A model to explain simultaneously the 222Rn and 220Rn emanation from thin electrodeposited sources

In thin radioactive sources, loss of radon by emanation is a very common phenomenon, especially in sources made by electrodeposition. A quantification of this effect in radium sources can be easily developed by using a simple model that assumes a radon diffusion term in the ingrowth equations. By measuring the corresponding Rn/Ra activity ratio, a constant diffusion factor can be determined which represents the Rn emanation from the whole source. However, this simple model cannot explain simultaneously the 222Rn and 220Rn diffusion produced in a thin source, because it gives diffusion factors that are different by many orders of magnitude for these two isotopes, while these values must be fairly close. In this paper, a new model of diffusion is proposed, which includes a linear dependence of the diffusion factor on the depth of Rn nuclides in the source. This new model has been applied to radium electrodeposited sources and allows us to explain satisfactorily both the 220Rn/224Ra and 222Rn/226Ra activity ratios observed in thin sources.