Enhancing the detection sensitivity of a high-resolution β - γ coincidence spectrometer.

A high-resolution β - γ coincidence spectrometry system has been set-up and calibrated at the UK CTBT Radionuclide Laboratory (known as GBL15) at AWE. The system has been configured specifically to measure the signatures of radioxenon isotopes that can be indicative of a nuclear explosion. The high purity germanium (HPGe) and PIPSBox detectors have been placed in an ultra-low-background lead shield to reduce the background count-rate and new software allows the combination of signals from four detectors (two HPGe detectors and two silicon-based detectors) to cover a larger solid angle. Measurements of samples of radioxenon isotopes are used to realise an improved detection efficiency and background acquisitions have demonstrated the achievable detection limits to reach 1.3 mBq for 133 Xe and ≤0.3 mBq for the metastable isomers 131m Xe and 133m Xe. Due to the improved energy resolution in both photon and electron detectors, the detection sensitivity remains high in the presence of interfering signals from other radioxenon isotopes, such as those that may be present due to the significant levels of atmospheric radioxenon in parts of the world. This paper summarises the detector setup, efficiency calibration measurements and determination of the limits of detection. This work demonstrates the benefits of high-resolution coincidence detector systems for re-measurement of samples from the CTBT International Monitoring System (IMS) - improved detectability of metastable isomers 131m Xe and 133m Xe in the presence of 133 Xe, compared to the current laboratory system.