiocesium measured in the soil samples. A vertical distribution map was created on the basis of this ratio using a simple equation derived from the abovementioned correlation. This technique can provide a novel approach for effective selection of high-priority areas that require decontamination.

After the Fukushima Daiichi Nuclear Power Station accident, the distributions of sediment-associated radiocesium were investigated to evaluate the dispersion and accumulation of radiocesium in the reservoir field. To develop an analytical method for measuring the horizontal and vertical distributions of radiocesium on a wide scale, we obtained 253 gamma-ray spectra at the bottoms of 64 ponds in Fukushima during 2014–2016 by using a NaI(Tl) scintillation detector. For visualizing horizontal distribution, correlation between detector counting rate and radiocesium concentration of the bottom sediment was confirmed. In estimating vertical distribution, the depth profile of sediment-associated radiocesium was found to be correlated to the intensities of scattered and photo peaks. Good agreement was observed between the results of in situ spectrometry and core sampling. These results indicate that the developed method is suitable for understanding the behavior of radiocesium and determining whether decontamination of reservoirs is required.