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Using PHITS to calculate ambient dose equivalent rates in radiocesium contaminated forests

Malins, A.  ; Imamura, Naohiro*; Niizato, Tadafumi ; Kim, M. ; Sakuma, Kazuyuki  ; Shinomiya, Yoshiki*; Miura, Satoru*; Machida, Masahiko 

About 72${%}$ of the $textsuperscript{134}$Cs and $textsuperscript{137}$Cs fallout in Fukushima Prefecture from the Fukushima Daiichi Nuclear Power Plant (FDNPP) deposited on forests. Future choices for the management of contaminated forests depend on estimates of radiation doses to workers and residents under different management scenarios. This talk describes research conducted with PHITS to understand the factors necessary for creating appropriate models for calculating ambient dose equivalent rates ($textit{.{H}}$*(10)) in forests from inventories of $textsuperscript{134}$s and $textsuperscript{137}$Cs. The models herein were based measurements from FFPRI and JAEA surveys of forest sites in Fukushima and Ibaraki Prefectures. These surveys provided measurements of densities, dimensions, moisture contents and $textsuperscript{134}$Cs and $$^{137}$$Cs inventories of crowns, trunks, organic layers and soil in the forests. Three geometry models were created for forests, varying from a simple model representing trees and the atmosphere as homogeneous layers of matter, to models with representations of individual tree trunks and crowns. A new PHITS function allows counter values to be set separately for source particles upon generation from each multi-source, meaning the contributions from radiation originating in different source regions to $textit{.{H}}$*(10) could be calculated easily. The counter function also allowed calculation of the contributions of scattered and unscattered photons to $textit{.{H}}$*(10) separately. Good correlation was obtained between simulated and measured $textit{.{H}}$*(10) values across nine sites for the 2011-17 period (Fig. 1). Despite the different levels of geometric detail, the results from all three geometry models were all within a few percent of each other. The elemental compositions chosen for materials had a negligible effect on the calculations.

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