Material radiopurity control in the XENONnT experiment

TL;DR

This paper details the selection, screening, and mitigation of radioactive materials for the XENONnT experiment, achieving significantly reduced background levels to enhance rare-event detection sensitivity.

Contribution

It presents an extensive radioassay program and cleanliness procedures that improve material radiopurity and reduce background noise in the XENONnT detector.

Findings

Materials background reduced by ~17% compared to XENON1T

Radon activity concentration lowered to 4.2 μBq/kg, three times less than XENON1T

Implementation of radon distillation further suppresses radon levels

Abstract

The selection of low-radioactive construction materials is of the utmost importance for rare-event searches and thus critical to the XENONnT experiment. Results of an extensive radioassay program are reported, in which material samples have been screened with gamma-ray spectroscopy, mass spectrometry, and $^{222}$Rn emanation measurements. Furthermore, the cleanliness procedures applied to remove or mitigate surface contamination of detector materials are described. Screening results, used as inputs for a XENONnT Monte Carlo simulation, predict a reduction of materials background ($\sim$17%) with respect to its predecessor XENON1T. Through radon emanation measurements, the expected $^{222}$Rn activity concentration in XENONnT is determined to be 4.2$\,(^{+0.5}_{-0.7})\,\mu$Bq/kg, a factor three lower with respect to XENON1T. This radon concentration will be further suppressed by means of the novel radon distillation system.