Measurement of ²²²Rn diffusion through sandy soil with solar cells photodiodes as the detector

An experimental system was developed to study the diffusion rate of radon (²²²Rn) gas through porous media as a function of soil porosity/grain size and soil water content. Columns with different grain sizes, soil water content and soil depths were used. The system used solar cells photodiodes as alpha (α) detectors. This new detector is highly efficient and low cost compared to other known detectors. Soil water content was found to be the most dominant factor affecting the ²²²Rn diffusion rate. A maximum diffusion rate value of (6.5 ± 0.07) × 10⁻⁶ m²/s was found in dry conditions. The minimum diffusion value of less than (3.9 ± 0.14) × 10⁻⁷ m²/s was found in 2% soil water content. The experimental results were compared with theoretical calculations done with the "GREEN equation". Two discrepancies were observed: the time to equilibrium state in the measurements was longer compare to the calculated values and the α count rates were lower in the experiment compared with the theoretical calculations. These results can be explained by the differences in the system geometry.