Patent Publication Number: US-10314151-B2

Title: Charged particle accelerators, radiation sources, systems, and methods

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/366,963, which was filed on Feb. 6, 2012 and will issue on May 12, 2015 bearing U.S. Pat. No. 9,030,134, which is a continuation of U.S. patent application Ser. No. 12/287,792, which was filed on Oct. 14, 2008 and issued on Feb. 7, 2012 bearing U.S. Pat. No. 8,111,025, which claims the benefit of U.S. Provisional Patent Application No. 60/998,691, which was filed on Oct. 12, 2007, and U.S. Provisional Patent Application No. 61/007,500, which was filed on Dec. 13, 2007, all of which are assigned to the assignee of the present application and are incorporated by reference herein. 
    
    
     STATEMENT OF GOVERNMENTAL RIGHTS 
     The U.S. Government has certain rights to this invention pursuant to Contract No. H92236-06-D-1004 with the U.S. Department of Defense. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to charged particle accelerators and radiation sources and, more particularly, to lightweight man-portable X-ray radiation sources, systems using such sources, and methods. 
     BACKGROUND OF THE INVENTION 
     X-ray scanning has been used to identify explosive materials, such as TNT, and wires of electronic control, timing, and/or detonation devices for explosive devices in suspect objects, for example. X-ray scanning may also be used to identify high atomic number material that may be special nuclear materials, such as uranium and plutonium, or shielding for such materials, such as tungsten and lead. X-ray scanning may also identify explosive devices that could be used to disperse radioactive, chemical, or biological materials. 
     Radiation having peak energies of about 0.5 MeV and higher typically comprise a particle accelerator, such as a linear radiofrequency (“RF”) particle accelerator, to accelerate charged particles, and a source of charged particles, such as an electron gun, to inject charged particles into the accelerator. The linear accelerator may comprise a series of linearly arranged, electromagnetically coupled resonant cavities in which standing or traveling electromagnetic waves for accelerating the charged particles are supported. The charged particles injected into the resonant cavities are accelerated up to a desired energy and directed toward a conversion target to produce radiation. Where the accelerated charged particles are electrons and the target is a heavy material, such as tungsten, Bremsstrahlung or X-ray radiation is generated. Electrons accelerated to a nominal energy of 1 MeV and impacting tungsten, will cause generation of X-ray radiation having a peak energy of 1 MeV, for example. 
     A microwave (RF) power source provides RF power to the cavities of the accelerator. The microwave source may be an oscillating microwave power tube, such as a magnetron, or an amplifying microwave power tube, such as a klystron. The microwave sources are powered by modulators, which generate high electric power pulses having peak electric powers of from 1 MW to 10 MW, and average powers of from 1 kW to 40 kW, for example. 
     Typical MeV radiation sources weigh several tons. Once set up at a location for scanning, they are not readily moved. Portable MeV radiation sources are known, which can be moved by truck or forklift, for example. They may be more readily moved to different locations. 
     One example of a portable MeV radiation source is a Mini-Linatron, which was available from Varian Associates, Palo Alto, Calif. As described in literature from Varian Associates, the Mini-Linatron comprised an X-ray head, a power module, a control case, and a modulator module that were connectable by transmission lines, cables and hoses. The X-ray head, which is said to have weighed from 100 pounds (45 kg) including a 2 MV-6 MV accelerator, and 300 pounds (136 kg) including a 9 MV accelerator, also contained an ion chamber and a collimator. The power module, which is said to have weighed 300 pounds (136 kg), contained a magnetron, a pulse transformer, and other RF components. The modulator module, which weighed 300 pounds (136 kg), is also said to have contained a pulse modulator, an electronic line-type chassis, and a power supply. The control case is said to have weighed 11 pounds (5 kg). A Mini-Linatron including a 2 MV accelerator therefore weighed about 711 pounds (323 kg). “MINI Field Portable X-Ray Equipment,” Varian Associates, Oct. 1997. 
     Another modular high energy source for mobile and fixed installations, available from Varian Medical Systems, Inc., Palo Alto, Calif. (“Varian”), is the Linatron®-M™. An X-ray head module including a 3 MV M3 Linatron® accelerator and RF unit, which includes a magnetron and pulse transformer, is said to weigh 1,950 pounds (886.3 kg). “Linatron®-M™ Modular high every radiation source,” Varian Medical Systems, Inc., September 2007. 
     Another portable system which was available from Varian is the Linatron-MP, in which an X-ray head module including a 4 MV accelerator weighs 150 pounds (68 kg), a modulator cabinet weighs 685 pounds (311 kg), and an RF unit weighs 340 pounds (155 kg). “VARIAN&#39;S LINATRON-MP: THE PORTABLE SYSTEM” for Field Radiography,” Varian Medical Systems Technology, Inc., 2003. 
     Russell G. Schonberg describes a 4 MeV traveling wave accelerator packaged with a 9.3 GHz magnetron r.f. source and a pulse transformer, weighing about 190 pounds (86 kg), in “A History of the Portable Linear Accelerator.” Schonberg states that “the total weight was marginal for two people at 190 pounds . . . .” Schonberg does not identify the weight of the modulator and power supplies, which, as described above, typically weigh many hundreds of pounds. “The History of the Portable Linear Accelerator”, Russell G. Schonberg, The American Association of Physicists in Medicine, Annual Meeting, 2001. 
     SUMMARY OF THE INVENTION 
     As used herein, the term “man-portable radiation source” means a radiation source with components that are arranged in subunits that may be carried by one or two people to a site of interest and set up, as compared to a “portable” radiation source, which has been used to refer to a source that is non-permanent and relocatable or movable by a forklift, a dolly, rolling on integral wheels, or lifting by multiple persons. Similarly, a “man-portable radiation scanning system” means a radiation scanning system with components that are arranged in sub-units that may be carried by one or two people to a site of interest and set up. 
     In accordance with an embodiment of the invention, a man-portable radiation generation system is disclosed comprising a first module containing at least one battery and a second module containing a modulator. The first and second modules are configured to be selectively electrically coupled to each other. The system further comprises a third module containing a charged particle accelerator. The second and third modules are configured to be selectively electrically coupled to each other and the at least one battery provides power to the first and second module when the first, second, and third modules are electrically coupled. Each module is portable by hand by one or two people. Each module may be portable by hand by one person. The system may weigh less than 300 pounds (136 kg), or less than 225 pounds (102 kg), for example. The first, second, and third modules may each weigh less than 100 pounds (34 kg) or less than 75 pounds (34 kg), and at least one of the first, second, and third modules may weigh less than 50 pounds (23 kg). At least one of the modules comprises a case with handles. 
     The first module may further comprises a controller to control operation of the source and a control device removably mounted to the first module, for remote control of the controller. The first module may further comprise a cable electrically coupling the control device to the controller, and a spool, around which the cable is selectively wound. An electrical plug may be provided for connection to an external power source. 
     The third module may further comprise an electron gun mounted to the accelerator, to inject electrons into the accelerator, a target coupled to the accelerator to generate X-ray radiation upon impact by accelerated electrons, a magnetron coupled to the accelerator to provide radiofrequency power to the accelerator, and the modulator, which in this example is powered by the at least one battery, provides power to the electron gun and to the magnetron. The third module may weigh less than 80 pounds (36 kg). 
     The third module may also comprise a rigid support coupled to at least one inner wall of the third module, and the accelerator may be coupled to the support. The support is coupled to the at least one inner wall by at least one resilient member. The accelerator and the magnetron may be suspended from the support, at a position such that respective spaces are provided between the accelerator and the magnetron, and an opposing wall of the case. The support may comprise a rigid plate connected to the at least one inner wall and at least one elastomeric member coupling the accelerator to the rigid plate. A second rigid plate may be coupled to the first rigid plate by the at least one elastomeric member and the accelerator may be connected to the second rigid plate. 
     The portable radiation generation system may be configured to generate radiation having a peak energy of about 1.0 MeV, for example. The system may be configured to generate radiation greater than 500 kHz and less than about 1 MeV, for example. 
     A plurality of fins may be coupled to an exterior surface of the accelerator. At least some of the plurality of fins are transverse to a long axis of the accelerator. A cover covering at least some of the plurality of fins may be provided, to form a cooling manifold having a first opening for air to enter the cooling manifold and a second opening for air to exit the cooling manifold. The third module has at least one wall defining at least one air inlet opening. At least one fan is proximate the at least one air inlet vent to move air through the third module and a guide is provided to direct air into the first opening. A duct may be provided to convey air from the at least one air inlet vent to the guide. A duct may convey air from the fan to the first opening. 
     In accordance with another embodiment, a man-portable radiation scanning system is disclosed comprising the man-portable radiation source described above and a detector. The system may further comprise a display to be coupled to the detector array. The detector may comprise radiographic film, for example. 
     In accordance with another embodiment of the invention, a man-portable radiation generation source is disclosed comprising a first module comprising a case containing at least one battery and a second module comprising a second case containing a source of charged particles, a charged particle accelerator, a target, a modulator, and a magnetron, wherein the first and second modules are configured to be selectively electrically coupled to each other. Each module is portable by hand by one or two people. 
     In accordance with another embodiment, a charged particle accelerator is disclosed comprising a source of charged particles and an accelerator comprising a buncher cell defining a buncher cell cavity. The charged particle source is coupled to the buncher cell to inject electrons into the buncher cell cavity, which captures and r.f. focuses the injected electrons into an electron beam. A plurality of linearly arranged cells defining periodic, linearly arranged accelerating cavities are downstream of the buncher cell, to receive and accelerate the electron beam. An output cell is downstream of the accelerating cells, to receive and output the accelerated electron beam. The cells further define a plurality of linearly arranged on-axis coupling cavities between respective cells. The buncher cell and a first periodic cell following the buncher cell are configured such that a field step ratio between the peak amplitude of the electric field in the first cell cavity and the peak amplitude of the electric field in the buncher cell cavity is greater than one (1), during operation. A cell period ratio between a cell length from a center of one periodic cell cavity to a center of next accelerator cell cavity, and half the free space wavelength of the accelerator during operation, is less than one (1). The field step ratio may be less than (2), during operation. The field step ratio may be from 1.2 to 1.5, or from 1.3 to 1.4. The cell period ratio may be greater than 0.78 and less than 0.82. The field step ratio may be 1.3, the cell length may be 12.5 mm, and the cell period ratio may be 0.78. A buncher cell ratio between a length of the buncher cell and half the free space wavelength of the accelerator may be less than one-half. The buncher cell ratio may be 0.3. The accelerator may comprise periodic coupling cavities between the periodic accelerating cavities. 
     The accelerator may be configured to define a particle beam having a spot size encompassing 75% of the beam on the target having a diameter of less than 2 mm, during operation. The accelerator may weigh seven pounds (3.2 kg) or less. As described above, a plurality of fins may be coupled to an exterior wall of the accelerator, and a cover may be provided to cover at least some of the plurality of fins to define a cooling manifold with openings for air to enter and exit the cooling manifold. A tube adjacent to an outer wall of the accelerator may be provided to provide cooling or heating fluid adjacent to the outer wall. 
     A magnetron may be coupled to the accelerator. The magnetron may drive the accelerator with radiofrequency energy having a frequency selected to excite the resonate cells with standing waves with π/2 radian phase between a coupling cell and a next accelerating cell. The magnetron may drive the accelerator at a frequency of 9.3 GHz, during operation, for example. The charged particle source may comprise an electron gun configured to operate at the same voltage as the magnetron, or lower voltage. The electron gun may comprise an anode plate coupled to the buncher cell. The buncher cell may define a half-cell and the buncher cell cavity may be defined by the half-cell and the anode plate. The anode plate may define an aperture with an entrance to the buncher cavity dimensioned to remove charged particles at a periphery of the charged particle beam. The diameter of the entrance may be dimensioned to remove at least half of the charged particles in the beam. 
     The accelerator may comprise ten periodic accelerator cavities between the buncher cavity and the output cavity, for example. A target may be coupled to the output cell, wherein impact of charged particles on the target generates radiation. 
     In accordance with another embodiment, a radiation generation source is disclosed comprising a linear charged particle accelerator, a source of charged particles coupled to the accelerator to inject charged particles into the accelerator, and a target coupled to an output of the accelerator. Impact of the accelerated charged particles on the target causes generation of radiation. A plurality of fins are coupled to an exterior surface of the accelerator, to air cool the accelerator, as described above. At least some of the plurality of fins may be transverse to a long axis of the accelerator and a cover covering at least some of the plurality of fins to define a cooling manifold having openings for air to enter and exit the cooling manifold may also be provided. The accelerator may be contained within a case with at least one wall defining an air inlet. A fan may be proximate the inlet to cause air to move through the case and a guide may direct air into the first opening, during operation. The guide may be coupled to the manifold, and a duct may convey air from the fan to the guide. A duct may convey air from the fan to the first opening, instead. The at least one wall of the case may further define at least one exhaust vent. 
     In accordance with another embodiment, a method of setting up a man-portable radiation source to examine an item of interest is disclosed, wherein the radiation source comprises at least a first module containing at least one battery to power the source and a second, separate module comprising an accelerator, an electron gun, and a target. The method comprises carrying by hand the at least first and second modules to a location proximate the item of interest, by at least one person, electrically coupling at least the first module to the second module by at least one electrical cable, and moving a safe distance from the radiation source, leaving the at least first and second modules at the site. The method may further comprise removing a control device from one of the modules and moving to the safe distance, with the control device, leaving the at least first and second modules at the site. The control device may be electrically coupled to a cable rolled around a spool in the one module and the method may further comprise unrolling a spool of cable in the one module and moving to the safe distance with the control device. The method may comprise moving to a safe distance, behind a dense structure. The method may further comprise activating the source to generate radiation and scanning the item of interest with the generated radiation. A detector may be carried by hand to the location and radiation interacting with the item of interest may be detected by the detector. A third module containing a modulator may be carried to the location and electrically coupled to the first and second modules. 
     In accordance with another embodiment, a battery operated radiation generation source is disclosed comprising at least one battery, a charged particle accelerator, a source of charged particles coupled to the accelerator to inject charged particles into the accelerator, and a target coupled to an output of the accelerator. Impact of the accelerated charged particles on the target causes generation of radiation. A radiofrequency power supply provides radiofrequency power to the accelerator. The at least one battery provides power to the source of charged particles and the radiofrequency power supply. A modulator may be coupled to the at least one battery, to convert direct current voltage from the battery to pulses of high voltage to be provided to the source of charged particles and to the radio frequency power supply. 
     In accordance with another embodiment, a radiation generation source is disclosed comprising an electron source and an accelerator comprising a buncher cell defining a buncher cell cavity. The electron source is coupled to the buncher cell to inject electrons into the buncher cell cavity and the buncher cell cavity captures and r.f. focuses the electrons injected by the electron source, into an electron beam. A plurality of linearly arranged cells define periodic, linearly arranged accelerating cavities downstream of the buncher cells to receive and accelerate the electrons. An output cell is downstream of the accelerating cells and a target is coupled to the output cell to receive and output the accelerated electron beam. A target is coupled to the output cell. Impact of accelerated electrons on the target causes generation of X-ray radiation. The buncher cell, the accelerating cells, and the output cell further define a plurality of linearly arranged on-axis coupling cavities between respective cells. The buncher cell and a first periodic cell following the buncher cell are configured such that a field step ratio between the peak amplitude of the electric field in the first cell cavity and the peak amplitude of the electric field in the buncher cell cavity is greater than one (1), during operation. A cell period ratio between a distance between from a center of one periodic cell to a center of next accelerator cell, and half the free space and length of the accelerator during operation, is less than one (1). The field step ratio may be less than two (2), during operation. A buncher cell ratio between a length of the buncher cell and half the free space wavelength of the accelerator may be less than one-half. The buncher cell ratio may be 0.3. The output cell may define an inwardly tapered passage from a cavity to a target. The target may comprise a copper substrate and a tungsten layer coupled to the copper substrate. The thickness of the tungsten layer may be less than 0.25 mm, less than 0.20 mm, less than 0.10 mm, or less than 0.05 mm. Other features described above may be incorporated in the accelerator in accordance with this embodiment of the invention, as described in more detail in the specification. 
     In accordance with another embodiment, a radiation generation source is disclosed comprising a charged particle accelerator, a source of charged particles coupled to the accelerator to inject charged particles into the accelerator, and a target coupled to an output of the accelerator. Impact of the accelerated charged particles on the target causes generation of radiation. The thickness of the tungsten layer is less than 0.20 mm. The thickness of the tungsten layer may be less than 0.10 mm. The thickness may be 0.05 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic block diagram of an example of a man-portable radiation source in accordance with one embodiment of the invention; 
         FIG. 2  is a perspective view of an example of a case for any or all of the modules; 
         FIG. 2 a    shows a view of the inner surface of a deflector that may be used in the case of  FIG. 2 ; 
         FIG. 2 b    shows an EMI shielded filter screen that may be provided in the inlet vents and the exhaust vents in the case of  FIG. 2 ; 
         FIG. 3  is an example of a man-portable radiation system incorporating the man-portable radiation source of  FIG. 1 , at a site of interest; 
         FIG. 4  is a perspective view of an example of an accelerator in accordance with an embodiment of the invention; 
         FIG. 5  is an axial sectional view of the accelerator of  FIG. 4 , excluding the electron gun and target assembly, to simplify illustration; 
         FIG. 6 a    is an enlarged sectional view of a buncher half-cell of  FIG. 4 ; 
         FIG. 6 b    is an enlarged sectional view of a half-cell connected to the buncher half-cell, of  FIG. 4 ; 
         FIG. 7  is an enlarged sectional view of a half-cell of  FIG. 4 ; 
         FIG. 8 a    is a perspective view of an example of a target assembly for use with the accelerator of  FIGS. 4 and 5 ; 
         FIG. 8 b    is a sectional view of the target assembly of  FIG. 8 a   , through line  8   b - 8   b;    
         FIG. 9  is a perspective view of another example of an accelerator with guides coupled to respective cooling fin assemblies; 
         FIG. 10 a    is a perspective view of an assembly comprising the magnetron and the accelerator, coupled to a strong back; 
         FIG. 10 b    is another example of the assembly supported by another strong back; 
         FIG. 11  is a graph of energy E (arbitrary) versus Z (cm) for the accelerator; 
         FIG. 12  is a sectional view of an example of the internal configuration of the X-ray head in the third module, including another strong back arrangement; and 
         FIG. 13  is a sectional view of another example of the internal configuration of the X-ray head in the third module, including another strong back arrangement. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     There may be times when it would be advantageous to quickly and easily set up an X-ray scanning system at a particular site by one or two people. For example, the ability to quickly and easily deploy a lightweight radiation source for object examination by one or two people could facilitate the identification of explosive devices hidden in suspect objects at crime scenes, actual or potential sites of terrorist attacks, or in combat or war-time situations. Hidden improvised explosive devices (“IEDs”) may thereby be identified, for example. Such a lightweight radiation source could also facilitate the identification of flaws and faults in infrastructure, such as bridges, as well as the examination of small or difficult to access locations, such as in an airplane or submarine, for example. A radiation source and radiation scanning system including such a source, which may be carried to a site by one or two people, would therefore be advantageous. 
       FIG. 1  is a schematic block diagram of an example of a man-portable radiation source  100  in accordance with one embodiment of the invention. As used herein, the term “man-portable radiation source” means a radiation source with components that are arranged in subunits that may be carried by one or two people to a site of interest and set up, as compared to a “portable” radiation source, which has been used to refer to a source that is non-permanent and relocatable or movable by a forklift, a dolly, rolling on integral wheels, or lifting by multiple persons. A man-portable radiation source may be used in a “man-portable radiation scanning system,” which, as used herein, means a radiation scanning system with components that are arranged in sub-units that may be carried by one or two people to a site of interest and set up. 
     In this example, the man-portable source  100  is designed to generate an X-ray radiation beam having peak energy of about 1 MeV (1 MeV+/−10%). In a particular example described herein, the peak energy is 0.93-0.94 MeV and the generated X-ray radiation has a half value layer (“HVL”) of 0.57 inches (14.5 mm)-0.62 inches (15.7 mm). The HVL is the length of steel required to reduce X-ray dose or intensity by half. The man-portable radiation scanning system  100  in one example may image an 18 gauge (7 mm) diameter copper wire through 3 inches (7.6 cm) of steel. As discussed above, electronic control, timing, and/or detonation electronics for explosive devices may include wires. These are just exemplary energies and higher energy (greater than about 1 MeV) man-portable radiation sources may be made in accordance with embodiments of the invention at other energies and HVLs. For example, man-portable radiation sources of 3 MeV or 6 MeV may also be provided. In addition, lower energy radiation sources, such as 500 KeV sources and higher may also be made in accordance with embodiments of the invention. 
     In this example, the X-ray source  100  comprises separate first, second, and third modules  102 ,  104 ,  106 , respectively, each light enough to be carried by one or two persons. In this example, each module  102 ,  104 ,  106  weighs less than 100 lbs (45 kg). Certain modules may weigh less than 75 lbs (34 kg) or less than 50 lbs (23 kg), for example. Each module  102 ,  104 ,  106  and thereby the source  100  are therefore man-portable, meaning that each module may be moved by one or two people without the assistance of a machine, such as a forklift. 
     The first module  102  in this example comprises a controller  108 , one or more batteries  110 , and a remote control (or pendant)  112 . The second module  104  in this example comprises a modulator  114 . The third module  106  in this example comprises an X-ray head  118 . The X-ray head  118  in this example comprises an electron gun  120 , an accelerator  122 , a target  124 , a magnetron  126 , and a pulse transformer  128 . The first module  102  may be coupled to the second module  104  by a first, control cable  120  and a second, power cable  122 . The second module  112  may be coupled to the third module  106  by a control cable  124  and a drive cable  130 . The drive cable may comprise two separate cables, one for the filaments of the electron gun  120  and magnetron  126 , and another for pulses provided to the electron gun and magnetron. The controller  108  controls operation of the source  100 , under the control of the pendant  112 , which is a portable remote control that may be physically mounted in the first module  102  when not in use. The batteries  110  provide DC power to the modulator  114 , which converts the DC power to pulses to drive the magnetron  126  and electron gun  120 . The pulse transformer  128  permits use of a lower voltage on the cable connectors. The magnetron  126  generates an electromagnetic field that is provided to resonant cavities within the accelerator  122 . Electromagnetic standing waves are supported within the accelerator  122 . Electrons provided by the electron gun  120  to the accelerator  122  are accelerated by the standing electromagnetic waves. The accelerated electrons impact the target  124  causing generation of X-ray radiation by the Bremsstrahlung effect. Alternatively, a traveling wave accelerator may be used. 
       FIG. 2  is a perspective view of an example of a case  170  for any or all of the modules  102 ,  104 ,  106  when resting on a surface. Two handles  171  are shown along different sides of the case  170 . Additional handles on a side or a longer handle may be provided to facilitate carrying by two people, if desired. Two exhaust air vents  172  are provided, one shown covered by a precipitation deflector  174   a  in  FIG. 2  and the other shown with the deflector  172   b  separated from the case  170  for illustrative purposes.  FIG. 2 a    shows a view of the inner surface of the deflector  174 . Returning to  FIG. 2 , two inlet air vents  176  are also provided, one covered by a deflector  174   c  and the other shown with the deflector  174   d  separated from the case  170  for illustrative purposes. Intake fans  178  are provided in the inlet air vents  176  to increase air flow through the vents and the case  170 , as described below. In this example, each case  170  for each of the modules  102 ,  104 ,  106  is identical, to decrease cost and for simplicity, but that is not required. An EMI shielded filter screen  180 , shown in  FIG. 2 b   , may be provided in the inlet vents  176  and the exhaust vents  172 , as well. 
     The case  170  may be a commercially available case or a custom designed case. The case  170  shown in  FIG. 2  is a commercially available Storm Case IM 2950, available from Hardigg Industries, South Deerfield, Mass., which weighs 20.8 lbs. (9.4 kg) (without foam). The internal dimensions of the IM 2950 are 29 inches (74 cm)×18 inches (46 cm)×10.5 inches (27 cm). The Storm Case IM 2590 does not include openings for vents. The inlet vents  176   a ,  176   b  and the exhaust vents  172  are therefore added. The Storm Case IM 2950 includes wheels, which may be removed to further decrease the weight of the modules, if desired. 
     Another commercially available case  170  is the Pelican 1650®, available from Pelican™ Products Inc., Torrance, Calif., which weighs about 29.1 lbs (13 kg), (without foam). The Pelican 1650® has internal dimensions of 28.5 inches (73 cm)×17.37 inches (44 cm)×105 inches (266 cm). The Pelican 1650® also does not include openings for vents and the inlet vents  176   a ,  176   b  and the exhaust vents  172  would need to be added. As above, wheels may be removed, if desired. 
     Another commercially available case  170 , which is lighter than the Pelican 1650®, is the Seahorse SE  1220 , available from Seahorse, Covina, Calif., which weighs about 24.44 lbs (11 kg). The internal dimensions of the SE  1229  are 25.52 inches (65 cm)×19.5 inches (50 cm)×13.08 inches (33 cm). The Seahorse SE  1220  also does not include openings for vents and the inlet vents  176   a ,  176   b , and the exhaust vents  172  would need to be added. As above, wheels may be removed, if desired. 
     To inspect an item of interest  150  with the man-portable X-ray source  100  in accordance with one embodiment of the invention, the modules  102 ,  104 ,  106  may be driven to a site near the item of interest by a vehicle, such as a car, jeep or truck, for example, and unloaded. The modules  102 ,  104 ,  106  may then be carried to and positioned proximate the item  150  by one or two people, as shown in  FIG. 3 . One person may carry one or two of the modules  102 ,  104 ,  106  by the handles  140 , at a time. If two handles  140  on a side or a long handle are provided, as discussed above, two people can carry one module at a time. 
     To assemble the source  100  proximate the item of interest  150 , the third module  106  is positioned a distance P from the item, as shown in  FIG. 3 , for example. The distance P may be any suitable distance. For example, the distance P may be about 1 meter. The first and second modules  102 ,  104  are positioned near the third module  106  and are coupled to each other via a control/power cable  120 / 122 , which may be a combined cable or separate cables. The second module  104  is coupled to the third module  106  by a combined or single control/drive cable  130 / 132 . The position of the first and second modules with respect to the third module may depend on the length of the cables  120 / 122 ,  130 / 132 . In this example, the cables  120 / 122 ,  130 / 132  are about 1 meter long, to reduce capacitive effects and still allow for flexibility in placement of the modules  102 ,  104 ,  106  at the scanning site. In one example, the man-portable X-ray source  100  may be assembled in about two minutes, for example. 
     One or more detectors  160  may be positioned a suitable distance behind the item of interest  150  to detect radiation transmitted through the item, as shown in  FIG. 3 , and/or one or more detectors  160  may be positioned to detect scattered radiation. The detector  160  may be an imaging panel, such as the HE4030 imager system, available from Varian Medical Systems, Inc., Palo Alto, Calif., for example, which weighs about 30 lbs (14 kg), or X-ray film. Film packets may weigh from about 10 lbs (4.5 kg) to about 12 lbs (5.4 kg), for example. Other types of detectors may be used, instead. 
     A processor  162  may be coupled to the detector  160  by a cable  163 , and a display  164  may be coupled to the processor. The processor  162  and the display  164  may comprise a laptop computer weighing about 5 lbs (2-3 kg) to about 20 lbs (9 kg), depending on the model, for example. The detector  160 , the processor  162 , and the display  164  may be carried to the site by one or two people in one or two trips. In this example, the cable is long enough for the processor  160  and the display  164  to be used by an operator about 30 m from the X-ray source, as discussed below, such as from about 30 m to about 40 m, for example The detector  160  may be coupled to the processor  162  wirelessly, if interference is not a concern. 
     The detector  160  may also comprise X-ray film, in which case a processor and display are not needed. Since film developers are quite large and heavy, a film developer is not incorporated in the system. The film may be carried from the site to a developer in another location. Imaging of an 18-gauge wire through 3 inches (76 mm) of steel could take from about 3 minutes of X-ray beam-time for some film types up to about one hour for others. Use of film requires shielding of unexposed film from the radiation field emitted by the X-ray head  118 , as is known in the art, to protect against premature exposure by radiation from the unshielded, non-collimated X-ray head  118 . Commercially available film, such as GAFCHROMIC® EBT film, available from International Specialty Products, Wayne, N.J., may be used, for example. 
     Other components as needed may also be carried to the site and coupled to the modules  102 ,  104 , and/or  106 . For example, the accelerator  122 , the modulator  114 , and/or the magnetron  126  may require water cooling and/or heating, as discussed below. A water supply and pump (not shown) may be carried to the site and coupled to the modulator  114 , the accelerator  122 , and/or to the magnetron  126 . The water supply and pump may be included within one of the modules, such as the first module  102 , or in a separate module, for example. 
     In the example described herein, no collimator is used and radiation R is emitted in all directions. Alternatively, the third, X-ray head module  106 , may include a recess (not shown) to receive a field deployable collimator, which could weigh about 30 lbs (14 kg). The collimator may be carried to the site of interest separately from the modules  102 ,  104 ,  106 , for example. 
     A major source of the weight in a radiation source is radiation shielding. To reduce the weight of the third module  106 , no shielding is provided around the X-ray head  118 . However, radiation is emitted in all directions, which could increase the risk of deleterious exposure to operators and others in the area. In the example described herein, radiation leakage may be as high as 1 R/m at 1 meter, or 60,000 mR/hour. 
     To protect the operator and other personnel from dangerous radiation exposure, after assembly of the X-ray source  100  at a site, distance is used to reduce exposure along with field expedient measures, if available, such as taking cover behind a masonry wall, an earthen berm, or other dense object. Personnel should move as far as possible from the third module  106 . They should be at least about 30 meters from the third module  106 , in this example. At 30 meters in the open, the X-ray dose could be as high as 70 mR/hour at 1 meter, corresponding to a “radiation area.” 
     An operator may remove the pendant  112  from the first module and carry the pendant to the safe location and/or behind a dense structure, such as a concrete wall or building, if in the vicinity. Such a dense structure may provide sufficient protection at less than 30 meters from the third module  106 . If the pendant  112  is connected to the controller  104  by the cable  130 , the cable may be wound on a spool  132  when stored in the first module  102  and unwound as the operator moves to the safe location. The cable may be 40 meters long, for example. If wirelessly controlled, the operator may similarly move to the safe location before activating the system  100 . 
     Personnel assembling and operating the source  100  may also carry personal dosimeters to monitor their exposure. An alarm bell may be provided to alert the personnel to a pre-set exposure. Perimeter access should be controlled to avoid exposure to others. Separate shielding slugs of shielding material (not shown) may be positioned around the third module  106  at a desired site, if desired, to enable personnel to be closer to the item of interest  150  during operation. 
     Where a digital imager is employed, the image may be analyzed immediately on a laptop, from the operator position. As discussed above, the digital imager may be an HE4030 imager from Varian Medical Systems, Inc., for example. It takes about 3-seconds for the HE4030 imager to generate an image. Since beam on time is reduced when using a digital imager, radiation exposure of personnel may be reduced by a factor of about 50 as compared to the use of film. Digital imaging also reduces battery usage, increasing battery life compared to use of film. The HE4030 digital imager, which weighs about 15 lbs, may be stored in the first module  102 . A laptop computer may be used to process and display the images, as is known in the art. 
     After imaging of the item of interest  150 , the modules  102 ,  104 ,  106  and the X-ray source  100  may be quickly disconnected and removed from the site, by one or two people carrying each module  102 ,  104 ,  106 . 
     The First Module 
     As discussed above, the first module  102  contains the controller  108 , the batteries  110 , and the pendant  112 , and related components. The controller  108 , which controls operation of the source  100 , may be a processor, such as a programmable logic controller (“PLC”), which may be a commercial off the shelf processor board. A battery operated, “wireless” radiation source, which is not limited to use near conventional power supplies, is more versatile than a source that must be plugged in to a conventional source. As discussed below, however, the man-portable radiation source  100  may be driven by conventional AC power in addition to or instead of batteries  110 . 
     As discussed above, the pendant  112  is a portable remote control that may be physically mounted in the first module  102  when not in use. In one example, a display screen is provided to display status information, such as warming up, beam on, exposure time and/or dose, and remaining battery life, for example. The pendant  112  may be coupled to the controller  104  by a cable  130  or where the application does not have sensitive electronics, wirelessly. If wirelessly controlled, electromagnetic/radio-frequency interference may need to be controlled. The cable  130  may be wound on a spool  132 . As discussed above, a 40 meter cable may be used, for example. The pendant  112 , controller  104 , and cables  120 ,  122 ,  130 ,  132  may weigh up to about 10 lbs (4.5 kg), for example. 
     Functions on the pendant  112  may include a red emergency off button to de-energize the system  10 , a yellow warning light for a fault causing the X-ray beam to turn off, and a manual override button to provide an instant “beam-on” and “beam-off”, for example. Alternatively, the pendant  112  may be mounted in any of the other modules  104 ,  106  where there is room. If the pendant  112  and the controller  108  are in different modules, additional cables may be required. The third module  106  may include an emergency off button instead or in addition to the emergency off button on the pendant  112 . 
     The batteries need to supply sufficient power to image for a desired period of time. In one example, the batteries provide sufficient power to scan for about 100 minutes continuously, at about 1 Rad/minute. In order to supply such power for such a period of time, the typical power requirements and operating levels of other components of the source, such as the modulator  114 , the electron gun  120 , and the magnetron  128 , need to be conserved. That requires changes in the typical design of the accelerator  122 , examples of which are described below. 
     The batteries  110  in the first module  102  generate DC voltage, such as 240 volts, which is provided to the modulator  116  in the second module  112  via the power cable  122 . In one example, the batteries need to store 640 kilojoules. The batteries  106  may comprise a pack of ten (10) 24 volt commercial batteries, for example. The battery pack may weigh about 20-25 pounds (9 kg-11 kg), for example. A separate compartment in the first module  102  may be provided for the cables  120 ,  122 ,  130 ,  132  and the pendant  110 . The batteries may be rechargeable and/or replaceable in the field. 
     The batteries may be a BA 5590 lithium/sulphur dioxide battery pack system from Saft Groupe SA, Bagnolet, France (“Saft”), which is said to comprise 10 LO26 SX cells connected in two groups of 5 cells in series, providing 2 nominal 12 volt sections at the connector, for example. The sections may be connected in series to provide 24 volts or in parallel to provide 12 volts. According to a specification provided by Saft, the typical operating control voltage (“OCV”) is 15.0 or 30.0 volts, the nominal voltage (at 500 mA) is 13.5 or 27.0 volts, and the cutoff voltage is 10.0 or 12.0 volts, depending on whether the sections are connected in series or in parallel. The typical capacity (at 70° F. (21° C.)), 250 mA discharge current is said to be 15 hours in a 12 volt mode and 24 hours in a 24 volt mode. The batteries are said to operate over a temperature range of from −40° F. (−40° C.) to 160° F. (71° C.). Each battery is said to weigh 2.25 pounds (1 kg), and the battery pack weighs about 22.5 pounds (10 kg). 
     Alternatively, lithium ion polymer batteries, such as LIP-5 (“LIP”) available from LINCAD, Ltd., Camberley, Surrey, England may also be used, for example. Lithium ion rechargeable batteries, such as the UBI-2590, available from Ultralife Batteries, Inc., Newark, N.J., for example, may also be used. Nickel metal hydride rechargeable batteries, such as those used in battery operated cars, may also be used. 
     An electrical plug  128  and cable  129  may be provided in the first module  102  for connection to a conventional source of AC power, such as a wall outlet providing 110 volts or a generator, for example. If the batteries  106  are rechargeable, the AC power may be used to recharge the batteries. The modulator  114  may also be powered by an AC power source (not shown) during use, if the cable  129  is long enough to reach it. 
     A fan (not shown) may be provided for further air circulation and cooling. 
     The weight of the first module  102  in this example is from about 50 lbs (23 kg) to about 80 lbs (36 kg), depending on the weight of the case  170 . If the Storm Case IM 2950 is used, for example, the first module  102  would weigh about 50 lbs (23 kg) to about 70 lbs (32 kg), for example, which may be readily carried by one person. 
     The Second Module 
     The second module  104  contains the modulator  114 , which converts the DC power provided by the batteries  106  to suitable pulses to drive the magnetron  126  in the X-ray head  118  in the third module  106 , as is known in the art. In one example, the modulator  116  converts the 24 volts provided by the batteries  106  to 2.2-2.4 microsecond pulses at about 29 kilovolts and 30 Amps. Alternatively, the modulator  104  may be included in the same module  106  as the X-ray head  118 . While increasing the weight of the third module  106 , fewer cables would be required, decreasing the risk of arcing. 
     To reduce the weight of the source  100 , the X-ray head  118  in this example is designed to operate under less power than typical X-ray heads (as discussed below), allowing for a smaller modulator  114 . With the X-ray head  118  described in this example, a 29 kV, 30 A modulator may be used. In this example, the modulator  114  is a commercially available modulator weighing about 75 lbs (34 kg) or less. 
     For example, the Stangenes Model SSM-3-3-M1, available from Stangenes Industries, Inc., Palo Alto, Calif., may be used. The SSM-3-3-M1, which weighs about 75 lbs (34 kg), is capable of 36 kV, 80 A at 0.001 duty with a 2-millisecond pulse. 
     Alternatively, the Scandinova Model Type M1, which also weighs about 75 lbs (34 kg), provides 48 kV and 110 A at 0.0012 duty, available from Scandinova AB, Uppsala, Sweden, may be used. 
     As in the first module  102 , the case  170  housing the second module  104  includes vents and one or more fans (not shown). 
     The weight of the second module  106  is about 75 lbs (34 kg) plus the weight of the case  170 . If the Storm Case IM 2950 is used, the second module would weigh about 96 lbs (44 kg), for example, which may be carried by one or two people. 
     The Third Module 
     As discussed above and shown in  FIG. 1 , the third module  106  contains the X-ray head  118 . The X-ray head  118  comprises an electron gun  120 , an accelerator  122 , a target  124 , a magnetron  126 , and a pulse transformer  128 . 
       FIG. 4  is a perspective view of an example of an accelerator  1000  in accordance with an embodiment of the invention. The accelerator  1000  comprises a biperiodic, standing wave electron beam linear accelerator body  1002 . The accelerator  1000  operates in the X-band at 9.3 GHz. X-band accelerators may be smaller than S-band accelerators, which operate at 3 GHz, as is known in the art. An S-band accelerator may be used in accordance with embodiments of the invention, if a larger and heavier X-ray radiation source  100  may be tolerated. An electron gun  1004  is coupled to one end of the accelerator body  1002  and a target assembly  1006  is coupled to the opposite end. The electron gun  1004  is coupled to the accelerator body  1002  via an anode plate  1008 . A waveguide window  1010  and a waveguide  1012  couple the accelerator body  1002  to the magnetron  126  (shown in  FIG. 9 a   ). In this example, the waveguide window  1010  defines a rectangular opening  1010   a . A vacuum pump  1013  is coupled to the waveguide  1012  to create a vacuum within the waveguide and the accelerator body  1002 . An optional cooling tube  1014  for water cooling or heating of the accelerator body  1002 , is also shown. Cooling fins  1016 , may also be provided instead of or along with the cooling tube  1014 , as discussed in more detail below. 
     The accelerator body  1002  shown in the example of  FIG. 4  weighs from about 6 pounds (2.7 kg) to about 7 pounds (3.2 kg). The accelerator  1000  has a length “L” of about 6 inches (about 15 cm) not including the electron gun  1004  but including the target assembly  1006 . The accelerator body  1002  has an outer diameter of about 35 mm without the cooling fins  1016  and about 96 mm with the fins. The dimensions of the cooling fins  1016  are based on providing stable operation over an ambient temperature range of from about 0° C. to about 56° C. The fins  1016  have been found to provide stable operation up to about 70° C. ambient. Smaller fins  1016  may be used if operating conditions are more tightly controlled. The accelerator  1000  would then have a smaller diameter. 
     Electron guns for many X-ray radiation sources are typically driven at a high voltage of about 20 kV to about 100 kV with a separate power supply or transformer. Higher voltage requires larger clearances (10 kV/inch, 254 kV/mm), and also adds to power supply weight. To reduce the weight of the X-ray head  118 , the accelerator  1000  is designed to allow operation of the electron gun  1004  at about the same voltage as the magnetron  126 , or less voltage. The accelerator  1000 , in this example, also accommodates a low accelerating gradient, which may be 6 MV/M, for example, required by the relatively low peak power available from the modulator  114  and the magnetron  126 . The electron gun  1004  is driven at a lower than typical voltage of 26 kV-29 kV. 
     The electron gun  1004  may be a commercially available diode gun with a perveance of 0.1 uperv. The electron gun voltage is at or below the magnetron voltage, which in this example is 28 kV. Voltage is provided to the electron gun  1004  via a high voltage connector  1004   a.    
     The vacuum pump  1013  may be a 0.2 liter/second ion pump, referred to as a Vacion pump, such as a mini ion pump with smaller magnets, Part Number 8130038, available from Varian Vacuum Technologies, Torino, Italy, for example. 
       FIG. 5  is an axial sectional view of the accelerator  1000  of  FIG. 4 , excluding the electron gun  1004  and target assembly  1006 , to simplify illustration. In this example, the accelerator body  1002  comprises a chain of cells  1020 - 1042  defining respective electrically coupled resonant accelerating cell cavities  1020   a - 1042   a . The first cell  1020  is a buncher cell, which defines a buncher cell cavity  1020   a  configured to bunch and focus the injected electrons to form a beam and to establish its size. Buncher cells are generally described in U.S. Pat. No. 6,864,633, for example, which is assigned to the assignee of the present invention and is incorporated by reference herein. Ten (10) full, in-line, periodic electrically coupled resonant accelerating cells  1022 - 1040  follow the buncher cell  1020  in this example. The term “periodic” as used herein means that the accelerating cavities  1022   a - 1040   a  defined by each respective cell  1022 - 1040  have the same dimensions. The waveguide  1012 , which couples the magnetron  126  to the accelerator body  1002 , is coupled to the sixth full accelerating cell  1032 , in this example. The final cell  1042  defines an output cavity  1042   a , from which accelerated electrons exit the accelerator body  1002 . 
     The buncher cell cavity  1020   a  is defined by the anode plate  1005  and the buncher cell  1020 , which is a half-cell. The anode plate  1005  defines an output of the electron gun  1004 , which in this example tapers to a narrow aperture  1056 . The aperture  1056  is inwardly tapered toward the buncher cell cavity  1020   a , in this example, and may have an diameter of 0.0050 inch (0.13 mm), for example. Such a small diameter facilitates a rapid creation of the electromagnetic field in the buncher cell. The small aperture  1056  has also been found to “scrape” off the outer electrons in the electron beam, reducing the electron beam current and diameter. About half of the electrons may thereby be removed. This reduces the peak power requirements of the accelerator  122  and introduces a smaller diameter electron beam to the buncher cell  1020 . Enlarging the diameter of the aperture  1056  to 0.080-0.100 inches (0.2 mm-2.5 mm) provides higher current and better transmission. If such a larger aperture  1056  is used, the buncher field step (discussed below) may need to be adjusted. 
     The buncher half-cell  1020  includes an iris or opening  1054 . The cross-section of the buncher half-cell  1020  is shown enlarged in  FIG. 6 a   . A shallow cavity  1055  is provided on an opposite side of the buncher half-cell  1020   a  as the cavity  1020   a . The iris  1054  electrically and physically couples the cavities  1020   a ,  1055 , allowing for the passage of RF energy and an electron beam, as is discussed further below. The cavity  1020   a  of the buncher half-cell  1020  faces the anode plate  1005 . The buncher half-cell  1020  is partially received within a recess  1005   a  in the anode plate  1005 . 
     In this example, the maximum diameter D 1  of the buncher cell cavity is 26.71 mm; the diameter D 2  of the iris  1054  is 6.52 mm; the maximum diameter D 3  of the coupling cavity  1055  is 26.65 mm; the depth De 1  of the buncher cell cavity  106  is 3.32 mm; the depth of De 2  of the coupling cavity is 0.49 mm; the depth De 3  of the iris of  1052  is 1.0 mm; and the length L b  of the buncher cell  1053  is 4.81 mm. 
     Each half-cell  1060  includes a first, deep cavity  1062 , a beam tunnel iris or opening  1064 , and a second, shallow cavity  1066  on an opposite side of the half-cell  1060  of the first, deep cavity  1062  and facing an opposite direction, as shown in  FIG. 7 . The full accelerating cavities  1022   a  are formed by identical facing cup shaped half-cells  1060 , one of which is shown enlarged and in cross-section in  FIG. 6 b   , and another of which is shown in  FIG. 7 . The shallow cavity  1055  of the buncher cell  1020  is attached to the shallow cavity  1066   a  of the first half-cell  1060   a  of the first resonant cell  1022  to form a full coupling cavity  1055 , as shown in  FIG. 6 b   . As shown in  FIG. 5 , the half-cells  180  are joined such that a first, deep cavity of one cell faces a first, deep cavity of an adjacent facing cell and a second, shallow cavity of one cell faces a second, shallow cavity of another adjacent cell. The matching larger cavities form the full cells  1022 - 1040  and accelerating cavities  1022   a - 1040   a , while the matching shallow, second cavities form the coupling cavities  1070 - 1088 . The irises  1064  and the coupling cavities  1070 - 1088  electrically and physically couples the cavities  1062 ,  1064 , allowing for the passage of RF energy and an electron beam, as is discussed further below. 
     In this example, each half-cell  1060  defines a deep cavity  1062  having a maximum diameter D 4  of 27.07 mm and a cavity depth De 4  of 4.78 mm; an iris  1064  having a diameter D 5  of 6.44 mm and an iris depth De 6  of 0.49 mm; and a coupling cavity  1066  having a maximum diameter D 6  of 26.65 mm and a cavity depth De 5  of 0.49 mm. 
     The irises  1054 ,  1064  of the buncher cell  1020 , the accelerating cells  1022 - 1040 , and the output cell  1042 , are aligned with the axis X of the aperture  1056  of the electron gun  1008  to form a tunnel for passage of an axial electron beam (not shown) through the accelerator body  1002 , as shown in  FIG. 5 . The full resonant cells accelerate the electrons injected by the electron gun while the coupling cells  1070 - 1088  electrically couple the accelerating cavities  1022   a - 1040   a  to each other. The sum of the accelerations in each cavity  1020   a - 1042   a  add in the aggregate to the desired energy of 0.93 MeV-0.94 MeV, in this example. 
     The output end  1065  of the accelerator body  1002  is defined by a full cell  1042 , which is formed in this example by another half-cell  1060  and a larger, deeper half-cell  1062  facing the half-cell  1060 . For example, the half-cell  1060  may have a depth of about 4.78 mm and the deeper half-cell  1062  may have a depth of about 7.39 mm. A tapered passage  1064  extends from the half-cell  1062  to the target assembly  1006 , which is coupled to the output end  1065 . The tapered passage  1064  is dimensioned to intercept outlying electrons where cavity tuning will be less affected by heat. 
     The target assembly  1006  (not shown in this view) fits within the recess  1062   a .  FIG. 8 a    is a perspective view of an example of the target assembly  1006 .  FIG. 8 b    is a sectional view of the target assembly  1006  of  FIG. 8 a   , through line  8   b - 8   b . In this example, the target assembly  1006  comprises a copper substrate  1072  supporting a tungsten button  1074  in a cavity  1072   a . The tungsten button  1074  is brazed to the copper substrate by a copper/gold braze  1076 . The braze  1076  may comprise 35% copper/65% gold, for example. Grooves  1078  may be provided in the copper substrate  1072  through which gas is pumped to create a vacuum and avoid a virtual leak, as is known in the art. In one example, the tungsten button  1074  is 2 thousandths of an inch (0.05 mm) thick and has a diameter of 0.3 inches (7.6 mm). Usually, tungsten target buttons are 10 thousandths of an inch thick (0.25 mm). It has been found, however, that a tungsten target button with a thickness of less than 10 thousandths of an inch (0.25 mm) provides higher radiation yield. For example, progressively better yield may be obtained with button thicknesses of less than 0.20 mm, 0.15 mm, and 0.10 mm, such as 0.05 mm. In this example, use of a tungsten button  1074  with a thickness of 0.05 mm increased the yield by about 50% compared to a tungsten button of 0.25 mm. The higher yield increases the radiation dose, enabling faster imaging. This is advantageous, especially where the total imaging time may be limited due to battery capacity. The braze is 1-2 thousandths of an inch thick (0.025-0.05 mm). The target button  1074  and other components of the target assembly  1070  may comprise other materials, instead of or in addition to those noted here, as is known in the art. The target assembly  1070  may also be mounted on a ceramic spacer to provide electrical insulation and to permit monitoring of target current, as is known in the art. 
     While it is common in accelerators for the cell cavity lengths to increase from cell to cell, in this example, the cell cavity length is kept the same, except in the buncher cell cavity  1020   a  and the output cell cavity  1042   a . This facilitates manufacture and assembly of the half-cells, since only one size half-cell  1060  is needed (besides the buncher cell  1020  and output cell  1042 ). All the half-cells  1060  are therefore interchangeable. However, cell lengths may be varied, if desired. 
     As shown in  FIGS. 4 and 5 , an optional cooling and/or heating tube  1014  extends along portions of the exterior surface of the accelerator body  1002 . If such a cooling tube  1014  is to be used, then a water pump may be set up next to the third module  108  at the site and coupled to the cooling tube, as discussed above. The water pump could weight about 100 lbs (45 kg) or less, which may be provided in a fourth module, or the first module  102 , if desired. The cooling tube  1014  may be made of copper and have an outer diameter of ⅜ inch (9.52 mm) and a wall thickness of 0.065 inch (1.65 mm). The pump may pump water at a rate of 1 to 2 Us, at 20° C.-40° C., for example. The cooling and/or heating tube  1014  may be used for testing of the accelerator  1000 , as well. 
     Instead of or in addition to the cooling tube  1014 , cooling fins  1016  may be provided around the accelerator body  1002  for cooling. In  FIG. 4 , two rear cooling fin assemblies  1150   a ,  1150   b  are shown. Two forward cooling fin assemblies  1150   c ,  1150   d  are shown in part, in phantom. In this example, each assembly comprises fourteen fins  1016  brazed to the accelerator body  1002  forming  13  ducts for air passage. The fins  1016  in each assembly  1150   a ,  1150   b ,  1150   c ,  1150   d  are covered by a respective solid outer casing  1154 . The fins  1016  are separated by a distance of 0.3 cm in this example. Each fin has an inner diameter of 36 mm, and an outer diameter of 96 mm. Each assembly may extend 120° around the accelerator body  1002 , for example. 
     One or more fans may be provided in the third module  106 , to draw air into and through the third module  106 , over the cooling fins  1016 . As discussed above with respect to  FIG. 2 , two inlet vents  176   a ,  176   b , each containing a fan  178  may be provided. One or more guides may be coupled to or adjacent to the cooling fin assemblies  1150   a ,  1150   b ,  1150   c ,  1150   d  to guide air drawn into the third module  108 , across the fins  1016 , as discussed below with respect to  FIG. 9 . Each fin  1014  may be made from 0.015 inch (0.38 mm) thick copper sheet. The fins  1014  may be assembled into the assemblies  1150   a - 1150   d  with a copper/gold braze and brazed to the accelerator body  1002  by a copper/silicon (Cusil) braze. The total weight of the four fin assemblies  1150   a - 1150   d  is about 1 lb (0.45 kg). Fins may be arranged longitudinally along the accelerator  1002 , instead. 
       FIG. 9  is a perspective view of another example of an accelerator  1000 , with respective guides  1151  coupled to respective cooling fin assemblies  1150   a ,  1150   d ,  1150   c . The guide coupled to the cooling fin assembly  1150   b  is not visible in this view. Air drawn into the third module  108  enters a first, open end of each guide  1151  along the arrows and exits the guides into the ducts between the fins  1016  at the lower portions of the cooling fin assemblies  1150   a ,  1150   b ,  1150   c ,  1150   d . The air exits the upper portions of the ducts at the top of the cooling fin assemblies  1150   a ,  1150   b ,  1150   c ,  1150   d , carrying away heat from the fins  1016  and accelerator  1002 . Air drawn into the third module  106  by one or more fans  178  may flow into the guides  1151 . Alternatively, the guides  1151  may be coupled to a fan or fans by ducting. In one example, a 3.5 inch (9 cm), 120 CFM fan draws air into a duct with a four way splitter. Four ducts extend from the splitter, one to each guide  1151 . An example of a duct  1152  connected to a fan  178  in a vent in a wall  106   a  of the third module  106  is shown in phantom, coupled to one of the guides  1151 . It is noted that in  FIG. 9 , the vacuum pump  1013  is rotated 90° with respect to the orientation of the pump  1013  in  FIG. 4 , to accommodate the guides  1151 .  FIG. 9  also shows a support  1153  bolted to the waveguide  1007  and the anode plate  1005 , to support the anode plate. 
     Louvers and/or vents may be provided on the third module  106  for additional cooling along with or instead the cooling tube  1014  and/or the cooling fins  1016 . The third module  106  may also comprise resistive heaters, if needed, for use in cold environments. Louvers and/or vents may also be used for heating in cold environments. 
     The magnetron  126  in this example, which provides microwave power to the resonant cells within the accelerator  1002 , is a modular, X-band (9.3 kHz) magnetron, with a motor activated mechanical tuner to adjust frequency, and filament leads powered to heat the cathode surface, permitting microwave emission. X-band magnetrons used with X-band accelerators generating X-ray radiation typically generate a power of 1-1.5 MW. To reduce weight in this example, the accelerator  1002  is designed to accelerate electrons to the desired energy (in this example 0.93-0.94 MV, 1 rad/min) with a lower power magnetron  126 . In this example, the magnetron  126  generates a peak output of less than 400 KW, an average power of 200 W, at a duty cycle of 0.0005. 
     The power of the magnetron  126  in this example is about 340 KW, at a voltage of 28 KV and a current of 29 Amps. Due to losses in the waveguide  1012 , the peak power at the accelerator  1002  is less than about 320 KW and the average output power is less than about 200 W. The magnetron  126  may weigh about 10 pounds (4.5 kg), for example. 
     The accelerator  1002  is designed to operate at about 290 KW, providing a wide margin that has been found to avoid the need for mechanical tuning. Prior art accelerators typically require mechanical tuning or polishing of cells to establish accurate an resonant frequency plane. Tuning may be provided in any particular configuration, if needed. 
       FIG. 10 a    is a perspective view of an assembly  1200  comprising the magnetron  126  coupled to the accelerator  1000 . A circulator  1210 , which controls the flow of microwave fields, is coupled to the magnetron  126  by an E-plane bend. The circulator  1210  is coupled to the accelerator body  1002  through a length of waveguide  1220  that is coupled to the waveguide window  1010  shown in  FIG. 4 , through a second E-plane bend. A dry load  1230  is coupled to the circulator  1210  through an H-plane bend. The dry load  1230  absorbs reflected waves from the circulator  1210 , as is known in the art. The circulator  1210  may be an Isolator RF System, 3 GHz, 240 kWp, 120 Wavg, WR 112, circulator from Advanced Ferrite Technologies, Germany, Part No. 1-0930020503, for example, which weighs about 5 lbs (2 kg). 
     In this example, the magnetron  126 , circulator  1210 , accelerator body  1002 , and associated components are coupled to a support or “strong back”  1240  of a rigid, light weight metallic or composite material, such as aluminum, by four brackets  1262 ,  1264 ,  1266 , and  1268 . Elastomeric isolators  1250 , such as metallic or plastic springs or elastomeric material, for example, are also provided to isolate vibrations when the assembly  1200  is mounted in the third module  106 . Suitable elastomeric isolators may be obtained from Lord Corporation, Cary, N.C. For example, 206 steel multiplane platform mounts, Part Number 206P-45, may be used. According to Lord Corporation, these platform mounts, which comprises an inner portion of specially compounded rubber and an outer portion of cold rolled steel, have a maximum axial rated load of 3/16 inch (4.80 mm) deflection of 45 lbs (200 N), and an axial spring rate of 240 lbs/in (42.0 N/mm).  FIG. 10 b    is another example of an assembly  1240  supported by another strong back configuration, in which three brackets  1262 ,  1266 , and  1268  connect the accelerator  1000  magnetron  126  and associated components to the strong back  1240 . Elastomeric isolators  1250  are also shown, having a different configuration than those in  FIG. 10 a   . The strong back  1240  and isolators  1250  may weigh from about 3 lbs (1.36 kg) to about 10 lbs (4.5 kg) in total, for example. 
     The magnetron  126  may be a VMX 3045 magnetron available from CPI Beverly Microwave Division, Beverly, Mass. According to Company specifications, the VMX 3045 weighs 9.9 lbs (4.5 kg), has a rated maximum output of 380 kW and is operable at a duty factor of 0.0005. 
     Other commercially available magnetrons that may be used include the VMX 1131 Magnetron available from CPI Beverly Microwave Division, Beverly, Mass. According to company specifications, the VMX 1131 has a rated peak output of 325 kW, and typical performance at a level of 400 KW. It is said to be rated an X-band coaxial magnetron operating over a frequency of 8.5 GHz-9.6 GHz. It is also said to be rated at a duty cycle of 0.001 and 3.5 milliseconds, an anode voltage of 29 KV, an anode current of 30 A, and a 9 volt heater with a power output of 14 A. It is air-cooled by a fan and is mechanically tunable. The VMX 1131 requires 30 A, 29 KV at 320 KW and weighs 17 lbs (7.7 kg). The VMX 1131 is said to be operable after 3 minutes warm-up at air into a matched load in the temperature range of from about −55° C. to about 270° C., 40 cfm air. It has been found to operate into an accelerator at about 5 to about 30 psi SF 6 . 
     The magnetron  126  may also be a CalTube PM-1100X, a CalTube PM-1000X, or a CalTube PM-325X, provided by CalTube Labs, a unit of L3 Communications Applied Technologies, Watsonville, Calif., for example, which weigh 35 lbs (16 kg). According to specifications provided by CalTube Labs, the CalTube PM-1100X is rated at 1.5 MW peak output at 36 kV and 80 A, with 0.001 duty cycle. It is tunable over +/−25 MHz, employs an integral permanent magnet. It requires a nominal 0.66 gpm water cooling. As discussed above, a water pump may be set up proximate to the third module  106  if needed. It has a 300-second warm-up time. Also according to specifications provided by CalTube Labs, the CalTube PM-1000X is rated at 1.2 MW for 32 kV-80 A and 0.0007 duty cycle; and the CalTube PM-325X provides 325 kW peak power with 28 kV-35 A at 0.001 duty cycle. 
     The pulse transformer  128 , shown schematically in  FIG. 1 , is coupled to the electron gun  1004  and the magnetron  126 , permitting use of a lower voltage on the cable connectors, improving their reliability and durability. A suitable pulse transformer  128 , weighing about 10 lbs (4.5 kg), is available from Stangenes Industries, Inc., Palo Alto, Calif., for example. 
     The power of the magnetron  126  and the electron gun  1004  may be selectively varied in this example to vary the dose rate of the radiation beam from about 0 to about 2 rads/min, at 1 meter, a depth of dose maximum (dmax). The power may be controlled by the controller  108  under the control of the pendant  110 , for example. 
     The weight of the X-ray head  118  in this example is from about 35 lbs (16 kg) to about 55 lbs (25 kg) or 60 lbs (27 kg). If the Storm Case IM 2950  170  is used, for example, the third module  106  would weigh from about 55 lbs (25 kg) to about 75 lbs (34 kg) or 80 lbs (36 kg), which may be readily carried by one or two people. 
     A man-portable radiation scanning source  100  in this example would therefore weight from about 200 lbs (91 kg) to about 250 lbs (113 kg), depending on the case  170 . A man-portable radiation scanning system  100  including such a man-portable radiation source  100  and a digital imager may therefore weigh from about 235 lbs (107 kg) to about 300 lbs (136 kg). 
     In another example, only two modules are provided, the first module  102  and a second module containing the modulator  114  and the X-ray head  118 . In this example, the second module could weigh from about 125 lbs (57 kg) to about 150 lbs (68 kg), which may be carried by two people to the site. Total system weight could be reduced by eliminating one case  107  and the cables necessary to couple the second module  104  to the third module  106 . Placing the modulator  114  closer to the X-ray head  118  also reduces power losses along long cables. 
     Operation 
     In operation, microwave energy generated by the magnetron  126  is provided to the cavities  1020   a - 1042   a  of the accelerator body  1002 , via the rectangular opening  1010   a  of the waveguide  1010 , which in this example is coupled to the sixth accelerating cavity  1032   a . (See  FIGS. 4 and 5 ). 
     The microwave energy propagates through the accelerator body  1002 , from one cavity  1020   a - 1042   a  to the next, through the coupling cells  1055 - 1088 , setting up alternating positive and negative portions of standing electromagnetic waves in the buncher cell  1020 , the full accelerating cell cavities  1022   a - 1040   a , and the output cell cavity  1042   a . The standing waves pass through zero in each coupling cell  1055 - 1088 . A high voltage pulse is applied to the electron gun  1004  by the modulator  114  in the second module  104  via a high voltage connector  1004   a , as is known in the art. 
     The aperture  1056  focuses electrons from the electron gun  120  as they enter the buncher cell  1020 . The electrons are accelerated by the time varying electromagnetic standing waves in the buncher cell cavity  1020   a . Since the electrons are only accelerated half the time in the field in the buncher cell cavity  1020   a , the electrons “bunch.” The phase at which they bunch, the capture fraction, and the radial focusing of the electrons are determined by the cell geometry, which is discussed in more detail, below. The electron beam converges as it enters the buncher cell  1020 . As the beam diverges within the buncher cell cavity  1020   a , it receives a focusing “kick” by radial forces generated by the standing electromagnetic waves in the buncher cell cavity  1020   a . The beam then passes through the iris  1064  into the first full cell cavity  1022   a , where it diverges again. Radial forces of the standing electromagnetic waves in the first cell cavity  1022   a  again focus the beam. The beam is also accelerated by longitudinal forces caused by the standing electromagnetic waves in the cell. The electron beam then passes through the downstream iris  1064  of the first full resonant cell  1022  into the second full cell cavity  1024   a . The diverging of the beam, the focusing of the beam, and the acceleration of the beam are repeated in each subsequent accelerating cavity  1022   a - 1040   a , and the output cavity  1042   a.    
     The phase of acceleration need not be perfect in the buncher cell cavity  1020   a , the first cell cavity  1022   a , and the subsequent cell cavities  1024   a - 1042   a . Instead, in one embodiment, the phase is optimized such that, for equal length cells, the net phase-error over the length of the accelerator body  1002  is minimized and the spectrum is thereby narrowed, providing for efficient conversion of microwave energy into X-rays. In the known prior art, in contrast, phase optimization is attempted in each cell cavity. This typically requires a multiplicity of unique parts, including a plurality of different sized cells, which increases design complexity and cost. Such accelerators may also be more sensitive to manufacturing and operating parameters. As discussed below, the structure of the parameters of the accelerator  1000  are adjusted to provide stable operations with low sensitivity to manufacturing and operating parameters. It is noted that focusing is achieved in this example without an external solenoid, reducing the size and weight of the accelerator  1000 . An external solenoid may be provided, however, if the additional size and weight of the accelerator  122  may be tolerated. 
     The standing waves accelerate the electrons as the electrons pass through each cell cavity  1022   a - 1042   a . The acceleration per cell cavity and number of cell cavities are arranged to provide the electrons with the desired peak acceleration. In this example, the cell cavities  1022   a - 1042   a  accelerate the electrons to the desired 0.93 MeV-0.94 MeV. Since low power is used to reduce the size and weight of the modulator  114  and the X-ray head  118 , including the accelerator  1002  and the magnetron  126 , the electrons in the electron beam are accelerated slowly. In this example, ten (10) full accelerating cells  1022   a - 1040   a  are required to accelerate the electrons to the desired energy. 
     The accelerated electrons exit the accelerator body  1002  through the output cell  1042  and the passage  1064 , toward the tungsten button  1074  in the target assembly  1006 . Impact of the accelerated electrons with the tungsten button  1074  generates radiation having a peak energy of about 0.93 MeV-0.94 MeV, by the Bremsstrahlung effect. Unless collimated, the generated radiation beam will be emitted from the tungsten button  1074  and out of the third module  108  in all directions. 
     The half value layer (“HVL”), which is the length of steel required to reduce an X-ray dose or intensity by one-half, is an indication of the energy of the X-ray beam and the quality of the X-ray spectrum. In this example, the X-ray radiation generated by the radiation source has an HVL (“HVL”) of from about 0.57 inches (14.5 mm) to about 0.62 inches (15.7 mm) with power peaking the spectrum at about 0.9 MV. Operating at 250 Hz and pulse-width of 2 us, for a duty cycle of 0.0005, the dose-rate output in a 10 cm×10 cm field at 1 m, with probe at d max  in solid-water, is in the range of 1 R/m. An 18 gauge (7 mm diameter) copper wire may be imaged through 3 inches (7.6 cm) of steel, with a wide variety of commercially available X-ray film, as well as a digital panel. 
     The HVL is affected by the “quality” of the X-ray spectrum, which refers to the spread of the energy spectrum. To achieve this HVL with the man-portable X-ray source  100  in this example, the electron beam has a relatively narrow energy spectrum. In this example, 40% of the electrons in the electron beam lie within 6% of the peak acceleration energy of 0.93 MV-0.94 MV. 
     A second figure of merit used to quantify the operation of an accelerator is the mean of the energy E n  raised to the 1/nth power (&lt;E n &gt; 1/n ), where E is the energy in MV and n=2.7, compared to the peak energy in the spectrum. This value determines the X-ray dose output, according to yield Y=0.07I avg E 2.7 , where I avg  is the average current in micro-amps, and the yield is expressed in Rad/min/microamp. This value has also been found to correlate well with the HVL figure for the X-ray beam, which is also an aggregate measure. Depending on the operating power of the magnetron  126  and the electron gun  1004 , this figure is over 0.64 MV or 72% of the peak. Considering the size, weight, and power constraints on the accelerator  1002 , this is a very “tight” radiation beam. 
     Another factor affecting the HVL is spot size of the electron beam on the target. In this example, the spot size of the radiation beam, which encompasses 75% of the electron beam on the target, has a diameter of less than 2 mm. 
     Three ratios related to the structure and operation of the accelerator  122  also contribute to achieving the desired HVL and spot size in this example. One is the ratio a between the peak amplitude of the field in the first full cell cavity  1022   a  to the peak amplitude of the field in the buncher cell cavity  1020 , referred to herein as the “field step ratio,” the second is the ratio between the length L b  of the buncher cell cavity  1020   a  and half the free space wavelength (λ/2), referred to herein as the “buncher cell ratio,” and the third is the ratio between the cell cavity period L p  and half the free space wavelength (λ/2), referred to herein as the “cell period ratio.” 
     The field step ratio a affects the balance between focusing and defocusing of the electron beam from the buncher cell  1020  to the first cell  1022 . The field step ratio a also affects the phase of the electrons exposed to the standing electromagnetic fields in the downstream cells. In one example, the peak field ratio is greater than one (1) and less than two (2). For example, the peak field in the buncher cell may be about 70% of the peak field in the first full cell cavity  1022   a , or the ratio may be from about 1.2 to about 1.5, such as from 1.3 to 1.4, for example. 
       FIG. 11  is a graph of energy E (arbitrary) versus Z (cm) for the accelerator  1000 . The energy is normalized to vary from 1.0 to −1.0. Z (cm) corresponds to the distance from the tapered aperture  1056 . The cell corresponding to the distance Z is also indicated. The peak energy in the buncher cell cavity  1020   a  is near the anode plate  1005 . The peak energy in each full cell cavity  1022   a - 1042   a  is found at a distance Z from the tapered aperture  1056  to the center of each full cell cavity. The energy passes through zero (0) in each coupling cell  1055  and  1070 - 1088 . As shown in  FIG. 11 , the peak energy of the field in the first buncher cell cavity is about 0.7 and the peak energy of the field in the first full cell cavity  1022  and subsequent cell cavities  1024   a - 1042   a  is +/−1.0. A field step ratio within these ranges has been found to launch the electron beam from the buncher cell cavity  1020   a  to the first full cell cavity  1022   a  at an appropriate phase for the selected cell cavity length (in this example from about 0.78λ/2 to about 0.82λ/2, where λ is the free space wavelength) and the overall length of the accelerator  1000  (14.3 cm in this example). Use of such a field step ratio a to provide simultaneous control of spectrum size and spot-size has in the past been accomplished by varying slot length in side coupled cavities, as in U.S. Patent Publication No. US 2005/0134203A1, which issued on Jul. 15, 2008 bearing U.S. Pat. No. 7,400,093, for example. 
     In one embodiment, the field step ratio a is controlled by the diameter of the buncher iris  1054 , which acts as a coupling element for the electromagnetic field propagating through the resonant cells  1022   a - 1042   a . With a buncher cell cavity  1020   a  having a first diameter D 1  and an iris  1054  having a second diameter D 2 , as indicated in  FIG. 6 a   , varying the iris diameter to a third diameter to vary the field step ratio a changes the steady state amplitude in the buncher cell cavity  1020   a . The inner diameter of the buncher cell cavity  1020  is therefore adjusted to correct the frequency shift resulting from the change in iris diameter. Alternatively, the field step ratio a may be controlled by introducing such a frequency error. In this example, the buncher cell iris diameter D 2  is 6.52 mm and the buncher cell cavity maximum diameter D 1  is 26.73 mm. The remaining cell cavities  1022   a - 1040   a  have maximum diameters D 4  of 27.07 mm and iris  1066  diameters D 5  of 6.44 mm, as indicated in  FIG. 7 . 
     The second ratio is between the length L b  (see  FIG. 6 a   ) of the buncher cell cavity  1020   a  and half the free space wavelength (λ/2), which is referred to as the buncher cell ratio. The buncher cell ratio affects the relation between the phase of the electron beam in the buncher cell cavity  1020   a  and the phase of the field in the first full cell cavity  1022   a , and focusing. The free space wavelength λ at 9.3 GHz is 32 mm. The buncher cell length L b  is in effect determined by the depth De 1  of the buncher cell cavity  1020   a , which, in this example, is less than the depth of the other cell cavities  1022   a - 1040   a . This has been found in simulation to facilitate the arrival of the slow moving electrons injected into the buncher cell cavity  1020   a , into the next cell cavity  1022   a  at optimal phase. In this example, where the gun voltage is low (29 kV-30 kV), the length L b  of the buncher cell cavity is about ⅔ the depth De 4  of the other cell cavities  1022   a - 1040   a . In one embodiment, this ratio is less than one-half (½). In one example, the length L b  of the buncher cell is 4.81 mm and the buncher cell ratio is 0.3 ((4.81 mm)/(½)(32 mm)=0.3). 
     The third ratio is between the cell period L p  and half the free space wavelength (λ/2), which is referred to as the cell period ratio L p  (λ/2), (where the cell period L p  is the distance between the center of one accelerating cell to the center of an adjacent accelerating cell, as shown in  FIG. 4 ). In one example, the cell period ratio L p /(λ/2) is adjusted to provide a sharp spectrum with the chosen field step ratio α and the second buncher cell ratio L b /(λ/2). Cell length L p  affects the quality Q of the accelerator body  1000 . Too short a cell length L p  spoils the Q, resulting in increased power requirements at a given energy and requiring a more powerful modulator  114  and magnetron  126 , increasing the size and weight of the X-ray head  118 . In this example, the cell period L p  between adjacent accelerating cavities  1022   a - 1040   a  is less than about one-half the free space wavelength (λ/2). With this relatively short cell length, electrons traveling well below the speed of light and accelerated in one cell will arrive at the next cell in the proper phase relative to the standing electromagnetic microwave field, for additional acceleration. The optimal available microwave cell period ratio will depend on the intended range of operating electron gun voltage, the desired energy of the electron beam, and the microwave power provided by the magnetron  126 . In this example, the cell period ratio is less than 1 and greater than 0.70. The cell period ratio may be about 80% of (λ/2), such as from about 0.78 to about 0.82 of (λ/2), for example. The ratio is typically 1.0 in known high energy accelerators. 
     The actual cell period L p  selected may depend on the field step ratio α. If the field step ratio is 1.3 in the accelerator  1002  of this example, the cell period is 12.5 mm, and the cell period ratio is 0.78 ((12.5 mm)/(½)(32 mm)=0.78). Adjustment of the cell length in the design facilitates phasing of the electrons with the standing electromagnetic waves in the accelerator  1002 . 
     The magnetron  126  is selected to drive the accelerator cavities  1020 - 1042  at the selected frequency. The frequency of the microwave energy is selected such that the chain of coupled resonant cells are excited by standing waves with less than π/2 radian phase between each coupling cell and adjacent accelerating or resonant cell (period length). In this example, the frequency is 9.3 GHz and the buncher diameter is 26.65 mm. 
     To provide a smaller and lighter accelerator  1000  with a good Q, such as 7700 in this example, cavity depth of the full resonant cell cavities  1022   a - 1040   a  is increased as much as possible at the expense of iris thickness and depth. In this example, the cavity depth is 4.78 mm in the half period cells and 3.32 mm in the buncher cell cavity  1020   a . The iris thickness of each half-cell is small in this example (about 1 mm). This reduces the number of accelerating cells required to accelerate the electrons to the desired energy, and therefore the length. 
     As discussed above, the aperture  1056  scrapes off about half of the electron beam as the beam is injected into the buncher cavity  1020   a . The capture fraction by the accelerator body  1002  is from about 10% to about 15%. The resulting lower beam current lowers the power requirements of the accelerator  1000 , facilitating the size and weight reductions discussed above, and the use of batteries  110 . 
     Summarizing certain dimensions and characteristics of components of the accelerator body  1002  in this example: 
     the accelerator body  1002  has an outer diameter of 35 mm; 
     the buncher cell cavity  1020   a  has a maximum diameter D 1  of about 26.71 mm; 
     the buncher cell iris  1054  has a diameter D 2  of about 6.52 mm; 
     the buncher cell cavity  1020   a  has a depth De 1  of 3.32 mm; 
     buncher cell length L b  is 4.81 mm; 
     each half-cell  1060  has an outer diameter of 35 mm; 
     the first full cavity  1022  has a maximum diameter D 4  of about 27.07 mm (matching the maximum inner diameter of the half-cell  1060 ); 
     the first full cavity  1022  has a depth DeF (see  FIG. 5 ) of about 9.56 mm (double the depth De 4  of the half-cell  1060 ); 
     the coupling cavities  1055  and  1070 - 1088  have a maximum inner diameter D 6  of about 26.65 mm (matching the diameter of the coupling cavities in the half-cells  1053 ,  1060 ); 
     the coupling cavities have depths of about 0.98 mm (double the depths De 3 , De 6  of the buncher cell  1053  and the half-cells  1060 ); 
     the iris  1064  have diameters D 5  of about 6.44 mm; 
     the circumferential edge of the iris in this example is radiused; 
     the thickness De 6  of the iris is about 1 mm; and 
     the cavities have radiused portions that are fully radiused. 
     As discussed above, the depth De 1  of the buncher cell cavity  1020   a  (3.32 mm) and the depths De 4  of the following half-cell cavities  1062  (4.78 mm) are different. The buncher cell cavity iris diameter D 2  (6.52 mm) is also different than the following half-cell iris diameters D 5  (6.44 mm). The smaller iris diameters D 5  of the half-cells  1060  provide wider modal separation. The accelerator  1000  is therefore less sensitive to thermal effects, decreasing problems during accelerator warm up, for example. 
     The Modules 
     The modules  102 ,  104 ,  108  protect the system components from dust, rain, and shock. The modules  102 ,  104 ,  108  may comprise stiff or flexible material. Each module may include recessed handles, recessed/protected vents, and/or recessed connectors with caps to protect contacts from dust. In one example, the modules  102 ,  104 ,  106  comprise polymers, such as polyurethane or glass filled polyethylene, which are durable, moldable, and lightweight. The modules  102 ,  104 ,  106  may be stackable. 
     As discussed above, components within a module, such as the third module  106 , may be coupled to a strongback of material, as discussed above with respect to  FIGS. 10 a  and 10 b   , to support and mechanically isolate the components and protect them from physical shock, movement, falling, etc. Aluminum may be used, for example. The strong back may be directly connected to the case or may be coupled to the case by elastomeric isolators  1250 , such as springs or resilient material. In addition, the strong-back facilitates field testing, eases maintenance, and facilitates field replacement of the superstructure. The first and second modules  102 ,  104  may also include a strong back and elastomeric isolation, if desired. The strong back may be a rigid, lightweight material, such as aluminum. 
     Electromagnetic interference (EMI) shielding may be provided in any or all modules  102 ,  104 ,  106 . EMI shielding may be provided by copper or silver paint, for example. EMI shielding may also be provided by vacuum deposited aluminum (VDA) on the inner and/or outer surface of the modules  102 ,  104 ,  106 , for example. Integrally molded wire mesh may be provided in the vents or other such openings, for example. 
       FIG. 12  is a cross-sectional view of another example of the internal configuration of the X-ray head  118  in the third module  106 . In this example, the module  106  comprises a case  2010  of a lightweight, deformable material, such as plastic. An aluminum bar  2020  is attached to an upper wall  2030  of the case  2010 . The bar may be about 3 inches (76 mm) wide and about ½ inches (12.7 mm) thick, for example. The aluminum bar  2020  may be clipped to the upper wall  2030  by clips  2035 , for example. The accelerator  1000 , magnetron  126 , and other components of the X-ray head  118 , are suspended from the aluminum bar  2020  by shock mounts, such as springs  2040 . Double springs may be used to decrease swaying. The accelerator  1000  and the magnetron  126  are suspended such that there are clearances  2050 ,  2060  below the accelerator  122  and the magnetron  126 , which allow for movement of these components within the case  2010  and deformation of the case  2010 . A second, rigid aluminum bar may be provided between the springs and the first bar. 
       FIG. 13  is a cross-sectional view of another example of the internal configuration of the X-ray head  118  in the third module  106  that is similar to the view of  FIG. 12 , except that a second aluminum bar  2021  is provided, coupled to the first bar by springs  2041 . The components of the X-ray head  118  are coupled to the second bar  2021 . 
     Two fans  2070 ,  2080  are attached to the aluminum bar  2020  adjacent to the air inlet openings through the case  2010 . A flexible duct  2090  extends from the fan  2070  to the magnetron  126 , to provide cooling air to the magnetron. A flexible duct  2100  also extends from the fan  2070 . Flexible tubes  2110  extend from the duct  2100  to various locations around the accelerator  1000 , including to the cooling fin assemblies  1150   a - 1150   d , to cool the accelerator body  1002 . Guides, such as guides  1151  coupled to the cooling fin assemblies  1150   a - 1150   d , are not provided in this configuration. 
     Air outlet openings  2120 ,  2130  are provided for air to flow out of the case  2000 . A plastic rain cover may be provided over the openings  2075 ,  2085  to the fans  2070 ,  2080 . Radio-frequency interference screens may be provided in the air openings  2120 ,  2130 . The inner and electromagnetic radio-frequency shielding, as well. 
     The power/control cables  130 / 132  may be coupled to the case  2010  via a ruggedized connector, such as those provided by Caton Connector Corporation, Kingston, Mass. 
     One handle  2140  is provided in this example. The handle  2140  folds in when not in use. 
     As mentioned above, the case may also include louvered vents and/or fans for thermal control, as well. Hot and/or cold kits could also be provided. 
     The case  2000  may be used underwater by attaching a snorkel to the air openings outlets  2120 ,  2130 , an inlet to attach an air tank hose to each air inlet opening  2075 ,  2085 , for providing cooling air, and gaskets at the case tab. High voltage hold off with moisture may be provided by additional potting, if needed. 
     One of ordinary skill in the art will recognize that changes may be made to the embodiments described above without departing from the spirit and scope of the invention, which is defined by the claims below.