Abstract
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.
Highlights
In 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred following a catastrophic earthquake and subsequent tsunami
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
aerial radiation monitoring (ARM) was performed for the entire study site to evaluate the pattern of radiocesium on the ground
Summary
In 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred following a catastrophic earthquake and subsequent tsunami. Many institutes and universities have investigated the deposition density of radionuclides and dose rate above the ground via ground measurements (e.g., [1]). The results of these measurements have elucidated the contamination situation for radionuclides with a relatively long half-life, such as 134 Cs (T1/2 : 2 years) and 137 Cs (T1/2 : 30 years), in the soil around FDNPP. More than six years have passed since the accident occurred, it is still necessary to evaluate the physicochemical properties of radiocesium in the soil (134 Cs and 137 Cs) in order to monitor the long-term behavior of radiocesium in the environment. Public Health 2017, 14, 926; doi:10.3390/ijerph14080926 www.mdpi.com/journal/ijerph
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