The Gerda muon veto Cherenkov detector

The GERmanium Detector Array, Gerda, is a new experiment designed to examine the neutrinoless double beta decay 0 ν β β of 76Ge which has a lifetime of at least 10 26 years and a single energy deposition of 2039 keV. To reach the goal of 10 - 3 background events/( keV kg y ), several background reduction techniques like anti-coincidence and pulse shape analysis will be used. Cosmic muons can produce background in form of particles and radioactivity. To reject them, two independent detector systems will be integrated in Gerda. One of these is a Cherenkov muon veto detector, that uses the water tank around the cryostat in which the crystals will be operated. It is equipped with 66 photomultipliers (PMTs) with 8 in. diameter. T distribution was found via extensive Monte Carlo studies to reach the highest efficiencies for dangerous muons (these are muons that cause an energy deposition of around 2 MeV in the germanium detectors), even though the PMTs cover less than 0.1% of the water tank surface. fficiencies depend strongly on the amount of detected photons. For this, as many surfaces as possible will be covered with ‘VM2000’, a highly reflective foil from 3 M. This foil has a high reflectivity in a wide range of wavelength and it also shifts photons from the UV into the optical range. It, more or less, doubles the amount of detectable photons, because the photomultipliers used, have an detection maximum between 370 and 400 nm. Thus, a detection efficiency of 98% should be easily achieved.