Radon permeability through nylon at various humidities used in the Borexino experiment

We have investigated the radon permeation through a nylon film at various humidities. The measurement method and the instrumentation applied, as well as a mathematical model of the 222Rn diffusion are described. The dependence of the radon diffusion, solubility and permeability coefficients for the nylon film on the relative ambient humidity and the nylon moisture content has been found. No significant change in these parameters was observed as the relative humidity was increased from 0% to 30%. However, in the range from 30% to 100% the radon permeability increases by about two orders of magnitude. In the range from 30% to 60%, which is of particular importance for Borexino, the 222Rn permeability is about one order of magnitude higher than it is through dry nylon.