Abstract:
An X-ray source is disclosed comprising a source of high energy electrons that travel along a longitudinal path. Target material lies along the longitudinal path and X-ray radiation is generated due to impact of the high energy electrons with the target. Shielding material is provided around at least a portion of the target. The shielding material defines a slot extending from the target to an exterior surface of the shielding material, to allow passage of generated radiation. The slot has an axis transverse to the longitudinal path. The axis may be perpendicular longitudinal path. The shielding material may define a plurality of slots having transverse axes. The source of high energy electrons may be a linear accelerator, for example. Scanning systems incorporating such sources are also disclosed. The scanning system comprises a conveying system having a longitudinal axis and the radiation source may be positioned so that the longitudinal path forms an acute angle with respect to the longitudinal axis, to decrease the size of the scanning unit as compared to a unit where the longitudinal axis is perpendicular to the longitudinal path. The longitudinal axis may be parallel to the longitudinal path, to form a more compact scanning system. A plurality of slots may be defined in the shielding material and a corresponding number of conveying systems may be provided to examine a plurality of objects concurrently. Methods of generating radiation and methods of examining objects are also disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    Radiation sources and radiation scanning systems. More particularly, X-ray radiation sources emitting radiation transverse to a longitudinal axis of the source and X-ray scanning systems using such sources for examining the contents of an object, for example.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases, and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example.  
           [0003]    [0003]FIG. 1 is a front view of one common X-ray scanning system  10 , referred to as a line scanner. The object  12  to be inspected is conveyed through a shielded tunnel  13  between a stationary source of radiation  14 , such as X-ray radiation, and a stationary detector array  16 , by a conveying system  18 . The radiation is collimated into a fan beam  20 . Windows  21   a ,  21   b  are provided in the walls of the tunnel  13  to allow for the passage of radiation to the object  12  from the source  14  and from the object  14  to the detector array  16 . The detector array  16  may also be provided within the shielded tunnel  13 , in which case only one window  21   a  would be required. The conveyor system  18  may comprise a mechanically driven belt comprising material that causes low attenuation of the radiation. The conveyor system  18  can also comprise mechanically driven rollers, with gaps in the rollers to allow for the passage of the radiation. Shielding walls  22  surround the source  14 , the detector  16  and a portion of the conveying system  18 . Openings (not shown) are provided in the shielding walls  22  for the object to be conveyed into and out of the scanning system  10  by the conveying system  18 . A second stationary source (not shown) may be provided above the conveying system  18  and a second stationary detector (not shown) may be provided below the conveying system (or vice-a-versa), to examine the object  10  from another angle.  
           [0004]    Radiation transmitted through the object  12  is attenuated to varying degrees by the object and its contents. The attenuation of the radiation is a function of the density and atomic composition of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object  12  are generated for inspection. The images show the shape, size and varying densities of the contents.  
           [0005]    The source  14  is typically a source of X-ray radiation of about 160 KeV to about 450 KeV. The X-ray source  14  in this energy range may be an X-ray tube. As shown in FIG. 1, the X-ray source  14  must be displaced a sufficient distance from the object  12  so that the fan beam  20  intercepts entire object. The fan angle  74  may be from about 30 degrees to about 90 degrees, for example. X-ray scanning systems, such as the system  10 , are generally large.  
           [0006]    X-ray radiation of 450 KeV will not completely penetrate large objects such as cargo containers. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain plural pieces of luggage stored in the body of an airplane, may range in size from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×96×118 inches (6.1×2.4×3.0 meters). In contrast, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40×0.60 meters. Only bags that fit through the tunnel may be inspected. Scanning systems for checked luggage have tunnel openings that are only slightly larger. Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers. The low energies used in typical X-ray luggage and bag scanners, described above, are too low to penetrate through the much larger cargo containers or collections of objects. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.  
           [0007]    To inspect larger cargo containers, X-ray radiation of at least about 1 MeV range is required. Linear accelerators may be used to generate X-ray radiation in the MeV range. Linear accelerators are long (about 12-18 inches). In addition, the intensity of the radiation is greatest in a forward direction, along the longitudinal axis of the electron beam. The uniformity of the emitted radiation decreases as the angle from the forward direction is increased. To maintain beam uniformity, at average energy distortions of about 9 MeV, for example, narrow beams having an arc up to about 30 degrees tend to be used. With average energy distributions of about 3 MeV, beams having an arc up to about 65 degrees may be used. The smaller the arc, the farther the source must be in order to intercept the entire object. The length of the high energy X-ray sources and the beam arc tend to make higher energy X-ray scanning systems large. Since the space occupied by an X-ray scanning system could often be used for other important purposes, a more compact X-ray scanning system would be advantageous.  
           [0008]    [0008]FIG. 2 is a schematic axial sectional view of an example of a prior art charged particle standing wave accelerator structure  50 , referred to as a linear accelerator. The linear accelerator  50  comprises a chain of electromagnetically coupled, doughnut shaped resonant cavities  52 ,  54 , with aligned central beam apertures  56 . An electron gun  57  at one end of the chain of cavities emits an electron beam  57  through the apertures  56 . A target  60  of tungsten, for example, is provided at an opposite end of the cavities  52 ,  54 . The cavities  52 ,  54  are electromagnetically coupled together through a “side” or “coupling” cavity  61  that is coupled to each of the adjacent pair of cavities by an iris  62 . The cavities are under vacuum.  
           [0009]    Microwave power enters one of the cavities along the chain, through an iris  66  to accelerate the electron beam. The linear accelerator is excited by microwave power at a frequency near its resonant frequency, between about 1000 to about 10,000 MHz, for example. After being accelerated, the electron beam  58  strikes the target  60 , causing the emission of X-ray radiation.  
           [0010]    Movable plungers or probes  68  extend radially into one of the coupling cavities  70 . One probe  68  is shown in FIG. 2. A corresponding probe is provided in the cavity  70  behind the probe  68  and cannot be seen in this view. The probes are moved under the control of a computer program to alter the magnetic fields within the cavity, to vary the energy of the accelerating electrons. The energy of the radiation generated by the electrons as the electron beam  57  impact the target is thereby varied. Such a linear accelerator  50  is described in more detail in U.S. Pat. No. 6,366,021 B1, which is assigned to the assignee of the present invention and is incorporated by reference, herein. Linear accelerators are also described in U.S. Pat. No. 4,400,650 and U.S. Pat. No. 4,382,208, which are also assigned to the assignee of the present invention and are incorporated by reference, herein.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with one embodiment of the invention, an X-ray source is disclosed comprising a source of high energy electrons that travel along a longitudinal path. Target material lies along the longitudinal path and X-ray radiation is generated due to impact of the high energy electrons with the target. Shielding material is provided around at least a portion of the target. The shielding material defines a slot extending from the target to an exterior surface of the shielding material, to allow passage of generated radiation. The slot has an axis transverse to the longitudinal path. The axis may be perpendicular to the longitudinal path. The shielding material may define a plurality of slots extending from the target to an exterior surface of the shielding material and the axis of at least some of the plurality of slots may be perpendicular to the longitudinal path, as well.  
           [0012]    The source of high energy electrons may comprise a source of electrons and an accelerating chamber. The chamber receives electrons from the source and accelerates the electrons. The accelerating chamber may be a linear accelerator, for example. The longitudinal path is defined in part by a tube extending from the source of high energy electrons, wherein the shielding material is around at least a portion of the tube.  
           [0013]    In accordance with another embodiment, an X-ray source is disclosed comprising a housing defining a chamber to accelerate electrons and an output of the chamber. The chamber has a first longitudinal axis and the output is aligned with the first longitudinal axis to allow passage of accelerated electrons from the chamber. A tube defining a passage having a second longitudinal axis has a proximal end coupled to the output of the housing such that the second longitudinal axis is aligned with the first longitudinal axis and accelerated electrons can enter the passage. A target material is provided within the tube, wherein impact of the target material by accelerated electrons causes generation of X-ray radiation. Shielding material is provided around at least a portion of the tube around the target. The shielding material defines a slot extending from the target to an exterior surface of the shielding material. The slot allows the generated radiation to exit. The slot has an axis transverse to the first and second longitudinal axes. The axis of the slot may be perpendicular to the first and second axes. The slot may define a fan beam or a cone beam, for example. The housing may be a linear accelerator, for example.  
           [0014]    The shielding material may define a plurality of slots extending from the target to the exterior surface of the shielding material. The slots may be transverse to the first and second axes. The slots may each have a respective axis perpendicular to the first and second axes.  
           [0015]    Two shielded targets comprising target material surrounded by shielding material defining a slot through the shielding material, may be provided and a bend magnet may selectively direct electrons to one or the other target. One target may be aligned with the longitudinal axis of the housing and a second bend magnet may be provided to direct electrons from the first bend magnet to the other shielded target. When used in a scanning unit, each slot may irradiate a different side of an object being examined.  
           [0016]    In accordance with another embodiment of the invention, a system for examining an object comprises a conveyor system to move the object through the system along a first longitudinal axis and a source of radiation. The source of radiation comprises a source of high energy electrons that travel along a longitudinal path. A target material lies along the longitudinal path. The target material generates X-ray radiation when impacted by the high energy electrons. Shielding material is provided around at least a portion of the target. The shielding material defines a slot extending from the target to an exterior surface of the shielding material, to allow passage of the generated radiation. The slot has an axis transverse to the longitudinal path. The radiation source is positioned with respect to the conveying system such that radiation emitted through the slot will irradiate an object for inspection on the conveying system. The source of radiation may be on a first side of the conveying system and a detector may be provided on a second side of the conveying system to detect radiation transmitted through the object. The source of radiation may be a source of X-ray radiation.  
           [0017]    The radiation source may have a second longitudinal axis and the first longitudinal axis and the second longitudinal axis may form an acute angle. The smaller the angle between the first and second longitudinal axes, the more compact the scanning system. For example, the acute angle may be less than or equal to 45 degrees. The acute angle may be less than or equal to 10 degrees, for a more compact system. The first longitudinal axis and the second longitudinal axis may also be parallel for an even more compact system.  
           [0018]    The shielding material may define a plurality of slots to form a plurality of radiation beams transverse to the longitudinal path. A corresponding plurality of conveying systems may be provided so that the plurality of radiation beams may be used to examine a plurality of objects concurrently. A corresponding number of shutters may be coupled to the system, to selectively close one or more of the slots when not needed.  
           [0019]    In accordance with another embodiment of the invention, a scanning system is disclosed comprising two targets surrounded by shielding material defining respective slots and one or two bend magnets to selectively direct the electrons to one or the other target. The slots in the shielded targets are positioned with respect to a conveying system to irradiate different sides of an object.  
           [0020]    In accordance with another embodiment, an X-ray scanning system to examine an object is disclosed comprising a conveyor system to move the object through the system along a first longitudinal axis and an elongated X-ray source having a second longitudinal axis. The X-ray source is capable of emitting X-ray radiation with an average energy of at least 1 MeV and is supported adjacent to the conveying system such that the first longitudinal axis is parallel to the second longitudinal axis. The X-ray source may be on a first side of the conveying system and a detector may be on a second side of the conveying system, to detect X-ray radiation transmitted through the object.  
           [0021]    A method of generating X-ray radiation is also disclosed comprising colliding high energy electrons traveling along a longitudinal path with a target surrounded by shielding material to generate radiation and collimating the generated radiation into a radiation beam transverse to the longitudinal path by a slot extending from the target through the shielding material.  
           [0022]    A method of examining contents of an object with a radiation source is also disclosed also comprising colliding high energy electrons traveling along a longitudinal path with a target surrounded by shielding material to generate radiation. The generated radiation is collimated into a radiation beam transverse to the longitudinal path by a slot extending from the target through the shielding material. The object is irradiated and radiation interacting with the object is detected. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a front view of one common X-ray scanning system, referred to as a line scanner;  
         [0024]    [0024]FIG. 2 is a schematic axial sectional view of a prior art charged particle standing wave accelerator structure, referred to as a linear accelerator;  
         [0025]    [0025]FIG. 3 is a schematic representation of an X-ray radiation source, in accordance with an embodiment of the invention;  
         [0026]    [0026]FIG. 3 a  is a schematic representation of a variation of the X-ray radiation source of FIG. 3;  
         [0027]    [0027]FIG. 4 is a front, cross-sectional view of the forward end of the X-ray source of FIG. 3, through line  4 - 4 ;  
         [0028]    [0028]FIG. 5 is a top view of a cargo scanning system in accordance with an embodiment of the present invention, incorporating the X-ray source of FIGS. 3 and 4;  
         [0029]    [0029]FIG. 6 is a front view of the scanning unit of FIG. 5, showing additional details of the scanning unit;  
         [0030]    [0030]FIG. 7 is a top view of a cargo scanning unit in accordance with another embodiment of the present invention, incorporating an X-ray source having first and second collimating slots transverse to a longitudinal axis L 3  of the source;  
         [0031]    [0031]FIG. 8 is a front view of the cargo scanning unit along arrow  8  in FIG. 7;  
         [0032]    [0032]FIG. 8 a  is a front view of the cargo scanning system of FIG. 8, slowing the X-ray source and the shutters in more detail;  
         [0033]    [0033]FIGS. 9 and 10 are cross-sectional views of shielded targets including three collimating slots and four collimating slots in accordance with the invention, respectively;  
         [0034]    [0034]FIGS. 11 and 12 are front views of X-ray scanning units comprising X-ray sources with the shielded targets of FIGS. 9 and 10, respectively;  
         [0035]    [0035]FIG. 13 is a perspective view of an X-ray source in accordance with another embodiment of the invention, where an electron beam from a linear accelerator body is selectively directed to one of two shielded targets by an electromagnetic bend magnet; and  
         [0036]    [0036]FIG. 14 is a perspective view of an X-ray source in accordance with the embodiment of FIG. 13, wherein the linear accelerator body is aligned with one of the shielded targets. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    [0037]FIG. 3 is a schematic representation of a radiation source  100  in accordance with an embodiment of the invention. In this embodiment, the radiation source  100  is an X-ray source comprising a linear accelerator body  102 , indicated schematically. The linear accelerator body  102  may have substantially the same configuration as the linear accelerator  50  of FIG. 2, or other configurations known in the art. The movable plungers to vary the energy of the X-ray radiation are optional. An electron beam  103 , shown in phantom, follows a path through the linear accelerator body  102  along a longitudinal axis L 1  of the body. In the linear accelerator body  102  used in this embodiment, the target  32  of the linear accelerator  50  is removed, leaving an open output end  103 . A proximal end of a tube  106 , referred to as a drift tube, is connected to the open end  104  of the linear accelerator body  102 , in communication with and extending from the open output end. The drift tube  106  may have a diameter of from about 6 to about 10 mm, for example. The drift tube  106  may be the same material as the linear accelerator body  102 , to facilitate connection of the drift tube  106  to the linear accelerator body. The drift tube  106  and linear accelerator body  102  may be metal, for example. The drift tube  106  may be other materials, as well. Both the cavities within the linear accelerator body  102  and the interior of the drift tube are under vacuum conditions. The linear accelerator body  102  may optionally include the probes  68 , or another such mechanism, to enable the selective generation of X-ray radiation of multiple energy distributions, if desired.  
         [0038]    A target material  108  of a metal with a high atomic number and a high melting point, such as tungsten or another refractory metal, is provided at distal end of the drift tube  106 . Shielding material  110 , such as tungsten, steel or lead, is provided around the drift tube  106 , the target material  108  and may extend over a distal portion of the linear accelerator body  102 , as well. The shielding material  110  may be in the shape of a sphere, for example, and the target material  108  may be at the center of the sphere, within the drift tube  106 . The shielding material  110  may have other shapes, as well. The drift tube  106 , the target material  108  and the shielding material  110  are referred to as a “shielded target  111 ”.  
         [0039]    A collimating slot  112  extends from the end of the drift tube  106 , through the shielding material  110 , transverse to the longitudinal axis L 1  of the linear accelerator body  102 . In the embodiment of FIG. 4, the slot  112  is centered about an axis  4 - 4  that is perpendicular to the longitudinal axis L 1 . The slot  112  is shaped to collimate the X-ray beam emitted by the target material into a desired shape, such as into a fan beam or a cone beam. The slot  112  may be formed by milling the shielding material, for example. The slot  112  may have an arc θ 1  ranging from less than 1 degree to about 5 degrees to define a fan beam and ranging from about 5 degrees to about 45 degrees to define a cone beam, for example. The slot  112  may have other shapes, as well.  
         [0040]    The electron beam  104  emitted by the linear accelerator body  102  along the longitudinal axis L 1  passes through the drift tube  106  and impacts the material  108 . Bremstrahlung X-ray radiation is emitted from the target material  108  in all directions. The radiation emitted in the direction of the collimating slot  112  is collimated into the desired shape and emitted from the device  100 . The shielding material  110  absorbs radiation emitted in directions away from the collimating slot  112 .  
         [0041]    As mentioned above, while the radiation emitted in the forward direction has the highest intensity, the intensity drops rapidly as the angle from the forward direction increases. While the intensity of the radiation emitted perpendicular to the direction of the electron beam impacting the target material  108  is much less than the intensity of the radiation emitted in the forward direction, it is very uniform and is sufficient for scanning objects such as cargo containers and luggage.  
         [0042]    [0042]FIG. 4 is a front, cross-sectional view of the forward end of the X-ray source  100  through the axis  4 - 4  in FIG. 3. The collimating slot  112  may extend over any arc θ 2  up to 360 degrees, depending on the configuration of the scanning system using X-ray source  100 . The linear accelerator body  102  is shown in phantom. Scanning systems using the X-ray source  100  are discussed further below.  
         [0043]    In this embodiment, the axis  4 - 4  of the slot  112  is perpendicular to the longitudinal axis L 1  of the X-ray source  100  (and perpendicular to the direction of the beam of electrons). The axis of the slot may be at other angles transverse to the longitudinal axis L 1 , as well. For example, FIG. 3 a  shows an X-ray source  100   a  where an axis  0  of a collimating slot  112   a  is at an oblique angle with respect to the longitudinal axis L 1  of the body  102   a . The angle θO may be 80 degrees with respect to the longitudinal axis L 1 , for example.  
         [0044]    While it is preferred to provide the drift  106  or other such passage from the output  109  of the linear accelerator body  102  to facilitate placement of shielding around the target material, that is not required. The target material  108  may be positioned at the output, as shown in FIG. 2. The shielding material  110  may then be provided forward of the output  109  and the collimating slot  112  defined through the shielding material. Additional shielding material  110  may be provided around a portion of the linear accelerator body  102  proximate the output  109 , to intercept radiation emitted behind the target material  108 . Additional shielding material may be provided in a scanning system incorporating such an X-ray source, as well.  
         [0045]    [0045]FIG. 5 is a top view of a cargo scanning system  200  in accordance with an embodiment of the present invention, incorporating the X-ray source  100  of FIGS. 3 and 4. A conveyor system  202  supports and conveys a cargo container  204  through the scanning system  200 , between the X-ray source  100  and a detector  205 . The conveyor system  202  may be a mechanically driven conveyor belt, a track or mechanically driven rollers, for example. The longitudinal axis L 1  of the X-ray source  100  is parallel to a longitudinal axis L 2  of the conveyor system  202 . The collimating slot  112  of the X-ray source  100  is directed towards the cargo container  204 . Shielding walls  206  surround the source  100  and the detector  205 . The conveyor system  202  extends through openings  207  though the shielded walls to allow for the entry and exit of the cargo container  204 .  
         [0046]    [0046]FIG. 6 is a front view of the scanning unit  200  of FIG. 5, showing additional details of the scanning unit. The cargo container  204  is conveyed by the conveyor system  202  through a shielded tunnel  208 . The detector is an L-shaped detector array  205 , with a first arm  210  behind the tunnel and a second arm  212  over the top of the tunnel. (In the top view of FIG. 5, the first arm  210  of the L-shaped detector array  208  and the shielded tunnel  206  are not shown to simplify the illustration.). The tunnel  206  has a first window  214  and a second window  216  to allow for the passage of an X-ray radiation beam R, as discussed above with respect to FIG. 1. The X-ray source  100  may be positioned so that the lower portion of the X-ray radiation beam is parallel to the top of the conveyor system  202 . If the radiation beam R intercepts the conveyor system  202  and the conveyor system  202  is a belt or track, a material that causes low attenuation of radiation may be used. If the conveyor system  202  comprises rollers, a gap may be provided among the plurality of rollers, where necessary. A window may be provided in the structure supporting the conveyor system  202 , if necessary, as well. Collimators (not shown) may be provided between the cargo container  204  and the detector array  208  to block scattered radiation from reaching the detector array  205 . The conveyor system  202  may be reversed to examine a portion or the entire cargo container  204  again, or to irradiate the cargo container  204  with a different energy distribution, for example. The cargo container  204  may also be irradiated with multiple energies by rapidly cycling between two or more energy levels as the cargo container is being conveyed through the scanning unit  200 .  
         [0047]    The L-shaped detector array  205  is electrically coupled to an image processor block  218 , which is coupled to a display  220 . The image processor block  218  comprises analog-to-digital conversion and digital processing components, as is known in the art. A computer  222  is electrically coupled to and controls the operation of one or more of the X-ray source, the detector array, the conveyor system, the image processor and the display. The connections between the computer and all the components are not shown, to simplify the Figure. The computer may provide the processing functions of the image processor.  
         [0048]    As shown in FIG. 6, the collimating slot  112  and the X-ray radiation beam R are directed towards the region above the conveyor system  202 , to irradiate the cargo container  204 . In this example, the X-ray beam  224  has an arc θ 2  of about 70 degrees, which is enough to illuminate the entire cargo container  204 , with a small separation between the X-ray source  100  and the cargo container. To examine a standard cargo container  204  having a height of about 8 feet (2.4 meters), the X-ray source  100  may be about 0.9 meters from the cargo container on the conveyor system  202 . The length and width of the cargo container  204  will not affect the desired position of the source. The width will, however, affect the energy distribution of the X-ray source  100 . In order to penetrate a standard cargo container having a width of 6-9 feet (1.8 to 2.7 meters), the energy distribution of the X-ray radiation beam R emitted by the source should be greater than about 1 MeV, as is known in the art.  
         [0049]    Since the longitudinal axis L 1  of the X-ray source  100  is parallel to the longitudinal axis L 2  of the conveyor system  202 , the X-ray scanning unit  200  of FIGS. 5 and 6 may have a shorter width W than a corresponding X-ray scanning unit  10  of the prior art. A scanning unit  200  of the present invention may therefore be more compact and take up less space than a corresponding prior art scanning unit  10  of similar energy to scan similarly sized objects, as shown in FIG. 1.  
         [0050]    While the size of the scanning unit is most compact when the longitudinal axis L 1  of the X-ray source  100  is parallel to the longitudinal axis L 2 , of the conveying system  202 , benefits may be obtained when the longitudinal axis L 1  is at an acute angle with respect to the longitudinal axis L 2 . The improvements increase as the angle decreases. Significant reductions in size may be obtained when the longitudinal axis L 1  is at an angle of 45 degrees or less with respect to the longitudinal axis L 2 . Even more of a size reduction may be obtained when the angle between the longitudinal axis L 1  and the longitudinal axis L 2  is 10 degrees or less. As mentioned above, the maximum improvement is obtained when L 2  is parallel to L 1 .  
         [0051]    [0051]FIG. 7 is a top view of a cargo scanning unit  300  in accordance with an embodiment of the present invention, incorporating an X-ray source  302  having first and second collimating slots  304 ,  306  transverse to a longitudinal axis L 3  of the source. The scanning unit  300  comprises first and second, parallel conveyor systems  308 ,  310  such as parallel conveyor belts, having parallel longitudinal axes L 4 , L 5 , respectively. One cargo container  311  is shown on the conveyor system  308  and another cargo container  313  is shown on the other conveying system  310 . The conveying systems  308 ,  310  convey the objects  311 ,  313  between the X-ray source  302  and detectors  316 ,  318 , respectively. Shielding walls surround the source  302 , the detectors  316 ,  318  and portions of the conveying shielded target of the systems  308 ,  310 . The conveying systems  308 ,  310  extend through openings in the shielding walls, to enable entry and exit of the cargo containers  311 ,  313 . The longitudinal axis L 3  of the X-ray source  302  is parallel to the longitudinal axes L 4 , L 5  of the two conveyor systems  308 ,  310 . The first collimating slot  304  is directed towards the region above the first conveyor system  308 , and the second collimating slot  306  is directed towards the region above the second conveyor system  310 .  
         [0052]    Shutters  312 ,  315  of shielding material, such as lead, steel or tungsten, may be pivotally or slidably attached to the shielding material  314 , the body of the X-ray source  302  or to the scanning unit  300 . The shutters selectively cover one or the other collimating slot  304 ,  306  when a respective side of the scanning unit  300  is not being used, as shown in more detail in FIG. 8 a . The shutters  312 ,  315  should be as close as possible to the focal point of electron beam on the target material  108 , to minimize its size.  
         [0053]    [0053]FIG. 8 is an end view along arrow  8  in FIG. 7, showing the cargo containers  311 ,  313  on each conveyor system  308 ,  310 , within shielded tunnels  320 ,  322 , respectively. Both the shutters  312 ,  315  are in open positions, allowing the exit of the radiation beams from the collimating slots  302 ,  304 . Two X-ray beams R 1 , R 2 , each being emitted by the X-ray source  100  through a collimating slot  304 ,  306 , respectively, are shown, passing through openings  324 ,  327  in the tunnels  320 ,  322 , respectively, to illuminate the cargo containers  311 ,  313 , respectively. Each X-ray beam R 1 , R 2  has an arc of about 70 degrees, as in the embodiment of FIG. 6, to fully illuminate the cargo container  311 ,  313 .  
         [0054]    [0054]FIG. 8 a  is a more detailed front view of the X-ray source  302  and the two shutters  312 ,  315 . Here, the shutters  312 ,  315  are pivotally attached to the source  302  or to the scanning unit  300  at respective points  312   a ,  315   a . The shutter  312  is an open position, so that radiation may be emitted from the collimating slot  304 . The shutter  315  is in a closed position, blocking the emission of radiation from the collimating slot  306 . To close the collimating slot  304 , the shutter  312  may be rotated about the pivot  312   a . Similarly, to open the collimating slot  306 , the shutter  315   a  may be rotated about the pivot point  315   a . A mechanism (not shown) may be coupled to the shutters  312 ,  315  to cause rotation. The mechanism may be controlled by the computer controlling operation of the system  300 , under the control of the user. As mentioned above, the shutters  312 ,  315  may also be moved along a rail in the direction of arrows A, B, respectively, to slide the shutters into and out of position to open and close each collimating slot  304 ,  315 , respectively, by a suitable mechanism. As discussed above, both collimating slots  304 ,  306  may be open at the same time to concurrently examine cargo containers on different conveyor systems.  
         [0055]    As above, the detectors  316 ,  318  are L-shaped. Openings  326 ,  328  are also provided in the far sides of the shielded tunnels  320 ,  322  to allow for passage of the radiation from the cargo containers  311 ,  313  to the detectors  316 ,  318 . Two image processors  340 ,  342  are electrically coupled to the detectors  316 ,  318  respectively. Two displays  344 ,  346  are electrically coupled to the image processors  340 ,  342 , respectively. A computer  348  controls operation of the scanning unit  300 . The cargo scanning unit  300  can examine twice as many cargo containers using a single X-ray device  302 , as in the embodiment of FIG. 6.  
         [0056]    To further increase number of cargo containers that can be examined at one time, three collimating slots  402  or four collimating slots  404  may also be provided in the shielded target material of the X-ray source  100  (FIG. 3), as shown in the cross-sectional views of the shielded targets  400 ,  403  in FIGS. 9 and 10, respectively. X-ray scanning units  410 ,  420  comprising three conveyor systems  412   a ,  412   b ,  412   c  or four conveyor systems,  422   a ,  422   b ,  422   c ,  422   d , respectively, may be constructed with the X-ray source of FIGS. 9 and 10, as shown in the front views of FIGS. 11 and 12, respectively.  
         [0057]    In these embodiments, the longitudinal axes of the X-ray sources  400 ,  403  and the three conveying systems  412   a ,  412   b ,  412   c  or the four conveying systems  422   a ,  422   b ,  422   c ,  422   d  are parallel. The arc of the beams emitted from each slot depends on the configuration of the system. The sum of the arcs of the beams cannot exceed 360 degrees. The arc of each beam in the three conveyor system  410  may be about 90 degrees to about 110 degrees, for example. The arc of each beam in the four conveyor system  410  may be about 75 degrees to about 90 degrees, for example.  
         [0058]    The arc of each beam need not be the same. For example, if each conveyor system is meant to handle different sized objects, the arcs of the respective beams directed to each conveyor system may be different. In addition, the axes of each of the slots need not be at the same angle with respect to the longitudinal axis of the X-ray source. For example, certain of the axes may be perpendicular and others at some other transverse angle. It is also noted that a single collimating slot extending 360 degrees may be used to illuminate cargo containers on all of the conveying systems, if desired. Extra shielding may then be provided in the scanning system, if needed.  
         [0059]    As above, mechanical shutters (not shown) may be provided to cover one or more of the collimating slots, as desired or required. Supporting structures for the source and the upper conveying systems, which are not shown to simplify the figures, may be readily provided by one of ordinary skill in the art.  
         [0060]    It is noted that in the lower sections of the scanning units  410 ,  420 , the L-shaped detectors  414 ,  424  have arm portions  416 ,  426  below the respective conveying systems  412   b ,  412   c ,  422   c ,  422   d.    
         [0061]    Separate image processor blocks and displays (not shown) may be provided for each conveying system in each scanning unit  410 ,  420 . Each scanning unit  410 ,  420  may be controlled by a single computer, also not shown. Other elements are common to the scanning unit  200  of FIGS. 5 and 6 and are not further discussed.  
         [0062]    [0062]FIG. 13 is a perspective view of an X-ray source  500  in accordance with another embodiment of the invention, where an electron beam from a linear accelerator body  502  is selectively directed to one of two shielded targets  504 ,  506  by an electromagnetic bend magnet  508 . A first drift tube  510  extends from the output end  511  of the linear accelerator body  502  to the bend magnet  508 . Two drift tubes  512 ,  514  extend at right angles from the bend magnet  508 , to the two shielded targets  504 ,  506 . The structure of the shielded targets  504 ,  506  may be the same as the structure of the shielded target of FIG. 3. The shielding material  520  in each shielded target has a collimating slot  522  defined therein, as described above.  
         [0063]    The two shielded targets  504 ,  506  are shown irradiating two perpendicular sides of a cargo container  530 . The remainder of the scanning unit, which may be the same as in the scanning unit of FIGS. 5 and 6, is not shown. In this embodiment, the shielded targets  504 ,  506  are positioned so that the X-ray beams emitted by the shielded targets irradiate different slices of the cargo container  530  in different parallel planes along the longitudinal axis L 5  of the cargo container  530 . This facilitates placement of the detectors (not shown) to receive X-ray radiation transmitted through the cargo container  530 , but is not required. The detectors may be L-shaped detectors, as above. In operation, the electromagnetic bend magnet, which is a well known device, is used to alternately deflect the electron beam into one or the other tube as the object is conveyed through the scanning unit.  
         [0064]    Depending on space constraints in the configuration of the scanning unit, it may be advantageous to align the linear accelerator body  502  with one of the shielded targets. FIG. 14 is a perspective view of an X-ray source  600 , comprising a linear accelerator body  602  is aligned with a first shielded target  604 . A first drift tube  606  couples the open end  608  of the linear accelerator body  602  to a first bend magnet  610 . A second drift tube  612  couples the first bend magnet  610  to the first shielded target  604 . A third drift tube  612  couples the first bend magnet  610  to a second bend magnet  614 . A fourth drift tube  616  couples the second bend magnet  614  to a second shielded target  618 . The first bend magnet  614  selectively allows the electron beam to pass to the first shielded target  604  or deflects the electron beam to the second shielded target  618 . The first bend magnet is an electromagnet. In this case, the second bend magnet  614 , which may always be on, may be a permanent magnet or an electromagnet. The configurations of the first and second shielded targets  604 ,  618  may be same as the shielded target in the embodiment of FIG. 3.  
         [0065]    The configuration of the detector or detector array may depend on the shape of the collimated radiation beam. For example, if the radiation beam is collimated into a fan beam, a one-dimensional detector array may be provided. A one dimensional detector array may comprise a single row of detector elements. If the collimated radiation beam is a cone beam, such as an asymmetric pyramidal cone beam, the detector array may be a two dimensional detector or detector comprising two or more adjacent rows of detector elements. The detector array may comprise a plurality of modules of detectors, each comprising one or more rows of detector elements supported in a housing.  
         [0066]    The L-shaped detector arrays may comprise conventional detectors. For example, the detectors may be a scintillator coupled to discrete photodiodes. The detectors may also comprise a scintillator coupled to a photomultiplier tube, for example, as is known in the art. X-ray photons impinging upon the scintillator cause the emission of light photons energies proportional to the energy of the X-ray photons. The light photons are detected by the photomultiplier tube, whose output is proportional to the energy of the detected light photons. A scintillator based detector may be particularly useful if the X-ray source selectively emits radiation having multiple energy distributions. The scintillator may be a cesium iodide scintillator, for example. Pulse Height Analysis (“PHA”) may be used to analyze the data from the detectors. The detector may also be amorphous silicon detectors available from Varian Medical Systems, Inc., Palo Alto, Calif., for example.  
         [0067]    Detectors may be positioned between the X-ray source and the cargo container to detect radiation scattered by the cargo container, in addition to or instead of detecting transmitted radiation.  
         [0068]    While the X-ray sources described above comprise from one (1) to four (4) collimating slots to form one (1) to four (4) radiation beams, additional collimating slots may be provided to form additional radiation beams. In any of the X-ray sources, the collimating slots may have the same or different arcs and define either fan beams or cone beams, or both in the same source. In addition, the transverse angle between the axis of each slot and the longitudinal axis of the X-ray source or the path of the electrons may be the same or different.  
         [0069]    The use of the term cargo container, above, encompasses pallets, which are comparably sized. In addition, while the scanning units described above are described as cargo scanning units to examine cargo containers, the scanning units may be used to examine other objects, such as luggage, bags, briefcases and the like.  
         [0070]    In addition, while the X-ray sources described above use a linear accelerator body as a source of high energy electrons, the X-ray source may use an X-ray tube or other such device, as well.  
         [0071]    One of ordinary skill in the art will recognize that other changes may be made to the embodiments described herein without departing from the scope of the invention, which is defined by the claims, below.