Radon progeny attachment to surfaces as a background source in nEXO experiment

nEXO is a next-generation, tonne-scale neutrinoless double beta decay experiment. The experiment utilizes a time projection chamber and 5 tonnes of xenon enriched in the mass-136 isotope. The projected 90% CL half-life sensitivity is 1.35·10²⁸ yr after 10 yr of exposure. Stringent radioactive background control and careful material selection are necessary to achieve such a sensitivity.

Radon daughter plate-out is a background source for nEXO. The exposure of detector materials to air will lead to the accumulation of ²¹⁰Po, whose α-decay can produce neutrons from (α, n) reactions. Some of these neutrons will capture on ¹³⁶Xe to form ¹³⁷Xe, whose β-decay has a Q-value of >4 MeV, above the double beta end point. The nEXO group at the University of Alabama conducted a comprehensive study of radon progeny attachment to surfaces of 9 materials relevant for nEXO. More than 1200 measurements were performed in several environmental conditions with careful monitoring and control of the exposure parameters. The impact of environmental parameters on attachment rates is studied. Attachment modelling, using the Jacobi model, is compared to data. The first measurement of the physical collection length for radon progeny on copper, corrected for chain equilibrium breakage, will be reported.