Characterization of GS20 and CLYC Detectors for Neutron Resonance Transmission Analysis in High Radiation Environments
Advanced reactor concepts based on the thorium fuel cycle offer several advantages over conventional uranium-fueled systems, but they also stress-test the existing NDA toolbox for international safeguards. In particular, the presence of 232U and its ~MeV gamma-emitting daughters in thorium-based spent fuel creates a harsh radiological environment that complicates gamma-based active interrogation safeguard techniques. NRTA has emerged as a promising safeguards technique due to its isotopic specificity in the epithermal range and its robustness against non-resonant shielding. However, deploying NRTA in thorium safeguards requires neutron detectors that maintain timing performance and quantitative accuracy in intense gamma fields. This paper reports a comparative characterization of two candidate detectors for portable NRTA: GS20 and CLYC. GS20 has already been demonstrated as an effective epithermal detector in portable NRTA systems but offers limited neutron--gamma discrimination. CLYC, by contrast, provides strong pulse-shape discrimination (PSD) but has a much longer scintillation decay time and includes 133Cs, whose resonances partially overlap with key actinide resonances in the epithermal region. Using a DT-driven NRTA setup with a 2 m flight path, we compare GS20 and CLYC in measurements of a 1.50 mm tungsten target under both ``clean'' conditions and in an artificially constructed high gamma-radiation environment produced by an auxiliary source as a way of emulating a highly radioactive 233U target. The results indicate that CLYC, despite its long decay time, provides significantly more precise NRTA measurements in high radiation environments than GS20. For thorium-based safeguards scenarios where 233U must be identified and quantified in the presence of intense gamma backgrounds, CLYC-like detectors with strong PSD appear to be the more reliable choice.