Sensitivity studies of CdZnTe semiconductor detectors for the COBRA experiment

Abstract

The COBRA Experiment searches for Neutrinoless Double Beta Decay. The observation of this decay would provide the absolute mass of the neutrino and clarify the question whether the neutrino is its own anti particle, a so called Majorana particle, or not. For the search COBRA applies CdZnTe room temperature semiconductor detectors. CdZnTe can be operated as a pixelated detector or with a Coplanar Grid (CPG) readout. Both detector types are a possible option for the COBRA Experiment, and in this work their prospects and suitability was investigated. Data measured with a large volume pixel detector was reproduced in Geant4 Monte Carlo simulations. With these simulations the veto capabilities and the most dangerous background contributions for COBRA were determined. It was shown that the background level can be reduced by several orders of magnitude by applying fiducial volume cuts. This is especially efficient for the reduction of background from alpha particles. The main remaining background for pixel detectors was found to derive from rare high energetic gamma radiation. For the COBRA CPG set-up at the Italian underground laboratory LNGS, a major contribution in the development of new readout electronics was made. Data taken with this new electronics was evaluated to determine the largest remaining background sources of the COBRA LNGS test set-up. As main background sources alpha-decays of Pt-190 and Po-210 were identified and their influence could be significantly reduced. For the rare alpha decay of Pt-190 a half-life of (5.8 +- 1.0 (stat.) +- 0.7 (syst.)) *10^11 years was determined. The search for small signals in sparse data requires special analysis methods. For the analysis of COBRA data a new unbinned extended maximum likelihood fitter was developed, thoroughly tested with Monte Carlo data and applied to LNGS data. With the achieved background reduction and the new analysis method, former COBRA half life limits for neutrinoless double beta decay were improved by more than a factor of two. For Cd-116 and Te-130 upper limits of 2.4*10^20 years and 1.1*10^21 years were obtained respectively.