Low Background Gamma Spectroscopy at the Boulby Underground Laboratory

Abstract

The Boulby Underground Germanium Suite (BUGS) comprises three low background, high-purity germanium detectors operating in the Boulby Underground Laboratory, located 1.1 km underground in the north-east of England, UK. BUGS utilises three types of detector to facilitate a high-sensitivity, high-throughput radioassay programme to support the development of rare-event search experiments. A Broad Energy Germanium (BEGe) detector delivers sensitivity to low-energy gamma-rays such as those emitted by 210Pb and 234Th. A Small Anode Germanium (SAGe) well-type detector is employed for efficient screening of small samples. Finally, a standard p-type coaxial detector provides fast screening of standard samples. This paper presents the steps used to characterise the performance of these detectors for a variety of sample geometries, including the corrections applied to account for cascade summing effects. For low-density materials, BUGS is able to radio-assay to specific activities down to 3.6 mBq/kg for 234Th and 6.6 mBq/kg for 210Pb both of which have uncovered some significant equilibrium breaks in the 238U chain. In denser materials, where gamma-ray self-absorption increases, sensitivity is demonstrated to specific activities of 0.9 mBq/kg for 226Ra, 1.1 mBq/kg for 228 Ra, 0.3 mBq/kg for 224Ra, and 8.6 mBq/kg for 40K with all upper limits at a 90% confidence level. These meet the requirements of most screening campaigns presently under way for rare-event search experiments, such as the LUX-ZEPLIN (LZ) dark matter experiment. We also highlight the ability of the BEGe detector to probe the X-ray fluorescence region which can be important to identify the presence of radioisotopes associated with neutron production; this is of particular relevance in experiments sensitive to nuclear recoils.

17 pages, 21 figures

References in corpus (4)

• Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment
• First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase xenon detector for dark matter detection
• The ZEPLIN-III dark matter detector: instrument design, manufacture and commissioning
• Calculation of neutron background for underground experiments

Cited by in corpus (10)

• The LUX-ZEPLIN (LZ) Experiment
• The XLZD Design Book: Towards the Next-Generation Liquid Xenon Observatory for Dark Matter and Neutrino Physics
• Evaluation and mitigation of trace ${}^{210}$Pb contamination on copper surfaces
• QUEST-DMC superfluid ${}^3$He detector for sub-GeV dark matter
• The upgraded low-background germanium counting facility Gator for high-sensitivity $\gamma$-ray spectrometry
• QUEST-DMC: Background Modelling and Resulting Heat Deposit for a Superfluid Helium-3 Bolometer
• Background Shielding by Dense Samples in Low-Level Gamma Spectrometry
• Ultra-Low Background Germanium Assay at the Boulby Underground Laboratory
• Contextual Isotope Ranking Criteria for Peak Identification in Gamma Spectroscopy Using a Large Database
• Volume reduction of water samples to increase sensitivity for radioassay of lead contamination