Performance Analysis and Operation of the EXO-200 Muon Veto System

The EXO-200 experiment implemented a single-phase time projection chamber (TPC) as the combined source and detector for a neutrinoless double-beta decay search in isotopically enriched $^{136}$Xe. Despite being emplaced roughly 655 m underground at the Waste Isolation Pilot Plant near Carlsbad, NM, atmospheric muons still represented a significant background. In order to tag incoming muons and prompt decays, a muon veto system comprising 29 large plastic scintillator panels surrounded the TPC cleanroom on four sides. Throughout the lifetime of EXO-200, the muon veto system was monitored continuously to ensure acceptable performance and deadtime contributions. These measures included semi-annual calibration campaigns using a sealed radioactive source, the fitting of Landau peaks in underground muon spectra, and a battery of studies on ROI background impact, trigger efficiency, and rate for each veto system channel. Methods and conclusions from these studies will be discussed.