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 235 U and 238 U. 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 3 He tube detectors. We demonstrate a new 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 3 He tubes, a capture-based heterogeneous composite detector consisting of scintillating Li-glass and polyvinyl toluene has been 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.

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Measurements of the geometric configuration of objects and their material composition are needed for nuclear treaty verification purposes. We experimentally demonstrate a simple method based on monoenergetic fast neutron transmission to realize crude imaging of the geometric configuration of special nuclear material, confirm its fissionable content, and obtain information on its approximate fissile mass. In the experiment, we used monoenergetic neutrons from D(d, n)3He and T(d, n)4He reactions and a linear array of liquid scintillation detectors to perform spectroscopic neutron imaging of up to 13.7 kg of highly enriched uranium in a spherical geometry. We also show an example of detection of material diversion and confirm the presence of fissionable material based on the measurement of high-energy prompt fission neutrons, including estimating the quantity of material from the comparison of measured and predicted fission neutron emission rate. The combination of crude imaging and fissionable material detection and quantification in a simple approach may be attractive in certain treaty verification scenarios.

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