222Rn and 220Rn diffusion in two mediums

In this paper diffusion of radon isotopes 222Rn and 220Rn is considered in diffusion chamber which is covered with radon soluble membrane. Diffusion is considered in two mediums, membrane and air inside of diffusion chamber. Analytical and numerical solutions are presented and time dependence of total activity inside of chamber is determined. Presented analytical solution can be of use for determining time for which equilibrium state can be reach in diffusion chamber. It is shown that thickness of membrane can strongly influence on radon concentration inside of chamber, especially for 220Rn isotope. Detectors for 222Rn measurements which discriminate 220Rn isotope can be devised by simply increasing of membrane thickness. For plain filter paper of 1cm thickness inside of chamber, there will be 0.5% of outside 220Rn concentration, while 222Rn concentration will be reduced on 50%. In this way, by placing two chambers with thin and thick membrane, concentrations of both 222Rn and 220Rn can be easily determined.