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 Ge-76. 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 epsilon(mu d) = (99.935 +/- 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(mu r) = (99.2(-0.4)(+0.3))% was found. Without veto condition the muons by themselves would cause a background index of BI mu = (3.16 +/- 0.85) x 10(-3) cts/(keV . kg . year) at Q(beta beta).