Tritiated methane reduction in the PandaX-4T experiment via purge and cryogenic distillation processes

TL;DR

This paper demonstrates an effective method combining purge and cryogenic distillation to remove tritiated methane impurities from xenon in the PandaX-4T dark matter detector, significantly reducing CH$_3$T levels to improve sensitivity.

Contribution

It introduces a novel purification process using cryogenic distillation optimized for removing ultra-trace levels of CH$_3$T from xenon in large-scale dark matter experiments.

Findings

CH$_3$T concentration reduced from 3.6×10⁻²⁴ to 5.9×10⁻²⁵ mol/mol

Purification of 5.7 tons of xenon achieved

Gas purging and distillation effectively remove ultra-trace impurities

Abstract

Tritium from tritiated methane (CH$_3$T) calibration is a significant impurity that restricts the sensitivity of the PandaX-4T dark matter detection experiment in the low-energy region. The CH$_3$T removal is essential for PandaX-4T and other liquid xenon dark matter direct detection experiments, as CH$_3$T serves as a critical component for low-energy calibration. To eliminate CH$_3$T, the xenon in the detector is suitably recuperated, leaving 1.8 bar of xenon gas inside, and the detector is flushed with heated xenon gas. Concurrently, leveraging the lower boiling point of methane relative to xenon, the PandaX-4T cryogenic distillation system is effectively utilized to extract CH$_3$T from xenon after optimizing the operational parameters. Following the commissioning run, 5.7 tons of xenon are purified via the distillation method. Recent data indicate that the CH$_3$T concentration reduces from $3.6\times10^{-24}$ mol/mol to $5.9\times10^{-25}$ mol/mol, demonstrating that gas purging and distillation are effective in removing CH$_3$T, even at concentrations on the order of $10^{-24}$ mol/mol.