Abstract
The Gerda experiment searches for the neutrinoless double beta (0νββ) decay in 76Ge. This lepton number violating process, if found, would prove the Majorana nature of neutrinos. The decay rate permits access to the effective neutrino mass provided accurate nuclear matrix elements are available. Therefore, different isotopes are investigated in various experiments. Gerda is located in the underground Gran Sasso laboratory of INFN and it employs germanium diodes made from isotopically enriched material which are immersed directly into liquid argon serving simultaneously as coolant and as radiation shield. Only screened materials have been selected for holders and preamplifier components as well as other constructional elements in the vicinity of the diodes in order to minimize the background; a muon detection systems reduces the part induced by cosmic radiation. After a successful commissioning, the measurements within Phase I started in November 2011. For the enriched detectors data have been accumulated with an exposure of 6.1kg⋅yr. Overall, a background index of about 0.02cts/(keV⋅kg⋅yr) is achieved which is smaller by about a factor of 10 with respect to previous experiments. For the neutrino accompanied double beta decay a preliminary half live of T1/22ν(76Ge)=(1.88±0.10)×1021yr is deduced.
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