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

• Two setups for low-level γ-ray spectrometry were built.
• The background count rate between 100 and 2700 keV is 5700 day−1.
• Sensitivities of 5 mBq/kg were achieved for 226Ra and 228Th.
• Sensitivities of less than 100 mBqkg−1 were achieved for 210Pb and 234Th.

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 5.6 kg piece of copper after 11 days of measurement we have reached a sensitivity for 226Ra and 228Ra/228Th of ~5 mBq kg−1 with the GEM setup. In the LoAx setup we have achieved limits of less than 100 mBq kg−1 for 234Th and 210Pb with a 156 g sample of PPO wavelength shifter after 18 days of measurement.