Abstract:
A standing wave electron beam accelerator and x-ray source is described. The accelerator has a plurality of on-axis resonant cells having axial apertures electrically coupled to one another by on-axis coupling cells having axial apertures. The accelerator includes a buncher cavity defined in part by an apertured anode and a half cell. The buncher cavity is configured to receive electrons injected through said anode aperture and r.f. focus them into a beam which is projected along the axis through said apertures. An x-ray target is supported in spaced relationship to said accelerator by a support having a smaller diameter than the accelerator.

Description:
BRIEF DESCRIPTION OF THE INVENTION  
         [0001]    This invention relates generally to x-ray sources employing standing wave electron beam accelerators, and more particularly x-ray sources employing compact high-energy electron beam accelerators having low-leakage x-ray radiation to minimize shielding requirement.  
         BACKGROUND OF THE INVENTION  
         [0002]    Standing wave type linear accelerators generate high-energy electron beams which strike metallic targets to generate x-rays. The linear accelerators have a series of linearly arranged cavity resonators separated by apertured walls. The apertures define a passage through which the electron beam travels to interact with standing waves supported in the cavities. The beam gains energy as it travels through successive resonant cavities. The electrons are injected into the first cavity at relatively low energy by an electron gun. The electron beam is accelerated as it travels through the cavities. Electrons which strike cavity walls during their travel through the accelerator not only reduce the electron current reaching the x-ray target but also generate undesirable leakage x-ray radiation. The electrons striking the target generate x-rays which are emitted in all directions. Forward traveling x-rays are intercepted by a beam blocker which includes an aperture which defines the shape of the desired beam. The accelerator and the target region are shielded to absorb the leakage x-ray radiation and the target radiation except for the desired radiated beam. The x-ray shielding adds weight and size to the x-ray source.  
         SUMMARY OF THE INVENTION  
         [0003]    It is a general object of an invention to provide a compact linear accelerator in which the beam energy is maximized and leakage x-ray radiation is minimized.  
           [0004]    It is another object of the invention to provide a buncher cell with an anode plate which incorporates rf focusing to establish beam size with good electron capture.  
           [0005]    It is another object for an invention to provide a linear accelerator with an extended x-ray target which enables shielding of reduced size and weight.  
           [0006]    It is a further object of the present invention to provide a linear accelerator having ultra-low leakage x-ray radiation.  
           [0007]    It is a further object of the present invention to provide on-axis coupling cells to insure undistorted circular beams by eliminating asymmetric perturbations caused by side cavity coupling holes.  
           [0008]    It is a further object of the invention to provide an accelerator having a large aperture beam tunnel to minimize electron interception and reduce leakage x-ray radiation.  
           [0009]    It is another object of the invention to provide a compact linear accelerator having low leakage radiation thereby reducing the amount of shielding required with the consequent reduction of the overall size and weight of the x-ray source.  
           [0010]    It is another object of the invention to provide an x-ray target that is moved away from the accelerator to simplify target shielding.  
           [0011]    It is still another object of the present invention to provide a compact linear accelerator which is simple in design and easy to manufacture.  
           [0012]    The foregoing and other objects of the invention are achieved by an x-ray source having a linear accelerator including an electron source that injects electrons into a buncher cell configured to capture and rf focus the injected electrons to establish an electron beam, linearly arranged resonant large-aperture cells that support standing waves through which the beam travels to interact with the standing waves and be further accelerated, and an extended target which generates x-rays in response to the electron beam. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention will be better understood from the following descriptions when read in conjunction with accompanying drawings in which:  
         [0014]    [0014]FIG. 1 is a longitudinal cross-sectional view of a standing wave electron beam accelerator and x-ray source;  
         [0015]    [0015]FIG. 2 is a longitudinal cross-sectional view of the standing wave electron accelerator and x-ray source taken at 90 degrees with respect to the cross-sectional view of FIG. 1;  
         [0016]    [0016]FIG. 3 schematically shows the shape of the electron beam as it is injected in to the buncher cavity and as it travels through the linearly-arranged resonant cavities;  
         [0017]    [0017]FIG. 4 shows a longitudinal cross-sectional view of an electron accelerator and x-ray source in accordance with another embodiment of the invention; and  
         [0018]    [0018]FIG. 5 is a longitudinal cross-sectional view of the accelerator details of still another embodiment of the invention; and  
         [0019]    [0019]FIG. 6 is a longitudinal cross-sectional view of an x-ray source and its shielding. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 is an axial sectional view of an x-ray source  7  including a standing wave electron beam accelerator structure  8  and extended target  9  in accordance with one embodiment of the present invention. It comprises a chain of electrically coupled resonant cells or cavities. The cells comprise a buncher cell  11  and in-line resonant cells  12 ,  13  and  14 . The cells are electrically coupled by on-axis coupling cells  16 ,  17  and  18  formed by joining facing half-cells. Electrons are injected into the buncher cell  11  by an electron gun  21 , which includes an anode plate  22  that forms one wall of the buncher cell  11 . The other walls of the buncher cell are formed by the cup-shaped half-cell  23  which includes an iris or opening  24 . The half cell includes an outer recessed region  26 . Each of the remaining cells  12 ,  13  and  14  are formed by identical cup shaped half cells  27  which include beam tunnel irises or openings  28  and outer recesses  29 . When the half-cell  23  and anode plate  22  are joined to one another they form the on-axis buncher cell  11 . On-axis resonant accelerating cells  12 ,  13  and  14  are formed by joining cup-shaped members  27 . Recesses  26  and  29  form the on-axis coupling cells  16 , while recesses  29  form coupling cells  17  and  18 . The axially aligned irises or openings  24 ,  28  are aligned with the axis of the electron gun and form a tunnel for passage of the axial electron beam  31 . The beam  31  strikes a tungsten target or button  32  at the end of an extended coaxial water-cooled target assembly  33 . Microwave energy is applied to the central resonant cell  13  through an iris  34  (of any shape) via a rectangular waveguide  36 , FIG. 2. Standing waves are induced in the resonant cells by the applied microwave energy. Operating voltages are applied to the electron gun via a high voltage connector  37 . The linear accelerator may be water cooled as illustrated by the tubing  38 .  
         [0021]    The extended water-cooled target-assembly  9  may be electrically isolated from the accelerator by a ceramic insulator  41 . The target button is supported by coaxial conducting members  42 . The ceramic members are protected by a metal shroud  43 . The target is water cooled via the water cooling lines  44 , FIG. 2. The cooling water flows between the coaxially arranged ceramic members  42 . The linear accelerator is evacuated via tubulation  46 . The accelerator may include electrical steering coils  47  for guiding the electron beam.  
         [0022]    The frequency of the microwave energy is selected such that the chain of coupled resonant cells are excited with standing waves with a π/2 radian phase between each coupling cell and adjacent accelerating or resonant cell. Thus, there is π radian shift between adjacent accelerating resonant cavities or cells  11 ,  12 ,  13  and  14 . The π/2 mode has several advantages. It has the greatest separation of resonant frequency from adjacent modes, which might be accidentally excited. Also when the chain is properly terminated there are very small electromagnetic fields in the coupling cells  16 ,  17  and  18  so that the power losses in these non-interacting cavities are small. The space between the resonant cavities is about one-half of a free space wavelength so that electrons accelerated in one accelerating cell will arrive at the next accelerating cell in the proper phase relative the microwave field for additional acceleration. After being accelerated the beam  31  strikes the x-ray target button  32 . Alternately, the linear accelerator may be provided with a thin metal window, which transmits electrons for other radiation purposes. The members  23  and  27  forming on-axis resonant coupling cells are of identical design and have mirror image symmetry whereby all of the resonant cavities will be substantially the same. Furthermore, the cup-shaped members  23  and  27  are easy to fabricate and the accelerator is easy to assemble.  
         [0023]    In accordance with one feature of the present invention, the buncher cavity  11  is configured to bunch and focus the injected electrons to form a beam and to establish its size while capturing the maximum number of electrons injected into the cavity. The electrons from the electron source are focused at location  51  within the anode aperture  52 . This aperture has a trumpet shape which bunches and captures the electrons as they are injected into the buncher cell  11 . To this end, the anode plate  22  has a thickness that places the electron waist, FIG. 3, at the optimum location  51 , for later rf focusing. Focusing is achieved without an external solenoid. The trumpet-shaped anode aperture  52 , FIGS. 1 and 2, opens into the buncher cell to establish rf fields within the buncher cell which cause the beam to be focused. The beam expands  53  within the trumpet and is focused by the large radial fields it then encounters (FIG. 3). The beam is then rf refocused  54  to establish the beam size  56 , FIG. 3, at the iris or aperture  24 , FIG. 1. The buncher cell length is designed to place the captured beam near the crest of the rf accelerating field within the buncher cavity. Plateau on shorting plate  57  formed on the wall of the anode compensates for detuning due to the trumpet. The combination of trumpet, plateau and cavity geometries provides a resonantly tuned, high Q cell necessary for low power operation and short cell length necessary for low voltage injection. The on-axis coupling cells  29  provide additional focusing. The bi-periodic design permits reduced sensitivity to tuning errors. Preferably, the irises and beam-passing tunnel are of large diameter to minimize stray radiation caused by interception of stray electrons. We have found that, at the design operating voltages, less than 0.6% of the injected beam is lost in the guide. The remainder of the beam is either rejected at the buncher cell or makes its way to the target. This results in reduced guide glow (stray radiation), which minimizes the required x-ray shielding required. Furthermore, the accelerator does not use external coupling cavities. As a result, the diameter of the accelerator is reduced, which enables shielding to be located close to the accelerator body, significantly reducing the volume and weight of the shielding material. The accelerator delivers a converging beam to the extended target.  
         [0024]    An alternate construction of the extended target is illustrated in FIG. 4 where like reference numerals have been applied to like parts. The extended target comprises a tapered extended x-ray target support  61  that is mounted to the accelerator by a mounting flange  62 . The target support may be a dense material such as Elkonite, for improved shielding, or copper. The target is conduction-cooled simplifying the manufacturing process and thereby reducing manufacturing costs. The tapered walls allow a gradual interception of outlying electrons and enables increasing thickness of shielding around the target button. The small radius of the extended target in comparison to that of the accelerator permits placing the x-ray shielding closer to the target and minimizes the weight and size of the accelerator and x-ray source and shielding assembly.  
         [0025]    Another embodiment of the present invention is illustrated in FIG. 5 where like reference numerals have been applied to like parts. The buncher cavity or cell  11  and the first cell or cavity  12  are 180 degrees or π radians apart in phase. Use of the π mode electron capture section or cell  12  coupled to the π/2 downstream cells permits a sharper energy spectrum for low injection voltage, while maintaining the high quality factor (Q) desired to minimize power requirements. The end result is bunching, phasing and focusing of injected beam electrons with minimal guide glow. Low injection voltage permits low radiation output at high energy.  
         [0026]    [0026]FIG. 6 schematically shows shielding associated with the embodiment of FIG. 4. The accelerator  10  is shown encased in shielding material  66 , and the extended target is shown in shielding material  67 . Shielding material  68  and any associated beam blocker shields against unwanted radiation other than desired radiation emitted in the forward direction. The shielding material can be lead or, to reduce size, a dense material well-known in the shielding art. Thus there has been provided a compact efficient low stray radiation linear accelerator and x-ray source.