An x-ray inspection system for identifying fissile material includes one or more sources of penetrating radiation that generate first, second, and third instantaneous spectra where the object is exposed to the second only if there is no penetration of the first and the object is exposed to the third only if there is no penetration of the second. Further, the source of the second and the third spectra is a source adapted to provide single pulses. Consequently, ambient levels of radiation may be held below cabinet levels while identifying objects containing fissile material.