Detection of uranium-based nuclear weapons using neutron-induced fission

The radiation emitted by weapons based entirely on highly enriched uranium can often be easily shielded. Verification of a treaty that limits the number of such weapons may require an active technique, such as interrogating the suspect assembly with an external neutron source and measuring the number of fission neutrons produced. Difficulties include distinguishing between source and fission neutrons, the variations in yield for different materials and geometries, and the possibility of non-nuclear weapons that may contain significant amounts of fissionable depleted uranium. The authors describe simple measurements that test the induced-fission technique using an isotopic Am-Li source, a novel energy-sensitive neutron detector, and several small assemblies containing /sup 235/U, /sup 238/U, lead, and polyethylene. In all cases studied, the neutron yields above the source energy are larger for the /sup 235/U assemblies than for assemblies containing only lead or depleted uranium. For more complex geometries, corrections for source transmission may be necessary.<>