Patent Number: 050769936
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear-based contraband detection systems, and more particularly to an apparatus and method for accurately detecting contraband concealed within a container, such as a suitcase, truck or other object. As used herein, the term "contraband" includes, but is not limited to, explosives and illicit drugs. Diagnostic nuclear techniques in general involve use of two highly penetrating radiations (e.g., neutrons and gamma rays) which enable one to detect concealed explosives or other contraband materials. The radiations act as follows: An appropriately fashioned primary radiation excites atomic nuclei within a designated volume of an object. The excited atomic nuclei subsequently relax, emitting electromagnetic or particle radiation in the process that is characteristic of the nuclear species. The analysis of the emitted spectrum thus facilitates the detection of a particular substance within the object, e.g., explosives or illegal drugs. In other words, if the emitted spectrum includes radiation of a given energy, then the presence of a particular element within the object can be inferred. Thus, a spectrum showing characteristic radiation lines of particular intensities serves as a "signature" that identifies the presence of a particular chemical element within the object being examined. Identifying the chemical elements and/or chemical compounds within an object thus involves identifying the corresponding signatures that are present in the radiations emitted from the material. See e.g., Gozani, Active Nondestructive Assay of Nuclear Materials, United States Nuclear Regulatory Commission, NUREG-CR-0602, SAI-FM-2585 (1981). It is common practice to use neutrons as the primary radiation and to measure the ensuing gamma-ray spectra for non-intrusive diagnostic purposes. U.S. Pat. No. 3,832,545 and patent application Ser. No. 07/053,950, filed 05/26/87, for example, disclose nuclear-based explosive detection systems that make use of neutrons of mainly thermal energies. In contrast, European Patent publication EP-O-227-497-A1 discloses a nuclear-based explosive detection system wherein fast neutrons of energies from 7 to 15 million electron volts (MeV) are employed. Disadvantageously, the thermal neutron based detection systems provide, for practical purposes, primarily only one signature of the four cardinal constituents of explosives, namely the signature of nitrogen (and possibly hydrogen). The fast neutron based detection system, on the other hand, may provide signatures of all four ingredients of explosives, or other contraband, thus enhancing the interrogating power of the fast neutron contraband detection systems. (The four cardinal chemical constituents of explosives are hydrogen, carbon, nitrogen, and oxygen.) It must be observed, however, that simply obtaining the signatures of the constituent elements of a specified contraband does not necessarily indicate that such contraband is present in the object under investigation. This is because many benign materials (non-contraband) also include such elements. A great diagnostic advantage may thus be obtained when a three-dimensional image of the distribution of element densities within the interrogated body is also formed, as such image of densities may help further distinguish contraband from non-contraband. A suitable three-dimensional image for this purpose may advantageously be obtained by performing a section-by-section neutron irradiation of the object, and by performing a computer-based analysis of the energy and intensity of the gamma rays that are produced in each section. Such analysis has in the past required the judicious positioning of gamma-ray detectors around the object, as taught in Applicants' earlier patent application, Ser. No. 07/053,950, filed 05/26/87. As previously indicated, neutron interrogation of objects for the detection of contraband, e.g., explosives, is known in the art. One of the most common forms of neutron interrogation, and the only form that has yet been commercialized, is thermal neutron activation (TNA). In the TNA techniques, the object being interrogated is exposed to low energy neutrons, causing gamma rays having an energy characteristic of the element(s) within the object to be generated. The gamma rays of a particular energy are detected and counted. From such count, a determination can be made as to the abundance of nitrogen within the object being interrogated. The ability of TNA techniques to reliably detect the explosives depends greatly on the large nitrogen content and density of the explosive. Another technique known in the art for detecting explosives is fast neutron activation (FNA). FNA techniques are similar to TNA techniques in that an object being interrogated is bombarded with neutrons. However, in the case of FNA, the neutrons have a higher energy, e.g., 14 MeV, and the gamma rays they generate allow the presence of additional elements to be detected. In particular, FNA allows the presence of hydrogen, carbon, and oxygen to be detected in addition to nitrogen. The relative concentrations of all of these elements thus comprise a "signature" that further helps to identify a particular substance, i.e., contraband. A still further technique for detection of explosives involves detection of the alpha particle generated in a T(d,n).sup.4 He reaction which produces a 14 MeV neutron. The neutron and alpha particle are emitted in opposite directions. A small particle detector near the tritium target detects the alpha particle. The corresponding neutron is emitted at 180.degree. within a solid angle equal to the solid angle subtended by the alpha detector from the target. This solid angle defines a "beam" of neutrons that is used to interrogate a sample, such as a suitcase or other container. A gamma ray detector is placed near the sample, detecting gamma rays in coincidence with the alpha particles. The time difference between the alpha particle detection and gamma ray detection can provide the position of the gamma ray source along the beam. By scanning the beam, a three-dimensional image of the gamma ray sources can thus be generated. Finally, as indicated in French patent document #EP 0 227 497 A1, and a paper recently presented in the 5th Asia/Pac Aviation Seminar in Kuala Lumpur (Aug. 17-21, 1987), it is also known in the art to combine both fast and thermalized neutrons in the same detection system. As indicated in these documents, a partially moderated 14 MeV pulsed neutron source is used with one or more well shielded germanium detector(s). Nitrogen and oxygen are determined through (n,x.gamma.) reactions during the bursts of the fast neutrons, and hydrogen and chlorine are determined between pulses through (n,.gamma.) reactions with thermalized neutrons. SUMMARY OF THE INVENTION In general and simple terms, the present invention provides a highly effective and direct manner for using neutrons to "look inside" a closed object or container, such as a cargo truck or a piece of luggage, to determine the abundance of particular atomic nuclei, i.e., elements, therewithin. Once such abundance is known, the identification of particular contraband is readily accomplished, as all contraband of interest contains specific atomic elements in identifiable proportions and densities. As with prior art approaches using nuclear techniques, the "looking inside" of the object is achieved by detecting gamma rays produced in nuclear reactions. The gamma rays have energies characteristic of the particular atomic nuclei, which nuclei represent the residuals of these nuclear processes. Unlike prior art approaches, in which the object is immersed in a bath of thermal (low energy) neutrons, thereby causing a large number of gamma ray sources to be produced in an extended volume, and thereby necessitating the use of a large number of gamma ray detectors and a rather complex processing procedure to reconstruct useful image data from the gamma ray data, the present invention uses a highly collimated short pulse of fast (high energy) neutrons to sequentially interrogate small volume elements ("voxels") of the object. In this manner, the present detection system is thus able to "see" right into a particular voxel and directly determine what elements are present therein based on the gamma rays that are detected. By looking in a controlled (and rapid) sequence into a sufficient number of voxels in this manner, a direct indication is thus obtained of the abundance of prescribed chemical elements within the object. In keeping with one aspect of the present invention, a rapid yet effective system for the reliable detection of designated types of contraband, including explosives, using pulsed fast neutron activation (PFNA), is provided. In one embodiment, the pulsed beams of fast neutrons are collimated to a very high degree, i.e., pencil beams, using the kinematics of a A(B,n)-nuclear endothermic reaction where M.sub.B &gt;&gt;M.sub.A. The pulse width is on the order of a nanosecond (preferably less). These pulsed neutrons are directed to the object under investigation and cause (n,x.gamma.) reactions (preferably with x.ident.n') in a limited small object volume that is defined by the intersection of the pencil beam and the screened object. By choosing appropriately the lateral section of the scanning neutron pencil beam, i.e., the reaction kinematics, if required augmented with an external collimation, and by measuring the time-of-flight of the interacting neutrons, a convenient subdivision of the object into a string of small volume elements, i.e., "voxels", is realized. By precisely controlling the time of occurrence including duration of the neutron burst and determining the time of gamma ray detection, it is thus possible, to determine the particular region, or voxel, from which the gamma ray is produced. Since the highly penetrating fast neutrons have a high probability for gamma ray production nuclear reactions with the elements oxygen, carbon, chlorine and nitrogen, the carbon, nitrogen, oxygen, and chlorine content of a particular voxel can advantageously be determined directly and precisely, which determination leads directly to an indication as to whether such particular voxel contains contraband. By combining this information from a substantial sample of the voxels that make up the object, the presence (or absence) of any type of contraband within the object can be inferred quantitatively with a high degree of reliability. In addition, by relaxing the fast timing condition, other chemical elements, such as hydrogen, can be determined advantageously through the neutron thermalization process and its subsequent radiative capture in the screened object. In an alternative embodiment of the present invention, the pulsed fast neutrons are produced in the A(B,n) reactions where M.sub.B .ltoreq.M.sub.A, i.e., reactions such as D(d,n), T(d,n) or Li(p,n). Subdivision of the interrogated object into voxels using this embodiment is obtained by using an external neutron beam collimator and may involve the positioning of gamma ray detectors. In a still further alternative embodiment, the interrogated object is irradiated from many directions using, e.g., multiple sources of pulsed fast neutrons, including appropriate transport of the ion beams and/or movement and/or rotation of the object. The sizes and loci of the voxels are derived in accordance with this embodiment mainly from the measurements of the time of flight of the neutrons and gamma rays. The present invention may thus be characterized as a contraband detection apparatus that includes: (1) scanning means for scanning an object under investigation with a pulsed beam of fast neutrons; (2) first detecting means for detecting gamma rays emitted from the object as a result of interactions between a neutron from the pulsed beam of fast neutrons and an atomic nucleus within the object; (3) identifying means for identifying the particular atomic element which gives rise to the detected gamma ray; (4) locating means for determining the approximate location within the object of the origin of each gamma ray detected by the detecting means; and (5) second detection means responsive to the scanning means, identifying means and locating means for detecting a distribution and concentration of at least one atomic element within the object indicative of the presence of contraband. Another embodiment of the invention may be characterized as a contraband detection system comprising: (1) means for generating a recurring short pulse of directed fast neutrons; (2) means for scanning an object under investigation for the presence of contraband with this recurring short pulse of fast neutrons, each of the fast neutrons possibly reacting with a particular atomic nucleus present within the object, thereby generating gamma rays having an energy characteristic of the particular atomic nucleus with which the fast neutrons react; (3) means for detecting the gamma rays produced by neutrons in reactions with atomic nuclei, the detection means including means for detecting the energy of a particular gamma ray and the time of its detection relative to the time of generation of the short pulse of fast neutrons; and (4) means for determining a particular volume element, or voxel, within the object from which a particular detected gamma ray originated, the detected gamma ray thereby providing a direct indication of the particular atomic nuclei in the particular voxel. Using this system, the particular atomic nuclei present in a sample of the voxels within the object advantageously provide a direct indication of the abundances and distributions of particular elements within the object. This determination, in turn, provides a direct indication of the presence or absence of contraband, as the presence of contraband is indicated by a prescribed abundance and distribution of the particular elements within the object. Further, the present invention may be characterized as a system for detecting contraband comprising: (a) means for scanning an object under investigation with a pulsed beam of fast neutrons by controllably directing the pulsed beam at a prescribed volume of the object; (b) means for detecting gamma rays having prescribed energies emitted from the prescribed volume of the object as a result of interactions between the pulsed fast neutrons and atomic nuclei of particular elements within the prescribed volume, the prescribed energies corresponding to atomic elements commonly found in contraband; and (c) means for ascertaining whether a distribution and concentration of at least one atomic element indicative of contraband exists within the prescribed volume. Using this system, the determination that contraband is present within the prescribed volume advantageously allows investigation of the object using pulsed fast neutron to be terminated, thereby reducing the amount of time required by the system to detect contraband within the object. That is, if contraband is found in a single voxel of the object, there is no need to continue searching through other voxels of the object. However, as desired and/or required, additional prescribed volumes, e.g. adjacent voxels, of the object can be similarly investigated in order to confirm the presence of contraband within the object. The invention also includes a method of detecting contraband comprising the steps of: (a) directing a pulsed beam of fast neutrons towards a prescribed volume of an object under investigation; (b) detecting gamma rays having prescribed energies emitted from the prescribed volume of the object as a result of interactions between pulsed fast neutrons and atomic nuclei of particular elements within the prescribed volume of the object, the prescribed energies corresponding to atomic elements commonly found in contraband; and (c) repeating steps (a) and (b) for a sufficiently large number of small prescribed volumes of the object under investigation so as to ascertain whether a distribution and concentration of at least one atomic element indicative of contraband exists within the object. It is a feature of the present invention to provide a contraband detection system that has improved sensitivity, i.e., an improved ability to reliably detect the elements that make up prescribed contraband, regardless of the form of the contraband or the manner in which the contraband may be arranged or hidden within the object. As a result of this improved sensitivity, the detection system of the present invention advantageously provides a higher probability of detection (PD) and a lower Probability of False Alarm (PFA) than has heretofore been possible with prior art contraband detection systems. It is another feature of the present invention to provide such a detection system wherein the requisite information upon which a contraband/non-contraband decision is based is obtained directly from scanning data obtained from the object under investigation without significant additional processing, and/or probabilistic assessments, thereby allowing a noticeable improvement in the throughput time, i.e., the time it takes to put an object through the system to determine if it contains prescribed contraband. It is still another feature of the invention to provide a contraband detection system that is available for use with more diverse types and forms of objects to be examined. For example, where the object being examined is luggage, the present invention may examine all sizes and types of luggage, from small carry-on parcels, to larger check-in luggage, to full luggage carts and other large containers. Where the object being examined is a cargo truck, all sizes of trucks or equivalent cargo-carrying vehicles can be readily examined using the system of the present invention. Another feature of the invention allows the detection of contraband to occur without using a large number of gamma-ray detectors, as has heretofore been required with nuclear-based detection systems. With prior art systems, such as is disclosed in U.S. Patent Ser. No. 07/053,950, it has been necessary to surround the object being examined with a large number of detectors so that the particular detector that sensed an emitted gamma ray could also provide some indication as to the location within the object from where the gamma-ray originated, thereby helping to form a density map of the contents of the object. With such prior art approach, it is necessary to not only determine that a gamma ray of a specified energy has been detected, but it is also necessary to know and track the particular detector (within an array of a large number of detectors) where the detection occurred. In contrast, the present invention only requires a few detectors because it is the time-of-flight measurement and the lateral size of the neutron beam that determines the origin of the gamma ray of a particular energy (and hence the location of a particular element), and this measurement is not significantly dependent upon the location of the detector. As a result, the present invention advantageously provides an image of the elemental distribution within the object directly without the need for a large detector array or complex mathematical reconstruction. Still further, it is a feature of the present invention to provide a detection system that exhibits an improved signal-to-noise ratio. This improved signal-to-noise ratio results largely from the pulsed nature of the highly collimated interrogating neutrons. That is, the collimated pulsed neutrons produce gamma rays only in desired regions or voxels of the object during a prescribed time window. Hence, the amount of background noise (i.e., gamma rays not of interest, such as those produced in the detector or in other locations or regions of the object or its surrounding environment) present in the time windows of interest is significantly reduced, thereby improving the signal-to-noise ratio. A further feature of the invention provides for the detection of contraband without the necessity of detecting associated particles, which associated particle detection is mandatory in some prior art systems. Advantageously, because associated particles, e.g., alpha particles, do not need to be detected, the intensity of the interrogating beam is not limited as it is in systems where associated particles are detected (in which systems the count rate capability of the associated particle detection channel severely limits the beam intensity). It is yet another feature of the invention to provide a reliable nuclear-based contraband detection system employing fast (high energy) neutrons that exhibits significantly reduced shielding requirements, geometry constraints and equipment specifications over prior art nuclear-based detection systems.