Detection of nuclear material with dual neutron — Gamma detector

Gamma-ray signatures are generally used for detecting nuclear materials. It is difficult to distinguish gammas from backgrounds and innocent radiological materials, which can result in high false alarm rate. Dual neutron-gamma detectors for detecting illegally trafficked nuclear materials have a potential to reduce the false alarm rate. Elpasolite scintillator Cs₂LiYCl₆ (CLYC) doped with Ce³⁺ presented in this work can detect both neutrons and gammas efficiently. In this work we will discuss results to show the ability of CLYC to detect thermal neutrons efficiently with very high gamma discrimination rate. Excellent energy resolution that can be obtained with CLYC is as good as 3.9 % for 662 keV (FWHM). On the other hand, the presence of ⁶Li which has an acceptable cross-section for thermal neutron capture, allows this material to detect thermal neutrons as well. In the energy spectrum, the full energy thermal neutron peak typically appears above 3 MeV gamma-equivalent energy (GEE). Thus very effective pulse height discrimination (PHD) can be implemented with these materials by rejecting gamma-ray events below 3 MeV. Apart from PHD, using pulse-shape discrimination (PSD) it is also possible to separate neutron and gamma events. Preliminary measurements using NIM modules were performed, the method frequently used for organic scintillators to estimate the gamma rejection ratio (GRR). Moreover, we have also tested this scintillator with Si-based optical sensors, such as avalanche photo diodes (APD), and solid state photo multipliers (SSPM). The results clearly show that CLYC can be used for thermal neutron detection with these devices. This gives the opportunity for developing efficient, compact, and low-cost radiation detectors that can be deployed in large numbers.