Stilbene-based Fast Neutron Multiplicity Counter for Nuclear Safeguards Applications

Traditionally, capture-based neutron multiplicity counters (i.e. He-3 based systems) are used for non-destructive assay of special nuclear material. Using capture-based detectors for non-destructive assay requires intervening moderating material that inhibits the ability to observe characteristic energy and angular signatures. Therefore, these systems rely on observing only the emitted neutron multiplicity to infer physical properties such as fissile mass, leakage multiplication, and contribution from non-fission neutrons (α-ratio). We have developed a fast neutron multiplicity counter using 24 - 5.08 cm x 5.08 cm stilbene detectors coupled to ETL 9214B photomultiplier tubes, and demonstrate that the system is sensitive to the emitted neutron energy and angular distributions, in addition to the multiplicity distribution. The system was used for passive assay of Pu-metal and Pu-oxide samples, and the detected neutron multiplicity, energy, and angular distributions are used to demonstrate that new correlated signatures exist related to the physical properties of the item. We also demonstrate the neutron cross-talk effects can be considered in order to yield more accurate estimates of the aforementioned physical properties. The final paper will present correlated signatures in energy, angle, and multiplicity that can potentially provide new quantities for characterizing special nuclear material when traditional techniques become unreliable.