The effect of water content on the radon emanation coefficient for some building materials used in Japan

Building materials used in Japan were collected from several companies and their radionuclide concentrations were measured. Fifteen granite samples with high activity concentrations were selected for the present study. To investigate the effect of water content on the radon emanation coefficient, the coefficient was measured under 3 different conditions (dry, normal, and wet). The emanation coefficients were then used to calculate the alpha equivalent dose (dose from indoor radon generated from building materials), assuming a simple room model. The radon emanation coefficient for the dry condition ranged from (3.7 ± 0.1)% to (27.2 ± 3.9)%, with an average value of (10.5 ± 1.4)%. The emanation coefficients were 2-5 times that size for the wet condition. Similarly, the alpha dose became larger, owing to its proportion to the emanation coefficient, indicating that water content in building materials is an important factor for the emanation coefficient as well as the radiation dose. The radon exhalation rate was also measured for the dry samples. Radon exhalation rate and radium concentration had a relatively low correlation (R2 = 0.40). However, the correlation between radon exhalation rate and 'emanated radon concentration' (radium concentration x emanation coefficient) was much higher (R2 = 0.84). Therefore, emanated radon concentration could be a useful index for exhalation rate and alpha equivalent dose, but radium concentration in building materials alone is not.