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The use of modem, low-dose X-ray imaging systems for threat and contraband detection dates back more than twenty years. These X-ray systems generally fall into two classes: those that use a fan beam of X-rays and a linear array of detectors to create an image via a line-by-line sampling technique; and those that use a mechanically or electronically generated sweeping pencil beam (a "flying-spot") of X-rays intercepted by a single, elongated detector to create an image as a time vs. position raster scan. Both techniques have found application in systems operating at a beam energy of 160 key or less, designed for the inspection of small parcels or baggage, hand-carried luggage, palletized cargo, and the like. Other systems use linear accelerators to produce more penetrating beams in the 2 -MeVrange, for the inspection of large cargo containers and trucks; these sources are suitable only for the fan beam/detector array method. The flying-spot technique makes possible the generation of images from both transmitted and scattered X-rays. For the latter, broad-area detectors are positioned to intercept radiation that has scattered from the inspected object, and an image is generated from this detected signal on the basis of the same time vs. position raster scan that is used for the transmission image. This ability to generate images from scattered radiation has several unique advantages: (1) it is "one— sided" - i.e., an image can be Obtained even if the object is not accessible from its far side or is too thick to penetrate; (2) because the scatter signal falls of quite rapidly with increasing depth into the object, backscatter images effectively represent a "slice" of the object characteristic of its side nearest the source; this image may be useful even when a transmission image representing the same scanned area is hopelessly confused by image clutter; (3) the underlying physical phenomenon that leads to scattered radiation is the Compton effect, which is a function not only of the density of the scattering material, but also of its atomic number; the scatter image can therefore yield information about the atomic number of the object. The current work describes the development and early results from a program that extends the flying-spot technique to over-the-road vehicles and large cargo containers, and which includes algorithms designed to facilitate the automatic detection of anomalies in vehicles or containers for which previously obtained baseline data are already on file. The system employs a pair of 450kilovoltX-ray sources and their corresponding detectors to provide two transmission images and two scatter images of the inspected vehicle during a single scan. The results of laboratory testing will be presented, including the ability to penetrate to a minimum 4 inches of steel; field tests results will be presented as available. A possible future upgrade to include a forward scatter imaging modality will be discussed.
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