Radon diffusion and the emanation fraction for NIST polyethylene capsules containing radium solution

The national standard for radon (Rn) measurements in the United States is based on Standard Reference Materials (SRMs) prepared at the National Institute of Standards and Technology (NIST), Gaithersburg, MD. NIST radon standards are hermetically sealed polyethylene capsules, filled with radium (Ra) solution. In preparation of standard reference materials for Rn, it is necessary to understand precisely the emanation fraction of radon from the internal radium solution, through the walls of the polyethylene container, to the surroundings. In preparation of a new radon SRM, it was found that the emanation fraction dependence on accumulation time is not accurately described by the standard ingrowth factor. A mathematical description of the emanation model is presented, which is in agreement with the empirical results for NIST radon emanation standards. It is shown that the radon accumulated inside the polyethylene walls of the capsule is significant and measurable. The radon diffusion coefficient of polyethylene is estimated to be 200 times less than the radon diffusion coefficient of water, while radon solubility in polyethylene is estimated as approximately 2/3 of the solubility of radon in water. This approach could be applied to describe the emanation of radon through other materials.