Application of topographical source model for air dose rates conversions in aerial radiation monitoring.

After the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in 2011, aerial radiation monitoring (ARM) using a manned helicopter was conducted to rapidly measure air dose rates and the deposition of radioactive nuclides over a large area. Typically, the air dose rate is obtained by conversion from the count rate using conventional flat source model (FSM). The converted dose rate obtained via aerial monitoring poorly matches the results of ground measurement in the mountain and forest areas because FSM does not consider topographical effects. To improve the conversion accuracy, we developed new methods to analyze aerial monitoring data using topographical source model (TSM) based on the analytical calculation of the gamma-ray flux. The ARM results converted using both FSM as well as TSM were compared with ground measurement data obtained after the FDNPS accident. By using TSM, the conversion accuracy was improved. In addition, to determine a parameter sensitive to topographical effects, we examined five parameters and it was clear that the difference between the elevation just below the helicopter and the mean elevation within the measurement area was the most influential.

Estimation of the Vertical Distribution of Radiocesium in Soil on the Basis of the Characteristics of Gamma-Ray Spectra Obtained via Aerial Radiation Monitoring Using an Unmanned Helicopter.

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.