A Measurement of the Absorption of Liquid Argon Scintillation Light by Dissolved Nitrogen at the Part-Per-Million Level

TL;DR

This study measures how dissolved nitrogen at ppm levels absorbs liquid argon scintillation light, providing data crucial for optimizing large LArTPC detectors.

Contribution

It quantifies the absorption cross section of nitrogen in liquid argon at ppm levels, informing detector design and data analysis.

Findings

Nitrogen absorption cross section is approximately 7.14 x 10^{-21} cm^2 per molecule.

Absorption length at 2 ppm nitrogen is about 30 meters.

Relationship between nitrogen concentration and absorption length established.

Abstract

We report on a measurement of the absorption length of scintillation light in liquid argon due to dissolved nitrogen at the part-per-million (ppm) level. We inject controlled quantities of nitrogen into a high purity volume of liquid argon and monitor the light yield from an alpha source. The source is placed at different distances from a cryogenic photomultiplier tube assembly. By comparing the light yield from each position we extract the absorption cross section of nitrogen. We find that nitrogen absorbs argon scintillation light with strength of $(1.51\pm 0.15)\times10^{-4} \;\mathrm{cm^{-1} ppm^{-1}}$, corresponding to an absorption cross section of $(7.14 \pm 0.74)\times10^{-21}\;\mathrm{cm^{2} molecule^{-1}}$. We obtain the relationship between absorption length and nitrogen concentration over the 0 to 50 ppm range and discuss the implications for the design and data analysis of future large liquid argon time projection chamber (LArTPC) detectors. Our results indicate that for a current-generation LArTPC, where a concentration of 2 parts per million of nitrogen is expected, the attenuation length due to nitrogen will be $30 \pm 3$ meters.