Year
2017
Abstract
We developed a fast-neutron multiplicity counter (FNMC) based on organic scintillators (liquid and stilbene). The system is able to detect correlated photon and neutron multiplets emitted by fission reactions. We tested the system both in passive and active operation mode, and present the results of the experiments, both carried out at the Zero Power Physics Reactor of Idaho National Laboratory. In passive mode, we assayed two types of plutonium metal plates, i.e. PAHN and PANN, with a 240Pueff mass of 4.7 g and 25.05 g per plate, respectively. The 239Pu mass percentage was 74% for the PAHN and 95% for the PANN series. We measured neutrons emitted in coincidence by several plate assemblies, having an overall 240Pueff mass in the 4.7 g to 500 g range. In active mode, we characterized several uranium samples. Six of them were certified reference material (CRM, set 149) samples, having a constant enrichment, i.e. 93.2 wt%, and mass in the range between 0.5 and 4 kg. We used two AmLi sources (~50000 neutrons emitted per second) to induce fission in the samples. In both configurations, the system succeeded in estimating the fissile mass of an unknown sample with a statistical uncertainty lower than 1% (4-minute assay) and 4% (10-minute assay), in passive and active mode, respectively. Both in passive and active mode, the statistical uncertainty is lower than the one achieved by standard 3He counters for samples of similar composition and mass.