Fast-neutron multiplicity counter for active measurements of uranium oxide certified material

We developed a fast-neutron multiplicity counter (FNMC) based on stilbene and EJ-309 organic scintillators. The system can detect and discriminate correlated photon and neutron multiplets emitted by fission reactions. We used the system to estimate the fissile mass of uranium oxide samples in active interrogation mode at the Zero Power Physics Reactor of Idaho National Laboratory (INL). Two sets of certified reference material (CRM) samples were characterized. The U-235 enrichment of the first set is constant at 93.2 wt%, and the UO₂ mass ranges between 0.5 and 4 kg. The second set includes samples of increasing enrichment (from 20 wt% to 97 wt%) and constant UO₂ mass of 230 g. We used two AmLi sources to induce fission reactions in the samples. Despite the intense gamma-ray background of the UO₂ and interrogating sources, the system could measure induced fission neutrons emerging from the interrogated samples without additional shielding surrounding the sample and only relying on pulse shape discrimination to classify neutron and gamma-ray pulses. The overall neutron count rate and time-correlated counts are well correlated with the sample fissile mass. We also proved that CRM samples can be used to build a calibration curve to assay the U-235 mass of unknown samples of different mass, geometry and enrichment, with an average bias error of 8%, for U-235 mass higher than 390 g.