  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.
