Search for double beta decay with HPGe detectors at the Gran Sasso underground laboratory

Abstract

Neutrinoless double-beta (0νββ) decay is practically the only way to establish the Majorana nature of the neutrino mass and its decay rate provides a probe of an effective neutrino mass. Double beta experiments are long-running underground experiments with specific challenges concerning the background reduction and the long term stability. These problems are addressed in this work for the Heidelberg-Moscow, Genius-Test-Facility and Gerda experiments. The Heidelberg-Moscow (HdM) experiment collected data with enriched 76Ge detectors from 1990 to 2003. An improved analysis of Heidelberg-Moscow data is presented, exploiting new calibration and spectral shape measurements with the HdM detectors. Genius-Test-Facility was a test-facility that verified the feasibility of using bare germanium detectors in liquid nitrogen. The first year results of this experiment are discussed. The Gerda experiment has been designed to further increase the sensitivity by operating bare germanium detectors in a high purity cryogenic liquid, which simultaneously serves as a shielding against background and as a cooling media. In the preparatory stage of Gerda, an external background gamma flux measurement was done at the experimental site in the Hall A of the Gran Sasso laboratory. The characterization of the enriched detectors from the Heidelberg-Moscow and Igex experiments was performed in the underground detector laboratory for the Gerda collaboration. Long term stability measurements of a bare HPGe detector in liquid argon were carried out. Based on these measurements, the first lower limit on the half-life of neutrinoless double electron capture of 36Ar was established to be 1.85·10^18 y (68% C.L).