Abstract:
An x-ray tube comprising an electron gun assembly having an electron gun container housing an electron generator for generating electrons in a first direction along a first axis. The beam of electrons impinges upon an anode which emits x-rays in response to the beam of electrons. The gun container is characterized by having a discharge end comprising a solid spherical shape.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an x-ray tube. More specifically, the present invention relates to an x-ray tube configuration capable of generating high intensity x-rays that emanate from a small focal spot without a loss of reliability. 
     2. Description of the Related Art 
     X-ray tubes have a number of applications which involve the treatment or analysis of a sample, for example, industrial imaging, analytical instruments and medical imaging. For such applications, it is often desirable to have an x-ray tube which has a long service life, which is capable of forming a small focal spot and which is also capable of generating a high intensity of x-radiation at the sample. 
     X-ray tubes generally include an electron gun and an anode. A beam of electrons generated by the electron gun is focused to a focal spot on the anode, and x-rays are generated by the interaction of the beam of electrons with the atoms of the anode. These x-rays are generated in all directions emitting from the anode in the region surrounding the focal spot. Typically, the anode is substantially surrounded by an evacuated housing in which a window is formed to permit some of the x-rays to pass out of the housing, the window typically comprising a thin foil of a low atomic number metal, such as beryllium or aluminum, having a high transmission coefficient for x-radiation. Illustrative prior art x-ray tube structures in this regard are described, for example, in U.S. Pat. Nos. 5,751,784 and 5,563,923. 
     A commercially available x-ray tube  1  is schematically illustrated in FIG. 1 which employs a gun container  2  having a flat surfaced discharge end  3 , i.e., gun snout, relative to the electron beam direction x, and x-rays emitted from anode  4  pass through an x-ray transmissive window  5  formed in a side of the hermetically sealed and evacuated envelope  6 . The gun container  2  houses a cathode and filamentary heater used to generate an electron beam in a generally known manner. The gun snout  3  is flat in the sense it is a planar surface oriented perpendiculary to the beam direction x with a central aperture (not shown in the view) for transmission of the electron beam out of the gun container  2 . Using the flat surfaced discharge end  3 , focal spot sizes are yielded on the anode  4  (i.e., target) which tend to be relatively large, e.g., exceeding 20 microns in largest diameter, and which tend to be heavily limited by chromatic aberration. One approach to reducing the spot size is to reduce the working distance between the gun snout and anode. Smaller working distances produce smaller magnification for the beam; hence, a smaller spot at the anode. However, investigations conducted by the present inventors have shown that a flat gun snout tends to produce high field emission and instability when positioned nearer a target due to high electric fields. Consequently, such x-ray tubes with flat gun container snouts are focus limited due to chromatic aberration and instability issues, especially if attempts are made to position the gun assembly in closer proximity to the target. 
     It would be desirable to be able to position a gun snout in very close proximity to the target in order to reduce beam size and provide small spot focusing yet without unduly increasing the structural or operational complexity of the x-ray tube. An object of this invention is to provide an x-ray tube endowed with enhanced electron beam focusing performance and capabilities, and, more particularly, provide an x-ray tube with an electron gun positionable in close proximity to the target at reduced surface fields to achieve smaller focal spots without incurring instability. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an x-ray tube is provided having an electron gun container having a spherical shaped snout for discharging electrons from the gun container towards an anode (target) where x-rays are generated. 
     More particularly, in one embodiment of this invention, there is an x-ray tube including: 
     an electron gun assembly comprising an electron gun container housing an electron generator for generating electrons in a first direction along a first axis, where the gun container has a gun snout with an egress aperture through which the electrons are discharged from the gun container; and 
     a target for generating x-rays upon being impinged by the electrons that are discharged from the gun snout; 
     and in which the gun snout of the x-ray tube is characterized by having a solid spherical shape. 
     The spherical surface of the gun snout has its concave (inner) side facing the electron generating means within the gun container and its convex (outer) side facing the anode (target) located outside the gun container. For purposes of this application, the term “spherical” refers to a gun snout having a surface profile all points of which are substantial equidistant from a common imaginary center of radius located inside the gun container, i.e. comprises a substantially constant radius of curvature. Thus, the edge profile of the spherical snout of the invention is curvilinear, and not a straight line such as would be the case with a conical shape. As a matter of course, the gun snout will be semi-spherical, as it does not form a complete sphere. Preferably, the spherical surface is arranged such the electron beam passes through the geometric center of the space defined within the solid spherical surface before exiting from the snout aperture. The spherical surface preferably has a decreasing cross-sectional diameter in the direction of the electron beam towards the exit aperture of the snout. In this way, the spherical gun snout presents an overall “lens” profile. The gun snout preferably is a substantially hemispherical dome-shape with the gun container having a separate cylindrical portion merged with the dome to form an overall tubular-shaped gun container with a rounded, aperture tip for discharging the electron beam towards the anode (target). 
     Advantageously, the x-ray tube also includes at least one accelerating electrode to provide a focus grid through which the electrons pass before reaching the snout situated within the gun container at a location between the electron generating means (e.g. a heated cathode), and the egress aperture of the gun snout of the gun container. The focus grid includes a central opening through which the electrons pass and accelerates the electrons emitted by the cathode and converges the electrons into an electron beam along the first axis. A separate electrode situated between the cathode and the focus electrode functions to control the beam current. 
     The inventive x-ray tube, as equipped with such a spherical discharge snout on the gun container, enhances the electron beam focusing capability of the x-ray tube. The inventive x-ray tube with the spherical gun can be operated in a highly stable manner with beam diameters generated at the x-ray target of relatively small dimension, e.g., less than approximately 10 micron focal spot sizes in largest dimension. The spherical shape provides the lowest possible surface field on the snout, thereby reducing field emission, even at very close distances and high field conditions. The inventive spherical gun snout configuration enables the closest possible spacing and approach of the electron gun to the high electric fields of the target (anode) without suffering focus aberration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of the invention will now be described in greater detail with reference to the accompanying drawings, which are provided by means of example only and in which: 
     FIG. 1 is a sectional view of a commercially available x-ray tube having a flat gun snout. 
     FIG. 2 is a sectional view of an assembled x-ray tube of an embodiment of the invention. 
     FIG. 3 is a sectional view of an x-ray tube of an embodiment of the invention during assembly of the body parts. 
     FIG. 4 is an exterior side view of the x-ray tube being assembled of FIG. 2, including a partial cut-away view of the mount of a vacuum line to the tube body. 
     FIG. 5 is a sectional of an x-ray tube of an embodiment of the invention during assembly when the electron gun is mounted in the body. 
     FIG. 6 is an an exterior side view of an x-ray tube of an embodiment of the invention during assembly when the electron gun is mounted in the body. 
     FIG. 7 is an exterior view of an x-ray tube of an embodiment of the invention during assembly when the target bulb assembly is mounted in the body, with a partial cut-away view provided to show the target assembly. 
     FIG. 8 is a bottom view of an x-ray tube of an embodiment of the invention during assembly when the x-ray window is mounted in the body. 
     FIG. 9A is an isolated sectional side view of the gun container with a spherical snout which is used in the x-ray tube of the invention. 
     FIG. 9B is an isolated section end view of the gun container with a spherical snout which is used in the x-ray tube of the invention. 
     FIG. 10 is a graphical illustration of a portion of an x-ray tube in which a gun container is used having a snout with a spherical configuration according to the invention based on a computer simulation. 
     FIG. 11 is a graphical illustration of a comparative x-ray tube in which a gun container is used having a snout with a stepped configuration based on a computer simulation. 
    
    
     It will be understood that the drawing is merely provided for illustrative purposes and that the depicted features are not necessarily drawn to scale. The same referencing numbering is employed throughout the various figures to designate the same features. 
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 2 illustrates an x-ray tube generally indicated by  10  comprising an anode  34  and a means for generating a beam of electrons. The electron gun assembly  11  and the anode  34  are both disposed inside an air evacuated housing or envelope body  12  including an x-ray transparent window  35 . The electron gun assembly  11  is provided with electrical connectors  31  for the supply of power to the electron gun assembly  11 . The target assembly  33 , the electron gun assembly  11 , and the window  35  are each welded to the envelope body  12  to provide air-tight seals. The beam current can range from a few microamps up to several milliamps. The electron emission source can be a dispenser type button cathode indirectly heated by a filament assembly. The focus grid electrodes  23  disposed between a cathode  36  (see FIG. 10) and an egress aperture  26  in gun container  11 , a plurality of which are used in the present invention, e.g., four, serve to accelerate electrons emitted from a cathode  36  in the gun container  11  while converging the electrons to produce an electron beam. The cathode  36 , egress aperture  26  of the spherical snout  27 , and the accelerating grid electrodes  23   1 - 23   4 , of the gun container  11  are indicated more clearly in FIG.  10 . The grid electrodes  23   1 - 23   4 , converge the electrons generated by the cathode  36  into an electron beam e along a first axis. The grid electrode  23   1  also functions to control the beam current. 
     The beam of electrons e is accelerated toward the anode  34  by the potential difference established between the focus grid electrodes  23  of the gun assembly  11  and the target assembly (electrode)  33 , and, in route, passes through a circular aperture  26  in the spherical gun snout  27  (see FIG.  9 B). The beam of electrons e generated by the electron gun assembly  23  has a potential typically between approximately −25 to −130 kV relative to the target assembly on exiting the electron gun  23  assembly. The target assembly  33  is coated on a lower inclined flat surface with a layer of tungsten (not shown)  34 , or other suitable material for generating x-rays upon electron beam exposure such as copper or molybdenum In one embodiment, the target angle is approximately 24 degrees from the plane of the target surface  34  relative to a direction (c—c) perpendicular to the window  35 . The x-rays x are generated in a range of directions from the anode  34  in the region surrounding the focal spot. X-rays x having a take-off angle at approximately a right angle to the electron beam e pass through window  35 . The beam of electrons e forms a focal spot having a diameter less than 10 μm, and as small as approximately 5 μm. The window  35  comprises a sheet of transparent x-ray transmissive material, such as beryllium (e.g., about 15 mm in diameter and about 0.13 mm thick). The position and dimension of the aperture  26  in the gun container  11  enables use of the x-ray tube  10  for imaging applications in which a high resolution and high x-ray flux is required. The high resolution is achievable because the small size of the focal spot. The envelope body  12  is preferably a conventional sealed metal-glass type. 
     An important aspect of the present invention is that the gun container  11  has a spherical snout  27 , and it is shown in more detail in FIGS. 9A-B. The snout has circular aperture  26 , and the gun container  11  also has a cylindrical section  28  that merges with the spherical snout  27 . The importance of the spherical shaped snout, as compared to other geometries, has been confirmed. Namely, other snout configurations were comparatively examined against the inventive spherical gun snout, but the comparative configurations were found to suffer from chromatic aberrations and instability due to field emission problems. For instance, a flat gun snout design discussed previously relative FIG. 1 was found to produce a 25 μm focal spot and it was severely focus limited due to chromatic aberration. Designs using a conical snout improved focusing but were found to be unstable due to field emission. This same problem was observed for a stepped, re-entrant gun snout  39  design having a shape as illustrated in FIG.  11 . That is, a stepped gun snout configuration also was constructed and tested and it had a close working distance capable of achieving a highly focused spot, e.g., about 6 μm, and which was generally less limited in chromatic aberration than the flat gun snout design. However, the re-entrant refined gun snout had relatively higher electrical surface fields on the gun snout than the inventive spherical gun design for the same gun/snout aperture size. None of the comparative flat, conical or stepped gun snout configurations could achieve the small spot focus and high voltage stability of the inventive spherical gun snout design. 
     Based on computer simulations conducted with commercially available software, the highest electric field point associated with the inventive spherical gun snout (FIG. 10) was reduced to 7700 volts/mil, which is significantly superior to the 11,800 volts/mil found for the re-entrant stepped snout design (FIG.  11 ). FIG. 10 shows the equipotential field lines or surfaces  40  associated with the spherical snout design, while FIG. 11 shows the equipotential field lines or surfaces  41  associated with the comparative stepped snout design. This comparative computer simulation that was graphically recorded in FIGS. 10-11 were premised on the same operational and dimensional conditions other than the differing snout geometries. As indicated above, actual experimental performance tests confirmed the spot performance and high stability at full voltage when using the spherical gun holder configuration of the invention. 
     The spherical gun snout of this invention enables the closest possible spacing of the electron gun to the anode (target). The smaller the spacing, the shorter the working distance for the electron optics. A spherical shape produces the lowest possible surface field on the snout, thereby reducing field emission, even at very close working distances (high field) conditions. As to aperture size, it is important to contour the physical size of the electron lens with regard to the beam diameter. The larger the ratio, the more uniform the lens, resulting in fewer aberrations, and these concerns are important in the context of small spot x-ray sources. The aperture size of the snout is important to control the physical size of the electron lens with regard to the beam diameter. The larger the ratio, the more uniform the lens resulting in fewer aberrations. This is an important consideration in small spot x-ray sources. 
     Referring to FIGS. 3-8, the assembly of an x-ray tube  10  according to one embodiment of the invention is generally shown. In FIGS. 3-4, gun container (holder)  11  is placed in an envelope body  12 . A tube flange  13  is assembled on body  12  with a plurality of counter sink holes facing the gun container  11 . A braze wire  14  is inserted into flange  13 . A braze sheet  15  is placed around a vacuum side of the gun container  11 . A stigmator  16  is optionally placed over the braze sheet and held in place with a brazing fixture. A body weld ring  17  is placed on the envelope body  12 , and a braze wire is placed around body  12  to seat the weld ring  17 . A window adapter  18  is inserted into the envelope  12  with placement of brazing wire, and the entire assembly is mounted on a window brazing fixture (not shown). Then, a vacuum line to be used to evacuate the envelope is installed as shown in FIG.  4 . In this regard, a tubulation elbow  19  is brazed to the envelope  12  using braze wire  14  around its circumference. The tube  20  is mounted in alignment with the centerline c—c of the envelope  12  and attached by brazing using braze wire  14 . 
     FIGS. 5-6 show the mounting of the electron gun assembly  21  in the envelope body  12 . A temporary gun alignment pin  22  is inserted into a focus grid of accelerating electrodes  23   1 - 23   4 , e.g., four, that are part of the electron gun assembly  21 . The electron gun assembly  21  includes a heater (e.g., a filamentary heater), a cathode, electrical power supply means (not shown), and the above-mentioned focus grid, which can be conventional in nature such as those described in U.S. Pat. No. 5,077,771 (Skillicorn), which teachings are incorporated herein by reference for all purposes. The gun locator alignment and support fixture  24  is placed inside body assembly  12  resting upon window adapter  18  and is temporarily fixed in place by screw pin  25 . The gun assembly  21  and the alignment pin  22  are inserted into the gun container  11  such that the alignment pin  22  goes through the aperture  26  of the gun container  11  and rests flush and perpendicular against a confronting flat  29  of the gun alignment support  24 . A header shield  30  is placed over header pins  31 , and then the gun assembly  21  is TIG welded in place. The temporary gun locator fixture  24  is not removed yet. 
     As shown in FIGS. 7-8, a corona guard  32  is slid onto anode (target) assembly  33  and is screwed into place on the anode assembly. The target assembly has an anode  34  which faces the spherical snout  27  of the gun container  11 . A target alignment structure is placed onto a target alignment support. The body assembly  12  is placed in the target alignment fixture such that the gun body  11  rests on the gun locator fixture  24  and the target locator fixture protrudes through the window hole of the body. The target bulb assembly is placed onto the body assembly  12 , and the target bulb assembly is rotated such that the target assembly fits in the slot on the target locator with the target oriented as shown. The target bulb assembly is TIG welded to the body assembly  12  at weld ring  17 . The window assembly, such as beryllium sheet, is then TIG welded to the body assembly  12  at window adapter  18 . 
     Accordingly, by the present invention, an advanced re-entrant gun snout has been developed with a close working distance capable of achieving a highly focused spot, e.g., about 6 μm, which was well limited by chromatic aberration. The enhanced focusing achieved with the inventive x-ray tube is derived from the large final aperture, namely, with a filling factor of approximately 0.125, and the ability to use a shorter working distance between the electron gun and the anode (target). The filling factor equals the beam diameter at the aperture divided by the aperture diameter, d. The spherical gun holder affords focus quality while having lower surface fields than other snout geometries such as flat, conical or stepped. In addition, with the inventive gun, the highest field point is reduced to approximately 7700 V/m. 
     The present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications could be made within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,