Isotopically modified Ge detectors for GERDA: from production to operation

The GERDA experiment searches for the neutrinoless double beta (0νββ) decay of 76Ge using high-purity germanium detectors made of material enriched in 76Ge. For Phase II of the experiment a sensitivity for the half life T1/20ν   ~ 2·1026 yr is envisioned. Modified Broad Energy Germanium detectors (BEGe) with thick n+ electrodes provide the capability to efficiently identify and reject background events, while keeping a large acceptance for the 0νββ-decay signal through novel pulse-shape discrimination (PSD) techniques. The viability of producing thick-window BEGe-type detectors for the GERDA experiment is demonstrated by testing all the production steps from the procurement of isotopically modified germanium up to working BEGe detectors. Comprehensive testing of the spectroscopic as well as PSD performance of the GERDA Phase II prototype BEGe detectors proved that the properties of these detectors are identical to those produced previously from natural germanium material following the standard production line of the manufacturer. Furthermore, the production of BEGe detectors from a limited amount of isotopically modified germanium served to optimize the production, in order to maximize the overall detector mass yield. The results of this test campaign provided direct input for the subsequent production of the enriched germanium detectors.