Operation and performance of a bare broad-energy germanium detector in liquid argon

The GERmanium Detector Array, GERDA, will search for neutrinoless double beta decay of 76Ge by operating bare high-purity germanium detectors, enriched in 76Ge, in liquid argon. To reduce the background to the required level below 10^(−3) cts/(keV·kg·y), it is necessary to employ active background-suppression techniques. Detectors based on the design of commercially available Broad-Energy Germanium (BEGe) detector are one of the two technologies included in research and development for the second phase of GERDA. BEGe detectors feature an enhanced capability to distinguish between an interaction of an electron from beta-decay and an interaction of a multiple-scattered photon inside the detector, via pulse-shape analysis. A GERDA Phase II prototype BEGe detector mounted in a low-mass holder was operated bare for the first time in liquid argon. The detector showed stable performance over more than one month, with an energy resolution of 1.9 keV (FWHM) at 1.3MeV, and a low leakage current of ≤ 20 pA. Periodic pulseshape analysis checks were performed and the results are equal to those obtained with the same detector in a vacuum cryostat