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 0 νββ 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 εμ 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 εμr=(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) × 10 - 3cts / (keV · kg · year) at Qββ.