Patent Publication Number: US-2005117683-A1

Title: Multiple energy x-ray source for security applications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      The present application is a continuation-in-part of a U.S. patent application with Ser. No. 10/750,178, which is a continuation-in-part application of a U.S. patent application with Ser. No. 09/818,987 filed Mar. 27, 2001, which claims priority from a U.S. Provisional Application with Ser. No. 60/192,425, filed Mar. 28, 2000 and is a continuation-in-part application of a U.S. patent application with Ser. No. 10/156,989, filed May 29, 2002, which claims priority from a U.S. Provisional Application with Ser. No. 60/360,854, filed Mar. 1, 2002. The present application is also a continuation-in-part of a U.S. patent application with Ser. No. 10/161,037, which is a continuation-in-part of a U.S. patent application with Ser. No. 09/502,093, filed Feb. 10, 2000, and of a U.S. patent application with Ser. No. 09/919,352, filed Jul. 30, 2001. The disclosures of all of the above applications are incorporated herein, in their entirety, by reference. This application claims priority from all of the aforementioned applications. 
    
    
     TECHNICAL FIELD  
      The present invention relates to systems and methods for inspecting objects, particularly cargo in containers, trucks, and trains, using penetrating radiation corresponding to multiple spectra and observing radiation transmission, backscatter, and initiation of photon-nucleus reactions.  
     BACKGROUND ART  
      X-ray inspection of containers is well established for many purposes including the search for contraband, stolen property and the verification of the contents of shipments that cross national borders. When an object enclosed within a container is detected, various characteristics can be assessed by its interaction with penetrating radiation. If low energy x-rays (i.e., less than 500 KeV) traverse the object, the object can be assumed to not incorporate high-atomic-number fissile materials associated with a nuclear or radioactive device. Observation of backscattered radiation can give more substantive information regarding organic content.  
      Upon probing of an object opaque to low energy x-rays with high energy x-rays (i.e., in a range up to approximately 3.5 MeV), regions of dense material are both more readily penetrated and more readily traversed. Regions opaque to high energy x-rays may be unusually dense fissile material. However, a container of dense material may still shield the characteristic x-rays emitted by such material from detection.  
      A determinative test for fissile material is exposure to x-rays of sufficient energy to initiate photon-nucleus reactions where the photoneutron products are detectable. One may expose the entire object to photon-nucleus reaction initiating (i.e., photoneutron-generating) radiation. However, this approach implicates the duration and flux of the x-ray pulse and may result in ambient levels of radiation in excess of acceptable standards.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the invention, an inspection system for inspecting an object comprises at least one source of penetrating radiation for irradiating the object where each source is characterized at each instant of time by an instantaneous energy spectrum and intensity, by a first detector for detecting the penetrating radiation after interaction with the object, by a second detector for detecting products of the interaction, and by a regulator for actuating at least one source to provide penetrating radiation for irradiating the object.  
      The penetrating radiation has a first instantaneous spectrum dominated by photons of energies less than a first fiducial energy in a first instance where photons of energies less than the first fiducial energy penetrate through the object and are registered by a detector. The penetrating radiation has a second instantaneous spectrum dominated by photons of energies exceeding a second fiducial energy and less than a third fiducial energy in a second instance where photons of energies less than the first fiducial energy do not penetrate through the object. Penetrating radiation of the second instantaneous spectrum penetrating through the object is registered by at least one of the detectors. The incident penetrating radiation has a third instantaneous spectrum dominated by photons of energies exceeding a fourth fiducial energy in a third instance where photons of energies less than the third fiducial energy do not penetrate through the object. In the course of irradiating the object, an average radiation dose is maintained below a specified level.  
      In certain embodiments, the first and second detectors may be coextensive.  
      In some embodiments, there may be a transmission detector disposed distally to the inspected object with respect to at least one of the sources and a neutron detector disposed between the inspected object and at least one of the sources.  
      In other embodiments, a first source may supply penetrating radiation having the first instantaneous spectrum, a second source may supply penetrating radiation having the second instantaneous spectrum, and a third source may supply penetrating radiation having the third instantaneous spectrum.  
      In further embodiments, the second source and the third source may be linear accelerators. A radio frequency switch may selectively couple microwave energy from a microwave source to the second source and to the third source of penetrating radiation. The second source of penetrating radiation may contain a first source of microwave energy and the third source of penetrating radiation may contain a second source of microwave energy.  
      In certain embodiments, a first source of penetrating radiation may supply penetrating radiation having the first instantaneous spectrum and a second source of penetrating radiation may supply penetrating radiation having the second instantaneous spectrum or the third instantaneous spectrum. The second source may be a linear accelerator that may include a mid-energy section that generates penetrating radiation having the second instantaneous spectrum in tandem with a high-energy section that generates penetrating radiation having the third instantaneous spectrum.  
      Further, a regulating power divider/phase shifter may divide microwave energy between the mid-energy section and the high-energy section and shift the phase of a microwave signal associated with the microwave energy directed to the high-energy section relative to the phase of a microwave signal associated with the microwave energy directed to the mid-energy section.  
      In additional embodiments, the inspection system may include at least one ambient radiation monitor for creating a signal based on radiation detected outside an exclusion zone and a controller for regulating the intensity of the sources based at least on the signal. The source may be pulsed and the controller may regulate the number of beam pulses per unit time based on the signal.  
      In certain embodiments, a single source may supply penetrating radiation having the first, the second, and the third instantaneous spectrum. The single source may be a linear accelerator that may include a mid-energy section that generates penetrating radiation having the first and the second instantaneous spectrum in tandem with a high-energy section that in combination with the mid-energy section generates penetrating radiation having the third instantaneous spectrum.  
      In other embodiments, the source of penetrating radiation may include a time-variable filter that may be a rotating element having a plurality of segments of distinct spectral absorption characteristics. Each of the plurality of segments may have a wedge-shaped cross-section, and the time-variable filter may include an absorber chosen from the group of heavy elements including uranium, tungsten, and lead.  
      In still other embodiments, the inspection system may include at least one ambient radiation monitor for creating a signal based on radiation detected outside an exclusion zone and a controller for regulating the intensity of the source based at least on the signal.  
      In still further embodiments, a first source of penetrating radiation may supply one or more pulses of penetrating radiation having the third instantaneous spectrum while another source of penetrating radiation may supply continuous penetrating radiation having at least one of the first and the second instantaneous spectrum. 
    
    
     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 reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic drawing of low-energy, high-energy, and reaction-initiating spectra.  
       FIG. 2  is a schematic diagram of a multiple energy inspection system where a single linear accelerator generates the low-energy, high-energy, and reaction-initiating spectra.  
       FIG. 3  is a schematic diagram of a multiple energy inspection system where a single linear accelerator generates the high-energy and the reaction-initiating spectra and a low-energy x-ray source generates the low-energy spectrum.  
       FIG. 4  is a schematic diagram of a multiple energy inspection system where two linear accelerators powered from the same source generate the high-energy and reaction-initiating spectra respectively and a low-energy x-ray source generates the low-energy spectrum.  
       FIG. 5  is a schematic diagram of a multiple energy inspection system where two separately powered linear accelerators generate the high-energy and reaction-initiating spectra respectively and a low-energy x-ray source generates the low-energy spectrum. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS  
      This invention takes advantage of the fact that the spectra of x-rays generated by accelerating electrons into a target, as provided by individual or multiple linear accelerators (“linacs”), may be tailored to cover distinct energy ranges. Use of such distinct spectra, as produced by a linac having a Shaped Energy™ option (see U.S. Pat. No. 6,459,761, “Spectrally Shaped X-Ray Inspection System,” issuing Oct. 1, 2002, hereby incorporated by reference) may allow for material identification within dense cargo while holding leakage dose rates to cabinet level specifications. A security system may also include backscatter recognition capability for organic recognition, as described, for example, in U.S. Pat. No. 5,313,511.  
      With a higher end of the linac energy range above a threshold of 7-10 MeV so as to be adequate for generating sufficient photo-neutron flux, reliable fissile material recognition capability may be provided by neutron detectors, even if the fissile material is concealed in an enclosure made of dense material (lead or tungsten, for example) that would otherwise obscure x-ray fluorescence, for example. A dense enclosure may reduce the flux of characteristic x-rays from the isotopes commonly used for nuclear weapons (such as  235 U,  239 Pu,  238 U,  232 U, or  241 Pu) or “isotope signatures”. For these listed isotopes, one expects to detect 186.7 keV and 205.3 keV, 375 keV and 413.7 keV, 1,001 keV, or 662.4 keV and 722.5 keV x-rays. (See U.S. Provisional Patent Application 60/192,425).  
      One embodiment of the present invention provides a combination of a high-energy irradiation spectrum transmission characterization of an inspected object, along with an optional high-energy operation to initiate photon-nucleus reactions in fissile material, if present. An indication of the presence of fissile material may be unusually dense matter in cargo, which cannot be easily or at all penetrated by x-rays at lower energy. An object in cargo may be considered to be composed of unusually dense matter if the object cannot be penetrated by a high-energy x-ray beam, which for example, is generated by an electron beam with energy of 3.5 MeV. A 3.5 MeV linac provides penetration of up to 300 mm of steel equivalent.  
      Upon observation of unusually dense matter not specified in a cargo manifest, a high degree of alert justifies use of higher energy x-rays (typically in vicinity of 10 MeV) to penetrate the observed dense matter. The described embodiments allow an operator or automated system to switch rapidly to a higher energy linac operation. If fissile material is present, high-energy photoneutrons are generated and detected by the neutron detectors, which are combined with moderators.  
      To reduce stray dose delivered to surrounding objects and personnel, the higher energy mode may be run with an extremely short duty cycle, corresponding in some cases to a single pulse or to a few pulses. Such an exposure would be sufficient to detect photo-neutrons while providing an average dose acceptable for a cabinet level system. Typical duration of the pulses may be from tens of nanosecond to microseconds.  
       FIG. 1  illustrates three spectra employed in distinguishing an object composed of fissile material. Low-energy spectrum  110  is characterized as dominated by x-ray energies less than first fiducial energy F 1 . That is, half of the x-rays in spectrum  110  have energies less than F 1 . High-energy spectrum  120  is characterized as dominated by x-rays with energies above second fiducial energy F 2  and less than third fiducial energy F 3 . Photon-nucleus reaction-initiating (i.e., photoneutron-generating) spectrum  130  is characterized as dominated by x-rays with energies above fourth fiducial energy F 4 . Each of the low-energy, high-energy, and photon-nucleus reaction-initiating spectrum is further characterized by an intensity.  
      There are a number of ways to produce the three spectra. For example, the low-energy spectrum  110  may be generated by a standard x-ray tube or as part of a Shaped Energy™ system (available from American Science &amp; Engineering, Inc., Billerica, Mass.) that also generates the high-energy spectrum  120  as a filtered output. A linac may also generate the photoneutron-generating spectrum  130 , either as part of a Shaped Energy™ system or individually.  
       FIG. 2  illustrates an embodiment of an inspection system  200  employing a single linac  250  to generate three spectra—low-energy, high-energy, and photoneutron-initiating. Linac  250  includes a mid-energy section  206  and a high-energy section  207  in tandem. The sections are powered by microwave energy that is generated by microwave power source  201  and that passes through circulator  204  and waveguide  203  before being directed to either or both sections by regulated power divider/phase shifter  205 . Electrons generated by electron gun  208  powered by high voltage power supply  209  are accelerated by passage through the mid- and high-energy sections ( 206  and  207 ) and generate x-rays in striking heavy metal target  210 . The x-rays are collimated by collimator  211  before exiting linac  250 .  
      To produce x-rays corresponding to low-energy and high-energy spectra, only mid-energy section  206  is powered. Collimated x-rays leaving linac  250  pass through absorber  221 . If the x-rays pass through open pie pair  222 , a low-energy dominated spectrum results. If the x-rays pass through absorbing pie pair  223 , a high-energy dominated spectrum results. Low- and/or high-energy x-rays passing through object  213 , itself transported on carrier  224  in a direction perpendicular to the path of the x-rays, may be detected by linear detector array  214 . Backscattered low-energy x-rays may be detected by backscatter detectors  218 .  
      To produce high-energy x-rays suitable for generating photoneutrons, a regulator or controller  216  directs regulated power divider/phase shifter  205  to energize both mid-energy section  206  and high-energy section  207 . At the same time, the console  216  causes the modulator  202  to modulate the microwave power source  201  and the high voltage power supply  209  to generate pulses of photoneutron-generating x-rays. Upon passage through the absorbing pie-shaped region  223  of the absorber  221 , the x-rays impinge upon the object  213 . Should the object  213  contain fissile material, neutron detector  215  detects photoneutrons generated by reactions within the fissile material initiated by the photoneutron-generating x-rays.  
      The object  213  is initially exposed to the low-energy x-ray spectrum  110  (for example, dominated by energies less than 500 KeV). If the low-energy x-rays penetrate the object  213 , backscatter detector  218  may identify organic content in the object  213 . If the object  213  is opaque to low-energy x-rays, object  213  may next be exposed to the high-energy x-ray spectrum  120  (for example, dominated by energies greater than 700 KeV and less than 3.5 MeV). If the object is opaque to high-energy x-rays, the object may be further exposed to a single pulse or to a few pulses of approximately tens of nanoseconds to microsecond duration of photoneutron-generating spectrum  130  (for example, dominated by energies greater than 5 MeV and less than 10 MeV). The neutron products from the pulse or pulses of radiation may be detected by neutron detector  215 , which may be coextensive with a detector of transmitted or scattered x-rays. It is to be understood that detection of other products of the interaction of penetrating radiation with the object are within the scope of this invention.  
      Use of the linac  250  to generate three spectra of x-rays permits identification of fissile material without shielding in addition to the shielding  220  immediately surrounding the linac  250 . Ambient radiation measured by ambient radiation detectors  225  is held below cabinet levels by a combination of employing a spectra containing higher energy x-rays only when observations based on a lower energy spectra are inconclusive—for example, if the object  213  is not totally penetrated by low-energy x-rays or, subsequently, if the object  213  is not totally penetrated by high-energy x-rays. Even beyond restricting photoneutron-generating x-rays to the latter case, exposure is further restricted by using only one or a couple of pulses to identify fissile material.  
       FIG. 3  shows a second inspection system  300  where a low-energy spectrum is furnished by a low-energy x-ray source  317 . Low-energy transmission through the object  213  may be detected by a transmission detector  319 . Further, backscattered low-energy x-rays may be detected by backscatter detectors  218 .  
      Linac  350  generates high-energy x-rays of spectrum  120  and photoneutron-generating x-rays of spectrum  130 . Low-energy x-rays are absorbed by low energy x-ray absorber  312 . Switching between the high-energy spectrum and the photoneutron-generating spectrum is accomplished in the manner described with reference to the inspection system of  FIG. 2 .  
       FIG. 4  shows a third inspection system  400  containing separate generators of low-energy, high-energy, and photoneutron-generating x-rays. Low-energy x-rays are generated by low-energy source  317  and detected by low-energy transmission detector  319  and backscatter detector  218 . High-energy x-rays are generated by mid-energy section  206  and transmission of high-energy x-rays through object  213  detected by linear detector array  214 . Photoneutron-generating x-rays are generated by high-energy section  207 . In this embodiment, a fast radio frequency switch  405  selectively directs power from microwave source  201  either to the mid-energy section  206  or to the high-energy (i.e., photoneutron-generating) x-ray section  207 . Whereas in inspection system  300 , mid-energy and high-energy sections share electron gun  208 , microwave power supply  201 , collimator  211 , and low-energy absorber  312 , in inspection system  400 , the mid- and high-energy sections have individual electron guns and low-energy absorbers and share microwave power supply  201  and collimator  211 . Linear detection array  214  detects high-energy x-ray transmission and photoneutron detector  215  detects photoneutrons.  
       FIG. 5  shows an inspection system  500  where generators of low-energy, high-energy, and photoneutron-generating x-rays are independent of each other. Low-energy x-rays are generated by  317  and detected by detectors  319  and  218  as described for  FIGS. 3 and 4 . High-energy x-rays are generated by a mid-energy section linac  350  and photoneutron-generating x-rays by an independent high-energy section linac  360 . Photoneutron-generating x-rays may be generated for short periods of time as a single pulse or as a series of single pulses while high-energy x-rays and low-energy x-rays are continuously generated.  
      Although various exemplary embodiments of the invention are disclosed above, it should be apparent that those skilled in the art can make various changes and modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.