Abstract: A system for interrogating a package, container, vehicle, or similar examination article for the presence of nuclear material. The system typically includes a source of photo-fission energy configured to irradiate the examination article and trigger fission of a fissile or a fissionable material present in the examination article and generate a plurality of fission products, wherein at least one of the plurality of fission products produces a plurality of fission neutrons. A neutron-to-gamma-ray-converter material may be configured to capture up to all of the plurality of fission neutrons and upon capture to emit internal gamma radiation. A gamma radiation detector is typically configured to detect at least a portion of the internal gamma radiation.