Abstract: A method and apparatus for detecting the mean atomic number of a mass of material, for example freight in a vehicle comprises: subjecting the mass to X-rays and determining the mean number N.sub.A passing therethrough; subjecting the mass to higher energy X-rays and determining the new mean number N.sub.B ; and determining the mean atomic number of the mass from look-up tables against the computed ratio N.sub.A /N.sub.B. The mass, such as a railway wagon, may be scanned by two linear accelerators arranged perpendicular to each other, detector arrays being disposed respectively opposite the accelerators, so that a three dimensional image can be built up of the mass.

Abstract: An industrial x-ray inspection system has detecting means comprising a linear array of scintillator crystals operable to emit light in response to incident radiation, a bundle of optical fibers arranged so that an input end thereof is positioned at the detecting means for receiving light therefrom and an output end thereof is positioned at an image intensifier or camera wherein the optical fibers are regrouped so that the aspect ratios of the input and output ends of the bundle of optical fibers match those of the detecting means and the image intensifier or camera respectively. One optical fiber is associated with each scintillator crystal.