The XENONnT Radon Removal System*

The XENONnT dark matter experiment uses about 8.5 tonnes of liquid xenon (LXe)  for the direct search of Weakly Interacting Massive Particles (WIMPs). The intrinsic radioactive contaminant Rn-222 in the LXe is the main background source. The noble gas can be removed by cryogenic distillation, employing the difference in vapor pressure between radon and xenon. Since radon continuously emanates from the detector components, a continuous removal is required. A new high-flow radon distillation system with LXe inlet and outlet was developed for XENONnT featuring a process flow of 200 slpm. This leads to an expected radon reduction by a factor of 2 for radon sources inside the detector. An additional factor of 2 can be achieved by extracting radon from the gaseous xenon part of the detector before it can enter the liquid. This way, a radon activity concentration of 1 µBq/kg comes into reach for the first time in a xenon-based dark matter experiment. Besides the separation performance, the main challenge for the radon distillation system is an efficient thermodynamic design. This talk will outline a general radon removal strategy with the focus on the principle, construction and commissioning measurements of the radon distillation system for XENONnT.