Active Measurements of Uranium Oxide Certified Material Using a Fast-neutron Multiplicity Counter and Well-characterized AmLi Sources

The verification of the U-235 content in fresh fuel assemblies has traditionally been performed using the Uranium Neutron Collar (UNCL), which is based on He-3 proportional counters. Recent efforts have included the use of fast neutron detection for safeguards applications. In this approach, neutrons from induced fission are detected without the need of thermalization, thus leading to much shorter gate windows and lower rates of accidental coincidences. We developed a fast-neutron multiplicity counter (FNMC) based on 16 organic scintillators and deployed it at Idaho National Laboratory (INL) for the characterization of uranium oxide certified reference material (CRM) samples. Two Americium-lithium (AmLi) sources were used to induce fission reactions in the samples. In this work, we present the Monte Carlo model of the system, including an accurate simulation of the AmLi neutron spectrum. The amount of beryllium impurities in the AmLi source and the corresponding neutron yield was derived from the AmLi gamma-ray spectrum, measured with an HPGe detector. Simulated results are validated against experimental data. Simulated light output spectra resemble well measured ones, while simulated neutron time-correlated counts tens to underestimate measured ones by approximately 13%.