Low-background gamma spectroscopy at the Boulby Underground Laboratory

The Boulby Underground Germanium Suite (BUGS) comprises three low-background, high-purity germanium detectors operating in the Boulby Underground Laboratory, located 11 km underground in the north-east of England, UK. BUGS utilises three types of detector to facilitate a high-sensitivity, high throughput radio-assay 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 Pb-210 and Th-234. 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 mBqkg(-1) for Th-234 and 6.6 mBqkg(-1) for Pb-210 both of which have uncovered some significant equilibrium breaks in the U-238 chain. In denser materials, where gamma-ray self-absorption increases, sensitivity is demonstrated to specific activities of 0.9 mBqkg(-1) for Ra-226, 1.1 mBqkg(-1) for Ra-228 0.3 mBqkg(-1) for Ra-224, and 8.6 mBqkg(-1) 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. 2017 Elsevier B.V. All rights reserved.