Back diffusion correction for radon exhalation rates of common building materials using active measurements

Abstract

The radioactivity content of various samples like soil and building materials are expressed in term of the activity concentration (Bq kg⁻¹) of uranium, radium, thorium and potassium present in them and the radon (²²²Rn) exhalation rate. The radon exhalation rate is an important parameter for expressing the radioactive contents of materials used for building. The contribution of radioactive gas radon to inhalation dose can be measured by exhalation rate. The exhalation rates measured may be free or bound, the free exhalation rates of materials were calculated by initial slope of radon growth curve in closed chamber test. While the measurement of the bound exhalation rates of materials is difficult due to back diffusion and leakage. Thus it is necessary to measure these two parameters and correction should be made for free exhalation rates. The leakage rate was found to be (4.76 ± 0.14) × 10⁻³ h⁻¹ (65 ± 2.1 cc h⁻¹) for 13.7 L chamber and (6.32 ± 0.9) × 10⁻³ h⁻¹ (216 ± 30 cc h⁻¹) for 34.2 L chamber. The free exhalation rates of building materials varied from 0.25 ± 0.02 Bq m⁻² h⁻¹ for fired brick to 1.79 ± 0.05 Bq m⁻² h⁻¹ for concrete while the back diffusion rates showed maximum value (5.49 ± 0.2) × 10⁻² h⁻¹ for unfired brick and minimum (1.05 ± 0.14) × 10⁻² h⁻¹ for concrete samples. The back diffusion rates of the samples under study were also measured by direct fitting of radon growth data and were in good agreement with the measured values.