We describe a new high-purity germanium (HPGe) detector setup for low-background γ-ray spectrometry. The GeMSE facility (Germanium Material and meteorite Screening Experi- ment) is dedicated to material screening for rare event searches in astroparticle physics as well as to the characterization of meteorites. It is installed in a medium depth (~620 m.w.e.) underground laboratory in Switzerland in a multi-layer shielding and is equipped with an active muon veto. We have reached a very competitive integral background rate of (246 ± 2) counts/day (100–2700 keV) and measured a sensitivity of ~0.5–0.6 mBq/kg for long-lived isotopes from the 238U/232Th chains in a ~1 kg sample screened for ~27 days. An extrapolation to higher sample masses and mea- surement times suggests a maximum sensitivity in the O(50) μBq/kg range. We describe the data analysis based on Bayesian statistics, background simulations, the efficiency calibration and first sample measurements.
