Diffusion of Pb 210 and Po 210 in nylon

Radon and its progeny constitute a major source of background in rare-event physics experiments, such as those searching for dark matter, neutrinos, and neutrinoless double β decay, due to their origin as unavoidable decay products of natural uranium. In particular, Rn 222 and its long-lived daughter Pb 210 can diffuse from detector material surfaces, resulting in sustained background contributions. To investigate this process, a system was developed using a controlled radon source, a vacuum chamber with a high electric field, and a thin nylon-6 film to enable deposition of radon progeny onto the film surface. Nylon-6 was selected for the initial measurement given its history in low-background experiments. We intend to systematically study diffusion in various polymers in the future. Our setup allowed for controlled study of the diffusion behavior of Pb 210 and its daughter Po 210 under varying humidity conditions. Our results show that both Pb 210 and Po 210 diffuse significantly in nylon under high relative humidity, which can potentially lead to internal contamination and increased background in low-background detectors. The diffusivity of Pb 210 was found to be lower than 1.14 × 10 − 15 cm 2 / s at 40% relative humidity (RH) and to be ( 4.03 ± 1.01 ) × 10 − 13 cm 2 / s at 95% RH. The diffusivity of Po 210 at 95% RH was measured to be ( 3.94 ± 0.98 ) × 10 − 13 cm 2 / s . These findings underscore the importance of controlling environmental