Year
2011
Abstract
Improved accuracy and uncertainty quantification for the mass determination of bulk unknown plutonium samples is important for international safeguards and nuclear material accountability. In typical multiplicity measurements, the rates of detected single, double and triple neutron events are used to determine the 240 Pu effective mass, the ratio of (a, n) neutrons to spontaneous fission neutrons , and sample self-multiplication or the induced fission rate caused by spontaneous fission or (a, n) neutrons in the sample. Sample multiplication is clearly affected by the shape and density of an item, and is particularly important when considering the possible range of masses and shapes (pucks, rods, spheres, and hollow spheres) that may be available in a plutonium research or weapon production environment. The Mini Epithermal Neutron Multiplicity Counter (miniENMC) utilized in this investigation was designed for high neutron detection efficiency and short die-away times, which provides enhanced statistical precision for the measurement of double and triple neutron events that is required for accurate and timely multiplicity measurements. This investigation seeks to quantify the performance of the conventional multiplicity approach on a variety of bulk plutonium metal and oxide samples. The application of Monte Carlo N-Particle Extended Transport Code (MCNPX) to improve measurement accuracy when the item shape is known is also explored.