A Monte Carlo Method for Estimating Secondary Photon Yields from Beta-emitting Radionuclides Concentrated in Environmental Soil.

Abstract: External exposure due to secondary photons (predominantly bremsstrahlung) generated from electron source emissions in environmental soil are of concern due to their ability to deposit significant amounts of ionizing energy to organs and tissues within the body. The "condensed history method" employed in many modern Monte Carlo (MC) codes may be used to simulate secondary photon yields (given as photons per beta decay) arising from electron source emissions with relatively few assumptions regarding the secondary photon spatial, energy, and angular dependencies. These yields may in turn be used to derive protection quantities such as secondary photon effective dose rate (DR) and risk coefficients for a variety of idealized external exposure scenarios. Use of the condensed history method is, however, computationally burdensome when simulating idealized external exposure scenarios even with available parallel computing resources. Consequently, use of the method was largely prohibitive for prior environmental dosimetry and risk assessment applications that required innumerable MC simulations for deriving secondary photon protection quantities. A MC method has herein been proposed for estimating secondary photon yields from electron source emissions in environmental soil with the condensed history method in a computationally feasible manner using the Monte Carlo N-Particle version 6.2 (MCNP6.2) radiation transport code. The proposed method was demonstrated with radiation transport models of idealized external exposure scenarios patterned after Federal Guidance Report (FGR) 15, and secondary photon yields determined using the proposed method and a previously adopted analytical method were compared.