Preliminary in situ observation and theoretical study of thoron exhalation rates from the gypsum board of a cavity wall: a comparison with radon and with thoron from a solid concrete wall

Building materials are the dominant source of indoor thoron. This study preliminarily investigates thoron exhalation rates (J_Tn) from a gypsum board as part of a cavity wall, comparing them to radon exhalation rates (J_Rn) from it and to J_Tn from a solid concrete wall. In situ measurements were conducted over 13 days in an unoccupied ground-floor apartment in Japan using an accumulation chamber and a dedicated thoron and radon monitor. The data showed no significant diurnal variation in J_Tn, unlike the notable variations observed in J_Rn; limited data, however, do not rule out such variation, which constitutes a study limitation. Theoretical calculations indicated that, similar to J_Rn, J_Tn was primarily driven by diffusion, with negligible advection due to typical pressure gradients across the gypsum board. Comparison between the gypsum board and solid concrete wall revealed material-specific differences, with noticeable differences observed in the daily averaged J_Tn and J_Rn ratios, suggesting that the properties of the gypsum board (or cavity wall) and concrete affect radon and thoron generation and transport to varying extents, depending on environmental conditions. The calculations also emphasized the importance of key material properties, particularly the radon/thoron diffusion coefficient and thoron concentration difference across the gypsum board. Further research is needed, including long-term monitoring, diverse building materials, and varying environmental conditions. These data are essential for understanding thoron exhalation from cavity walls, improving thoron source characterization, and refining indoor thoron modeling, particularly in environments where thoron contributes significantly to radiation exposure.