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Metal shield alarm in a nuclear quadrupole resonance/x-ray contraband detection systemMetal shield alarm in a nuclear quadrupole resonance/x-ray contraband detection system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070035295, Metal shield alarm in a nuclear quadrupole resonance/x-ray contraband detection system. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application claims the benefit of U.S. Provisional Application No. 60/635,527, filed Dec. 13, 2004, which is incorporated in its entirety as a part hereof for all purposes. TECHNICAL FIELD [0002] This invention relates to the activation of an alarm to signal the possible presence of metal shielded contraband in a detection system that combines nuclear quadrupole resonance sensors and an X-ray detection system. BACKGROUND [0003] X-rays are currently used to scan luggage in security systems. Typically, a dual-energy X-ray system is used to distinguish organic, inorganic and metal materials. Most explosives, biological agents that can be used for bioterrorism, and drugs (controlled substances) fall within the broad organic materials category. Since the X-ray system does not specifically identify chemical compositions, detection of organic materials can result in false positives and the need for further examination. [0004] The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives, drugs and other contraband has been recognized for some time; see, e.g., T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing 39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance, and NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies. Since NQR provides this specificity it can identify particular compositions, e.g. specific explosives, biological agents that can be used for bioterrorism, and drugs. [0005] A NQR detection system can have one or more coils that serve as both excitation and receive coils, or it can have separate coils that only excite and only receive. An excitation, i.e. transmit, coil of a NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals that the receive coil, i.e. sensor, detects. [0006] It can be especially advantageous to use a sensor made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a quality factor Q of the order of 10.sup.3-10.sup.6. The NQR signals have low intensity and short duration. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise ratio is proportional to the square root of Q so that the use of a HTS self-resonant coil as a sensor results in an increase in S/N by a factor of 10-100 over that of a copper coil. Therefore, the use of a high temperature superconductor coil with a large Q as the sensor provides a distinct advantage over the use of an ordinary conductor coil. [0007] A combined nuclear quadrupole resonance and X-ray detection system provides the existing detection capabilities of the X-ray system with the specific compound detection capabilities of the NQR system. Particular contraband can be unequivocally detected by NQR. This eliminates the uncertainty connected with false positives of the X-ray system. The detection of sheet explosives is one of the capabilities of the NQR system. [0008] The metal shielding of contraband such as explosives, biological agents that can be used for bioterrorism, and drugs can present a problem to both X-ray and NQR detection systems. A thick metal shield will be detected by an X-ray system and prompt further examination. However, a thin metal shield will be essentially transparent to X-rays. For a NQR system, a metal shield as thin as a 25.mu. thick aluminum foil will prevent detection by identification of a particular NQR frequency that is characteristic of a particular target substance. Sheet explosive is one type of contraband for which thin metal shielding might be used. [0009] An object of the present invention is to provide a combined nuclear quadrupole resonance and X-ray detection system with a metal shield alarm that will signal the existence of metal not detected by X-ray and of sufficient area to shield contraband, e.g. sheet explosives, from NQR detection. SUMMARY [0010] One embodiment of this invention is a detection system to scan an object that includes (a) at least one pair of sensors to detect nuclear quadrupole resonance in the object, (b) an X-ray system to determine the presence of metal in the object, and (c) an alarm that is activated when the area of metal in the object, as determined by resonance frequency shifts of the sensors, exceeds the area of the metal in the object, as determined by X-ray, by at least a pre-selected amount. [0011] The combined nuclear quadrupole resonance and X-ray contraband detection system is typically comprised of a tunnel through which the object to be scanned passes. Preferably, each sensor is placed along a side of the tunnel with the normal to the plane of the sensor perpendicular to the axis of the tunnel, each pair of sensors is arranged directly opposite one another with one sensor of a pair placed on one side of the tunnel, and the other sensor of a pair placed on the opposite side of the tunnel, and the normals to the planes of the two sensors of a pair are collinear and the planes of the sensors are parallel. [0012] Preferably, the sensors are high temperature superconductor self-resonant coils. Most preferably, the sensors are high temperature superconductor self-resonant planar coils. [0013] Preferably, the sensors only detect NQR signals, and separate transmit coils are provided that only excite nuclear quadrupole nuclei. [0014] Another embodiment of this invention is a method for scanning an object to determine the presence therein of a target substance shielded by metal, by: [0015] (a) scanning the object with X-rays to determine the area of metal contained in the object; [0016] (b) scanning the object to determine the area of metal contained in the object as measured by the resonance frequency shifts of at least one pair of nuclear quadrupole resonance sensors; [0017] (c) comparing the size of the area of metal determined in step (a) with the size of the area determined in step (b); and [0018] (d) activating an alarm if the area determined in step (b) exceeds the area determined in step (a) by at least a pre-selected amount. DETAILED DESCRIPTION [0019] This invention addresses the problem of the detection of contraband shielded by thin metallic coverings in an object being scanned in a combined nuclear quadrupole resonance and X-ray detection system. The detection system will typically have a tunnel through which the object to be scanned passes. [0020] As used herein, "tunnel" means an opening in a device that performs the activities of scanning a sample for the detection of the presence of nuclear quadrupole resonance. The sample is passed into, down the length of the longitudinal axis of, and out of the opening that forms the tunnel. The object is scanned for the detection of nuclear quadrupole resonance while it is in the tunnel. The cross-section of the tunnel, looking down its length, can have various shapes. Typically, the tunnel will have a rectangular cross-section, but the cross-section may be in other shapes such as circular, substantially circular, elliptical or polygonal. Typically, there will be a conveyor belt or other means to transport the object to be scanned through the tunnel, i.e. from one end of the tunnel to the other. [0021] A dual-energy X-ray system is used to distinguish organic, inorganic and metal materials. Thicker metallic items do not transmit X-rays, and the shapes of their opaque, two-dimensional images can aid identification. Guns, knives and other weapons can be identified in this way, even when large portions are made of plastics. Organic materials prove more of a challenge for a dual-energy X-ray detection system, and an object that includes organic material being scanned with an X-ray system may require additional examination for positive identification. The addition of nuclear quadrupole resonance detection capabilities complements those of the X-rays. NQR can identify particular compositions, e.g. specific explosives, biological agents that can be used for bioterrorism, and drugs. [0022] Contraband may, however, be shielded with metal sufficiently thin to be essentially transparent to X-rays but sufficiently thick to prevent detection of the chemical composition by nuclear quadrupole resonance sensors, and this presents a severe detection problem for the combined NQR/X-ray contraband detection system. A shield of 25.mu.-thick aluminum foil will create the type of problem described above. This invention solves the problem by providing a combined nuclear quadrupole resonance and X-ray contraband detection system with a metal shield alarm that is activated under appropriate circumstances. [0023] The combined nuclear quadrupole resonance and X-ray contraband detection system is comprised of at least one pair of sensors. As used herein, "sensors" refer to NQR detectors. Each sensor is placed so that the normal to the plane of the sensor is perpendicular to the longitudinal axis, i.e. the centerline, of the tunnel. The sensor is placed along the side of the tunnel wall. [0024] The reference to a side of a tunnel is used here in the sense of distinguishing the placement of one sensor (or excitation coil) in a certain location about the wall or perimeter of the tunnel that is different from the location in which another sensor (or excitation coil) is placed. A tunnel that has a rectangular-shaped, or even polygonal-shaped, cross-section will have flat surfaces that clearly qualify as "sides" of the tunnel in this sense. In a tunnel with a rectangular cross-section, for example, "sides" may refer to the top and bottom walls of the tunnel as well as to the two vertical walls. Even a tunnel with a circular, substantially-circular or elliptical cross-section, however, despite not having flat surfaces in the shape of its cross-section, may be thought of as having "sides" in the sense that the location in which one sensor (or excitation coil) is placed may be distinguished from the location in which another sensor (or excitation coil) is placed by reference to other features or attributes that enable distinguishing the location of one "side" from that of another. Continue reading about Metal shield alarm in a nuclear quadrupole resonance/x-ray contraband detection system... 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