Two-Color Neutron Detection for Zero-Knowledge Nuclear Warhead Verification

We have previously proposed an inspection system for nuclear warhead verifi- cation combining 14-MeV neutron radiography with a zero-knowledge protocol. We confirmed that transmission radiography is particularly sensitive to geometric and elemental differences of inspected items. Distinguishing different isotopic compositions of the same element, how- ever, can be more challenging. In this paper, we propose an upgraded system that uses neutrons in the 100-keV range to illuminate a test object. Fissile isotopes, like plutonium- 239 and uranium-235, exposed to these neutrons undergo fission and produce neutrons in the MeV-range. These neutrons can be counted using detectors that are blind to lower en- ergy neutrons in the direct beam. We model the neutron source for this setup based on the 7Li(p,n)7Be reaction in a lithium target coupled to the open source Monte Carlo particle transport toolkit Geant4 to simulate neutron interactions in the inspected item and neutron detection in the detector bank. Preliminary results for this “two-color setup” using neutron detectors with different energy thresholds (100 keV and 500 keV) demonstrate a good intrin- sic discriminability for different materials, which could make this a promising method for a nuclear warhead verification system.