Study of thoron exhalation from soil under different temperature and humidity conditions

Radon (222Rn) and thoron (220Rn) exhalation from soil is governed by the soil properties as well as the ambient meteorological parameters. The temperature and humidity cause significant changes in the radon and thoron flux from the earth's crust. Exhalation rate measurements can be made rapidly for thoron due to its short half-life leading to speedy attainment of steady state under closed conditions. In the present study, thoron exhalation from beach sand rich in 232Th, is studied in the controlled environment of a (0.5 m3 volume) radon chamber. The chamber is equipped with air-heater and moisture generator with automated temperature and humidity controller. Experiments were conducted in the temperature and relative humidity (RH) ranges 35-50°C and 40%-60%, respectively. For different combinations of temperature and RH, the steady-state thoron concentration near the floor was measured and the exhalation rate was estimated by solving the mass balance equation. Common to all levels of RH, the thoron exhalation rate was found to steadily increase with temperature. For a total of 15°C (in steps of 5°C) temperature increase, the average percentage increase in exhalation rate is 34%. At all temperatures, when the RH increased from 40% to 45%, an increase in the exhalation rate was observed. When the RH is further increased to 50%, the exhalation rate shows a dip. Above RH 50%, there is further drastic decrease in exhalation rate, and this was explained by formulating a modified mass balance equation accounting for the decreased concentration gradient in the higher layers of air due to the restricted diffusion of thoron.