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

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.

Development of an Analytical Method for Estimating Three-Dimensional Distribution of Sediment-Associated Radiocesium at a Reservoir Bottom

“…It is suitable to quantify the vertical distribution of radiocesium in soil in accordance with soil characteristics, such as water content and soil density. However, the vertical distribution of radiocesium was affected by inversion tillage and presence of wild animals in the land field. ,, To cope with these situations, Matsuda and Saito proposed a distribution parameter, effective relaxation mass depth, β eff . β eff is calculated as follows where D is the air kerma rate at 1 m above surface soil (μGy h –1 ), A m, 0, eff is the effective radiocesium concentration (wet weight) in surface soil (Bq kg –1 ), ζ is the mass depth (g cm –2 ), I γ is the branching ratio of gamma ray from each nuclide, C (ζ) is the conversion factor proposed by Saito and Jacob, z ′ is the depth from surface soil (cm), ρ is the soil density (g cm –3 ), and A inv.…”

“…However, the vertical distribution of radiocesium was affected by inversion tillage and presence of wild animals in the land field. 36,39,40 To cope with these situations, Matsuda and Saito 39 proposed a distribution parameter, effective relaxation mass depth, β eff . β eff is calculated as follows…”

“…is the measured inventory (Bq m −2 ). 36,39 We determined the soil density based on the mass (wet weight) and volume of each layer of sediment sample. It was not possible to collect core samples from layers containing rocks.…”

“…We developed an analytical method for aerial radiation monitoring (ARM) based on the characteristics of gamma-ray spectra to estimate the vertical distribution of radiocesium in soil. 36 In the present study, we developed an analytical method for estimating three-dimensional distribution of sediment-associated radiocesium at reservoir bottoms without collecting core sediment. To confirm the validity of the obtained in situ result, core sediments were collected in parallel.…”

“…For estimating the vertical distribution of radiocesium, the peak-to-vary (PTV) method is generally used to estimate the position of the radiation source from the surface ground by examining the ratio of the photo peak counting rate to the vicinity valley peak counting rate. − Tyler et al investigated the vertical distribution of 137 Cs in a saltmarsh environment using a NaI­(Tl) detector. We developed an analytical method for aerial radiation monitoring (ARM) based on the characteristics of gamma-ray spectra to estimate the vertical distribution of radiocesium in soil …”