Development of dose rate simulation code in consideration of displacement in a forest of radioactive cesium and development of high precision measurement technique for low-concentrated environmental radioactivity samples

This study aimed to develop/improve the simulation tool (1) to quantitatively evaluate the concentration distribution and displacement of radioactive cesium, judging from changes in the spatial dose rate and surface contamination density obtained by measurement, and to develop a technique to streamline the measurement by back-calculating the radiation source from the distribution of the spatial dose rate. In addition, it aimed to develop/improve the simulation tool (2) capable of evaluating the spatial dose rate in and out of a forest, in consideration of the scattering and shielding due to sunshine, landform (area dimensions, slope, etc.), and trees. At the same time, the authors performed the analysis of spatial dose rate through the Monte Carlo method aiming at the verification of the model and the expansion of application scope. As for the simulation tool (1), its validity was verified for the calculation model where finite-infinite correction was added to the finite evaluation model. The simulation tool (2) was proven to be able to express decontamination effect through the calculation using Phits (particle and heavy ion transport system, Monte Carlo Simulation), which enables examination on the implementation of an efficient decontamination. In addition, the authors examined a quantification method for low-level radioactive Cs. (A.O.)