Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

Abstract

The GERDA experiment searches for the neutrinoless double beta decay of76Ge using high-purity germanium detectors enriched in76Ge. The analysis of the signal time structure provides a powerfultool to identify neutrinoless double beta decay events and to discriminate themfrom gamma-ray induced backgrounds. Enhanced pulse shape discriminationcapabilities of Broad Energy Germanium detectors with a small read-outelectrode have been recently reported. This paper describes the full simulationof the response of such a detector, including the Monte Carlo modeling ofradiation interaction and subsequent signal shape calculation. A pulse shapediscrimination method based on the ratio between the maximum current signalamplitude and the event energy applied to the simulated data shows quantitativeagreement with the experimental data acquired with calibration sources. Thesimulation has been used to study the survival probabilities of the decays whichoccur inside the detector volume and are difficult to assess experimentally.Such internal decay events are produced by the cosmogenic radio-isotopes68Ge and 60Co and the neutrinoless double beta decay of76Ge. Fixing the experimental acceptance of the double escape peakof the 2.614 MeV photon to 90%, the estimated survival probabilitiesat Qββ = 2.039 MeV are (86±3)% for 76Geneutrinoless double beta decays, (4.5±0.3)% for the 68Gedaughter 68Ga, and (0.9+0.4−0.2)% for 60Codecays.