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 beta decay, due to their origin as unavoidable decay products of natural uranium. In particular, 222Rn and its long-lived daughter 210Pb 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 210Pb and its daughter 210Po under varying humidity conditions. Our results show that both 210Pb and 210Po 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 210Pb was found to be lower than 1.14 × 10−15 cm2/s at 40% relative humidity (RH), and to be (4.03 ± 1.01) × 10−13 cm2/s at 95% RH. The diffusivity of 210Po at 95% RH was measured to be (3.94 ± 0.98) × 10−13 cm2/s. These findings underscore the importance of controlling environmental humidity and material exposure to radon in the design of ultra-low background experiments.
@article{arxiv.2509.11464,
title = {Diffusion of $^{210}\text{Pb}$ and $^{210}\text{Po}$ in Nylon},
author = {P. Adhikari and M. G. Boulay and R. Crampton and M. Perry and D. Sinclair},
journal= {arXiv preprint arXiv:2509.11464},
year = {2026}
}