Photofissions were induced in samples of highly enriched uranium (HEU) with masses up to 22 kg using bremsstrahlung photons from a pulsed 10-MeV electron linear accelerator (linac). Neutrons were detected between pulses by large ~3He detectors, and the data were analyzed with the Feynman variance-to-mean method. The effects of shielding materials, such as lead and polyethylene, and the variation of the counting rate with distance for several configurations were measured. For comparison, a beryllium block was inserted in the beam to produce neutrons that were also used for interrogation. Because both high-energy photons and neutrons are very penetrating, both approaches can be used to detect shielded HEU; the choice of approach depends on the details of the configuration and the shielding.
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