The Effects of Dissolved Methane upon Liquid Argon Scintillation Light

TL;DR

This study investigates how dissolved methane affects liquid argon scintillation light, revealing significant suppression at very low concentrations and providing insights into absorption and quenching mechanisms.

Contribution

It provides the first detailed measurements of methane's impact on liquid argon scintillation, including absorption effects and absence of visible re-emission in the liquid phase.

Findings

Methane causes significant scintillation light suppression at 10 ppb.

Loss is primarily due to absorption, with some quenching at higher concentrations.

No visible re-emission observed in liquid argon methane mixtures.

Abstract

In this paper we report on measurements of the effects of dissolved methane upon argon scintillation light. We monitor the light yield from an alpha source held 20 cm from a cryogenic photomultiplier tube (PMT) assembly as methane is injected into a high-purity liquid argon volume. We observe significant suppression of the scintillation light yield by dissolved methane at the 10 part per billion (ppb) level. By examining the late scintillation light time constant, we determine that this loss is caused by an absorption process and also see some evidence of methane-induced scintillation quenching at higher concentrations (50-100 ppb). Using a second PMT assembly we look for visible re-emission features from the dissolved methane which have been reported in gas-phase argon methane mixtures, and we find no evidence of visible re-emission from liquid-phase argon methane mixtures at concentrations between 10 ppb and 0.1%.