Thanks to its scintillation properties, Liquid Argon (LAr) is a commonly used detector material in neutrino physics and other rare event searches. The chemical purity of LAr is critical, as it directly correlates with its light emission properties. In the LEGEND experiment, LAr serves as a cooling medium for the germanium detectors and an instrumented shield. The LAr was purified during the LEGEND-200 cryostat's initial filling to achieve optimal detector performance.
The LEGEND LAr purification System (LLArS) performance was monitored in real-time during purification and filling. The monitoring was done by probing the argon scintillation properties by triplet state lifetime measurements. Simultaneously, a direct measurement of the concentrations of impurities, such as water, oxygen, and nitrogen, with a sensitivity of 0.1 ppm was performed. Additionally, the purity and light-emitting properties of argon inside the cryostat are constantly monitored and studied by the LEGEND Liquid Argon Monitoring Apparatus (LLAMA) device. The achieved argon triplet lifetime (τ3) measured inside the 70%-filled LEGEND-200 cryostat was 1.3 μs. It dropped to 1.1 μs after filling some hundred liters of LAr of worse quality; the delivered LAr had high nitrogen content, which was not fully removed by LLArS. For the fully filled cryostat, τ3 has been kept at 1.15 μs since September 2021.
Here, the design, construction, and performance of the LLArS system capable of purifying 65 $m^3$ of liquid argon to a sub-ppm level for the LEGEND-200 experiment will be presented.