Abstract:
A sleeve or cover for preventing the production of secondary x-ray signal contamination from an analytical x-ray tube is disclosed. The x-ray tube includes an evacuated enclosure in which is disposed a cathode and anode. The sleeve or cover is useful in applications such as x-ray fluorescence spectroscopy for improving the spectral purity of the primary stream of x-rays produced by electron bombardment of the anode target surface by the cathode. In one embodiment, the sleeve is disposed about a portion of the anode substrate, and is comprised of beryllium. Electrons back-scattered from the target surface are attracted to the anode substrate and impact the beryllium sleeve, producing secondary x-rays that are not detected by spectroscopic detectors and are therefore not contaminating to the primary x-ray stream.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention generally relates to x-ray tubes. More specifically, the present invention relates to an apparatus for reducing contaminating secondary x-ray emission from stationary anode x-ray tubes. 
     2. The Relevant Technology 
     X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor fabrication, joint analysis, and non-destructive materials testing. While used in a number of different applications, the basic operation of an x-ray tube is similar. In general, x-rays are produced when electrons are accelerated and impinged upon a material of a particular composition. 
     X-ray generating devices typically include an electron source, or cathode, and an anode disposed within an evacuated enclosure. The anode includes a target surface that is oriented to receive electrons emitted by the cathode. In operation, an electric current is applied to a filament portion of the cathode, which causes electrons to be emitted by thermionic emission. The electrons are then accelerated towards the target surface of the anode by applying a high voltage potential between the cathode and the anode. Upon striking the anode target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays. 
     The specific frequency or wavelength of the x-rays produced depends in large part on the type of material used to form the anode target surface. Anode target surface materials with high atomic numbers (“Z” numbers), such as tungsten, are typically employed. The x-rays ultimately exit the x-ray tube through a window in the x-ray tube, and interact in or on various material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic applications. 
     One application for which x-ray tubes are well suited is referred to as x-ray fluorescence spectroscopy (“XRF”). XRF is typically used to determine the elemental composition of a selected material. An XRF instrument setup typically includes an analytical x-ray tube (AXT), a specimen to be analyzed, a collimator, a diffracting crystal, and an x-ray detector. To analyze the composition of the specimen, the x-ray tube is activated and x-rays are directed at the specimen. The interaction of the x-rays with the atoms in the specimen causes the atoms to emit, or fluoresce, a second group of excited x-rays having energies characteristic of the elements in the specimen. Once emitted by the sample, the fluoresced x-rays are dispersed into a spectrum by a diffracting crystal, and are then collimated towards a detector and associated instrumentation, which quantifies and correlates the results. The intensities of the various energy peaks in the spectrum are roughly proportional to the concentration of the corresponding elements that comprise the specimen. In this way, the elemental composition of a variety of materials may be ascertained. 
     Many x-ray tubes employ a rotary anode that rotates portions of its target surface into and out of the stream of electrons produced by the cathode. However, analytical x-ray tubes, such as those used for XRF applications, typically use a stationary anode. The stationary anode typically includes a substrate portion, comprised of copper or similar material, and a target surface comprised of rhodium, palladium, tungsten, or other suitable material. For an XRF procedure to yield superior results when assaying a specimen, it is highly desirable that the x-ray tube produce a stream of primary x-rays that is spectrally pure, i.e., the spectrum is comprised of the continuous spectrum and the characteristic peaks of the target material. This spectrally pure stream of primary x-rays is produced by those electrons that impact the target surface of the anode and produce x-rays having a characteristic wavelength corresponding to the material deposited on the target surface. 
     Unfortunately, many of the electrons that impact that target surface do not produce primary x-rays. Most of the kinetic energy that results from the impact is released in the form of heat. Also, a significant number of electrons simply rebound from the anode target surface and strike other non-target surfaces within the x-ray tube, such as the anode substrate or other components within the tube. These electrons are often referred to as “back-scatter” or secondary electrons. These back-scattered electrons retain a significant amount of their original kinetic energy after rebounding. As such, these secondary collisions with non-target surfaces can produce secondary or off-focus x-rays having a wavelength that is characteristic of the material impinged, such as copper. These secondary x-rays are emitted from the x-ray tube along with the primary x-rays created at the target surface of the stationary anode. In XRF spectroscopy, these secondary x-rays are considered an undesirable contamination of the primary x-ray stream because they interfere with the measurement of the fluorescing x-rays emanating from the specimen under analysis. In other words, an XRF detector may mistake a contaminating secondary x-ray having, for example, a characteristic copper wavelength produced by the copper anode substrate as having been produced by a fluorescing copper atom present in the specimen under analysis. Thus, to optimize the quality of the signal, it is critical to reduce or eliminate these secondary x-rays from the x-ray emissions of an x-ray tube. 
     Several attempts have been made to eliminate secondary x-ray contamination from primary x-ray emissions. One approach involves the process of chemically coating the anode substrate with the same material as is deposited on the target surface. This method has met with only limited success due to the difficulty in getting a sufficiently thick plating to adhere to the anode substrate. Moreover, during tube operation, the high temperatures present in the anode substrate often cause the coating to intermingle with the substrate material, leading to the eventual production of contaminating secondary x-rays. 
     Another approach has involved the use of a graphite layer to cover a portion of the anode substrate where back-scattered electrons typically impact. Though this approach reduces the amount of contaminating x-rays that are emitted, it gives rise to other problems. In particular, the approach results in serious outgassing and particle creation problems during tube operation because of differing thermal expansion rates between the graphite layer and the anode substrate, and because of the extensive machining and handling steps required for assembly and attachment of the graphite layer. Outgassing and particle creation within the evacuated environment of an x-ray tube are highly detrimental to its performance and operating lifetime 
     A need therefore exists for a stationary x-ray tube that reduces or eliminates the production of secondary x-rays. This need is especially acute in x-ray tubes employed in XRF spectroscopy operations, which require spectrally pure x-ray signals. Further, any solution to enable the creation of spectrally pure x-ray streams should do so without creating ensuing problems, such as outgassing and particle creation that are detrimental to the operation of the tube. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention have been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available x-ray tubes. In general, embodiments of the present invention are directed to a cover, or sleeve, that reduces or eliminates the emission of secondary x-ray contamination in stationary anode x-ray tubes. In addition, the sleeve is implemented in a manner so as to prevent other problems within the tube, such as contamination and outgassing. In preferred embodiments, the sleeve is sized and configured to cover a portion of a component disposed within the x-ray tube evacuated enclosure that is susceptible to being impinged by secondary or back-scattered electrons. For example, in one embodiment, the sleeve is affixed to a portion of the stationary anode substrate that is adjacent to the target surface. In particular, the sleeve is positioned so that it prevents errant electrons back-scattered from the target surface from impacting the anode substrate, thereby preventing the production of secondary x-rays. Instead, the back-scattered electrons that would otherwise impact the anode substrate impact the anode sleeve, and produce x-rays that are within a wavelength range that do not negatively impact the analysis being undertaken. 
     Preferred embodiments of the anode sleeve generally comprises a shape necessary to cover a portion of the outer surface of the anode substrate. For example, in one embodiment, the anode sleeve is formed in the shape of a hollow cylindrical body. The anode sleeve has a cylindrical length sufficient to cover those portions of the anode substrate that are susceptible to impact by back-scattered electrons. For example, in some applications, the anode sleeve covers a small portion of the anode substrate adjacent to the target surface. Alternatively, if the application dictates, the length of the sleeve may be greater so as to cover a greater portion of the anode substrate. The thickness of the anode sleeve wall(s) need only be thick enough to prevent penetration of electrons to the anode substrate material. The sleeve is preferably comprised of a material that does not create contaminating x-rays as detected by analysis instrumentation when impacted by electrons. For example, in a preferred embodiment, the anode sleeve comprises beryllium. Other suitable materials could be used depending on the functional requirements of the x-ray tube and the analysis being performed. 
     Embodiments of the present invention use the anode sleeve on a stationary anode in an x-ray tube having an end-window configuration. Alternative embodiments use the sleeve in x-ray tubes having a side-window configuration. Indeed, the anode sleeve of the present invention may be adapted in size and shape to fit a variety of anodes and types of x-ray tubes. Also, a sleeve or a cover could be fitted to other interior x-ray tube components to prevent secondary x-ray emissions from those components as well. An example of this would include a cathode shield comprising beryllium that is positioned so as to prevent secondary x-rays from being produced from portions of the cathode. 
     The present anode sleeve makes possible the production of spectrally pure primary x-ray streams by reducing or eliminating the production of secondary x-ray signals. Inaccuracies created by such contamination in sensitive analysis procedures, such as XRF spectroscopy, are significantly reduced or eliminated. Therefore, the composition of samples subjected to XRF spectroscopy may be determined with greater precision that what was before possible. Additionally, forming the sleeve from beryllium or similar materials avoids the problems associated with outgassing and particle creation encountered with prior art solutions. 
     These and other objects, features and advantages of the present invention will more fully apparent from the following description and appended claims, or may be by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a simplified cross-sectional side view of a stationary anode x-ray tube configured with the anode sleeve of the present invention; 
     FIG. 2 is a cross-sectional side view of a preferred embodiment of the anode sleeve of the present invention; 
     FIG. 3 is a perspective view of the anode sleeve of FIG. 2; 
     FIG. 4 is a cross-sectional side view of the present anode sleeve in accordance with an alternative embodiment thereof; and 
     FIG. 5 is a cross-sectional side view of a side-window x-ray tube incorporating an alternative embodiment of the present anode sleeve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of presently preferred embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale. 
     Reference is first made to FIG. 1, which depicts one example of an analytical x-ray tube  10  having a stationary anode, such as might be used in XRF spectroscopy applications. The x-ray tube  10  includes an outer housing  12  forming a vacuum enclosure. Disposed within the vacuum enclosure is a cathode structure  14 , and a stationary anode structure  16 . The anode structure  16  includes an anode substrate  17  and a target surface  18  disposed at one end of the substrate. The target surface preferably comprises a material having a sufficiently high “Z” number, such as rhodium, palladium, tungsten or the like. However, it will be appreciated that various other target surface materials could be used as required to achieve one or more desired results of affects, and depending on the type of analysis to be performed. The anode substrate  17  is formed of a material having a high thermal conductivity, such as copper or a copper alloy. The high thermal conductivity of the substrate  17  facilitates dissipation of at least some of the heat produced at the target surface  18  resulting from the interactions between the electrons  20  and the target surface  18 . 
     In operation, an electrical current is supplied to a filament coil portion of the cathode  14 , which causes a beam of electrons (depicted at  20 ) to be emitted from the cathode  14  by way of thermionic emission. A high voltage potential difference is applied between the cathode  14  and the anode  16 , which causes the electrons  20  to accelerate to a high velocity. The electrons  20  possess a large amount of kinetic energy, and when they impinge upon the target surface  18  a portion of some of this kinetic energy is converted to x-rays including the characteristic peaks of the target material. The x-rays are directed through a window  24  defined in the housing  12  as is depicted at  22 , and directed towards the specimen being analyzed (not shown). X-ray tubes having windows situated at the end of the vacuum enclosure (as depicted in FIG. 1) are known as end window x-ray tubes, while tubes having windows disposed in the side of the vacuum enclosure are referred to as side window x-ray tubes. 
     In the illustrated embodiment, a shield  26  is disposed within the vacuum enclosure  12  so as to prevent electrons  20  emitted from the cathode  14  from impacting other interior tube parts before impacting the target surface  18 . 
     As mentioned above, a small percentage of the electrons striking the anode target surface  18  actually stimulate the production of x-rays  22 . Many of the electrons merely generate heat at the target surface. Also, a substantial portion of the electrons rebound off of the target surface while retaining a large portion of their original kinetic energy. These back-scattered electrons may strike other areas of the x-ray tube, such as the anode substrate  17  and produce contaminating secondary x-rays—i.e., having wavelengths that differ from that of the primary x-ray signal  22 . 
     With continuing reference to FIG. 1, one embodiment of an anode sleeve  30  is shown. As can be seen in cross section, the anode sleeve  30  is sized and configured to circumferentially fit about at least a portion of the outer surface of the anode substrate  17 . As is better shown in FIG. 3, the anode sleeve  30  is formed with a hollow main cylindrical body portion  32  in order to fit over a corresponding cylindrical portion of the anode substrate  17 . As is shown, the anode sleeve  30  is preferably disposed about a portion of the substrate  17  adjacent to the target surface  18 , where back-scattered electrons are known to impact the substrate. Of course, the anode sleeve  30  could be sized and configured to cover more or less of the anode substrate  17 , as discussed further below. 
     Referring now to both FIGS. 2 and 3, the hollow cylindrical body  32  defines an aperture on both a first end  34  and a second end  36 . The body  32  near the second end  36  is flared to an increased diameter relative to the first end  34  in order to accommodate the shape of the outer surface of the substrate  17  shown in FIG.  1 . It will be appreciated that the sleeve could be implemented with other shapes and configurations. 
     The outer wall of the hollow cylindrical body  32  is preferably of a sufficient thickness to prevent penetration by back-scattered electrons. Factors that determine the minimum thickness of the wall of the body  32  include the atomic number of the element from which the sleeve is manufactured, and the kinetic energy of the electrons incident upon the surface of the body, which depends on the operating power of the x-ray tube. For example, for the illustrated sleeve  30 , when used in a typical XRF spectroscopy application, the thickness may be about 0.01 inches. 
     The anode sleeve  30  is composed of a material satisfying several requirements. First, the anode sleeve should be composed of a material that does not produce contaminating secondary x-rays as detected by detector instrumentation used in connection with the x-ray tube. The selected material should also be able to withstand the extreme operating temperatures present within an operating x-ray tube, which can exceed temperatures of 700° C. Preferably, the selected material should be amenable to machining or manufacturing processes without creating an increased likelihood for particle creation or flaking after the sleeve is installed on the anode  16 . Finally, the material used should have minimal outgassing characteristics once it is disposed within the evacuated housing in the tube. 
     In regards to the first requirement, the selected material for the anode sleeve  30  should be selected from those substances that produce characteristic x-rays that have wavelengths not within the range of detection of the detector instrumentation used in conjunction with the x-ray tube  10 , such as in XRF spectroscopy. Otherwise, the secondary x-rays produced as a result of the interaction between the sleeve and the back-scattered electrons will contaminate the primary stream of x-rays and provide inconclusive results to the detector equipment. Most detector instruments used in conjunction with stationary anode x-ray tubes are designed not to recognize x-rays characteristic of elements with atomic numbers less than approximately 11, such as sodium. 
     One preferred material for the sleeve  30  is beryllium, which has an atomic number of 4 and is thus out of the designated sensitivity range of most x-ray detector instruments. Any secondary x-rays produced by a sleeve composed of beryllium will not be considered as contaminating to the primary stream of x-rays. Beryllium also meets the other desired characteristics of an anode sleeve material. In particular, it is capable of enduring high temperatures, is easily machinable, and is not susceptible to particle creation or outgassing after installation in, or during the use of, a stationary anode x-ray tube. 
     Other materials could be used for the anode sleeve. Diamond is an example of such a material. Also, for certain applications it may be desirable to manufacture the anode sleeve from the same material as the anode target material, such as rhodium or palladium. A sleeve composed of the same material as the target surface does not produce contaminating secondary x-rays because any x-rays that are produced are of a frequency that is accounted for by the detector instrumentation used in conjunction with the tube. 
     The anode sleeve  30  depicted in FIGS. 2 and 3 may be manufactured by known manufacturing processes. One method for manufacturing the preferred anode sleeve  30  includes providing a rod comprising beryllium and machining a portion thereof such that a hollow cylindrical body  32  is formed including a first end  34  having a first diameter and a second end  36  having a second diameter. The first end  34  is defined such that its diameter is sufficient to cooperatively fit about the outer circumference of the target surface  18 , while the second end  36  is defined to receive a portion of the anode substrate  17 . The machined sleeve  30  is then cleaned to remove any particles before being affixed to the anode  16  by known means, such as brazing. Other attachment schemes could also be used, including use of intermeshing threads, or a detent and nub arrangement disposed on the anode  16  and sleeve  30 . 
     When installed, the first end  34  of the anode sleeve  30  is preferably disposed directly adjacent to the target surface  18  such that a snug fit exists between the outer circumference of the target surface and the inner circumference of the aperture defined in the first end  34  of the sleeve. In this way, any back-scattered electrons that rebound off the target surface  18  may not cause secondary x-ray contamination by infiltrating any spacing that might otherwise exist between the target surface  18  and the sleeve  30  and impacting the anode substrate  17 . 
     During tube operation, the anode sleeve  30  of the present invention advantageously prevents contamination of the primary stream of x-rays emitted by the target surface  18  by reducing or eliminating the production of secondary x-rays by the anode substrate  17 . As explained above, many back-scattered electrons do not produce primary x-rays when they impact the target surface  18 , but instead rebound. These back-scattered electrons can be re-attracted not only back to the target surface  18 , but also to a portion of the anode substrate  17  near the end upon which the target surface is deposited. The anode sleeve  30  is sized to cover this portion of the substrate  17  that would otherwise be impacted by these errant electrons, as shown in FIG.  1 . With the sleeve  30  attached, the back-scattered electrons do not impinge the surface of the copper substrate  17 , but rather impact the beryllium sleeve. Any secondary x-rays created by the electrons&#39; impact with the beryllium sleeve  30  possess a wavelength characteristic of beryllium which, as explained above, is not recognized by attached detection equipment and is therefore not considered secondary x-ray contamination of the primary x-ray stream. In this way, a spectrally pure primary x-ray stream is produced by the x-ray tube  10 , with the stream collectively possessing a continuous spectrum with characteristic peaks of the target. 
     The anode sleeve  30  is but one example of a means for preventing the production of x-rays by the substrate  17 . It should be understood that this structure is presented solely by way of example and should not be construed as limiting the scope of the present invention in any way. 
     FIG. 4 illustrates one alternative embodiment of the anode sleeve, designated generally at  30 ′. As in the previous embodiment, the anode sleeve  30 ′ comprises a hollow cylindrical body  132  having a first end  134  and a second end  136 . The hollow body  32  in this embodiment is manufactured to have a longer length as may desired or needed to suit the particular application with which the sleeve  30 ′ is used. The longer length of the sleeve  30 ′ as depicted in FIG. 4 may be necessary, for example, to cover a greater portion of the anode substrate  17  in order to ensure that no back-scattered electrons impact the substrate. In such a case, the sleeve  30 ′ of this alternative embodiment may define an axial cavity having more than one diameter, as is shown in FIG. 4, in order to cooperatively fit over the outer surface of the anode substrate  17 . In fact, the anode sleeve could be sized to any one of a variety of length, thickness, and/or axial cavity dimensional configurations. 
     Yet another embodiment of the anode sleeve is depicted in FIG. 5, which illustrates in cross section a side window x-ray tube  50 , in contrast to the end-window x-ray tube depicted in FIG.  1 . The x-ray tube  50  comprises a housing  52  defining a vacuum enclosure, which has disposed within it a cathode  54  and an anode  56 . The anode  56  includes a target surface  58  disposed on a substrate  60 . The substrate  60  comprises a hollow cylindrical portion  60 A, which also forms part of the vacuum enclosure, and a supporting portion  60 B on which is disposed the target surface  58 . A window  62  is disposed in the side of the vacuum enclosure  52 . An anode sleeve  70  is shown disposed between the inner surface of the hollow cylindrical portion  60 A and the outer surface of the supporting portion  60 B of the anode substrate  60 . In one embodiment, the anode sleeve  70  comprises beryllium and covers that portion of the substrate  60  that is susceptible to impinging back-scattered electrons within the vacuum enclosure  52 . The anode sleeve  70  is formed as a hollow cylindrical body  72  of sufficient thickness to prevent the complete penetration of back-scattered electrons therethrough, a first end  74 , and a second end  76 . A portion of the sleeve has an aperture  71  formed through it to allow x-rays to pass through to the window  62 . The anode sleeve  70  covers the desired portions of the substrate  60  without interfering with the production of primary x-rays on the target surface or the emission thereof through the window  62 . 
     The operation of the anode sleeve  70  is similar to that of the anode sleeve  30  installed in the end-window x-ray tube  10 . The sleeve  70  covers those portions of the anode substrate  60  that may be impacted by back-scattered electrons. The electrons impact the anode sleeve  70  instead, and non-contaminating x-rays are thus produced. This prevents secondary x-ray contamination of the primary x-ray stream produced by the target surface and increases the performance of the x-ray tube. 
     An alternative means by which the production of secondary x-rays may be reduced or eliminated within an x-ray tube involves the use of covers disposed over components, other than the anode, that are located within the interior of the vacuum enclosure. Such covers may be desirable to prevent the production of secondary x-rays resulting from the incidence of back-scattered electrons on other non-target components. These covers are preferably composed of beryllium, though other suitable materials could alternatively be used in place of beryllium, as explained above. An example of an intra-tube component that could benefit from such a cover is the shield  26  shown in FIG.  1 . This shield  26  (designed to prevent the electrons  20  emitted from the cathode  14  from impacting other interior tube parts before impacting the target surface  18 ) will emit only non-contaminating secondary x-rays should any back-scattered electrons impinge upon it. In this way, other intra-tube components may be eliminated as sources of secondary x-ray contamination during tube operation, thereby providing superior spectral quality in the primary x-ray stream emitted from the tube. 
     In summary, the anode sleeve of the present invention enables the production of high quality, spectrally pure primary x-ray emissions free from the contaminating x-rays otherwise produced at the anode substrate. Such x-ray streams allow for more precise measurements by attached detector instrumentation because they are free from x-ray impurities that may provide inclusive results in such applications as specimen analysis in XRF spectroscopy. The utilization of a sleeve that fits over a portion of the anode substrate is easier to install than known substrate plating techniques, and use of beryllium (or similar material) as the sleeve material provides a sleeve that will not suffer from outgassing or particle creation problems. 
     The present claimed invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.