A <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si81.svg" display="inline" id="d1e976"> <mml:mi mathvariant="normal">γ</mml:mi> </mml:math> – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si81.svg" display="inline" id="d1e981"> <mml:mi mathvariant="normal">γ</mml:mi> </mml:math> coincidence spectrometry detector system in the Boulby Deep Underground Laboratory

Radioactive aerosols are monitored in the atmosphere for many applications, including detecting the radionuclide signatures of clandestine nuclear testing. Current best methods involve collecting particulates onto a filter paper using a high-volume air sampler, then measuring it using high-resolution gamma ( γ )-ray spectroscopy. An underground nuclear explosion can prevent the release of many of the radionuclides produced during a nuclear explosion, so there is a drive to push the sensitivity of measurement systems to the limit, which requires a reduction in the background signal. One of the limits to low-background γ spectroscopy is the background signal associated with cosmic ray-induced radiation, which increases the spectral count-rate and thus decreases the measurement sensitivity of sample air filters. Deep underground laboratories, such as that at Boulby Mine in the UK, operate with ∼ 10 6 times lower radiation than on the surface, meaning a drastic reduction in the background count-rate so that detectors can achieve much lower (improved) detection limits. Here we provide a first look at a new detector system installed in the Boulby Underground Laboratory (Yorkshire, UK), with 1.1 km overburden. A dual high-purity germanium (HPGe) detector system built using low-background materials has been installed within a low-background lead shield. The system is designed for coincidence-based spectroscopy, which is far more selective, and can be more sensitive than traditional singles measurements. The detection limits of key radionuclides for nuclear test monitoring (such as 140 Ba) are compared with surface laboratory capabilities and demonstrate an order-of-magnitude improvement on current operational systems. The system performance is described, along with the impact of such a system on the ability to verify the Comprehensive Nuclear-Test-Ban Treaty (CTBT) through enhanced laboratory capabilities.