Gradient chamber for measuring the exhalation rate of radon and thoron from surfaces

A novel instrument (gradient chamber) and method for measuring the exhalation rates of radon (²²²Rn) and thoron (²²⁰Rn) from surfaces (e.g., soil, rocks, building materials) have been developed and tested. Determining radon and thoron exhalation rates is essential both for protecting human health from radon/thoron exposure and for providing valuable data relevant to climate change research. The chamber is cylindrical with open bases: the upper base is covered with a porous material that blocks atmospheric turbulence while remaining permeable to radon and thoron, and the lower base is placed in direct contact with the test surface. Within the chamber, radon and thoron activity concentrations form a diffusion profile, measured by detectors for alpha particles, installed at different heights inside the chamber walls. Each detector comprises a Kodak Pathe LR-115/II alpha track detector covered by activated carbon fabric (ACC-5092-10), which efficiently adsorbs radon and thoron. The detectors are compact (≈ 1 cm) and locally sensitive, enabling high-resolution profiling of radon and thoron distributions. Exhalation rates are determined from the diffusion gradient using Fick's law. Results from theoretical modeling and pilot field experiments confirm the high sensitivity of this method, sufficient to quantify even low environmental radon exhalation rates from the earth's surface.