LiF/ZnS neutron multiplicity counter

Abstract

The availability of 3He in recent years is becoming restricted with an order of magnitude price increase for this material. Alternatives to the use of 3He for the detection of thermal neutrons are under investigation. One of the most challenging applications for 3He alternatives is in neutron multiplicity counters that provide rapid assay of samples which contain an unknown amount of plutonium in a potentially unknown configuration. With appropriate detector design that has minimal gamma-ray sensitivity and a high detection efficiency even for triple coincidence events, the neutron single, double, and triple coincidence events can be used to extract three unknown parameters such as the 240Pu-effective mass, the sample self-multiplication, and the (α,n) rate. This project is aimed at determining if commercially available 3He alternatives can satisfy this challenging application. Using MCNP modeling the best alternative identified used LiF/ZnS neutron-scintillator sheets and wavelength shifting plastic for light pipes. A four-panel demonstrator module has been constructed, tested, and compared with detailed modeling results. However, to attain that desired high-level of performance two primary design challenges must be addressed. They include building a fast electronics system and robust neutron/gamma-ray discrimination based on pulse shape analysis at high rates. A review of the current effort and the most recent findings will be presented.