Abstract:
A detection system for detecting sheets of material includes a device for moving along a path a container which can harbor a sheet of material sought to be detected; an X-ray scanner having a beam for scanning across the path of the container through a predetermined angle and a device for moving the scanner to shift the origin of the scanning beam to align during at least a portion of the scan the scanning beam with the sheet for producing a high projected density contrasted with its surroundings.

Description:
FIELD OF INVENTION 
     This invention relates to a detection system for detecting thin sheets of material, and more particular to a system for detecting sheets of organic material including contraband materials such as drugs and explosives. 
     BACKGROUND OF INVENTION 
     There are a number of different techniques for detecting objects in closed containers such as suitcases and boxes carried by airplanes which involve conveying the suitcases past an X-ray scanner. In particular, contraband such as drugs or explosive materials are sought to be detected by discerning their densities and/or atomic number using dual energy approaches. The dual energy atomic number approach relies on the fact that when an X-ray beam strikes material the energy of the beam is diminished either because of absorption (the photoelectric effect μ pe ) or because of scattering (the Compton scattering effect μ cs ) and that the probability of the photoelectric effect, μ pe , changes markedly with increased energy while the compton scattering, μ cs , does not. Since μ pe  is a function of atomic number/energy and μ es  is a function of atomic mumber, these expressions can be solved for atomic number by using two different X-ray energy levels, e.g., 40 Kev and 90 Kev. In the case of explosives the materials sought are organic, containing carbon, nitrogen and oxygen, and have an atomic number of around 7. Heavier metals such as iron and chromium often found in luggage have atomic numbers of 28 or higher, and aluminum and chlorine have atomic numbers of around 12. Therefore, there is a comfortable margin for detection of the organic explosives. See “Device and Method for Inspection of Baggage and Other Objects”, Krug et al., U.S. Pat. No. 5,319,547. 
     Density is also used to detect explosives because they typically have a density of 1.2-1.9 gm/cm 3  for military and 1-1.4 gm/cm 3  for commercial grade explosives which are well separated from the densities of other materials commonly found in luggage. Since a single dimension X-ray system can only produce a two dimensional or areal density, that is, weight per unit area related to the projected area of an object, it is not entirely reliable: the projected density is a composite of all densities in the line of the X-ray beam and one material can mask another. To overcome this and other shortcomings a three-dimensional scanner was developed. See “Three-Dimensional Reconstruction Based on a Limited Number of X-Ray Projections”, Bjorkholm et al., U.S. Pat. No. 5,442,672. 
     But even this approach is subject to failure when thin sheets of explosive or other contraband are imaged perpendicularly or transversely relative to the sheet. A sheet imaged on edge, i.e., aligned with a scanning beam, is highly contrasted and detectable but when it is crosswise or wholly perpendicular to the scanning beam its thin dimension gives a very low areal density, e.g., less than 1 gm/cm, easily obscured when combined with the other objects in the line of sight. Such sheets of material are most likely to be disposed or secreted in the broad sides of a suitcase, not in the narrower ends or top and bottom, so they are not likely to be seen on edge. The only present technique for detecting these sheets with good reliability are computerized axial tomography systems which are large, complex and expensive. 
     SUMMARY OF INVENTION 
     It is therefore an object of this invention to provide an improved detection system which can detect thin sheets of material. 
     It is a further object of this invention to provide such a detection system which can detect thin sheets of organic material. 
     It is a further object of this invention to provide such a detection system which can detect thin sheets even when they are aligned in the broad sides of a container or luggage. 
     It is a further object of this invention to provide such a detection system which is simple and inexpensive and requires no complicated solutions. 
     The invention results from the realization that a truly effective detection system capable of exposing even a thin sheet of contraband such as explosives or drugs hidden density in a container can be achieved by shifting the X-ray source as it scans so that at at least one point the X-ray beam will align with a contraband sheet in one of its possible orientations in the container producing a high contrast, highly detectable edge-on view. 
     This invention features a detection system for detecting sheets of material. There are means for moving along a path a container which can harbor a sheet of material and an X-ray scanner having a scanning beam for scanning across the path of the container through a predetermined angle. There are means for shifting the origin of the scanning beam to align during at least a portion of the scan the scanning beam with the sheet for producing a high projected density contrasted with its surroundings. 
     In a preferred embodiment the X-ray scanner may include an X-ray source and a spaced detector and the means for shifting may include a movable member for supporting the source and the detector. Alternatively, the X-ray scanner may include an X-ray source and a spaced detector and may include the means for shifting may include a movable member for supporting the source. The X-ray scanner may include an X-ray source including a plurality of individual sources and said means for shifting may include means for sequentially enabling the individual sources. The X-ray scanner may include a linear X-ray anode and said means for shifting may include means for sweeping an electron beam across the anode for generating a series of X-ray scanning beams. The X-ray scanner may include a detector for detecting X-ray energy transmitted by the sheet. The X-ray scanner may include means for determining whether the X-ray energy detected from the sheet represents an areal density within a target envelope of areal densities. The X-ray scanner may include a threshold detector for determining whether the areal density representative of the sheet exceeds a predetermined level. The detector may include a dual energy detector for detecting high and low X-ray energies. The means for determining may include a look-up table of stored areal densities within the target envelope. The X-ray scanner may include a storage device for storing areal densities representing a set of scans of the sheet. The X-ray scanner may include an envelope comparator for determining whether the areal density which exceeds the threshold represents an atomic number indicative of a sheet of the particular material sought. The X-ray scanner may include an angular response circuit for determining the angular response of a set of scans. The angular response circuit may include means for determining symmetry in the areal density of scans surrounding a scan which exceeds the threshold level to confirm the presence of a sheet. The angular response circuit may include means for determining the slope of the angular response of the areal densities of a set of scans indicative of the presence of a sheet of material. The X-ray scanner may provide a fan beam of X-ray energy. 
    
    
     DISCLOSURE OF PREFERRED EMBODIMENT 
     Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
     FIG. 1 is a schematic side elevational view of a suitcase harboring a sheet of material being imaged by an X-ray scan generally perpendicular to the sheet, wherein detection is unlikely; 
     FIG. 2 is a view similar to FIG. 1 with the sheet being imaged end-on by the X-ray scan where detection is most likely; 
     FIG. 3 is a view similar to FIGS. 1 and 2 with the sheet being imaged at an angle to two differently oriented X-ray scans wherein detection is unlikely; 
     FIG. 4 is a view similar to FIGS. 1-3 in which the X-ray source is moved to align at some point the X-ray beam with the sheet, according to this invention; 
     FIG. 5 is a view similar to FIGS. 1-4 illustrating the spacing between scans to ensure interception of a sheet material; 
     FIG. 6 is a view similar to FIGS. 1-5 illustrating the timing considerations to insure sheet interception of the X-ray scan with the movable X-ray scanner according to this invention; 
     FIG. 7 is a schematic end view of a sheet detection system according to this invention in which both the X-ray source and the detectors are moved; 
     FIG. 8 is a functional block diagram of a sheet detection system according to this invention; 
     FIG. 9 is an illustration of the low energy and high energy output waveforms of the dual energy detector of FIG. 8; 
     FIG. 10 is a graphical illustration of projected or areal densities in the target envelope stored in the look up table of FIG. 8; 
     FIG. 11 is an illustration of the variation in areal density over a number of scans of a compact mass of material sought to be detected; 
     FIG. 12 is an illustration of the variation in areal density over a number of scans of a sheet of material sought to be detected; 
     FIG. 13 is an illustration of the variation of amplitude with scan angle for a set of scans comprising a frame; 
     FIG. 14 is a view similar to FIG. 7 in which the detectors are stationary and the X-ray source is movable; 
     FIG. 15 is a view similar to FIGS. 7 and 14 in which the detectors are stationary and a plurality of X-ray sources are fired in sequence to move the X-ray scanning beam origin; and 
     FIG. 16 is a view similar to FIGS. 7,  14  and  15  in which the detectors are stationary and an X-ray anode is sequentially energized by a sweeping electron beam to move the scanning X-ray beam origin. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     There is shown in FIG. 1 a typical suitcase  10  having two hingeably connected halves  12  and  14 , a handle  16  and feet  18 . Contained in suitcase  10  is a sheet of material  20  which is to be detected. Sheet  20 , which may be drugs or an explosive, would typically have a projected or areal organic thickness of less than 0.8 gm/cm 2  when viewed by beam  22  oriented perpendicularly or at least transversely to sheet  20 . Normally a suitcase would have approximately 10 gm/cm 2  organic material or more and there would be large variations. Thus the low areal density which occurs when thin sheet  20  is viewed by a transverse X-ray beam  22  is easily hidden amongst the other material in the suitcase and is not likely to be detected. However, if an X-ray beam  24 , FIG. 2, is oriented so that it is end-on to sheet  20 , then the projected density could be more than 30 gm/cm 2  which would likely result in a detection. However, this is a very low probability occurrence even when the X-ray scanning system is a three-dimensional or Z-axis scanning system such as disclosed in U.S. Pat. No. 5,542,672, for sheet  20  may not always be oriented parallel to the broad flat sides of the suitcase but may be inclined as shown in FIG. 3 so that both X-ray beams  22  and  24  strike it transversely and neither produces the end-on high contrast view which is likely to be detected. 
     In accordance with this invention, as suitcase  10 , FIG. 4, moves along an axis either into or out of the paper, and source  26  is shifted or moved up and down in the direction of arrow  28 , at some point a beam of energy  32  from source  26  will align for an end-on view with sheet  20 . The typical threshold detection level is around 15 gm/cm 2 ; the typical X-ray beam power is from 40-150 Kev and the scan beam is typically 90°. 
     Typically the conveyor carrying the suitcases will move at a rate of 20-40 cm/sec as shown by arrow  31 , FIG.  5 . Thus the reciprocating motion of source  26 , FIG. 4, in the direction  28  must occur frequently enough so that at least one scan of sheet  20  will occur before the suitcase gets past the vertically reciprocating source. For example, scans occurring at  36  and  38 , FIG. 5, would suffice. The criteria for determining this is shown in FIG. 6 where the path  40  of source  26  is shown as a sawtooth when viewed by the suitcase  10  which is passing by source  26 . In that case, if sheet  20  has a length l then the maximum length between scans across the width of the suitcase l max  can be no more than the length of the sheet l sheet .                l   max     =         V   conv     ×     τ     time                 of                 reciprocation         2             (   1   )               or   ,                             1   τ     =       f     scan                 freq       =     v     2      lsheet                 (   2   )                                
     which typically turns out to be about 1 cycle per second for the reciprocation of the source for a conveyor that is moving at approximately 20-40 cm/sec. 
     A detection system  50 , FIG. 7, according to this invention includes some means, such as conveyor  52  driven by motor  54  for moving a container or suitcase  10  along a path past an X-ray scanner. The X-ray scanner may include a source  26  which provides a fan-shaped beam  30  and a detector  56  which includes a plurality of individual detector elements  58 . Source  26  and detector elements  58  are both mounted on a support or frame  60  which has some means for shifting or moving frame  60  up and down, for example, a rack  62  engaged with pinion  64  driven by motor  66 . In this way, at some point in the reciprocating motion  28  of frame  60  a beam of X-ray energy from source  26  will align with sheet  20 . 
     Typically detector  56  is a dual energy detector  56   a , FIG. 8, as is known which detects two different energy levels of incoming X-rays, for example, one at 40 Kev and one at 90 Kev, which are provided on lines  70  and  72 . These two signals would appear as low energy  74  and high energy  76  waveforms, FIG. 9, which are composed of, for example, 512 data points  78  from 512 individual detector cells  58 . The low energy  70  and high energy  72  signals are presented to look-up table  80 , FIG. 8, which in turn produces an organic areal density corresponding to those energy levels if they are within a target envelope. 
     The target envelope  90 , FIG. 10, which defines the values stored in look-up table  80 , is the area  92  between the organic boundary  94  obtained empircally by passing X-rays through a lucite sample, and an inorganic boundary  96  obtained by passing X-rays through an iron sample. All values between these two extremes can be considered to be a combination of some amount of lucite with some amount of iron. This is called basis vector decomposition as explained in Alvarez et al., U.S. Pat. No. 4,029,963. Low energy level  98  and high energy level  100  define a point  102  which represents the total transmitted energy of the object. The line connecting points  104  and  106  and intersecting  102  represent all of the low energy and high energy signals which have the same sum (low plus high or total energy transmitted). However, each point on this line represents a different amount of overlapping lucite and iron. Point  104  represents a low energy and high energy combination that can only be reached by a totally organic target. Point  106  can only be reached by a totally iron target. Point  102  can only be reached by a combination of lucite and iron. The output values at each location in the lookup table are those projected amounts of iron and lucite that can make up that combination of high energy and low energy. The output of  80  that goes to the threshold detector  116  of FIG. 8 is simply the lucite component and is referred to as the projected organic density. In addition Look Up Table  80  has for each point an effective atomic number which is another equivalent representation of the high energy and low energy signals. Within target envelope  90  the atomic number of the material increases from the organic boundary to the inorganic boundary as indicated by vector  108  and an increasing thickness of the material detected increases generally parallel to the organic boundary  94  as indicated by vector  110 . Within target envelope  90  different define specific materials. For example, the cross-hatched area  112  represents plastic explosives whereas section  114  represents hypochloride based drugs such as cocaine and heroin. 
     Having determined the particular areal density, this value is delivered to threshold detector  116 , FIG. 8, which determines whether the value exceeds a predetermined threshold. If it does, a threshold alarm is provided at output  118 . 
     Another alarm can be derived by determining whether the atomic number of the material detected matches that of a particular contraband or material sought to be detected. For example, envelope comparator  119  can be triggered upon the detection of an areal organic density exceeding a predetermined threshold to provide a comparison between the effective atomic number of that thresholded signal with that of the atomic numbers in the explosive sector  112 , FIG.  10 . This is accomplished by using the store frame equivalent atomic number circuit  122  which stores the equivalent atomic number output from look-up table  80  for each scan in the frame. Thus when threshold detector  116  indicates that it has seen a threshold exceeded, a signal on line  124  causes envelope comparator  119  to compare the equivalent atomic number of the signal that exceeded the threshold with the explosive sector  112  of FIG. 10, as provided by a signal on line  120  shown in FIG.  8 . If the effective atomic number of that detected signal is within the explosive sector then an atomic number alarm is provided on line  126 . In some cases the measured atomic number will need to be corrected for the background on either spatial side of the thresholded peak. This can be done because the store frame circuit contains the full scan. 
     A third alarm can be generated using an angular response circuit  130 , FIG. 8. A scan symmetry comparator  132  compares the signal from each scan in a frame, where a frame includes all the scans for one excursion of the source movable with frame  60 , FIG.  7 . Typically a massive explosive in the form of a ball or a lump, FIG. 11, has a similar areal density profile along the pixels of the detector for each of the scans. For example, a frame including seven scans, shows an areal density profile  134 - 1  through  134 - 7  for each scan. However, when the explosive is in the form of a sheet the detector produces a profile which begins low and broad  136 - 1 , becomes somewhat narrower and taller  136 - 2  in the second scan, even taller and narrower in the third scan  136 - 3 , and finally peaks sharply  136 - 4  when the edge-on view occurs. Then as the source continues to move and the edge-on view dissipates, the profile begins to drop and broaden as shown at  136 - 5 ,  136 - 6  and  136 - 7  so that the leading and lagging scans appear generally symmetrical. Scan symmetry comparator  132  compares these profiles  136 - 1 ,  136 - 2  and  136 - 3  with profiles  136 - 7 ,  136 - 6  and  136 - 5 , respectively, and if symmetry is found a symmetry alarm is provided on line  138 . A separate alarm can be generated by angular response circuit  130  using the thresholded scan slope circuit  140 . Thresholded scan slope circuit  140  calculates the slope of the scans  136 - 1  through  136 - 7  as shown in FIG. 13, where the characteristic  142  of amplitude versus scan angle is shown. If the slope at  144  as determined by slope comparator  146  has a predetermined value, for example, 1/sinθ, then a slope alarm signal is provided on line  148 . The slope alarm and symmetry alarm may be used conjunctively by means of AND circuit  150  to provide an angular response alarm on line  152  when both the slope and symmetry alarms are present. Scan symmetry comparator  132  and thresholded scan slope circuit  140  may be triggered to operate only upon the receipt of a signal on line  124  indicating that a signal has exceeded the threshold as determined by threshold detector  116 . 
     Although thus far the means for shifting the scanner to move the origin has been shown as including a frame which moves both the detectors and the source, this is not a necessary limitation of the invention. For example, as shown in FIG. 14, the detector  56  may be stationary and frame  60 a may contain only the X-ray source  26  which is driven by means of a rack  160  and pinion  162  operated by motor  164 . The detector  56  can be stationary and the X-ray source  26  may be composed of a number of individual X-ray sources  26   a-n , FIG. 15, which are fired in sequence (shifted) by firing circuit  170  operated by timer  172 . In another construction detector  56  and X-Ray source  26 ′ may be stationary and source  26 ′ may be implemented using a linear anode  180  in the face of an electron beam scanner such as CRT  182  which provides a vertically scanning electron beam  184  driven (shifted) by coils  186  operated by sweep circuit  188 . 
     Although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. 
     Other embodiments will occur to those skilled in the art and are within the following claims: