Discriminating Uranium Isotopes Using the Time-Emission Profiles of Long-Lived Delayed Neutrons

In nuclear nonproliferation and safeguards, detecting and accurately characterizing special nuclear material remains one of the greatest challenges. Uranium enrichment determination is typically achieved by measuring the ratio of characteristic γ-ray emissions from U235 and U238. Fission also produces β-delayed neutrons, which have been used in the past to determine uranium enrichment from the time dependence of the long-lived delayed-neutron emission rate. Such measurements typically use moderated He3-tube detectors. We demonstrate an alternative measurement technique that employs a fast neutron active-interrogation probe and a scintillation detector to measure the enrichment of uranium using both the buildup and decay of β-delayed-neutron emission. Instead of He3 tubes, a capture-based heterogeneous composite detector consisting of scintillating Li glass and polyvinyl toluene is constructed and used, offering a prospect to scale delayed-neutron measurements to larger detector sizes. Since the technique relies on the existing tabulated nuclear data, no calibration standards are required. It is shown that the buildup of delayed-neutron emission can be used to distinguish between uranium samples and infer the uranium enrichment level, with accuracy that rivals the method that employs the time-dependent decay of delayed-neutron emission.