Passive assay of plutonium metal plates using a fast-neutron multiplicity counter

We developed a fast-neutron multiplicity counter based on organic scintillators (EJ-309 liquid and stilbene). The system detects correlated photon and neutron multiplets emitted by fission reactions, within a gate time of tens of nanoseconds. The system was used at Idaho National Laboratory to assay a variety of plutonium metal plates. A coincidence counting strategy was used to quantify the ²⁴⁰Pu effective mass of the samples. Coincident neutrons, detected within a 40-ns coincidence window, show a monotonic trend, increasing with the ²⁴⁰Pu-effective mass (in this work, we tested the 0.005–0.5 kg range). After calibration, the system estimated the ²⁴⁰Pu effective mass of an unknown sample (²⁴⁰Pu_eff>50 g) with an uncertainty lower than 1% in a 4-min assay time.