A system for inspecting an enclosure. A source generates a beam of penetrating radiation that is characterized, at each instant of time, by a power spectrum of intensity as a function of energy. A first module conveys the source along the length of the enclosure while a detector, coupled so as to move in coordination with the first module, detects the beam of penetrating radiation after the beam interacts with the object and generating a detector signal. A beam shaper modifies the instantaneous power spectrum of the beam of penetrating radiation. The first module and the detector may be coupled by a gantry straddling the enclosure. Other modules may be coupled to move with the first module.