LArGe : A liquid argon scintillation veto for Gerda

LArGe is a Gerda low-background test facility to study novel background suppression methods in a low-background environment, for possible applications in the Gerda experiment. Gerda searches for the neutrinoless double-beta decay in ⁷⁶Ge, by operating naked germanium detectors submersed into 65 m³ of liquid argon. Similarly, LArGe runs Ge-detectors in 1 m³ (1.4 tons) of liquid argon, which in addition is instrumented with photomultipliers to detect argon scintillation light. The light is used in anti-coincidence with the germanium detectors, to effectively suppress background events that deposit energy in the liquid argon. This work adresses the design, construction, and commissioning of LArGe. The background suppression efficiency has been studied in combination with a pulse shape discrimination (PSD) technique for various sources, which represent characteristic backgrounds to Gerda. Suppression factors of a few times 10³ have been achieved. First background data of LArGe (without PSD) yield a background index of (0.12-4.6)·10⁻² cts/(keV·kg·y) (90% c.l.), which is at the level of the Gerda phase I design goal. Furthermore, for the first time we measure the natural ⁴²Ar abundance (in parallel to Gerda), and have indication for the 2νββ-decay in natural germanium