Development of a lithium fluoride zinc sulfide based neutron multiplicity counter

The feasibility of a full-scale lithium fluoride zinc sulfide (LiF/ZnS) based neutron multiplicity counter has been demonstrated. The counter was constructed of modular neutron detecting stacks that each contain five sheets of LiF/ZnS interleaved between six sheets of wavelength shifting plastic with a photomultiplier tube on each end. Twelve such detector stacks were placed around a sample chamber in a square arrangement with lithiated high-density polyethylene blocks in the corners to reflect high-energy neutrons and capture low-energy neutrons. The final system design was optimized via modeling and small-scale test. Measuring neutrons from a ²⁵²Cf source, the counter achieved a 36% neutron detection efficiency (ϵ) and an 11.7μs neutron die-away time (τ) for a doubles figure-of-merit (ϵ²∕τ) of 109. This is the highest doubles figure-of-merit measured to-date for a ³He-free neutron multiplicity counter.