Characterization of inverted coaxial $$^{76}$$Ge detectors in GERDA for future double-$$eta $$ decay experiments [Characterization of inverted coaxial Ge-76 detectors in GERDA for future double-beta decay experiments]

Neutrinoless double-beta decay of Ge-76 is searched for with germanium detectors where source and detector of the decay are identical. For the success of future experiments it is important to increase the mass of the detectors. We report here on the characterization and testing of five prototype detectors manufactured in inverted coaxial (IC) geometry from material enriched to 88% in Ge-76. IC detectors combine the large mass of the traditional semi-coaxial Ge detectorswith the superior resolution and pulse shape discrimination power of point contact detectors which exhibited so far much lower mass. Their performance has been found to be satisfactory both when operated in vacuum cryostat and bare in liquid argon within the Gerda setup. The measured resolutions at the Q-value for double-beta decay of Ge-76 (Q(beta beta) = 2039 keV) are about 2.1 keV full width at half maximumin vacuum cryostat. After 18months of operationwithin the ultra-low background environment of the GERmanium Detector Array (Gerda) experiment and an accumulated exposure of 8.5 kg center dot year, the background index after analysis cuts ismeasured to be 4.9(-3.4)(+7.3) x10(-4) counts/(keV center dot kg center dot year) around Q(beta beta). This work confirms the feasibility of IC detectors for the next-generation experiment Legend.