The liquid-phase xenon purification system of the XENONnT dark matter experiment

Abstract

The XENONnT dark matter experiment utilizes a dual-phase time projection chamber (TPC) housed in a cryostat filled with 8.5 t of ultra-pure liquid xenon. To achieve optimal sensitivity, free electrons generated by particle interactions are required to drift across the 1.5-meter detector under an electric field. Dissolved impurities, particularly oxygen (O$_2$), can attenuate the resulting charge signal due to electron attachment. XENONnT demands an O$_2$-equivalent concentration below 100 parts per trillion (ppt) to enable its dark matter sensitivity reach. A novel liquid-phase xenon purification system was developed that continuously circulates liquid xenon through custom-designed, packed-bed sorbent purifier units at a volumetric flow rate of $\sim$2 liters per minute (L/min) at a cryogenic temperature around 175 K. With a purification time constant of approximately 25 h, the system reached the required purity within five days. The purification strategy employs a two-step approach: For an initial cleanup, a purifier based on a copper catalyst on high-surface-area alumina support (Q5) removes impurities from the liquid xenon bulk with nearly 100% efficiency, ideal for initial detector filling or after long-term maintenance. However, the high $^{222}$Rn emanation rate of the Q5 catalyst leads to increased background in the detector. In a second step, a purifier with sintered pellets of non-evaporable getter alloy (St707) ensures sustained purity with negligible background impact. The St707-based purifier exhibits a 55% efficiency, sufficient for normal operation. Following system commissioning, XENONnT achieved an O$_2$-equivalent concentration in liquid xenon below 19 ppt during its first science run. With this exceptional purity, over 86% of generated electrons survive a full drift across the TPC.