It is desirable to be able to determine the presence of objects, such as contraband, weapons, or explosives, that have been concealed in an enclosure, such as luggage or a shipping container. Conventional x-ray techniques provide measures either of attenuation, in the case of transmission techniques, or of scatter, in the case of scatter techniques.
The measurement of the intensity of x-rays backscattered from an object has been used extensively to give a measure of the effective atomic number of the object. Various methods of identifying a backscatter signal with a position within the illuminated object employ scanned pencil beams of x-rays, as described, for example, in U.S. Pat. Nos. 4,809,312 and 4,825,454 which are hereby incorporated herein by reference. In practice, the backscatter intensity may give only a crude measure of the atomic number of the object since the backscatter intensity is a function of several variables: the effective atomic number of the object; the object's geometry, including its distance from the x-ray source and the detectors; and the presence of material interposed between the object and the x-ray source/detector arrangement. In particular, current backscatter techniques do not provide a measure of the density of the scattering object.