The Performance of the Muon Veto of the GERDA Experiment

Abstract

Low background experiments need a suppression of cosmogenically induced events. The GERDA experiment located at LNGS is searching for the neutrinless double beta decay of ${}^{76}$Ge. It is equipped with an active muon veto the main part of which is a water Cherenkov veto with 66 PMTs in the watertank surrounding the GERDA cryostat. With this system 806 live days have been recorded, 491 days were combined muon-germanium data. A muon detection efficiency of ${\epsilon }_{\hat{I}¼d}=(99.935\pm 0.015)$ \% was found in a Monte Carlo simulation for the muons depositing energy in the germanium detectors. By examining coincident muon-germanium events a rejection efficiency of ${\epsilon }_{\hat{I}¼r}=(99.2_{-0.4}^{+0.3})$ \% was found. Without veto condition the muons by themselves would cause a background index of ${\text{BI}}_{\hat{I}}¼=(3.16\pm 0.85)\times 10^{-3}$ cts/(keV$\cdot$kg$\cdot$yr) at $Q_{{\hat{I}}^2{\hat{I}}^2}$.

11pages, 14 figures