Oxygen contamination in liquid Argon: Combined effects on ionization electron charge and scintillation light

A dedicated test of the effects of Oxygen contamination in liquid Argon has been performed at the INFN-Gran Sasso Laboratory (LNGS, Italy) within the WArP R&D program. Two detectors have been used: the WArP2.3 lt prototype and a small (0.7 lt) dedicated detector, coupled with a system for the injection of controlled amounts of gaseous Oxygen. O₂ contamination in LAr leads to depletion of both the free electron charge (via attachment process) and the scintillation light (via quenching and absorption mechanisms) available for ionization signal detection. Purpose of the test with the 0.7 lt detector was to detect the reduction of the long-lived component lifetime of the Argon scintillation light emission and of the overall light yield at increasing O ₂ concentration. Data from the WArP prototype were used for determining the behavior of both the ionization electron lifetime and the scintillation long-lived component lifetime at decreasing O₂ concentration by the purification process activated in closed loop during the acquisition run. The electron lifetime measurements allowed to infer the O ₂ content of the Argon and correlate it with the long-lived scintillation lifetime data. The effects of Oxygen contamination on the scintillation light have been thus extensively measured over a wide range of O₂ concentration, spanning from ∼ 10^-3 ppm up to ∼ 10 ppm.