Instrumentation of the guard ring electrode of coplanar-grid CdZnTe detectors for the COBRA experiment

A novel method to suppress alpha induced background in the COBRA experiment is presented here. The experiment's aim is to search for neutrinoless double beta-decay (0νββ) using CdZnTe semiconductor detectors with a coplanar-grid (CPG) readout. The next phase of COBRA will be the installation of the extended demonstrator (XDEM) consisting of CdZnTe detectors with a volume of (2 x 2 x 1.5) cm³ and applying an adjusted coplanar quad-grid (CPqG) readout. The CPqG anodes are surrounded by an outer electrode called guard ring. It is used to reduce leakage currents as well as to increase the homogeneity of the weighting potential. In the standard case, the guard ring is left on a floating potential. Recent results from laboratory measurements showed that setting the charge-collecting ground potential to it can lead to a great suppression of alpha induced events in the order of 99.9 %. Current recorded physics data indicate that the main background contribution is due to alpha contaminations on the detector surfaces. Therefore, the instrumentation and readout of the guard ring electrode can be highly beneficial in terms of background reduction for the COBRA experiment.