Annual modulation of the muon flux in the GERDA experiment

The Gerda collaboration aims to determine the half life of the neutrinoless double beta decay (0νββ) of ⁷⁶Ge. In Phase I, the experimental background was reduced to 10^-2 cts/(keV.kg.yr) in the region around Q_ββ. For Phase II we want to reduce the background contribution by one order of magnitude. Cosmic muons induce part of this dangerous background and must be vetoed. The muon veto consists of a water Cherenkov detector with 66 PMTs in the water tank surrounding the Gerda cryostat which contains the germanium crystals. The muon veto operated stably for 806 days where only 2 PMTs were lost. The rate however is modulated by the Cngs neutrino beam and the atmospheric temperature effect, both will be presented in this talk.