Prototype fast neutron counter for the assay of impure plutonium

For plutonium-bearing materials that undergo (α,n) reactions, such as oxides, metal with oxide coatings, salts, and residues, the present generation of thermal neutron coincidence counters often gives inaccurate assays. This is because thermal counters that measure only coincident and total neutrons do not uniquely specify the plutonium mass for samples with unknown self-multiplication and (α,n) reaction rates. Currently, a fast neutron counter using liquid scintillators and gamma-ray/neutron pulse-shape discrimination is being developed and evaluated for the nondestructive assay of impure plutonium materials. This type of counter can provide a direct solution to the measurement problems associated with impure plutonium by collecting single, double, and triple coincidence events. The goal of a fast neutron counter is to correctly determine the plutonium content of any sample without prior knowledge of its self-multiplication or (α,n) reaction rate. The present prototype meets this goal to within 3 to 5% RSD for many in-plant materials containing 10g 240Pu or more