Radiation source

Field of the Invention

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

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.

<figref idref="f0001">Fig. 1</figref> 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 21a, 21b 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 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.

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.

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 <figref idref="f0001">Fig. 1</figref>, 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 θ may be from about 30 degrees to about 90 degrees, for example. X-ray scanning systems, such as the system 10, are generally large.

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; 1 foot = 0.3048 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 x 21 x 21 inches (0.89 x 0.53 x 0.53 meters) up to about 240 x 96 x 118 inches (6.1 x 2.4 x 3.0 meters; 1 inch = 0.0254 meters). In contrast, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40 x 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.

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. A linear accelerator itself may range from about 12-18 inches. With associated equipment, the X-ray source may extend from about 30 to about 48 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 peak X-ray energy of about 9 MeV, for example, narrow beams having an arc up to about 30 degrees tend to be used. With peak energy 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.

<figref idref="f0002">Fig. 2</figref> 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 58 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. 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. Movable plungers or probes 68 extend radially into one of the coupling cavities 70. One probe 68 is shown in <figref idref="f0002">Fig. 2</figref>. 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 <patcit id="pcit0001" dnum="US6366021B1"><text>U.S. Patent No. 6,366,021 B1</text></patcit>, which is assigned to the assignee of the present invention. Linear accelerators are also described in <patcit id="pcit0002" dnum="US4400650A"><text>U.S. Patent No. 4,400,650</text></patcit> and <patcit id="pcit0003" dnum="US4382208A"><text>U.S. Patent No. 4,382,208</text></patcit>, which are also assigned to the assignee of the present invention.

<patcit id="pcit0004" dnum="US6009146A"><text>US6009146</text></patcit> discloses scanning X-ray apparatus comprising a high energy electrostatic generator and particle accelerator for producing a high energy electron beam. Programmable magnets are provided for selectively varying the trajectory of the electron beam and a target is provided for converting the electron beam into a photon beam. A group of slots are linearly and angularly offset from each other for collimating the photon beam into a pencil beam and a transmission detector is provided for detecting the collimated pencil photon beam to allow a plurality of <patcit id="pcit0005" dnum="EP0768699A"><text>EP0768699</text></patcit> discloses an X-ray tube comprising a means for generating a beam of electrons, a magnetic coil for deflecting the electron beam to focus the same onto an anode located adjacent a side wall of a tube. The electrons impinge the anode between 0-25 degrees relative to the anode surface to create x-rays and scattered electrons. A housing encloses the anode and the housing has a window through which the X-rays can pass. A barrier is provided between the anode and the window to allow X-rays to pass but to prevent scattered electrons from reaching the window. <patcit id="pcit0006" dnum="US5111494A"><text>US5111494</text></patcit> discloses a magnet for use in a drift tube of an X-ray tube. The improved magnetic core for focusing electroncs leaving the accelerating anode of an X-ray tube has rounded poles. Non-magnetic sealing material is placed between the poles and the magnetic core is sealed to the target assembly, so that no separate drift tube is required.

<patcit id="pcit0007" dnum="US4192998A"><text>US4192998</text></patcit> discloses neutron based radiotherapy apparatus comprising a linear charged particle accelerator which delivers a beam of accelerated electrons, a target intended to be bombarded by the beam of accelerated electrons for emitting neutrons under the impact of the beam, means for cooling the target, means for eliminating parasitic radiations and a collimation system for the useful beam of neutrons.

Summary of the Invention

In accordance with the present invention there is provided an X-ray source according to claim 1.

Description of the Drawings

<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a front view of one common X-ray scanning system, referred to as a line scanner;</li><li><figref idref="f0002">Fig. 2</figref> is a schematic axial sectional view of a prior art charged particle standing wave accelerator structure, referred to as a linear accelerator;</li><li><figref idref="f0003">Fig. 3</figref> is a schematic representation of an X-ray radiation source;</li><li><figref idref="f0003">Fig. 3a</figref> is a schematic representation of a variation of the X-ray radiation source of <figref idref="f0003">Fig. 3</figref>;</li><li><figref idref="f0004">Fig. 4</figref> is a front, cross-sectional view of the forward end of the X-ray source of <figref idref="f0003">Fig. 3</figref>, through line 4-4;</li><li><figref idref="f0004">Fig. 5</figref> is a top view of a cargo scanning system incorporating the X-ray source of <figref idref="f0003">Figs. 3</figref> and <figref idref="f0004">4</figref>;</li><li><figref idref="f0005">Fig. 6</figref> is a front view of the scanning unit of <figref idref="f0004">Fig. 5</figref>, showing additional details of the scanning unit;</li><li><figref idref="f0006">Fig. 7</figref> 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 L3 of the source;</li><li><figref idref="f0007">Fig. 8</figref> is a front view of the cargo scanning unit along arrow 8 in <figref idref="f0006">Fig. 7</figref>;</li><li><figref idref="f0008">Fig. 8a</figref> is a front view of the cargo scanning system of <figref idref="f0007">Fig. 8</figref>, slowing the X-ray source and the shutters in more detail;</li><li><figref idref="f0009">Figs. 9 and 10</figref> are cross-sectional views of shielded targets including three collimating slots and four collimating slots in accordance with the invention, respectively;</li><li><figref idref="f0010">Figs. 11</figref> and <figref idref="f0011">12</figref> are front views of X-ray scanning units comprising X-ray sources with the shielded targets of <figref idref="f0009">Figs. 9 and 10</figref>, respectively;</li><li><figref idref="f0012">Fig. 13</figref> is a perspective view of an X-ray source where an electron beam from a linear accelerator body is selectively directed to one of two shielded targets by an electromagnetic bend magnet; and</li><li><figref idref="f0013">Fig. 14</figref> is a perspective view of an X-ray source wherein the linear accelerator body is aligned with one of the shielded targets.</li></ul>

Description of the Preferred Embodiments

<figref idref="f0003">Fig. 3</figref> is a schematic representation of a radiation source 100. 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 <figref idref="f0002">Fig. 2</figref>, or other configurations known in the art. The movable plungers to vary the energy of the X-ray radiation are optional. An electron beam 104, shown in phantom, follows a path through the linear accelerator body 102 along a longitudinal axis L1 of the body. In the linear accelerator body 102 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 103 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.

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".

A collimating slot 112 extends from the end of the drift tube 106, through the shielding material 110, transverse to the longitudinal axis L1 of the linear accelerator body 102. In the embodiment of <figref idref="f0004">Fig. 4</figref>, the slot 112 is centered about an axis 4-4 that is perpendicular to the longitudinal axis L1. 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 θ1 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.

The electron beam 104 emitted by the linear accelerator body 102 along the longitudinal axis L1 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.

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.

<figref idref="f0004">Fig. 4</figref> is a front, cross-sectional view of the forward end of the X-ray source 100 through the axis 4-4 in <figref idref="f0003">Fig. 3</figref>. The collimating slot 112 may extend over any arc 02 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.

The axis 4-4 of the slot 112 is perpendicular to the longitudinal axis L1 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 L1, as well. For example, <figref idref="f0003">Fig. 3a</figref> shows an X-ray source 100a where an axis 0 of a collimating slot 112a is at an oblique angle with respect to the longitudinal axis L1 of the body 102a. The angle θO may be 80 degrees with respect to the longitudinal axis L1, for example.

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 <figref idref="f0002">Fig. 2</figref>. 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.

<figref idref="f0004">Fig. 5</figref> is a top view of a cargo scanning system 200 incorporating the X-ray source 100 of <figref idref="f0003">Figs. 3</figref> and <figref idref="f0004">4</figref>. 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 L1 of the X-ray source 100 is parallel to a longitudinal axis L2 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.

<figref idref="f0005">Fig. 6</figref> is a front view of the scanning unit 200 of <figref idref="f0004">Fig. 5</figref>, 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 205 is an L-shaped detector array, with a first arm 210 behind the tunnel and a second arm 212 over the top of the tunnel. (In the top view of <figref idref="f0004">Fig. 5</figref>, 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 <figref idref="f0001">Fig. 1</figref>. 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 205 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.

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.

As shown in <figref idref="f0005">Fig. 6</figref>, 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 θ2 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 peak energy of the X-ray radiation beam R emitted by the source should be greater than about 1 MeV, as is known in the art.

Since the longitudinal axis L1 of the X-ray source 100 is parallel to the longitudinal axis L2 of the conveyor system 202, the X-ray scanning unit 200 of <figref idref="f0004">Figs. 5</figref> and <figref idref="f0005">6</figref> may have a shorter width W than a corresponding X-ray scanning unit 10 of the prior art. A scanning unit 200 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 <figref idref="f0001">Fig. 1</figref>.

While the size of the scanning unit is most compact when the longitudinal axis L1 of the X-ray source 100 is parallel to the longitudinal axis L2, of the conveying system 202, benefits may be obtained when the longitudinal axis L1 is at an acute angle with respect to the longitudinal axis L2. The improvements increase as the angle decreases. Significant reductions in size may be obtained when the longitudinal axis L1 is at an angle of 45 degrees or less with respect to the longitudinal axis L2. Even more of a size reduction may be obtained when the angle between the longitudinal axis L1 and the longitudinal axis L2 is 10 degrees or less. As mentioned above, the maximum improvement is obtained when L2 is parallel to L1.

<figref idref="f0006">Fig. 7</figref> 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 L3 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 L4, L5, 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 L3 of the X-ray source 302 is parallel to the longitudinal axes L4, L5 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.

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 <figref idref="f0008">Fig. 8a</figref>. 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.

<figref idref="f0007">Fig. 8</figref> is an end view along arrow 8 in <figref idref="f0006">Fig. 7</figref>, 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 304, 306. Two X-ray beams R1, R2, 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 R1, R2 has an arc of about 70 degrees, as in the embodiment of <figref idref="f0005">Fig. 6</figref>, to fully illuminate the cargo container 311, 313.

<figref idref="f0008">Fig. 8a</figref> 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 312a, 315a. 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 312a. Similarly, to open the collimating slot 306, the shutter 315a may be rotated about the pivot point 315a. 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. The shutters 312, 315 may also be curved and can slide around the outer surface of the shielding material, by a suitable mechanism, to selectively cover and uncover a respective slot 304, 306. As discussed above, both collimating slots 304, 306 may be open at the same time to concurrently examine cargo containers on different conveyor systems.

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 <figref idref="f0005">Fig. 6</figref>.

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 (<figref idref="f0003">Fig. 3</figref>), as shown in the cross-sectional views of the shielded targets 400, 403 in <figref idref="f0009">Figs. 9 and 10</figref>, respectively. X-ray scanning units 410, 420 comprising three conveyor systems 412a, 412b, 412c or four conveyor systems, 422a, 422b, 422c, 422d, respectively, maybe constructed with the X-ray source of <figref idref="f0009">Figs. 9 and 10</figref>, as shown in the front views of <figref idref="f0010">Figs. 11</figref> and <figref idref="f0011">12</figref>, respectively.

In these embodiments, the longitudinal axes of the X-ray sources 400, 403 and the three conveying systems 412a, 412b, 412c or the four conveying systems 422a, 422b, 422c, 422d 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.

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.

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.

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 412b, 412c, 422c, 422d.

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 <figref idref="f0004">Figs. 5</figref> and <figref idref="f0005">6</figref> and are not further discussed.

<figref idref="f0012">Fig. 13</figref> is a perspective view of an X-ray source 500 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 <figref idref="f0003">Fig. 3</figref>. The shielding material 520 in each shielded target has a collimating slot 522 defined therein, as described above.

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 <figref idref="f0004">Figs. 5</figref> and <figref idref="f0005">6</figref>, is not shown. 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 L5 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.

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. <figref idref="f0013">Fig. 14</figref> 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 615 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 <figref idref="f0003">Fig. 3</figref>.

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.

The L-shaped detector arrays may comprise conventional detectors. For example, the detectors may comprise scintillators coupled to discrete photodiodes. The detectors may also comprise scintillators coupled to photomultiplier tubes, 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 photodiodes or photomultiplier tubes. A scintillator/photomultiplier tube 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, California, for example.

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.

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.

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.

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.

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.