Active Interrogation of Highly Enriched Uranium for Detection with a Dual-Particle Imager

The detection of highly enriched uranium (HEU) presents a challenge in nuclear security and treaty verification. Detection of emitted gamma-rays is difficult because the primary emissions are low energy. Neutron emission is also low because of the small spontaneous fission probability for 235U. A well-established technique for increasing the emission from HEU is to induce fission through interrogation with photons or neutrons. The induced fission reactions will emit both prompt and delayed neutrons and gamma-rays at a greatly increased rate. While the theory of active interrogation is well understood, the breadth of experimental data is small compared to passive measurements. In this work, we use a dual-particle imaging system to detect HEU that is actively interrogated using neutrons from AmLi sources. Experiments were performed at the Device Assembly Facility located within the Nevada National Security Site using a 13.7-kg sphere and a 10.0-kg hollow sphere of HEU. The resulting emission allowed for localization of the HEU and for spectroscopy using both particle types. To further characterize the HEU samples, Rossi-alpha distributions using fast neutron singles, detected by the organic liquid scintillators, were created. The distributions showed the relative difference in decay of neutron arrival times in the system, highlighting a difference in the multiplication between the two masses.