High Event Rate Studies on a CLYC-Based Neutron Detector Prototype

To prevent diversion of nuclear materials at facilities within the nuclear fuel cycle, a series of controls are implemented to account for material quantity and location. One particular concern is Pu in a closed fuel cycle, which separates Pu from spent fuel and can be used for a weapon. One component in material accounting is measuring neutrons, where pressurized He-3 tubes are the detector of choice, yet 3He is becoming prohibitively expensive, motivating the use of other materials for neutron counting. One isotope that has a large neutron cross-section is 6Li, and RMD has developed a scintillation material CLYC (Cs2LiYCl6) that is enriched with 6Li. CLYC can provide a similar or better sensitivity to thermal neutrons as a He-3 tube, depending on the pressure. CLYC responds to both neutrons and gamma rays, yet the material exhibits unique pulse shapes with respect to the incident radiation. The 6Li(n,a)t reaction results in an energetic alpha and triton, whereas a gamma ray excites electrons. The charged ions generate light pulses associated to some degree with the linear energy transfer of the charged particles in CLYC, and due to the differences in the excited states produced from the heavier ions compared to the electrons, the pulse shapes differ. The pulse generated from CLYC is complicated, consisting of multiple decay times ranging from tens of nanoseconds to microseconds. As an example where Pu is mixed with waste material, such as 241Am, the gamma ray event rate can be significantly high with respect to the neutron event rate for a detector system. For any gamma ray sensitive detector, such as CLYC, the pulse pileup is a concern. The existing method for gamma-neutron detection provided pulse discrimination for event rates surpassing 1 MHz with marginal results. The processing algorithm has been revised, providing a significant improvement to the gamma-neutron discrimination up to 1.7 MHz event rates. Even though a significant amount of the scintillator pulse is not processed, the energy resolution at 662 keV stays below 20% up to 1.4 MHz. These results are based on measurements done with an AmBe neutron source, while additional testing was conducted at Idaho National Laboratory with sealed Pu sources.