Performance and stability tests of bare high purity germanium detectors in liquid argon for the GERDA experiment

GERDA will search for neutrinoless double beta decay of 76Ge by using a novel approach of bare germanium detectors in liquid argon (LAr). Enriched germanium detectors from the previous Heidelberg-Moscow and IGEX experiments have been reprocessed and will be deployed in GERDA Phase-I. At the center of this thesis project is the study of the performance of bare germanium detectors in cryogenic liquids. Identical detector performance as in vacuum cryostats (2.2 keV FWHM at 1.3 MeV) was achieved in cryogenic liquids with a new low-mass detector assembly and contacts. One major result is the discovery of a radiation induced leakage current (LC) increase when operating bare detectors with standard passivation layers in LAr. Charge collection and build-up on the passivation layer were identified as the origin of the LC increase. It was found that diodes without passivation do not exhibit this feature. Three month-long stable operation in LAr at 5 pA LC under periodic gamma irradiation demonstrated the suitability of the modifed detector design. Based on these results, all Phase-I detectors were reprocessed without passivation layer and subsequently successfully characterized in LAr in the GERDA underground Detector Laboratory. The mass loss during the reprocessing was 300 g out of 17.9 kg and the exposure above ground 5 days. This results in a negligible cosmogenic background increase of 5x10-4 cts/(keV kg y) at 76Ge Q for 60Co and 68Ge