Design, construction and commissioning of a high-flow radon removal system for XENONnT

TL;DR

This paper presents the design, construction, and commissioning of a high-flow cryogenic distillation system to significantly reduce radon backgrounds in the XENONnT liquid xenon detector, enabling more sensitive rare event searches.

Contribution

It introduces a novel high-flow radon removal system with custom heat exchangers and an energy-efficient heat pump, achieving stable operation at 30% above design flow.

Findings

Achieved a process flow of 91 kg/h, 30% above design.

Expected radon activity concentration below 1 μBq/kg.

Demonstrated stable operation and effective radon reduction.

Abstract

A high-flow radon removal system based on cryogenic distillation was developed and constructed to reduce radon-induced backgrounds in liquid xenon detectors for rare event searches such as XENONnT. A continuous purification of the XENONnT liquid xenon inventory of 8.4 tonnes at process flows up to 71 kg/h (200 slpm) is required to achieve a radon reduction by a factor two for radon sources inside the detector. To reach such high flows, the distillation column's design features liquid xenon inlet and outlets along with novel custom-made bath-type heat exchangers with high liquefaction capabilities. The distillation process was designed using a modification of the McCabe-Thiele approach without a bottom product extraction. The thermodynamic concept is based on a Clausius-Rankine cooling cycle with phase-changing medium, in this case the xenon itself. To drastically reduce the external cooling power requirements, an energy efficient heat pump concept was developed applying a custom-made four cylinder magnetically-coupled piston pump as compressor. The distillation system was operated at thermodynamically stable conditions at a process flow of (91$\pm$2) kg/h ((258$\pm$6) slpm), 30 % over design. With this flow, a $^{222}$Rn activity concentration <1 $\mu$Bq/kg is expected inside the XENONnT detector given the measured radon source distribution.