出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：The determination of two important properties of the neutrino is still pending: its rest mass and its nature - Dirac or Majorana particle. The observation of neutrinoless double beta decay would clarify both, but the expected half-life of more than 1025 years for this decay is an ambitious challenge. Recently, first experiments have been commissioned that will be able to probe this magnitude of half-lifes. If they succeed, it will be important to verify the results using different isotopes. If they do not succeed, new experimental approaches are required. The COBRA experiment is an excellent candidate for both: it uses CdZnTe room-temperature semiconductor detectors that contain several double beta decay candidate isotopes, among them also b+ emitters and two of the most promising isotopes, Te-130 and Cd-116. Cd-116 has a decay energy well above the most energetic naturally occurring gamma lines reducing its background drastically compared to isotopes with lower Q-values. In addition, CdZnTe can be used as a solid-state TPC combining the large advantages of a source-equals-detector approach with the possibility of decay identification via particle track reconstruction. The COBRA collaboration comprises 12 institutes from 8 countries and currently operates detectors with two readout techniques: The commercially available Co-Planar Grid (CPG) technology was developed specifically for CdZnTe and allows the operation of fairly large crystal sizes of the order of some cm3 with only one readout channel. It reaches energy resolutions better than 2% FWHM at 662 keV. COBRA operated several R&D-arrays of CPG-type detectors in ultra-low background mode at the Italian underground laboratory LNGS. Half-life limits above 1020 years for several isotopes and decay modes have been determined with these setups. New readout electronics optimized for low-rate spectroscopy and allowing background reduction by pulse-shape analysis was developed. Using semiconductor pixel detectors is unique for a double-beta decay experiment. Three different pixelated CdZnTe/CdTe detector types have been operated in an ultra-low background environment and have demonstrated the power of background suppression with this detector type by enabling fiducial cuts, vertexing and coincidence techniques.
