Measurement of the attenuation length of argon scintillation light in the ArDM LAr TPC

TL;DR

This paper measures the attenuation length of scintillation light in a large liquid argon detector, revealing the presence of impurities affecting light transmission, which is crucial for optimizing dark matter detection experiments.

Contribution

It provides the first measurement of the attenuation length in a tonne-scale liquid argon detector using full light ray tracing and impurity analysis.

Findings

Attenuation length is approximately 0.5 meters.

Impurities in liquid argon significantly affect light transmission.

Analysis of gas purity and molecular cross sections informs future improvements.

Abstract

We report on a measurement of the attenuation length for the scintillation light in the tonne size liquid argon target of the ArDM dark matter experiment. The data was recorded in the first underground operation of the experiment in single-phase operational mode. The results were achieved by comparing the light yield spectra from 39-Ar and 83m-Kr to a description of the ArDM setup with a model of full light ray tracing. A relatively low value close to 0.5 m was found for the attenuation length of the liquid argon bulk to its own scintillation light. We interpret this result as a presence of optically active impurities in the liquid argon which are not filtered by the installed purification systems. We also present analyses of the argon gas employed for the filling and discuss cross sections in the vacuum ultraviolet of various molecules in respect to purity requirements in the context of large liquid argon installations.