Construction of a low background facility for the COBRA experiment and its performance

Abstract

The COBRA experiment is investigating neutrinoless double-beta decays of Cd, Zn and Te isotopes with an array of CdZnTe semiconductor detectors. The current development stage consists of 64 CPG-type detectors of 1 ccm size each that will be arranged in form of a 3-dimensional 4 × 4 × 4-array. At the time of writing, the first 16-detector layer was installed and has been collecting data since January 2007. The remaining three layers are currently being assembled.

Within the course of this work, key experimental components for the COBRA setup have been developed, among them several versions of custom-built preamplifiers, shapers and large parts of COBRA’s VME-based ADC. Furthermore, the detector suspension and contacting were completely re-designed and the shielding concept was significantly enhanced.

Based on roughly 4.2 kg · d of data collected with the first operational 16-detector layer, a coincidence-based analysis has been performed focusing on neutrinoless double-beta decays of Cd-116, Te-128 and Te-130 into excited states. For one of the decays, previously published limits on the half-life could be improved. Moreover, the power of the coincidence analysis method was demonstrated for decays involving gamma emission.