Patent Number: 059303140
Section: summary

BACKGROUND OF THE INVENTION The technical field of this invention is elemental detection and imaging and, in particular, methods and apparatus for the detection of the elemental composition of objects by nuclear interaction analysis. The invention is useful in the detection of contraband concealed within cargo containers, suitcases, parcels or other objects. As used herein, the term "contraband" includes, but is not limited to, explosives, drugs, and alcohol. During the past ten years, the Federal Aviation Administration (FAA) of the US Department of Transportation has funded considerable research into the prevention of illegal transportation of explosives and drugs. One goal of this research is to create a detection system for airports that will screen passengers' luggage for explosives, as well as other contraband. Once this system is implemented, it is likely to be applied to other inspectional purposes, such as the screening of cargo containers at custom stations, ports, etc., as well. The probability of the existence of explosives in a piece of luggage at an airport is approximately 1 in 10 million. To avoid lengthy delays at airport security check points, a practical contraband detection system at an airport requires a high detection speed, e.g. 6-8 seconds per piece of luggage and an acceptable false alarm rate. The false alarm rate can be defined as m/n, where n equals the number of the suitcases that the system determines to contain contraband, and m equals the number of suitcases that, upon inspection, do not in fact contain explosives. A false alarm rate of 10-20% or less is preferable. Similar processing constraints apply to inspection of truck and rail cargo containers at border crossings and other security check points. In both applications, nondestructive detection is required. Damaging effects, such as the activation of the objects under examination, must be minimized. Furthermore, spatial resolution on the order of several centimeters in each dimension is highly desirable. Various techniques are known for detecting contraband. Metal detectors are routinely used in airports to screen carry-on luggage. While metal detectors are useful in detecting metal weapons they are not imaging systems and most often can not distinguish between weapons and other metallic objects. X-ray imaging systems provide a rudimentary view of objects within a suitcase or container, but suffer from a general inability to image low atomic weight objects (e.g., plastic weapons, explosives and drugs). Moreover, images from conventional X-ray detectors can be stymied by materials, such as metal foils or coatings, that absorb the relatively low energy X-ray radiation and thereby shield the contents from view. Further, X-ray systems determine density or average atomic number but not the existence of explosives, per se. Ideally, a method of detecting contraband should be capable of distinguishing illegal materials from the typical objects found in luggage or cargo based on distinctive characteristics of the contraband. Thus, elemental analysis of the object undergoing inspection is an important goal for state-of-the-art inspection systems. Typically, explosives have a high nitrogen content, a low carbon-to-oxygen ratio, and high nitrogen and oxygen densities. Drugs, such as cocaine and heroin, have been shown to have high carbon-to-oxygen ratios, high carbon and chlorine contents, and little nitrogen. Included within nuclear interaction analysis are nuclear emission detection techniques. Nuclear emission detection techniques are based on the realization that characteristic elemental composition data can be obtained from the induced emission of radiation, e.g. gamma-rays, or particles, from the nuclei of the atoms of an object undergoing inspection. According to these techniques a source of radiation, e.g. a particle beam, such as a neutron beam, or a source of hard X-rays or gamma rays, bombards an object under investigation, triggering the nuclei of the object to emit characteristic radiation. In these techniques, referred to generally as "nuclear emission" analyses, different contraband molecules are identified based on their unique nuclear emissions in response to such high energy interrogation. The related term nuclear fluorescence is most commonly used to describe the emission of X-ray radiation by nuclei in response to excitation by X-rays. However, for the purposes of this application nuclear fluorescence will indicate the emission of photons by nuclei in response to excitation by radiation (electromagnetic or particulate). The emissions are analyzed for characteristic energy profiles that indicate the elemental structures present in the object. Advantageously, nuclear methods can detect the general properties of contraband by identifying and localizing (imaging) the chemical constituents of an object under investigation. One technique of particular interest at present is known as "fast neutron" analysis. In this approach, fast neutrons (e.g., having energies greater than about 1 MeV, preferably greater than a few MeV) are generated and used to interrogate the object undergoing inspection. The neutrons strike the nuclei of the object and induce gamma ray emissions. Fast neutrons are used because they have high penetration capability and large activation cross-sections with elements of interest, e.g. carbon, nitrogen, and oxygen. Simple neutron spectroscopy systems merely analyze the spectrum of radiation induced by fast neutrons to detect characteristic emissions. Unfortunately, such data are often insufficient for detection of contraband when the volume of the object is large because the telltale signatures of contraband will be scrambled with the emissions from all the other contents of the object. Considerable research has been directed towards the development of position-sensitive detection systems for fast neutron and other nuclear emission analyses. Radiographic techniques can be used to construct images. By employing a two-dimensional array of detectors (or a scanning one-dimensional array) a two-dimensional distribution of neutron interaction cross-sections of the object under examination can be obtained. For greater spatial resolution, tomographic approaches can be employed (e.g., using multiple projections from orthogonal arrays of detectors) to construct a three-dimensional image of the emissions. In another approach to acquiring three dimensional data, pulsed neutron beams have been proposed for use with detector arrays, whereby the timing of the detected emissions can provide a degree of depth resolution. All of the known techniques for nuclear emission detection suffer from one or more deficiencies which make them unattractive for large scale implementation. The spatial resolution of such systems is often compromised by the need to minimize the dose to each object, the limited neutron source strength, and the desire to maintain rapid throughput of objects. Present techniques require strong sources of interrogating radiation. These sources are generally expensive and unreliable. With respect to the requirement of rapid throughput, multiple projection arrays and synchronous timing of such arrays (or pulsed neutron beams) add to the computational overhead and likewise limit throughput. Moreover scanning systems that require moving parts often introduce artifacts that degrade the spatial resolution of the system. There exists a need for better methods and systems for remote inspection of objects, in general, and for detection of contraband in containers, in particular. A simplified remote inspection system that can provide practical spatial resolution while making efficient use of an interrogating radiation source would satisfy a long-felt need in the art. SUMMARY OF THE INVENTION Methods and apparatus for detection of the elemental composition of objects by nuclear interaction analysis are disclosed employing coded aperture detection systems. Coded aperture systems provide a simplified apparatus for rapid spatial resolution of radiation produced as a result of nuclear interrogation given a relatively weak source of interrogating radiation. In one aspect of the invention, an apparatus is disclosed for analyzing radiation emitted by an object. The apparatus can include: 1) a radiation detector array for detecting at least a portion of the radiation emitted by the object in response to nuclear interrogation and for producing detection signals responsive to the radiation; 2) a coded aperture having a predetermined configuration disposed between the detector array and the object such that the emitted radiation is detected by the detector array after passage through the coded aperture; and 3) a data processor for characterizing the object based upon the detection signals from the detector array and based upon the predetermined configuration of the coded aperture. This invention further provides a method of analyzing radiation emitted by an object in response to nuclear interrogation. The method includes the steps of: 1) disposing a coded aperture in selected proximity to the object; 2) interrogating the nuclei of the object with an energy source, the interrogation resulting in emitted radiation; 3) detecting at least a portion of the emitted radiation with a detector that produces detection signals responsive to the emitted radiation, the detector being disposed so that the coded aperture is situated between the detector and the object and such that emissions are detected by the detector after passage through the coded aperture; and 4) processing the detection signals to characterize the object based upon the detected emitted radiation and based upon the predetermined configuration of the coded aperture. In a preferred embodiment, the nuclei of the object under investigation are excited by neutron bombardment. The systems and methods of the present invention are based on the discovery that coded apertures provide a simple mechanism for obtaining position-sensitive nuclear emission data from a two-dimensional array of detectors. Essentially, a coded aperture filters or encodes an image data set in a manner that allows decoding on a unit volume ("voxel") basis, i.e. a three dimensional (tomographic) reconstruction. In one embodiment, the present invention employs a fast neutron beam to bombard the object under examination, and gamma rays emanate because of neutron capture or neutron inelastic scattering with the nuclei of the object. Neutron activation usually designates only a neutron capture (n,.gamma.) reaction, in which a nucleus captures a neutron and emits a gamma ray. For the purposes of this application neutron activation describes any neutron-nucleus interaction that results in the nuclear emission of a gamma-ray. Neutron activation analysis is an analysis technique that quantitatively determines the nuclear elemental densities in the object under neutron bombardment based on the precise measurement of the neutron-induced gamma rays. According to this embodiment, different gamma-ray energy spectra correspond to the nuclei of different nuclear elements in the object; thus, the emitted gamma rays that form the energy spectra are characteristic to the nuclear elements and are called signature gamma rays. By precisely measuring these gamma-ray spectra (energy versus intensity), it is possible to determine the elemental composition of the source. The signature gammaray intensity (i.e. the number of counts) is proportional to the multiplication of the neutron interaction cross-sections with the nuclear elemental density. Typically, only signature gamma rays of high energies (E&gt;1 MeV) are considered. These gamma-rays emanate from the object under examination with very little attenuation. By providing a coded aperture between the object under investigation and the detector array, the imaging system, according to one embodiment of the invention, can use a data processor to substantially correlate particular gamma-ray spectral information with a particular unit volume or voxel based on the predetermined configuration of the coded aperture. Importantly, the sterradian subtended by a coded aperture and an associated detector array is typically large compared to a single collimated detector. Thus, coded aperture systems are superior to collimated detectors in that they utilize a large portion of the radiation emitted by the target object, while concurrently providing image data. As a consequence, the cost of the required neutron source is reduced, and small sealed tube neutron sources developed and produced for the oil exploration industry can be used rather than large expensive accelerators. Thus, a relatively inexpensive and mobile unit can be produced for field use. In sum, the invention improves the sensitivity of detection by a factor of as much as thirty as compared to previous techniques. Further, the increase in sensitivity is accomplished with improved spatial resolution. The coded aperture detection methods of the present invention can take the form of a planar imaging technique and can operate with only one projection; accordingly the system is simple and the detection time is small because no mechanical rotations are involved. The methodology is similar to planar radiography, but the reconstructed images have depth information; thus this technique has tomographic capability. The term "nuclear interrogation" is intended to encompass various techniques for interrogating the nuclei of an object undergoing inspection, including but not limited to techniques using high energy X-rays, gamma rays, neutrons and other high energy particles. The interrogation beams useful in the present invention can be of narrow or broad bandwidths and energy spectra. Moreover, the beams can be focused, collimated or divergent, as well as continuous or pulsed, depending upon the application. As used herein the term "emitted radiation" is intended to encompass gamma rays, photons and high energy particles either induced by the interrogation beam or caused by the scattering thereof. Thus, according to one embodiment of the invention, a coded aperture imaging system can utilize X-rays scattered by the object to image the object. The invention will next be described in connection with certain illustrated embodiments. Although, the illustrations that follow are directed to the application of contraband detection, it should be clear that the invention can be applied to various other remote inspection applications, including, for example, assaying of ore during mining operations, elemental analysis during metallurgy (e.g., steel making) and monitoring manufacturing processes, generally, when the homogeneity or composition of a material must be monitored. Any material having a characteristic radiation signature (e.g. resulting from characteristic relative densities of elements) as a result of inelastic scattering of fast neutrons, can be identified using the apparatus and methods described herein. Moreover, while the systems are largely described in terms of two dimensional coded aperture and detector arrays, the same principles can be applied to construct one-dimensional systems.