Procurement and Purification of Liquid Argon for the LEGEND-200 Experiment

Abstract

LEGEND-200 requires high-purity liquid argon for effective background discrimination. In this paper, we present the design, construction, and performance of a dedicated liquid argon purification system, along with the procurement and purification of liquid argon for filling the LEGEND-200 cryostat to its total capacity of 91 t. The purifier is based on copper catalyst and molecular sieve to remove oxygen and water. Starting with liquid argon of 5.5 quality, featuring an effective scintillation light triplet lifetime $τ_t$ of about 0.9 $μ$s, we achieved a final purity corresponding to $τ_t$ = 1.3 $μ$s. After complete filling of the LEGEND-200 cryostat, the measured effective triplet lifetime was 1.16 $μ$s. The notable reduction is caused by a residual nitrogen impurity introduced by an accidentally spoiled liquid argon delivery. An excessive nitrogen influx was prevented by the LEGEND Liquid Argon Monitoring Apparatus (LLAMA), which served as one of the three independent purity monitors during the filling campaign.