The present invention relates to x-ray inspection of containers, and, more particularly, to x-ray inspection employing the detection of backscatter radiation by means of a plurality of backscatter detectors in order to derive information including spatial and material information with respect to contents of the containers.
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, or that are concealed behind a surface such as a wall. Additionally, it is desirable to obtain information regarding the geometrical and material characteristics of such objects. Conventional x-ray techniques provide measures either of attenuation, in the case of transmission techniques, or of scatter, in the case of scatter techniques.
Various methods of identifying a backscatter signal with a position within an illuminated enclosure employ scanned 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. It is desirable to obtain and process further scatter data so as to resolve a portion of the ambiguity inherent in backscatter measurements.
In accordance with one aspect of the invention, in a preferred embodiment, there is provided an inspection system for characterizing an object concealed by a concealing surface. The system has a source of penetrating radiation for emitting a beam that has an orientation and is incident upon the concealing surface at a plane of incidence. The source of penetrating radiation is characterized by a source position. The system also has a first scatter detector having a specified position with respect to the source position and beam orientation, for generating a first signal corresponding to penetrating radiation that has been scattered by the object. Additionally, the system has a second scatter detector having a field of view. The second scatter detector also has a specified position with respect to the source position and beam orientation and generates a second signal corresponding to penetrating radiation that has been scattered by the object. Finally, the system has a controller for determining an effective depth of the object with respect to the plane of incidence on the basis of at least the first and second signals.