Investigation of key parameters influencing 222Rn exhalation rate from selected industrial and environmental samples using AlphaGUARD monitor

Abstract Radon-222 ( 222 Rn) exhalation from natural and building materials is a significant source of indoor radiation exposure, yet the effects of key physical parameters controlling radon release remain insufficiently understood. This study systematically investigated the influence of exposed surface area, sample mass, and temperature on the 222 Rn exhalation rate of phosphate, granite, and sand using an AlphaGUARD monitoring system. Measurements were performed on controlled grain-size fractions (< 80 μm for phosphate and granite and 200–425 μm for sand) under standardized laboratory conditions. Phosphate exhibited the highest exhalation rates and the strongest dependence on exposed surface area, showing an approximately linear increase attributed to its high porosity and enhanced radon emanation efficiency. In contrast, granite showed weak and irregular surface-area dependence, whereas sand exhibited the lowest exhalation rates and a non-monotonic response due to its coarse-grained structure. No systematic relationship was observed between sample mass and radon exhalation rate for any material, indicating that sample mass alone is not a reliable predictor of radon release. All materials exhibited a biphasic temperature response, with exhalation rates increasing from 30 to 60 °C and decreasing thereafter to 100 °C, reflecting enhanced radon transport at moderate temperatures and transport limitation caused by thermally induced microstructural changes at higher temperatures. Phosphate achieved the maximum area exhalation rate (277.33 Bq m −2 h −1 ), approximately three times higher than granite and forty times higher than sand. These findings provide practical guidance for standardized radon exhalation measurements and improved radiological assessment of natural and construction materials.