Analysis of scintillation light in a liquid argon veto system for neutrino-less ββ decay detectors*

Liquid argon (LAr) is used as a scintillation material for active shielding for the Large Enriched Germanium Experiment for Neutrino-less ββ Decay (LEGEND), while simultaneously serving to cool the germanium detectors. It is essential to have an efficient veto to suppress extraneous background events, as T_1/2 > 5.3 x 10²⁵ yr for neutrino-less ββ decay in germanium-76. To improve the effectiveness of a LAr system, we study how impurities affect the LAr scintillation light. The light is detected by photomultiplier tubes immersed in the LAr. Both the number of photons and the lifetime of the triplet excited state in LAr have been shown to be reduced by the amount of impurities in the argon and can therefore affect the efficiency of a LAr veto system. We have demonstrated the effect of impurities in argon and the removal of impurities with a simple argon re-circulation system. We also plan to investigate xenon doping as a method for further improving the veto system.