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Abstract

An apparatus and method non-invasively interrogates an object to detect contraband. The apparatus irradiates the object with fast neutrons of energy greater than 6.7 MeV, and measures the ensuing gamma-ray spectra. Irradiation of the object is accomplished by producing a neutron beam and moving the interrogated object stepwise or continuously relative to the beam. The nuclear interactions of highly penetrating neutrons within the irradiated volume of the object give rise to energetic nuclear-species characteristic gamma-rays. The energy and intensity of the emitted gamma-rays provide information relative to the spatial and density distributions of the gamma-ray sources, i.e., the specific atomic nuclei within the object. From this information, three-dimensional images of the atomic nuclei spatial and density distributions are inferred. Such three-dimensional images allow a decision to be made as to the presence of contraband. Measurements are made using arrays of gamma-ray scintillator detectors. A decision analysis based on statistical methods (e.g., discriminant analysis) or an artificial neural system, or other expert system, facilitates a low false alarm rate. One embodiment of the apparatus includes neutron beam collimator means to define and limit the irradiation volume of the object, and auxiliary neutron shielding means to attenuate the neutrons in other than desired neutron beam direction(s). Another embodiment includes an X-ray system and combines electron density information obtained from an X-ray image of the object with the spatial and density distribution information of the gamma-ray sources in order to further enhance the decision analysis.

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