Patent Publication Number: US-9852875-B2

Title: X-ray tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 14/849,647, filed Sep. 10, 2015, which claims the benefit of U.S. Provisional Patent Application 62/051,303, filed Sep. 17, 2014, whose disclosure is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to X-ray tubes, and particularly to X-ray tube anodes. 
     BACKGROUND OF THE INVENTION 
     X-ray beams can be used for characterizing a large range of materials including materials used in the semiconductor industry. Various methods have been developed for generating X-ray beams. For example, U.S. Pat. No. 7,257,193, to Radley, et al., whose disclosure is incorporated herein by reference, describes an X-ray source assembly having enhanced output stability using tube power adjustments and remote calibration. A control system is provided for maintaining intensity of the output X-rays dynamically during operation of the X-ray source assembly, notwithstanding a change in at least one operating condition of the X-ray source assembly, by changing the power level supplied to the assembly. 
     European Patent 2,050,100, to Boulee, et al., whose disclosure is incorporated herein by reference, describes a system for delivering an X-ray beam, comprising a source block that emits a source X-ray beam and conditioning means for conditioning the source beam sent towards a specimen. The system includes stabilization means designed to thermally stabilize a region of the system lying downstream of the source block, in order to limit heat transfer towards the conditioning means for the purpose of preventing thermal perturbations in the conditioning means. 
     U.S. Pat. No. 6,282,263, to Arndt, et al., whose disclosure is incorporated herein by reference, describes an X-ray generator which produces an X-ray source having a focal spot or line of very small dimensions and which is capable of producing a high intensity X-ray beam at a relatively small point of application using a low operating power. 
     U.S. Pat. No. 6,788,633, to Loxley, et al., whose disclosure is incorporated herein by reference, describes a method and apparatus for prolonging the life of an X-ray target. An X-ray generator comprises an evacuated and sealed X-ray tube, containing an electron gun and an X-ray target. An electron beam is produced by the electron gun in which the cathode is at negative high voltage, the electron gun consisting of a filament just inside the aperture of a Wehnelt grid which is biased negatively with respect to the filament. 
     U.S. Pat. No. 4,675,890, to Plessis, et al., whose disclosure is incorporated herein by reference, describes an X-ray tube for producing a high-efficiency beam and especially a pencil beam as applicable to the field of radiology and more especially digital radiology, comprises an anode provided with a rectilinear bore and a cathode for generating an electron beam which enters the bore. The internal walls of the bore constitute an anode target which is bombarded by the electron beam in order to produce at least one x-ray beam which emerges from one end of the bore. 
     U.S. Pat. No. 5,148,462, to Spitsyn, et al., whose disclosure is incorporated herein by reference, describes formation of high thermal conductivity X-ray anode sources for production of high intensity X-rays. The anode sources are structures containing diamond (passive element) and desired target material(s) consisting of metal(s) and (or) their alloys for the generation of high intensity X-radiation of the desired wavelength. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention that is described herein provides an X-ray tube including a cathode and an anode. The cathode is configured to generate an electron beam. The anode has at least one hole that faces the electron beam, the hole having sidewalls and a floor. The electron beam impinges on one or more of the sidewalls of the at least one hole so as to emit a first X-ray beam at angles that are not orthogonal to a surface of the anode. The electron beam also impinges on the floor of the at least one hole so as to emit a second X-ray beam, at least some of which is emitted at an angle that is orthogonal to the surface of the anode. 
     In some embodiments, the floor has a curved depth profile. In an embodiment, the at least one hole includes at least first and second holes. In an example embodiment, the first and second holes differ in diameter. In a disclosed embodiment, the tube includes a film that covers a surface of the anode facing the electron beam, including the at least one hole, so as to protect the anode from the impinged electron beam and to isolate the cathode in a vacuum environment. 
     In another embodiment, the anode includes at least one cooling channel, which is configured to transport gas through the at least one hole so as to control a temperature of the anode. In yet another embodiment, the anode includes stacked layers of two or more materials, and is configured to emit the first X-ray beam at one or more wavelengths corresponding to the materials. 
     In some embodiments, a cross-section of the first X-ray beam has an annular shape. In an embodiment, a cross-section of the at least some of the second X-ray beam covers at least a center of the annular shape. 
     In yet other embodiments, the anode includes a tape, which covers a distal end of the at least one hole, such that the electron beam impinges on the tape so as to emit an additional X-ray beam. The tape may be configured to move relative to the electron beam. In an embodiment, the tape includes one or more materials, and is configured to emit the additional X-ray beam at wavelengths corresponding to the materials. 
     There is additionally provided, in accordance with an embodiment of the present invention, an X-ray tube including a cathode, a round anode and an array of gas pipes. The cathode is configured to generate an electron beam. The round anode is configured to rotate such that the electron beam impinges on a rotating surface of the anode so as to emit an X-ray beam. The array of gas pipes is configured to direct gas onto the surface so as to cool the anode. 
     There is also provided, in accordance with an embodiment of the present invention, a method including, in an X-ray tube, generating an electron beam by a cathode. The electron beam is directed onto an anode having at least one hole that faces the electron beam, the hole having sidewalls and a floor. The electron beam impinges on one or more of the sidewalls of the at least one hole so as to emit a first X-ray beam at angles that are not orthogonal to a surface of the anode. The electron beam also impinges on the floor of the at least one hole so as to emit a second X-ray beam, at least some of which is emitted at an angle that is orthogonal to the surface of the anode. 
     There is further provided, in accordance with an embodiment of the present invention, a method for producing an X-ray tube. The method includes providing a cathode, which is configured to generate an electron beam, and an anode. At least one hole is formed in the anode, facing the electron beam. The hole has sidewalls and a floor such that, in response to the electron beam, the sidewalls are configured to emit a first X-ray beam at angles that are not orthogonal to a surface of the anode, and the floor is configured to emit a second X-ray beam, at least some of which is emitted at an angle that is orthogonal to the surface of the anode. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that schematically illustrates an X-ray source assembly, in accordance with an embodiment of the present invention; 
         FIGS. 2-8A  are schematic sectional views of anodes used in X-ray generating tubes, in accordance with embodiments of the present invention; 
         FIG. 8B  is a schematic bottom-view of a tape in an anode that is used in an X-ray generating tube, in accordance with another embodiment of the present invention; and 
         FIG. 9  is a schematic illustration of an X-ray rotational anode, in accordance with an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Compact, micro-focus X-ray tubes are used in a variety of measurement systems, including X-ray characterization and metrology tools for the semiconductor industry. An X-ray tube typically comprises a cathode and a metal anode (e.g., copper) that typically operate in vacuum environment within the tube. The cathode is configured to generate a high-energy electron beam which is accelerated and directed to impinge on the anode. The beam interacts with the copper atoms of the anode so as to generate an X-ray beam. 
     Using a high-energy electron beam may be advantageous for generating the X-ray, but may overheat the anode and cause sublimation of the anode. As a result, the vacuum within the tube may suffer rapid degradation that negatively affects the quality of the electron beam and reduces the lifetime of the tube. To reduce the damage that the electron beam causes to the anode, it is possible in principle to use a broader electron beam having reduced energy density. A broader electron beam, however, typically results in X-rays of larger spot size, which is generally undesired. 
     Embodiments of the present invention that are described hereinbelow provide an anode to be used in an X-ray tube. The disclosed anode configurations enable the electron beam to impinge on a large surface area at maximum power loading, so as to achieve high brightness X-rays, and at the same time retain a small X-ray spot size. 
     In some embodiments, the anode comprises a bulk with one or more holes that face the electron beam, such that the electron beam impinges on sidewalls of the holes so as to emit the X-ray beam. The sidewalls are typically made of an x-ray emitting material such as copper, molybdenum, or tungsten. In other embodiments, the hole may comprise a thin walled hollow tube surrounded by high thermal conductivity material such as diamond, or sapphire. The holes do not penetrate through the entire bulk, such that the remaining “floors” of the holes serve as additional anode surfaces. The floor emits the X-ray beam orthogonally to the lower surface of the anode in a transmission mode while the sidewalls emit the X-ray beam at angles that are oblique relative to the lower surface of the anode at a reflective mode. 
     The hole geometry increases the surface area of the anode impinged by the electron beam, and thus allows using a maximum power of the electron beam so as to increase the intensity of the emitted X-ray from the anode. The X-ray beam emitted from the sidewalls typically has an annular cross-section, which is impractical or at least sub-optimal in some applications. The additional X-ray radiation emitted from the floor essentially fills the center of this annular cross-section. 
     In some embodiments, the anode may comprise multiple holes, each hole functioning as a separate anode. The electron beam impinges on a single hole at a given time. In an embodiment the diameter of the holes is uniform (e.g., 50 μm) across the anode, and thus, all the holes emit substantially similar X-ray beams. For example, when a given hole is blocked or damaged, the electron beam may be steered to impinge on another hole of the anode. 
     In an embodiment, the holes may have different respective diameters so as to enable generating X-ray beams having different properties (e.g., angle or intensity). This capability may be useful for specific inspection and measurement applications. 
     In an embodiment, the anode may comprise more than one type of metal. Each metal type emits X-ray photons at a different spectrum of wavelengths corresponding to a different energy-gap associated with the metal atoms. Using multiple different bulk metals in the anode allows producing a wide spectrum of wavelengths. For example, by directing the electron beam to holes formed in selected locations of the anode, a user may be able to control the spectrum of the emitted X-rays. 
     In some embodiments, the metal bulks may be arranged horizontally (side-by-side) so that directing the electron beam on a hole of a given bulk provides the user with a narrow and well-defined spectrum. In alternative embodiments, arranging the metals in a vertical stack and directing the electron beam on the sidewalls of the holes may result in X-ray photons emitted from multiple metal elements at a wide range of wavelengths corresponding to the metal layers. 
     The electron beam spot and the average diameter of the holes are typically on the same range (e.g., 50 μm). Electron beam that is not aligned to the hole may result in high density of electrons impinging on the surface of the anode, generating local overheat and may result in a sublimation of the metal atoms. The evaporated metal atoms may degrade the vacuum in the tube and thus, degrade the electron beam, reduce the intensity of the X-rays and may shorten the lifetime of the cathode. 
     In some embodiments, a thin protective film made of a superior heat-conductive material, such as diamond, may be used to protect the anode from sublimation and to seal the cathode from vapors that are emitted from the anode. The protective layer allows utilization of higher electron beam energy and thus, to generate X-rays having high intensity without risking the cathode. Higher electron-beam energy allows operating the anode at an atmospheric environment rather than in vacuum. In an embodiment, the anode comprises internal cooling channels patterned in the anode bulk so as to allow cooling of the anode with a suitable gas, such as air. The cooling air may flow through the hole so as to cool the sidewalls of the hole (in addition to known cooling methods at the perimeter of the anode, such as water cooling), and thus, enable higher intensity of X-ray emission from the anode. 
     In alternative embodiments, the anode may comprise a moving tape (typically made of copper), which is cooled by forced air flowing through the channels. Thus, a fresh piece of tape can be introduced by rolling the tape, such that the electron beam impinges on a fresh section of the tape while the recently used sections move to the cooling area. 
     Operating the anode in atmospheric environment enables efficient control of the anode temperature. For example, rotating the anode in air is significantly easier and more efficient compared to applying the same operation in vacuum. In an embodiment, the cathode is sealed in a vacuumed housing and coated with a thin diamond film that allows the electron beam to pass through and reach the anode. The anode comprises a round-shaped copper bulk located in close proximity to the cathode and configured to rotate while the electrons are impinging on the surface of the anode. In addition, an array of air pipes positioned around the anode bulk improves the cooling efficiency by using forced cooling air in addition to water cooling in the perimeter of a conventional rotating anode. 
     The above techniques enable improved efficiency of the X-ray tube by allowing higher power of the electron beam, which results in higher intensity of the X-ray beam, without compromising the lifetime of the cathode or the beam quality. Furthermore, the above techniques provide the user of an X-ray machine with a controllable spectrum of the X-ray photons, and thus, achieve high flexibility and wide-range functionality for inspection and measurement applications in material characterization. 
     System Description 
       FIG. 1  is a block diagram that schematically illustrates an X-ray source assembly  20 , in accordance with an embodiment of the present invention. Assembly  20  comprises a cathode  21  that generates electrons accelerated towards a transmission metal anode  23 , in the form of a high energy electron beam  22 , by a high potential difference of several tens of kV. Cathode  21  is mounted on an anode flange  27  that comprises an aperture for the electron beam and is electrically isolated from the anode flange by an insulator  29 . 
     Two sets of beam deflection coils  16 , which are configured to center the beam from cathode  21  on the aperture in anode flange  27 , are deployed in two planes and mounted between cathode  21  and a focusing coil  15 . Alignment coils  14 , which are configured to align beam  22  with the aperture in anode  23 , are located between coil  15  and anode  23 . Anode  23  may be formed from any suitable metal, such as copper, molybdenum, or tungsten. 
     Cathode  21  and anode  23  are enclosed in a high vacuum envelope, which comprises a flight tube  25 , anode flange  27  and insulator  29 . Flight tube and anode flange are typically made of a UHV compatible stainless steel. Insulator  29  is typically made of glass or ceramic and provides an electrical insulation between cathode  21 , anode flange  27  and anode  23 . High energy electrons of beam  22  interact with metal atoms of anode  23 , and generate an X-ray beam  28 . In some embodiments, anode  23  may comprise a cylindrical hole  32  that ends at a copper film  34  so that the electron beam impinges on walls of the hole and on film  34  so as to generate X-rays as will be described below in details. 
       FIG. 2  is a schematic sectional view of an anode  24  and an imager screen  30 , in accordance with an embodiment of the present invention. Anode  24  can be used, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  24  comprises a round bulk  40 , typically made of 2 mm thick copper disk, and copper film  34  that is typically less than 10 μm thick. Cylindrical hole  32  of about 50 μm in diameter is formed in the bulk comprises sidewalls  36  and optionally, a floor  38 , which serves as a transmission anode located on the upper surface of film  34 . 
     In some embodiments, the cathode and the anode are held within an X-ray tube in vacuum (e.g., 10 −5 -10 −8  Torr) and film  34  seals the tube from the environment so as to prevent vacuum loss. Electron beam  22  impinges on sidewalls  36  and floor  38  (also denoted a film anode) so as to interact with the copper atoms of the bulk and to generate X-ray beams  42  and  44 , respectively. Beams  42  are formed by electron beam  22  impinging on sidewalls  36  and are typically emitted through the lower surface of film  34  at non-orthogonal angles resulting in a spot  53  having an annular cross-section. Beam  44  is formed by electron beam impinging on floor  38  and emitted at a wide range of angles that may cover the entire area of screen  30 . 
     An inset  33  provides a view of screen  30  from the X-ray tube. Annular spot  53  represents an area where both beams  42  and  44  impinge on screen  30 . In addition, a first portion of beam  44  may emit from floor  38  at a substantially orthogonal angle relative to the lower surface of film  34 , so as to impinge on screen  30  at an area  55 . 
     A second portion of beam  44  may emit from floor  38  at an angle  31  (relative to film  34 ), which is smaller than the emitting angles of beam  42 , thus the second portion may impinge on screen  30  at an area  57 . The combination of beams  42  and  44  covers the entire area of screen  30  (as opposed to beam  42  that covers only annular spot  53 ). In some applications the floor is required to complement the annular spot of beams  42  with X-ray intensity (i.e., beam  44 ) in areas  55  and  57 . 
     In other embodiments, film  34  below floor  38  is sufficiently thin to enable X-rays generated in sidewalls  36  to pass through, but still block electron beam  22  so that the energy of the beam electrons is used for generating X-ray beam  44 . 
     Floor  38  typically comprises a flat surface, yet, in alternative embodiments, the floor may have a paraboloid, ellipsoidal or other curved depth profile so as to increase the overall reflections of the X-ray beam from the walls of the hole. In addition, the paraboloid or ellipsoidal depth profile may assist in focusing beams  42  on screen  30  and/or in directing the X-rays orthogonally, or at a low angle such as angle  31  to the lower surface of film  34  (so as to impinge on areas  55  and  57  of screen  30 . 
     Sidewalls  36  typically comprise a flat vertical surface, yet, in some embodiments, the sidewalls may have a curved depth profile, such as a paraboloid or ellipsoidal, so as to optimize distribution of electrons impinging along the sidewalls of the anode. For example, a curved depth profile may be used for distributing of electrons uniformly along the sidewalls. 
       FIG. 3  is a schematic sectional-view of an anode  35  in accordance with another embodiment of the present invention. Anode  35  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  35  comprises bulk  40  having multiple holes  32 . In some embodiments, the holes may have same or different diameters across the anode. In the example of  FIG. 3 , the left hole has smaller diameter than the middle and right holes. In other embodiments, the holes may have substantially similar diameter (e.g., 50 μm across the anode). The structure of anode  35  is substantially similar to that of anode  24  except for the multiple holes in anode  35  rather than a single hole in anode  24 . 
     The electron beam typically impinges on a single hole at any given time. In an embodiment, multiple holes of the same diameter may provide the user with redundancy (e.g., in case holes become damaged). For example, a first hole in an array of similar holes may degrade by long exposure to the electron beam. As a result, the first hole will no-longer be able to generate the X-ray at the required quality. The user may apply a steering circuitry, such as electron optics incorporating electrostatic and or magnetostatic fields (not shown), to steer the electron beam to a second (undamaged) hole, which is substantially similar to the first hole, so as to impinge the electron beam on the second hole and to generate X-rays that meet the required specifications. In an embodiment, the user may steer the electron beam from the right hole of anode  35  to the left hole so as to modify the properties of the emitted X-rays. 
       FIG. 4  is a schematic sectional-view of an anode  37  in accordance with another embodiment of the present invention. Anode  37  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  37  comprises two or more bulks, in the example of  FIG. 4  sub-bulks  46  and  48  are inter-connected horizontally and are made of different respective materials, such as but not limited to copper, molybdenum, aluminum, cobalt, iron, rhodium, silver, and tungsten. The above-referenced materials may be used in any desired combination and are separated by a dotted line, which represents the interface between the bulks. Each of the materials listed above has a different energy gap between electrons around its nucleus. When electrons of the material are excited by beam  22  to a higher energy level and then return to their original energy band, the material emits photons whose wavelength depends on the respective energy gap. 
     In some embodiments, using an anode with two or more sub-bulks that are made of different materials, may provide the user with a possibility to switch from emitting X-rays of a given spectrum to a different respective spectrum by switching from a first hole located in sub-bulk  46  to a second hole located in sub-bulk  48 . 
       FIG. 5  is a schematic sectional-view of an anode  39  in accordance with another embodiment of the present invention. Anode  39  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  39  comprises multiple layers (or sub-bulks)  50 ,  52 ,  54  and  56 , in an embodiment each layer may comprise a different material from the list described in  FIG. 4 . Alternatively, two or more of layers  50 ,  52 ,  54  and  56  may comprise a similar material from the same list. For example, layers  54  and  56  are made of copper, similar to film  34  shown in  FIG. 2 . 
     Beam  22  impinges on the sidewalls of holes  32 , each sidewall comprises layers  50 ,  52 ,  54  and  56  that emit X-ray photons at a range of wavelengths as described in  FIG. 4 . In an embodiment, a user of the X-ray machine may control a spectrum of the emitted X-rays by directing beam  22  to impinge on selected layers along the sidewalls of holes  32 . In another embodiment, holes  32  may comprise paraboloid or ellipsoidal cross-section profiles, as described in  FIG. 2 , so as to control the spectrum, the intensity and/or the angle of the emitted X-rays. 
       FIG. 6  is a schematic sectional-view of an anode  41  in accordance with another embodiment of the present invention. Anode  41  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  41  comprises a thin protective film  60 , typically made of diamond, which is deposited on the upper surface of bulk  40  and seals hole  32 . When impinging on anode  41 , the spot of beam  22  undesirably generates local heating and sublimation of anode bulk  40 . For example, at temperatures of about 600° C. the copper anode typically starts to evaporate into a gas that may damage the vacuum within the tube and thus, may reduce the quality of beam  22  and shorten the life time of the cathode. The thickness of film  60  depends on the anode material, the required X-ray intensity and the planned power of beam  22 . For minimal absorption of the electron beam by the diamond film, the thickness of film  60  should be on the range of 10-300 μm. 
     In some embodiments, film  60  is made of a highly heat-conductive material, such as diamond, to dissipate the local heat developed in bulk  40  and to prevent potential vapors sublimated from the anode materials to interfere with the vacuum above the upper surface of film  60 . This capability allows operating beam  22  at higher accelerating voltages (e.g., 100 keV rather than 50 keV that is used conventionally) and higher power density. 
     Operating the X-Ray Anode at Atmospheric Environment 
     Traditional X-ray tubes typically operate in vacuum environment (such as 10 −8  Torr) so as to protect the cathode and to enable tight control of beam  22 . By protecting the anode from vapors of the anode, film  60  enables the use of high accelerating voltage, which allows beam  22  to reach sidewalls  36  and floor  38  at an atmospheric environment. For example, electrons accelerated at 100 keV in air can reach as far as about 10 cm according to the completely slowing down approximation (CSDA) Range method of Berger and Seltzer (1964). The depth of hole  32  is typically about 2 mm, thus even for a 3 mm to 4 mm hole depth, at 100 keV, the energy loss of beam  22  when traversing the hole is sufficiently small even when hole  32  is filled with air. 
       FIG. 7  is a schematic sectional-view of an anode  43  in accordance with another embodiment of the present invention. Anode  43  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  43  comprises film  60 , which seals the cathode in vacuum as described in  FIG. 6 . Anode  43  further comprises cooling channels  62  and  64  patterned in the anode bulk so as to allow cooling of anode  43  with air or any other suitable gas. 
     In the example of  FIG. 7 , the cooling air enters the anode from the left side of channel  62  and flows in the right direction into hole  32  where it flows up so as to enter channel  64  from which it outflows to the right side of the anode. In some embodiments, the heat convection by air may operate in addition to other heat dissipation techniques, such as heat conduction by the metal bulk of the anode (from the center to the edge of the anode), or heat convection by water (or any other suitable liquid or gas) flowing at the perimeter of the anode. 
       FIG. 8A  is a schematic sectional-view of an anode  45  in accordance with another embodiment of the present invention. Anode  45  may serve, for example, as anode  23  in assembly  20  of  FIG. 1  above. Anode  45  comprises film  60 , which seals the cathode in vacuum. Alternatively, using high accelerating voltage eliminates the need for sealing hole  32  in vacuum. In the example of  FIG. 8A , cooling air flows from right to left through channel  64 , via hole  32 , down to channel  62  and outflows from right to the left edge of anode  45 . 
     Anode  45  further comprises a thin tape  66 , typically made of copper, but may be made of any other suitable material or a combination of materials such as but not limited to molybdenum, aluminum, cobalt, iron, rhodium, silver, and tungsten. Beam  22  impinges on tape  66 , which is a transmission target, similar to floor  38  shown in  FIGS. 2, 6 and 7 . The tape, however, does not have to be fixed to bulk  40  and can in fact move, thus increasing the power capacity of the tape. One possibility for such a moving tape encompasses a thin continuous copper tape around rollers (not shown), translated at high speed through the emerging electron beam. Tape  66  recirculates around the rollers at high speed, similar in concept to a liquid metal jet tube or any alternative suitable rotating anode tube. 
       FIG. 8B  is a schematic bottom-view of tape  66  in anode  45 , in accordance with an embodiment of the present invention. Tape  66  is a heat-transmission target and thus, is heated by beam  22  and cooled by forced air flowing in channel  62 . The power of beam  22  may ware out tape  66  over time as shown in a location  70 . In some embodiments, a new piece of tape can be introduced by rolling the tape and thus, moving the tape with respect to beam  22  such that beam  22  impinges on a new section of the tape while the degraded location, such as location  70 , moves to a cooling process (not shown but represented by an arrow  68 ). Strip  72  represents the new sections in tape  66  after the tape is moved by the rollers or by any other suitable moving technique. 
     In other embodiments, anode  45  comprises a motion element  76 . Once the entire area of strip  72  is exposed to beam  22 , element  76  is configured to move tape  66  by a vertical continuous raster type oscillation so as to move the exposed location of tape  66  from location  70  to another location  74 , and thus, to improve the cooling efficiency of the tape. In yet other embodiments, the tape may revolve several hundreds of times through the cooling process before any part of the tape is impinged again by beam  22 . 
     For example, for a tape that is 1 m long and 30 mm wide it is possible to use about 500 tracks by moving the tape up by 50 μm (a typical diameter of beam  22 ) every time the tape makes one pass through the electron beam. This sequence is equivalent to a rotating anode circumference of one (1) meter times 500 (i.e., 500 m) or to a rotating anode diameter equivalent of 159 m. 
     Tape  66  should at least 10 μm thick so as to allow the emission of X-rays from hole  32 , and yet, to have sufficient mechanical strength to withstand the fast motion in the rollers and in element  76 . 
       FIG. 9  is a schematic illustration of an X-ray anode  47 , in accordance with an alternative embodiment of the present invention. The cathode (not shown) is located above beam  22  and is sealed in vacuum environment by a housing  49  that comprises a protective diamond window  51 . The window has a substantially similar structure and functionality as diamond film  60  described above. Window  51  allows beam  22  to pass through from housing  49  towards anode  47 , which is located in atmospheric pressure, at a typical accelerated voltage of 100 keV. 
     Anode  47  comprises a round-shaped copper bulk  78  located in close proximity (e.g., 3 mm) to window  51  so as to allow beam  22  to reach bulk  78  at desired illumination conditions and to impinge on bulk  78  so as to generate X-ray photons. Anode  47  further comprises an array of gas pipes  79  located around bulk  78 , which are configured to blow forced cooling air  80  on the surface of bulk  78 . In some embodiments, bulk  78  is configured to rotate so as to reduce the exposure time of any given point on the surface of bulk  78 , to beam  22 . Operating the anode in atmosphere allows to increase the operating temperature without causing sublimation to the bulk material. The disclosed techniques also allow using a simplified rotation mechanism (compared to rotation in vacuum) and improve the cooling efficiency by using forced air (or any other suitable gas) in addition to water cooling of a conventional rotating anode. 
     The examples of  FIGS. 1-9  refer to a specific X-ray tube configuration. This configuration, however, is chosen purely for the sake of conceptual clarity. In alternative embodiments, the disclosed techniques can be used, mutatis mutandis, in various other types of X-ray tubes and X-ray sources. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the following claims are not limited to what has been particularly shown and described hereinabove. Rather, the scope includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.