Simulation of diffusion and decay of radon/thoron exhaled from a wall and its newly created progeny. Response of a bare LR-115 detector placed on the wall

Monte Carlo techniques were used to simulate the air diffusion of exhaled radon/thoron atoms from a wall, the formation and decay of different radioactive species during transport, and the response of a bare LR-115 detector placed on the exhaling surface. The spatial distributions of radionuclide decays and the detector's partial sensitivities to radon/thoron and their progeny were determined. The simulation results were numerically validated through comparisons with published theoretical and experimental data. From the simulated experiments, the contributions of different species to the total track density and what the detector might measure in the studied configuration and assumed parameters were inferred. It was shown that near the wall, where the effective volumes of all species are located, the number of radon atoms was approximately constant, while that of thoron decreased to about 30% relative to the number of exhaled atoms, and the equilibrium factors of both gases were very low. A negligible contribution of ²²⁰Rn and ²¹⁶Po to the track density is expected if the distance between detector's edges and support exceeds 5–6 times the thoron characteristic diffusion length. For similar detector and support sizes, these atoms can significantly contribute to the track density if thoron exhalation rate from wall is high. The advantages and limitations of the detector exposure method for estimating indoor radon concentrations are analyzed. Finally, a simple method is suggested for more accurate radon measurements using the bare LR-115 detector placed on a wall.