Low-level γ-ray spectrometry at the underground laboratory Garching

Two low-background setups for material screening based on HPGe detectors were built in the Garching Underground Laboratory with an overburden of ~10. m.w.e. They include several layers of passive shielding as well as an active muon veto. The first setup (GEM) comprises a 150% efficiency HPGe detector which can optionally be surrounded by a NaI(Tl) scintillation detector that serves as anti-Compton veto. The second setup (LoAx) consists of two smaller HPGe detectors which are arranged face-to-face to cover a larger solid angle around the sample and to allow coincidence measurements. For a 5.6kg piece of copper after 11 days of measurement we have reached a sensitivity for ²²⁶Ra and ²²⁸Ra/²²⁸Th of ~5mBqkg^-1 with the GEM setup. In the LoAx setup we have achieved limits of less than 100mBqkg^-1 for ²³⁴Th and ²¹⁰Pb with a 156g sample of PPO wavelength shifter after 18 days of measurement.