The performance of the Muon Veto of the GERDA experiment

Low background experiments need a suppression of cosmogenically induced events. The Gerda experiment located at Lngs is searching for the \(0\nu \beta \beta \) 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 water tank 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 \(\varepsilon _\mathrm{\upmu d}=(99.935\pm 0.015)\) % 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 \(\varepsilon _{\upmu r}=(99.2_{-0.4}^{+0.3})\) % was found. Without veto condition the muons by themselves would cause a background index of \(\text {BI}_{\mu }=(3.16 \pm 0.85)\times 10^{-3}\) \(\mathrm{cts/(keV}\cdot \mathrm{kg}\cdot \mathrm{year)}\) at \(Q_{\beta \beta }\).