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[STT58-01] Estimation of depth profile of radiocesium in soil based on characteristics of gamma-ray spectra obtained by airborne radiation monitoring
Keywords:Fukushima Daiichi Nuclear Power Plant accident, radiocesium, airborne radiation monitoring, depth profile, decontamination
The extended farm land of National Livestock Breeding Center in Fukushima Prefecture was selected for verifying this method. This farm is located on approximately 100km south west of FDNPP in Nishigo-village. Dose rate in the farm was measured with three LaBr3:Ce scintillators (3.8cmΦ×3.8cmH) using unmanned helicopter, R MAX G1 (YAMAHA Co., Ltd) at 6-10, Jun. 2016. The Spectra of LaBr3:Ce scintillators showed the best resolution of the three systems, able to clearly distinguish the 605keV energy peaks of 134Cs from the 662keV energy peak of 137Cs. However, background of spectra of LaBr3 were highly affected by self-contamination of the nuclides such as daughter nuclide of 227Ac and 138La in the detecter. Self-localization of the helicopter was controlled by flight programs based on differential GPS (Gloval position system). When used for monitoring, the flight altitude(altitude above grand level) of the helicopter was 20-30m and its velocity was approximately 8.0m/s. The distance from one measurement line to the other was 20-30m. The γ-ray spectra were measured per 1s continuously with position data. In addition, ratio of peak-compton (RPC) was defined by the ROI (region of interest) ratio of scattered area (50-450keV) and photo peak area (450-760keV) on γ-ray spectrum for evaluating of influence with the depth profile of radiocesium in soil. The deeper radiocesium exist in soils, the more γ-ray was scattered by soil particles compared with direct γ-ray. Thus, value of RPC changes by depth profile of the radiocesium in the soil.
Soils were sampled by liner soil sampler (0-30cm) and root auger (30-60cm) for measurement of depth profile of radiocesium. Quantitative analyses of radiocesium in the samples in the containers were conducted at the Institute for University of Tokyo using NaI(Tl) scintillators. In addition, the parameters (Depth: D20-90) about depth profile of radiocesium were calculated for evaluating of influence with scattered γ-ray. For examples, D90 at that the soil contains 90% of the inventory of radiocesium. It is estimated that the higher value of the parameters, the deeper radiocesium exist in soils.
Result of aerial monitoring indicated that relationship between RPC and D90 has good correlation. It is suggested that feature on gamma-ray spectra of LaBr3:Ce scintillators were affected by depth profile of radiocesium. Thus, it supported the hypothesis that the deeper radiocesium exist in soils, the more γ-ray was scattered by soil particles compared with direct γ-ray. Furthermore, result of quantitative analyses suggested that the depth profile of radiocesium in the farm were irregular. The irregular profiles of radiocesium in soil were result in the decontamination called reversal tillage. It was expected that the depth profile of radiocesium will show an exponential distribution with depth in many cases. However, the maximum concentration progressively moved from the surface layer to deeper ground layers when the decontamination was performed. In summary, it is hoped that this method will help in rapidly selecting of area that is needed decontamination with high priority by focusing on the feature on gamma-ray spectra. This research was supported by grants from the Project of the NARO Bio-oriented Technology Research Advancement Institution (the special scheme project on regional developing strategy).