Not So Fast: Tailoring 14-MeV Neutrons for Zero-Knowledge Isotopic Comparison of Uranium Objects

A physical zero knowledge protocol using 14 MeV neutron differential transmission radiography has previously been demonstrated to be sensitive to geometric and material diversions from a valid test configuration. Zero-knowledge verification of nuclear assemblies can be extended to include isotopic sensitivity by using sub-MeV neutrons to probe the difference in the fission cross-sections of fissile and fissionable isotopes of uranium and plutonium. Through MCNP neutronic simulations, we present the concept of a neutron collimator for experimental zero-knowledge isotopic verification using various moderator materials to reduce the energy of neutrons from 14 MeV DT generator and a unique “inverted” polyethylene collimator. This neutron beam illuminates a test object configuration, and the emission signature is captured by shielded detector arrays at wide angles. Using 2-inch cubes of depleted, low-enriched (19.75%), and highly enriched (93%) uranium, we then demonstrate the ability to distinguish between valid and invalid test object configurations.