Moisture content effect on radon emanation in porous media

It is impractical to establish the detailed recoil range distribution and to measure the rough recoil surface of the porous media experimentally. Thus a computer based modelling approach is followed. The ion collision theory of nuclear physics provides the basics for calculating the recoil range distributions for radon within solid, water and air. The collision history is established by the modified Monte Carlo TRIM program. The recoil emanation power can also be calculated from the recoil range distributions. The recoil surface areas of the porous media are calculated through the fractal quantification of the synthetic porous media. The embedding effects are numerically calculated for solid, water and air, the three coexisting phases. The moisture distribution in the porous media is discussed based on the capillary theory. The simulation results indicate that the radon emanation rate is positively correlated with the moisture saturation in the porous media. The greater the moisture saturation is, the greater the possible radon emanation rate is. With moisture contents from 10% up to 30%, the recoil emanation rates quickly reach the emanation rate of the saturated condition. As the moisture reaches 30%, a universal thin film on the pore surface is formed. This thin film is sufficient to stop the recoil radon from embedding into another part of the pore wall. The existing experimental data support this result.