DOE OSTI2012
This report details the results of the work accomplished for the „Neutron Detection without Helium-3‟ project during the 2011 fiscal year. The primary focus of the project during this year was to develop models using alternatives to helium-3 to investigate the possibilities and optimizations for multiplicity counters in safeguards applications by computer simulations. The multiplicity counter with significant capability, the Epithermal Neutron Multiplicity Counter (ENMC), was used as the baseline requirement for alternative technologies, which, if a suitable alternative can be found, should also satisfy systems with less stringent requirements, including coincidence counters. The project is in collaboration with Los Alamos National Laboratory (LANL), and the baseline model of the ENMC that had been developed at LANL with helium-3 as the detector material was used as a starting point. The model was modified to support optimization investigations, and verified with the helium-3 base material. The model was then modified to incorporate boron-trifluoride (BF3) gas, which can be used as a proportional gas similar to helium-3. The BF3 gas however, cannot be operated at very high pressure, and therefore optimization were performed with lower pressures and involved changing the tube size and number. The simulation of this model is very similar in approach to the model with helium-3 where the simulation ends when the neutron is captured in the boron. The reaction products are easily detected in the proportional gas, and the simulation results of counting the number of reactions matches well with experimental data. Next, the tubes were replaced with boron-lined proportional tubes, where the neutron capture material is a thin boron lining inside the tube wall, and which use standard proportional counting gas. For this alternative, the simulation needs to extend beyond the neutron capture, and track the reaction products, which must exit the boron-lining into the counting gas. The reaction products do not always end up in the counting gas, depending on the boron-lining thickness and neutron capture depth. Methods to track the reaction products have been recently implemented into MCNPX and were verified by using several different approaches. The tube number and size were also varied for the boron-lined case. Finally, work has started on a model of an alternative using lithium-6, another material with a high cross-section for thermal neutron capture. The most promising approach is with lithium fluoride in a zinc sulfide (silver activated) scintillator medium. The neutron is captured in the lithium, and the reaction products trigger scintillation light in the zinc sulfide, which is subsequently detected by photo-multipliers by the use of light guides or fibers. The simulations were performed with the focus on determining the efficiency, which is critical when detecting 2 or 3 neutrons in coincidence, as well as the die-away time, which is a measure of the time required to capture neutrons in the system. The die away time is related to the coincident time gate needed for the system which drives the accidental coincident rate, the primary background or noise in this type of measurement. The usual figure of merit for coincidence type systems, the efficiency squared divided by the die-away time, was calculated for each variation simulated.