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

TL;DR

Tritium from CH3T calibration is a significant low-energy background in PandaX-4T. The authors implement a hybrid approach combining hot xenon gas flushing with a cryogenic distillation tower to remove CH3T from xenon. They report CH3T concentrations dropping from $3.6×10^{-24}$ mol/mol to $5.9×10^{-25}$ mol/mol, an ~84% reduction, demonstrating effective removal at ultralow concentrations. This work validates purge plus distillation as a practical method for ultra-low impurity control in xenon-based detectors and informs future online purification strategies.

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.

Figures (6)

• Figure 1: Flow diagram of the PandaX-4T cryogenic circulation system with heating belts.
• Figure 2: Photo of PandaX-4T distillation tower.
• Figure 3: Flow diagram of PandaX-4T distillation system.
• Figure 4: Schematically represented setup of the HYSYS simuation ( note: AL300 means the cooling power supplied by the cryocooler installed at the condenser).
• Figure 5: Left: Fixed flow rate at 10 kg/h with varying reboiler heating power and feeding points; Right: Fixed heating power at 180 W (reflux ratio = 220) with varying flow rate and feeding points.