A measurement of the absorption of liquid argon scintillation light by dissolved nitrogen at the part-per-million level

Abstract

We report on a measurement of the absorption length of scintillationlight in liquid argon due to dissolved nitrogen at the part-per-million (ppm)level. We inject controlled quantities of nitrogen into a highpurity 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 extractthe absorption cross section of nitrogen. Wefind that nitrogen absorbs argon scintillation light with strength of (1.51±0.15) × 10−4 cm−1ppm−1, correspondingto an absorption cross section of (4.99±0.51) × 10−21 cm2molecule−1.We obtain the relationship between absorption length and nitrogenconcentration over the 0 to 50 ppm range and discuss the implicationsfor the design and data analysis of future large liquid argon time projection chamber (LArTPC)detectors. Our results indicate that for a current-generation LArTPC, wherea concentration of 2 parts per million of nitrogen is expected, the attenuationlength due to nitrogen will be 30±3 meters.