X-ray tube

Abstract

To provide an X-ray pipe.SOLUTION: The present invention relates to an X-ray pipe for analyzing an X-ray. The X-ray pipe includes an anode with a target surface and a cathode. The cathode includes a discharge loop. The discharge loop extends around the shaft through the anode, and the anode and the cathode are located separately from each other along the shaft. Electrons discharged from the cathode are applied on the target surface of the anode so that X-rays are generated. The X-ray pipe further includes an electronic beam guide. The electronic beam guide guides electrons discharged from the cathode so that the electrons are applied to one region of the anode. The irradiated region is surrounded by a single boundary.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an X-ray tube. [Background technology]

[0002] X-rays are produced by an X-ray source, often in the form of a vacuum tube (i.e., an X-ray tube) that contains a cathode and an anode. Electrons from the cathode are accelerated by an electric field towards the anode and produce X-rays upon collision with the anode. These X-rays exit the X-ray tube through a window. A high voltage (e.g., in the range of 1 kV to 100 kV) is applied to the anode while the cathode is held at ground; that is, there is a high potential difference between the anode and cathode of the X-ray tube.

[0003] The current associated with the beam of electrons emitted from the cathode and directed towards the anode is called the emission current. Generally, the higher the emission current, the greater the number of electrons directed towards the anode per unit time.

[0004] The area of ​​the anode that is illuminated by the electrons from the cathode is called the "focal spot" of the X-ray tube. In some X-ray analytic applications, especially those requiring high resolution, it is preferable to have small focal spots. In general, smaller focal spots have higher brightness and are used in X-ray analytic instruments to achieve relatively high efficiency at the detector.

[0005] Typically, where a small focal spot is required, an X-ray tube is provided that includes a coiled tungsten filament. Electrons are emitted by thermionic emission from the cathode by resistively heating the filament. These X-ray tubes tend to have poor spectral stability when operated at low high voltage settings (e.g., less than 10 kV).

[0006] To maximize the power at low high voltage settings, it is necessary to increase the current applied to the filament (thereby increasing the emission current). However, this can increase the amount of tungsten that evaporates from the filament and deposits on the anode, preventing x-rays from emitting from the anode. This can reduce the spectral stability of the x-ray tube.

[0007] Some other applications have used X-ray tubes containing coated emission loops, which have traditionally been used only in applications that do not require high resolution because these loops produce a large, ring-shaped focal spot.

[0008] It would be desirable to provide a compact x-ray tube that is capable of achieving superior performance in terms of providing a small focal spot, high emission current at low high voltage settings, and excellent spectral stability (ie, low power drift). Summary of the Invention

[0009] According to one aspect of the present invention there is provided an X-ray tube comprising: an anode that emits x-rays, the anode having a target surface; a cathode having an emitting loop for emitting electrons, the emitting loop extending about an axis passing through the anode, the cathode and the anode being spaced apart from one another along a forward axis; an electron beam guide configured to allow electrons emitted by the emission loop to irradiate an area of ​​a target surface of an anode, the area of ​​the target surface being irradiated by the electrons being surrounded by a single boundary; has.

[0010] The electron beam guide is configured to affect the trajectory of the electrons from the cathode to the anode, so that the electrons irradiate a "solid" region of the anode rather than a "hollow" region. More formally, the electron beam irradiates an area surrounded by a single boundary rather than an area surrounded by two separate boundaries (e.g., an annulus is an area surrounded by two separate boundaries).

[0011] This arrangement facilitates the use of the emission loop to irradiate a small area of ​​the anode. At the same time, the cathode filament is an emission loop rather than a conventional coiled tungsten filament. Providing this arrangement helps ensure that the x-ray tube can be operated at high emission currents with minimal impact on the output stability of the x-ray tube.

[0012] The maximum linear dimension of the irradiated area of ​​the anode may be between 0.1 mm and 2.0 mm, preferably between 0.9 mm and 1.5 mm. If the irradiated area is circular, the diameter of the irradiated area may be between 0.1 mm and 2.0 mm, preferably between 0.9 mm and 1.5 mm.

[0013] The x-ray tube further includes a first wall portion between the anode and the emission loop, the first wall portion including a ring extending about the axis.

[0014] In some embodiments, the ring is positioned to block at least one direct line of sight from the emitting loop to the anode, such that, during use, electrons emitted from the emitting loop travel along a curved trajectory from the emitting loop to the anode.

[0015] In use, electrons emitted by the emitting loop are accelerated towards the anode. The first wall portion interrupts the shortest path from the emitting loop to the anode, so that the electrons take a curved trajectory. The anode and cathode are spaced apart from each other along the axis, so that the first wall portion partially blocks the anode. This arrangement helps to avoid the formation of a "hollow" focal spot. Providing this arrangement facilitates the formation of a small, solid, focal spot.

[0016] The axis passes through the centre of the area enclosed by the first wall portion.

[0017] In some embodiments, the ring has an inclined outer surface facing the emission loop, the outer surface being inclined relative to the axis and defining an angle between the outer surface and the axis of between 30 and 60 degrees, preferably the angle is between 40 and 50 degrees.

[0018] In some embodiments, the exterior surface of the first wall portion defines a frustoconical surface centered about the axis.

[0019] In some embodiments, the x-ray tube further comprises a housing surrounding the anode and the cathode, the first wall portion being integrally formed with the housing.

[0020] The first wall portion forms part of an inner wall of the housing, or alternatively, the first wall portion can be separate from the housing.

[0021] In some embodiments, the emission loop extends about a longitudinal axis of the housing, in these embodiments, the longitudinal axis passes through the target surface of the anode.

[0022] In some embodiments, the emission loop is centered on an axis and the angle between the target surface and a line extending from a point on the emission loop through the center of the target surface is between 30 degrees and 60 degrees.

[0023] Preferably, the angle is between 40 and 50 degrees. The distance between the target surface of the anode and the center of the emission loop is 10 mm or less. In some embodiments, the perimeter of the emission loop is greater than the perimeter of the target surface.

[0024] In some embodiments, the x-ray tube further has a second wall portion extending about the axis, the emission loop being disposed between the first and second wall portions, the second wall portion having a sloped inner surface defining a tapered volume that tapers from the target surface of the anode along the axis in a direction away from the anode.

[0025] The shape of the inner surface of the second wall portion can assist in forming a symmetric focal spot. The tapered volume tapers in a direction from the anode towards the emission loop.

[0026] The inner surface of the second wall portion may define a frusto-conical surface centered about the axis.

[0027] In some embodiments, the x-ray tube further comprises a window that allows x-rays to exit the x-ray tube, and the anode, emission loop and window are spaced apart along the axis.

[0028] In some embodiments, the x-ray tube further comprises a third wall portion, the third wall portion being ring-shaped and the axis extending through a center of the ring shape.

[0029] In some embodiments, the ring or ring-shaped third wall portion has an inner surface facing the anode, the inner surface extending in a plane parallel to the target surface of the anode.

[0030] In some embodiments, the emission loop is disposed between the third wall portion and the anode in the axial direction.

[0031] The annulus extends around a window of the x-ray tube. The emission loop and the third wall portion are centered around an axis passing through the target surface of the anode.

[0032] In some embodiments, the cathode comprises a first wire of a refractory metal extending between a first end and a second end, the first wire having an emissive loop; a spiral of a second wire of a refractory metal extending around and over the first wire; and a coating over the second wire spiral, the coating having a work function of less than 4 eV. has.

[0033] The coating can reduce the temperature at which the emission loop can emit electrons by thermionic emission. Providing this cathode helps ensure that the X-ray tube can operate at high emission currents with minimal impact on the output stability of the X-ray tube. In some embodiments, the coating is a barium oxide coating.

[0034] In an aspect of the invention, there is provided an X-ray tube comprising an anode for emitting X-rays, the anode having a target surface, a cathode having an emitting loop (11) for emitting electrons, the emitting loop extending about an axis passing through the anode, the cathode and the anode being spaced apart from each other along the axis (10), and an electron beam guide having a first wall portion (19) between the anode and the emitting loop, the first wall portion having a ring extending about said axis, the ring being positioned to block at least one direct line of sight from the emitting loop to the anode such that, in use, electrons emitted by the emitting loop travel along a curved trajectory from the emitting loop to the anode.

[0035] The electron beam guide is configured to affect the trajectory of the electrons from the cathode to the anode such that the electrons irradiate a "solid" region of the anode rather than a "hollow" region. More formally, the electron beam irradiates an area surrounded by a single boundary rather than an area surrounded by two separate boundaries (e.g., an annulus is an area surrounded by two separate boundaries).

[0036] This arrangement facilitates the use of an emission loop to irradiate a small area of ​​the anode, and at the same time, the cathode filament is an emission loop rather than a conventional coiled tungsten filament, which allows the x-ray tube to operate at high emission currents with minimal impact on the x-ray tube's output stability.

[0037] The maximum linear dimension of the irradiated area of ​​the anode may be between 0.1 mm and 2.0 mm, preferably between 0.9 mm and 1.5 mm. If the irradiated area is circular, the diameter of the irradiated area may be between 0.1 mm and 2.0 mm, preferably between 0.9 mm and 1.5 mm.

[0038] The x-ray tube further includes a first wall portion between the anode and the emission loop, the first wall portion including a ring extending about the axis.

[0039] In some embodiments, the ring is positioned to block at least one direct line of sight from the emitting loop to the anode, such that, during use, electrons emitted from the emitting loop travel along a curved trajectory from the emitting loop to the anode.

[0040] In use, electrons emitted by the emitting loop are accelerated towards the anode. The first wall portion interrupts the shortest path from the emitting loop to the anode, so that the electrons take a curved trajectory. The anode and cathode are spaced apart from each other along the axis, so that the first wall portion partially blocks the anode. This arrangement helps to avoid the formation of a "hollow" focal spot. Providing this arrangement facilitates the formation of a small, solid, focal spot.

[0041] The axis passes through the centre of the area enclosed by the first wall portion.

[0042] In some embodiments, the ring has an inclined outer surface facing the emission loop, the outer surface being inclined relative to the axis and defining an angle between the outer surface and the axis of between 30 and 60 degrees, preferably the angle is between 40 and 50 degrees.

[0043] In some embodiments, the exterior surface of the first wall portion defines a frustoconical surface centered about the axis.

[0044] In some embodiments, the x-ray tube further comprises a housing surrounding the anode and the cathode, the first wall portion being integrally formed with the housing.

[0045] The first wall portion forms part of an inner wall of the housing, or alternatively, the first wall portion can be separate from the housing.

[0046] In some embodiments, the emission loop extends about a longitudinal axis of the housing, in these embodiments, the longitudinal axis passes through the target surface of the anode.

[0047] In some embodiments, the emission loop is centered on an axis and the angle between the target surface and a line extending from a point on the emission loop through the center of the target surface is between 30 degrees and 60 degrees.

[0048] Preferably, the angle is between 40 and 50 degrees. The distance between the target surface of the anode and the centre of the emission loop is at most 10 mm.

[0049] In some embodiments, the perimeter of the emission loop is greater than the perimeter of the target surface.

[0050] In some embodiments, the x-ray tube further has a second wall portion extending about the axis, the emission loop being disposed between the first and second wall portions, the second wall portion having a sloped inner surface defining a tapered volume that tapers from the target surface of the anode along the axis in a direction away from the anode.

[0051] The shape of the inner surface of the second wall portion can assist in forming a symmetric focal spot. The tapered volume tapers in a direction from the anode towards the emission loop.

[0052] The inner surface of the second wall portion may define a frusto-conical surface centered about the axis.

[0053] In some embodiments, the x-ray tube further comprises a window that allows x-rays to exit the x-ray tube, and the anode, emission loop and window are spaced apart along the axis.

[0054] In some embodiments, the x-ray tube further comprises a third wall portion, the third wall portion being ring-shaped and the axis extending through a center of the ring shape.

[0055] In some embodiments, the ring has an inner surface that faces the anode, the inner surface extending in a plane parallel to the target surface of the anode.

[0056] In some embodiments, the emission loop is disposed between the third wall portion and the anode in the axial direction.

[0057] The annulus extends around a window of the x-ray tube. The emission loop and the third wall portion are centered on an axis passing through the target surface of the anode.

[0058] In some embodiments, the cathode comprises a first wire of a refractory metal extending between a first end and a second end, the first wire having an emissive loop, a spiral of a second wire of a refractory metal extending around and over the first wire, and a coating covering the spiral of the second wire, the coating having a work function less than 4 eV.

[0059] The coating can reduce the temperature at which the emission loop can emit electrons by thermionic emission. Providing this cathode helps ensure that the X-ray tube can be operated at high emission currents with minimal impact on the output stability of the X-ray tube. In some embodiments, the coating is a barium oxide coating. [Brief description of the drawings]

[0060] Embodiments of the invention will now be described in detail with reference to the accompanying drawings. [Figure 1] FIG. 1 is a schematic cross-sectional view of an X-ray tube according to an embodiment of the present invention. [Figure 2A] FIG. 2 shows a perspective view of a cathode having an emission loop. [Figure 2B] FIG. 13 shows a close-up view of the emission loop. [Diagram 3] FIG. 1 shows a "ring-shaped" focus and a "point-shaped" focus. [Figure 4] FIG. 2 is a diagram showing the path of an electron beam in the X-ray tube of FIG. [Diagram 5] FIG. 2 illustrates an electric field inside an X-ray tube according to an embodiment. [Figure 6] FIG. 2 illustrates the intensity variation with position in the irradiated area of ​​an exemplary anode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] The present invention provides an X-ray tube having a cathode and an anode. The cathode includes an emission loop for emitting electrons. The X-ray tube also has an electron beam guide configured to cause electrons from the emission loop to irradiate an area of ​​the anode surrounded by a single boundary (solid area). The cathode has an emission loop that has traditionally only been used to form a large, hollow focal spot (e.g., and 12 mm diameter).

[0062] FIG. 1 shows an X-ray tube 1 in cross section. The X-ray tube 1 has a cathode 3 and an anode 5 arranged in a housing 7. The housing 7 provides a vacuum tube. In use, the cathode 3 is heated and emits electrons by thermionic emission. The electrons from the cathode 3 form an electron beam, which is directed towards the target surface 6 of the anode 5. The cathode 3 is held at ground and a high voltage is applied to the anode 5, thus creating a potential difference between the anode 5 and the cathode 3. The electrons from the cathode 3 accelerate towards the anode 5. When the electrons from the cathode 3 collide with the anode 5, they cause X-rays to be emitted from the anode 5. The emitted X-rays exit the X-ray tube 1 through a window 9 of the tube 1. The window is arranged at the end of the X-ray tube, facing the target surface 6 of the anode 5. The cathode is arranged between the anode 5 and the window 9.

[0063] 1 , the housing 7 has a longitudinal axis 10. The cathode 3 and the anode 5 are spaced apart from one another in a direction along the longitudinal axis 10 of the vacuum tube 7. The cathode 3 includes an emitting loop 11 that extends about the longitudinal axis 10 such that the emitting loop 11 is centered about the axis 10. The perimeter of the emitting loop is greater than the perimeter of the target surface 6, and thus the emitting loop 11 also extends about the target surface 6 of the anode 5. The longitudinal axis 10 passes through the target surface 6 of the anode 5, through the center of the emitting loop 11, and through the window 9.

[0064] Electrons emitted by the emission loop are guided by an electron beam guide that at least partially determines the path taken by the electrons from the cathode to the anode.

[0065] In the embodiment shown in Figure 1, electron beam guidance is provided by the inner walls of the housing 7. The inner walls are shaped such that, in use, the electric field within the x-ray tube 1 will direct electrons emitted from the cathode 3 towards the target surface 6 of the anode 5 to illuminate solid areas of the target surface (rather than hollow areas). The shape of the inner walls affects the electric field inside the x-ray tube and therefore the trajectories 35 of the electrons emitted by the cathode 3. During operation, the inner walls are held at ground and a high voltage is applied to the anode 5.

[0066] The inner wall of the x-ray tube comprises a first wall portion 19. The first wall portion is disposed between the emission loop 11 and the anode 5 so as to physically block the shortest straight line path from the emission loop to the target surface 6 of the anode 5. The first wall portion 19 has an outer surface 13 that faces outward from the axis 10 towards the emission loop 11. The outer surface is inclined with respect to the axis 10. In FIG. 1, the outer surface is at a 45 degree angle with respect to the axis 10. In this configuration, the first wall portion 19 partially blocks the target surface 6 of the anode from the emission loop. Electrons emitted by the emission loop are forced to bend over the first wall portion 19 before they reach the target surface 6 of the anode. Because the anode is spatially separated from the emission loop along the axis 10, the electrons travel along an extended path towards the target surface. This helps to reduce the size of the illumination area of ​​the anode.

[0067] The inner wall also has a second wall portion 21 that is disposed between the first wall portion and the window 9 in a direction along the axis 10. The second wall portion 15 comprises a ring extending around the axis 10. The inner surface of the second wall portion is inclined and defines a tapered volume that tapers from the target surface 6 of the anode towards the window 9.

[0068] A third wall portion of the inner wall surrounds the window 9 and defines the neck of the X-ray tube. A second wall portion 21 of the inner wall of the X-ray tube is disposed below the third wall portion.

[0069] The second wall portion and the third wall portion together define a shoulder extending from the neck above the cathode 3. The lower surface of the third wall portion facing the anode defines an upper portion of the shoulder and the inner surface of the second wall portion 21 provides a lower portion of the inner wall shoulder.

[0070] Below the shoulder and below the cathode, a first wall portion of the inner wall projects inwardly toward the axis and upwardly, away from the target surface of the anode.

[0071] In Fig. 2A the cathode 3 is shown in more detail. The cathode 3 comprises a single length of tungsten wire extending between adjacently disposed first and second ends. The cathode has the form of a circular emitting loop 11, with first and second thermal loops 12 formed between the emitting loop 11 and the first and second ends. Each of the first and second thermal loops is in the form of a U-shaped loop of wire, with the legs of the U extending parallel to the emitting loop, i.e. continuing in a circle. The term "thermal loop" is used because the function or action of the loop is to provide some thermal resistance between the emitting loop and the ends of the wire.

[0072] FIG. 2B shows the emission loop in more detail. The cathode includes a first tungsten wire 4, a second tungsten wire 30, and an emitter coating 32 that form a circular emission loop. The second tungsten wire 30 is arranged in a spiral around the first tungsten wire 4. The emitter coating 32 is disposed on the composition of wires. In this embodiment, there are small gaps between the individual turns of the spiral wire, and the coating extends into and over these gaps. This is believed to create a strong bond and good chemical contact between the coating 32 and the wires 4, 30.

[0073] The emission loop has a maximum linear dimension (i.e. diameter if circular) of 0.5 mm to 5 mm. The coating thickness can be from 0.5 μm to 50% of the diameter of the emission loop. The second wire can be tightly bounded to the first wire or can be spaced apart, for example from 0 to 20% of the diameter of the emission loop. The support wire can be, for example, 20 to 500 μm in diameter and of any suitable length, for example from 2 mm to 30 mm. The support wire can in particular be 20% to 80%, or 20% to 50% in diameter of the inner wire.

[0074] The emitter coating 32 contains a material that produces thermionic emission at a lower temperature than tungsten. For example, the coating contains barium oxide and / or strontium oxide. The coated emission loop 11 produces electron emission at a lower temperature, so that evaporation of the material is reduced or avoided. Thus, the cathode can achieve a stable x-ray output over time.

[0075] The cathode provides a very uniform x-ray spot due to the good bonding between the coating and the coiled wire and the uniform temperature distribution.

[0076] The inventors have discovered that by providing an x-ray tube with an emission loop located above the anode, it is possible to provide a complete / solid (i.e., not hollow) focal spot while simultaneously achieving stable high power output at low high voltage settings.

[0077] Table 1 shows several low kV settings for an X-ray tube according to the invention and two comparative X-ray tubes. One of the comparative X-ray tubes has an uncoated tungsten filament in the form of a coil. It can be seen that the lowest kV setting for this comparative X-ray tube without unacceptable power drift is 10 kV. To preserve the life of the cathode, the power is limited to 500 W at this low high voltage setting. Also note that the focal spot using a 12 mm emission loop is annular.

[0078] X-ray tubes according to the present invention can also be used at higher mA settings, for example they can be operated at currents up to 12 mA while still achieving good spectral stability.

[0079] [Table 1]

[0080] 3 shows an annular focal spot 36 and a solid focal spot 38. An annular focal spot has a hollow shape and is defined between two distinct boundaries. Traditionally, x-ray tubes that use an emission loop produce focal spots of this shape. These focal spots are large (typically about 12 mm in diameter).

[0081] The solid focal spot in Figure 3 is surrounded by a single continuous boundary. It is not hollow. It has a diameter of 1.5 mm.

[0082] Figure 4 is a schematic diagram of the path of the electron beam, during use, of the x-ray tube 1 of Figure 1. As shown in Figure 4, in use, electrons are emitted from the cathode 3 and travel under the influence of an electric field within the x-ray tube 1 towards the anode 5. An electron beam guide is arranged to shape the electron beam from the cathode such that the electrons irradiate a solid area of ​​the target surface 6 of the anode 5. The inner wall is shaped so that electrons from the cathode are forced to take a curved trajectory above the first wall portion. The shape of the inner wall helps ensure that the electrons from the cathode spread out above the anode so that they irradiate a solid area of ​​the anode's surface. As shown in Figure 4, electrons from cathode 3 are directed towards anode 5 and irradiate an area of ​​the anode that is surrounded by a single boundary.

[0083] FIG. 5 shows the electric field inside the X-ray tube 1 when the X-ray tube is in use.

[0084] Figure 6 shows how the intensity of the focal region varies with position. This is used to determine whether the focal region is "solid" (a region surrounded by a single boundary) or "hollow" (a region surrounded by multiple boundaries). To assess the shape of the focal region, it is possible to determine the shape of the focal region by capturing an image of the anode and processing the image.

[0085] The portions of the anode that are illuminated by the electrons have a higher intensity in the image of the anode. One way to identify whether a central area of ​​the region of the anode is illuminated by the electrons is to analyze the image of the anode.

[0086] For example, a region of the anode surrounded by a single boundary is selected. The selected region surrounds the focal region. A peak intensity of the selected region is identified. Then, an average intensity of a central area of ​​the selected region is identified. For example, the central area is the area in the middle of the selected region and is approximately equal to 10% of the total selected area. If the average intensity of the central area is less than or equal to 5% of the peak intensity, the focal region is a hollow focal region. If the average intensity of the central area is greater than 5% of the peak intensity, the focal region is a solid focal region.

[0087] In Figure 6, an image of a selected area of ​​the anode is shown on the left side of the page. The graph on the right side of the page shows how the measured intensity varies with position on the image of the selected area of ​​the anode. In this example, the central area has an intensity that is greater than 5% of the peak intensity. Thus, the focal area is a "solid" focal area.

[0088] In some embodiments, the first wall portion is a ring. The ring is not necessarily circular.

[0089] In some embodiments, the cathode does not include any thermal loop.

[0090] In some embodiments, the first wall portion is not integrally formed with the housing, i.e., the first wall portion is a separate body relative to the housing.

[0091] In some embodiments, the first wall portion and the second wall portion are separate entities. In other embodiments, the first wall portion and the second wall portion are integrally formed.

[0092] In some embodiments, the second wall portion and the third wall portion are separate components. In other embodiments, the second wall portion and the third wall portion are integrally formed.

[0093] In some embodiments, the electron beam guide is provided by an inner surface of the vacuum tube, while in other embodiments the electron beam guide is separate from the vacuum tube.

[0094] The area of ​​the anode that is irradiated by the electron beam may be circular or may have a different shape, for example the area irradiated by the electron beam may be elliptical.

[0095] The anode may be of any material suitable for producing x-rays.

[0096] Those skilled in the art will appreciate that x-ray tubes are not strictly tubular.

Claims

1. 1. An X-ray tube comprising: an anode having a target surface and configured to emit X-rays; a cathode comprising: a first wire of a refractory metal extending between a first end and a second end, the first wire having an emissive loop; a spiral of a second wire of a refractory metal extending around and over the first wire; and a coating over the spiral of the second wire, the coating having a work function of less than 4 eV, the emissive loop extending about an axis passing through the anode, the anode and the cathode being spaced apart from one another along the axis; an electron beam guide configured so that electrons emitted by the emission loop irradiate an area of ​​the target surface of the anode, the area of ​​the target surface irradiated by electrons being a solid spot surrounded by a single boundary; and Equipped with The emission loop is centered on the axis, and the angle between the target surface and a line extending from a point on the emission loop through a center of the target surface is between 30 degrees and 60 degrees.

2. the electron beam guide having a first wall portion between the anode and the emitting loop; The first wall portion includes a ring extending about the axis.

2. The X-ray tube of claim 1.

3. The electron beam guide further includes a second wall portion extending about the axis; the emission loop is disposed between the first wall portion and the second wall portion; the second wall portion having an inclined inner surface defining a tapered volume that tapers from the target surface of the anode along the axis in a direction away from the anode.

3. The X-ray tube according to claim 2.

4. the ring is positioned to block at least one direct line of sight from the emitting loop to the anode such that, in use, electrons emitted by the cathode travel along a curved trajectory from the cathode to the anode.

4. The X-ray tube according to claim 2 or 3.

5. the ring has a sloped outer surface facing the emission loop; the outer surface is inclined with respect to the axis to define an angle between the outer surface and the axis of between 30 and 60 degrees; 5. The X-ray tube according to claim 4.

6. The angle between the outer surface and the axis is between 40 degrees and 50 degrees.

6. The X-ray tube according to claim 5.

7. an outer surface of the first wall portion defines a frustoconical surface centered about the axis; 7. The X-ray tube according to claim 2.

8. The X-ray tube further includes a housing surrounding the anode and the cathode; the first wall portion is integrally formed with the housing; 8. An X-ray tube according to any one of claims 2 to 7.

9. the circumference of the emission loop is greater than the circumference of the target surface; 9. An X-ray tube according to any one of claims 1 to 8.

10. the x-ray tube further comprising a window for allowing x-rays to exit the x-ray tube; the anode, the emission loop, and the window are spaced apart along the axis; 10. An X-ray tube according to any one of claims 1 to 9.

11. the x-ray tube further comprises a third wall portion; the third wall portion is annular; The axis extends through a center of the annular shape.

11. The X-ray tube according to any one of claims 7 to 10.

12. the annular shape has an inner surface facing the anode; the inner surface extends in a plane parallel to the target surface of the anode; 12. The x-ray tube of claim 11.

13. the emission loop is disposed between the third wall portion and the anode in a direction along the axis.

13. The X-ray tube according to claim 11 or 12.

Images