The International Atomic Energy Agency is exploring the use of an Unattended Cylinder Verification Station (UCVS) to provide independent verification of the declared relative ~(235)U enrichment, ~(235)U mass and total uranium mass of the declared UF_6 cylinders moving through modern centrifuge enrichment plants. The Hybrid Enrichment Verification Array (HEVA) method is a candidate nondestructive assay method for inclusion in a UCVS. Modeling and measured data from several field campaigns have demonstrated the potential of the HEVA method to assay relative cylinder enrichment with a precision comparable to or potentially better than today's high-resolution handheld devices. The HEVA instrument is comprised of an array of sodium iodide gamma-ray detectors that measure two primary spectral components. One of these components is the traditional, direct, weakly-penetrating ~(235)U gamma-ray signature, which contains the 186-keV photopeak. The other spectral component is a nontraditional, high-energy (above ～3 MeV) gamma-ray signature, which is generated indirectly from neutrons emitted from within the UF_6 cylinder. These neutrons are more penetrating than the ~(235)U gamma-ray signature and create high-energy gamma rays through neutron capture reactions in the steel collimators surrounding the detectors and within the detector crystals. This paper will present results from Monte Carlo simulations and analyses of the HEVA method with a focus on the origins of this high-energy signature and the ability of this nontraditional signature to reveal partial-defect scenarios wherein material is missing or substituted in the interior of the cylinder.
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