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 $\varepsilon_{ \mu d}=(99.935\pm0.015)$ \% 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 $\varepsilon_{ \mu r}=(99.2_{ -0.4}^{ +0.3})$ \% was found. Without veto condition the muons by themselves would cause a background index of $\textrm{ BI}_{ \mu}=(3.16 \pm 0.85)\times10^{ -3}$ cts/(keV$\cdot$kg$\cdot$yr) at $Q_{ \beta\beta}$.
Categories: physics.ins-det hep-ex