Abstract:
A system and a method for determining the depth of an object with respect to a surface behind which the object is concealed. The intensity of x-rays backscattered from the object is measured by at least two backscatter detectors disposed at different positions with respect to the scattering object. The depth of a scattering source within the volume penetrated by the x-rays is derived from the ratio of scattered x-rays measured by the detectors.

Description:
The present application claims priority from U.S. Provisional Application No. U.S. Provisional Application No. 60/113,412, filed Dec. 22, 1998, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     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. 
     BACKGROUND OF THE INVENTION 
     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&#39;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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of the invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 depicts a schematic cross-sectional representation of an x-ray system employing multiple backscatter detectors for obtaining depth information with respect to concealed objects in accordance with a preferred embodiment of the present invention; 
     FIG. 2 provides a schematic representation of an x-ray system employing backscatter detectors asymmetrically disposed with respect to an illuminating beam in accordance with a preferred embodiment of the present invention; 
     FIG. 3 provides a schematic representation of an x-ray system employing backscatter detectors asymmetrically disposed with respect to an illuminating beam in accordance with a further embodiment of the present invention; and 
     FIG. 4 is a schematic cross-sectional representation of an x-ray system incorporated within a self-propelled vehicle, the x-ray system employing multiple backscatter detectors for obtaining depth information with respect to objects concealed within a large enclosure, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A principle of operation of preferred embodiment of the present invention, whereby geometrical and material information with respect to a concealed object may be derived by using multiple or segmented backscatter detectors to measure the intensity of x-rays backscattered from the object, is described with reference to FIG. 1. A beam  10  of penetrating radiation is incident upon one or more objects  12  and  20  which may be concealed from view, such as by surface  30  which may be the surface of a wall or may be a surface of an enclosure or container  14 . A volume  2  posterior to surface  30  or contained within enclosure  14  may be referred to, herein, without limitation, simply as “enclosure  14 .” “Penetrating radiation” refers to electromagnetic radiation of an appropriate range of energy and intensity as to penetrate container  14  and objects  12  and  20 , and will be referred to, without limitation, in the following description as x-ray radiation. Beam  10  will similarly be referred to, without limitation, as an x-ray beam. Beam  10  is generated by a source (not shown) of penetrating radiation which may, for example, be an x-ray tube or a radioactive source. Plane  30  tangential to a point at which beam  10  penetrates surface enclosure  14  is referred to as the “plane of incidence.” 
     X-rays  10  are scattered by objects  12  and  20 , giving rise, for example, to scattered x-ray paths  16 ,  18 ,  22 ,  24 , and  26 . Backscatter detectors  3 ,  4 ,  5 , and  6  are disposed on the same side of container  14  as source  46 , with detectors  3  and  5  on one side of beam  10  and detectors  4  and  6  on the opposite side of the beam. X-rays  10  are preferably in the form of a pencil beam that is raster scanned in the plane perpendicular to the line of the detectors Other shapes of beam  10  may also be employed within the scope of the present invention. Backscatter detectors may be any detectors known in the art for detection of the penetrating radiation scattered by objects  12  and  20 , with the choice of particular detectors governed by design considerations with respect to a particular system and application. Backscatter detectors  3 ,  4 ,  5 , and  6  may include, without limitation, an array of x-ray detectors arranged in a linear or planar configuration. The detectors may be segmented scintillators or other solid state detectors, for example, or photomultipliers or liquid scintillators which may be doped with tin or other metal. The use of cesium-iodide on PIN diodes and of room-temperature CdZnTe semiconductors are examples of detector technologies which may be employed. Energy resolution of backscatter detectors  3 ,  4 ,  5 , and  6  is within the scope of the present invention and advantageously allows a determination of material characteristics of the object according to algoritms well-known in the art. 
     The position and relative sizes of backscatter detectors  3 ,  4 ,  5 , and  6  may be chosen, in accordance with preferred embodiments of the invention, to optimize the efficiency of the system in discriminating among x-rays scattered from various selected regions of the space penetrated by beam  10 , and to obtain images that enhance scattering features located at different depths into container  14 . Radiation scattered from more distant scattering sources such as object  20  will be detected preferentially by exterior detectors  5  and  6  relative to interior detectors  3  and  4  since the detected flux is substantially proportional to the solid angles (depicted in projection in the plane of the paper) designated respectively as Ω 6   far  and Ω 3   far , subtended by the respective detectors. The collection area of exterior detectors  5  and  6  may be increased relative to the collection area of the interior detectors  3  and  4  in order to enhance the magnitude of Ω 6   far  relative to Ω 3   far  for the more distant scattering sources  20 . By way of contrast, for nearer object  12 , the ratio of solid angles (depicted in projection in the plane of the paper) designated respectively as Ω 6   near  and Ω 3   near , subtended by exterior detectors  5  and  6  relative to interior detectors  3  and  4 , favors detection by the interior detectors. 
     As enclosure  14  is scanned in lateral direction  8  with respect to beam  10 , whether by motion of the enclosure on a conveyor  34 , or, equivalently, by motion of beam  10 , the contents of enclosure  14  may be imaged or otherwise processed using techniques known in the art of x-ray inspection. Images obtained using exterior detectors  5  and  6  will emphasize more distant objects  20 , whereas images obtained using interior detectors  3  and  4  will emphasize objects  16  nearer the plane of the detectors. 
     The generalization to a larger number of detectors of the principles described in the foregoing paragraph will readily be apparent to persons skilled in the art of imaging, and is within the scope of the invention as described herein and as claimed in any appended claims. 
     In accordance with an alternate embodiment of the present invention, one or more collimators  32  may be provided for restricting the field of view of particular detectors, as shown for detector  6 , thereby enhancing the selectivity of those detectors in favor of scattering originating at specified depths into enclosure  14 . 
     Referring to FIG. 2, comparison, by processor  15 , of the scattered radiation flux detected at detectors  3  and  6  disposed with lateral asymmetry with respect to beam  10  may advantageously provide a quantitative measure of the distance from the plane of the detectors to scattering object  20  making reasonable assumptions regarding the isotropy of any medium ambient to object  20  through which scattered radiation  40  and  42  propagates to the respective detectors. Scattered radiation  44  scattered from a nearby scattering source  12  may be shielded from detection by one or more of the backscatter detectors  3  and  6 . 
     As shown in FIG. 2, backscatter detectors  3  and  6  are disposed asymmetrically with respect to beam  10 . Detector  3  subtends an angle, in the plane shown, of 6.1° with respect to object  20 , whereas detector  6  subtends an angle, in the plane shown, of 5.2° with respect to object  20 . The further detector  6  gets a fraction less than 1 of the counts recorded by near counter  3 . The ratio of counts detected by the respective counters approaches unity as the distance to object  20  increases (as measured with respect to the separation between detectors  3  and  6 ). In this discussion, it is assumed, for simplicity, that propagation effects with respect to scattered beams  40  and  42  may be neglected. Knowledge of the orientation of beam  10  may allow the location of object  20  to be derived using straightforward algorithms. 
     An embodiment of source  46  of beam  10  of penetrating radiation is shown. A beam  48  of electrons emitted by cathode  50  is accelerated toward anode  52 . Electron beam  48  may be scanned with respect to anode  52  such that the orientation of beam  10  may be varied. 
     Referring now to FIG. 3, an alternate embodiment of the invention is depicted in which backscatter detectors  3  and  6  are disposed at different distances with respect to concealing surface  30 . As discussed with reference to FIG. 2, the difference in counts received by detectors  3  and  6  may be used to determine the distance between concealing surface  30  and scattering object  20 . Again, the ratio of scatter flux detected by the respective detectors approaches unity as the distance to scattering object  20  increases. 
     In accordance with a further alternate embodiment of the invention, a source  60  of scanning x-rays may be mounted on a moving platform such as a self-propelled vehicle  62 , as shown in FIG.  4 . Source  60  may include a scanning chopper wheel as known in the art for the production of a flying spot beam  64 . Vehicle  62  may be driven in direction  66  past a large object  68 , such as a truck or sea cargo container, in a manner described in detail in U.S. Pat. No. 5,764,683, which is herein incorporated by reference. Interior backscatter detectors D 1  and D 1 ′ preferentially detect radiation scattered from near scattering source A, whereas exterior backscatter detectors D 2  and D 2 ′ preferentially detect radiation scattered from far scattering source B, as described in the foregoing discussion. The respective scatter images derived from interior and exterior detector sets may be displayed, for example, as images on display devices  70  and  72 , or otherwise processed as known in the art. 
     In accordance with other embodiments of the present invention, it is possible to simultaneously measure the effective atomic number of an object, using known techniques, as well as the density of the object so as to give a more precise characterization of the object that can be obtained from each property alone. In some cases, it is possible to reduce or eliminate the effects of the objects geometry with respect to the x-ray source/detector arrangement as well as effects of interposed material. 
     Transmission of penetrating radiation through the inspected object may also be measured and combined with backscatter data to provide additional characterization of the object concealed within an enclosure. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.