Argon purification studies and a novel liquid argon re-circulation system

Abstract

Future giant liquid argon (LAr) time projection chambers (TPCs) require a purity of better than 0.1 parts per billion (ppb) to allow the ionised electrons to drift without significant capture by any electronegative impurities. We present a comprehensive study of the effects of electronegative impurity on gaseous and liquid argon scintillation light, an analysis of the efficacy of various purification chemicals, as well as the Liverpool LAr setup, which utilises a novel re-circulation purification system. Of the impurities tested -Air, O 2 , H 2 O, N 2 and CO 2 in the range of between 0.01 ppm to 1000 ppm -H 2 O was found to have the most profound effect on gaseous argon scintillation light, and N 2 was found to have the least. Additionally, a correlation between the slow component decay time and the total energy deposited with 0.01 ppm -100 ppm O 2 contamination levels in liquid argon has been established. The superiority of molecular sieves over anhydrous complexes at absorbing Ar gas, N 2 gas and H 2 O vapour has been quantified using BET isotherm analysis. The efficiency of Cu and P 2 O 5 at removing O 2 and H 2 O impurities from 1 bar N6 argon gas at both room temperature and -130 • C was investigated and found to be high. A novel, highly scalable LAr re-circulation system has been developed. The complete system, consisting of a motorised bellows pump operating in liquid and a purification cartridge, were designed and built in-house. The system was operated successfully over many days and achieved a re-circulation rate of 27 litres/hour and high purity.

Triplet lifetime in gaseous argon

“…We know of no systematic effect in our analysis that could account for the difference. We speculate that the functional form is altered at large impurities due to nitrogen which is known to only significantly quench the triplet state for concentrations above 1 ppm [12].…”

“…The dominant impurity species in MiniCLEAN are oxygen and nitrogen at the operating temperatures (<140 K). The quenching effects from nitrogen diminish with impurity level less than 1 ppm [12], therefore we assume the quenched light yield is mainly due to oxygen when the impurity level is below 1 ppm. We note that the χ 2 of both fits are large, due to a discrepancy in the high impurity level region.…”

“…However, even with an impurity level at 1 ppm, the density effect alone can not explain the variation of triplet decay rates and lifetimes. A more complete explanation requires inclusion of the effects of impurities, which have been investigated by many groups [11] [12][13] [14]. Table 1.…”

Sterile Neutrinos in Cold Climates

“…As shown in [206], [207], quenching becomes significant at 10 ppb-level concentrations of water and oxygen. This is far above the allowed concentration for TPC operation, so no scintillation quenching due to these molecules is expected in such detectors.…”

Studies with a Liquid Argon Time Projection Chamber

“…The removal of impurities from noble gases and liquids has already been studied extensively [6,7,[42][43][44][45][46]. The main contaminants of concern in these media are atoms or molecules with a high electronegativity as they have a larger tendency to attach an electron to form a negative ion.…”