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 0νββ decay of ⁷⁶Ge. 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 ε_μd = (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 ε_μr = (99.2_-0.4^+0.3)% was found. Without veto condition the muons by themselves would cause a background index of BI_μ = (3.16 ± 0.85) x 10^-3 cts/(keV . kg . year) at Q_ββ. (orig.)