JP6407591B2 - Fixed anode X-ray tube - Google Patents

Description

  Embodiments relate to a fixed anode X-ray tube.

  The fixed anode type X-ray tube is a vacuum including a cathode (cathode) including a filament that emits an electron beam, an anode (anode) including an anode target that receives the emitted electrons and emits X-rays, and the cathode and the anode. And a vacuum envelope that is hermetically sealed. The vacuum envelope is made of glass and / or ceramics. The electron beam emitted from the filament is focused by the focusing electrode of the cathode and enters the target, and X-rays are emitted from the anode. Some of the incident electrons are scattered to the cathode side as recoil electrons by elastic scattering. Further, some of the recoil electrons collide with the vacuum envelope. The energy of electrons when colliding with the vacuum envelope is usually several tens of KeV. Therefore, cracks may occur in the vacuum envelope after continuous use for a long time.

Japanese Utility Model Laid-Open No. 3-110753

  In Patent Document 1, an anode hood is provided around the anode target so that recoil electrons do not fly to the vacuum envelope in order to prevent cracking of the vacuum envelope. However, depending on the shape and size of the anode hood, recoil electrons may jump out of the anode hood and collide with the vacuum envelope. When a crack occurs, the gas generated from the crack deteriorates the degree of vacuum of the X-ray tube, and discharge easily occurs, and a glass piece may adhere to the cathode surface and the like, which may reduce the voltage resistance.

  The problem to be solved by the present invention is to provide a fixed anode type X-ray tube in which withstand voltage is maintained during long-term continuous use.

  A fixed anode type X-ray tube according to the present embodiment includes a cathode having an electron emission source that emits an electron beam, an anode target that is disposed opposite to the cathode and emits X-rays when the electron beam is incident thereon, A cylindrical first shielding member that has the same potential as the anode target and surrounds the anode target, and is formed in a lid shape at the tip of the first shielding member facing the cathode, and is emitted from the cathode to the anode target. A vacuum enclosure that holds the second shielding member having an opening which is a through hole through which the electron beam passes, the cathode, the anode target, the first shielding member, and the second shielding member in a vacuum-tight state. And a container.

FIG. 1A is a schematic diagram of an internal structure of an X-ray tube according to an embodiment. 1B is a cross-sectional view taken along line AA in FIG. 1A. FIG. 2 is an enlarged view of the anode cover according to the embodiment. FIG. 3A is a view showing an anode cover having a minimum protruding length for shielding recoil electrons. FIG. 3B is an explanatory diagram of the principle of an anode cover having a minimum protruding length for shielding recoil electrons. FIG. 4A is a cross-sectional view taken along line AA of an X-ray tube showing a modification. FIG. 4B is an enlarged view of a cross section of the anode cover shown in FIG. 4A. FIG. 5 is a cross-sectional view taken along the line AA of the X-ray tube. FIG. 6 is a schematic view of an X-ray tube related to the modification.

Embodiments will be described below with reference to the drawings.
(First embodiment)
FIG. 1A is a schematic diagram of the internal structure of the X-ray tube 1 according to the embodiment. 1B is a cross-sectional view taken along line AA in FIG. 1A. As shown in FIG. 1A, an X-ray tube 1 includes a cathode (cathode) 10 that emits an electron beam, an anode (anode) 20 that is disposed to face the cathode 10, and a cylindrical shape that is sealed in a vacuum atmosphere. And a vacuum envelope 30. The X-ray tube 1 is a fixed anode type X-ray tube. The X-ray tube 1 is formed in a substantially cylindrical shape having a tube axis TA as a central axis. The cathode 10 and the anode 20 are accommodated in a cylindrical vacuum envelope 30 sealed in a vacuum atmosphere. Further, an external power source, for example, a high voltage plug (not shown) or the like is connected to the cathode 10 and the anode 20, and a high voltage (tube voltage) is applied between the cathode 10 and the anode 20. The X-ray tube 1 may include a cooling mechanism for cooling the anode 20 that becomes high temperature during operation.

  The cathode 10 has a filament 11 as an electron emission source that emits an electron beam, and a focusing electrode 12 for focusing the emitted electrons toward an anode target. Hereinafter, for convenience of explanation, the electron beam may be expressed as electrons. The cathode 10 is formed in a substantially cylindrical shape, and the cathode 10 is installed such that the tube axis TA passes through the center of the circle of the cathode 10. A negative tube voltage is applied to the cathode 10 relative to the anode 20.

  The filament 11 has, for example, two terminals. The filament 11 is heated by the supply of electric current, and electrons (thermoelectrons) are emitted from the heated filament 11 toward the anode target 20 described later (thermoelectron emission).

  The focusing electrode 12 is arranged around a trajectory through which electrons pass and focuses the electrons emitted from the filament 11 with respect to the anode target 20. When a terminal of an external power source, for example, a high voltage plug or the like is attached to the X-ray tube 1, a high voltage is applied to the filament 11 and the focusing electrode 12. The current generated by the applied voltage is supplied to the filament 11 as described above and is emitted as thermoelectrons. The emitted electrons are focused by the focusing electrode 12 so as to collide with an anode target 21 described later. For example, the electrons emitted from the cathode 10 are focused toward the center position of the surface of the anode target 21.

  The anode 20 includes an anode target 21, an anode base material 22, and an anode (anode) hood (first shielding member) 23. Hereinafter, the anode hood 23 is referred to as an anode hood 23. The anode target 21 is disposed to face the filament 11 of the cathode 10. The anode target 21 is embedded in the center position of the surface of the anode base material 22 at the tip of the anode base material 22 described later in the direction of the cathode 10. The anode target 21 is made of, for example, molybdenum (Mo) or tungsten (W). A positive tube voltage is applied to the anode 20 relative to the cathode 10. Due to the potential difference between the anode target 21 and the cathode 10, electrons (incident electrons) emitted from the cathode 10 are accelerated toward the anode target 21, focused by the focusing electrode 12, and collide with the anode target 21. When the accelerated and focused electrons collide with the anode target 21, X-rays are emitted from the anode target 20 by bremsstrahlung. Hereinafter, in the anode target 21, the point where the electron beam collides may be referred to as a focal point.

  The anode base material 22 is formed in a substantially cylindrical shape with the tube axis TA as a central axis. Specifically, the anode base material 22 has an outer periphery formed in a circular shape, and the surface facing the cathode 10 is inclined. The anode base material 22 dissipates heat generated in the anode target 21. The anode base material 22 is formed of copper having a high thermal conductivity. For example, the anode base material is made of copper.

  The anode hood 23 includes an anode (anode) cover (second shielding member) 23 </ b> A and a radiation window 24. The anode hood 23 is formed of a metal member. The anode hood 23 shields recoil electrons scattered (or reflected) by the anode target 22. The anode hood 23 is formed in a substantially cylindrical shape centering on the tube axis TA so as to surround the anode 20. The anode hood 23 is joined to the outer periphery of the anode base material 22 and is formed so as to protrude from the tip of the anode base material 22 in the direction of the cathode 10 with a predetermined length. A metal member 25 is joined to the anode 20 on the tip surface opposite to the tip surface on the cathode 10 side. The metal member 25 is made of metal. The metal member 25 is sealed to the anode base material 22 and the vacuum envelope 30. Hereinafter, the anode cover 23A is referred to as an anode cover 23A.

  The anode cover 23A is formed in a lid shape at the tip of the anode hood 23 on the cathode 10 side. As shown in FIG. 1B, the anode cover 23A is formed in a disk shape, for example, when viewed in the AA cross section perpendicular to the tube axis TA. The anode cover 23A has a circular opening having the tube axis TA as a central axis. The opening of the anode cover 23A is a through hole through which electrons emitted from the cathode 10 pass. The anode cover 23A is formed to protrude vertically from the outer periphery of the anode hood 23 toward the tube axis TA at the tip of the anode hood 23 on the cathode 10 side. Here, the protruding length from the outer periphery of the anode hood 23 of the anode cover 23A toward the tube axis TA is referred to as “the protruding length of the anode cover 23A”. The anode cover 23A prevents recoil electrons from the anode target 21 from being scattered in the vacuum envelope 30 described later. The anode cover 23 </ b> A is installed with the disk-shaped surface facing the cathode 10. The anode cover 23 </ b> A is formed to have a protruding length that prevents recoil electrons scattered by the anode target 21 from reaching the vacuum envelope 30. A detailed structure of the anode cover 23A will be described later.

  The radiation window 24 is an opening for radiating X-rays radiated from the anode target 21 from the anode hood 23 to the outside. A metal member may be fitted to the radiation window 24 so that recoil electrons do not fly to the vacuum envelope 30. For example, a beryllium plate is fitted into the radiation window 24.

  The vacuum envelope 30 is formed in a substantially cylindrical shape with the tube axis TA as a central axis, and has a cathode 10 and an anode 20 inside. The vacuum envelope 30 that houses the cathode 10 and the anode 20 is hermetically sealed in a vacuum-tight manner. That is, the vacuum envelope 30 is sealed and the inside is maintained in a vacuum state. The vacuum envelope 30 is made of, for example, glass and / or ceramics.

(Anode cover structure)
Next, the structure of the anode cover 23A will be described with reference to FIGS. 2, 3A, and 3B. FIG. 2 is an enlarged view of the anode cover 23A of the present embodiment. FIG. 3A is a view showing an anode cover 23A having a minimum protruding length for shielding recoil electrons. FIG. 3B is an explanatory diagram of the principle of the anode cover 23A having a minimum protruding length for shielding recoil electrons.

  In FIG. 2, D1 indicates the width (width) of the opening of the anode cover 23A. D2 indicates the outer diameter of the anode cover 23A. L1 indicates the distance from the intersection (the focus of the electron beam) between the tube axis TA and the surface of the anode target 21 to the tip surface of the anode cover 23A on the cathode 10 side. L2 indicates the distance between the tip surface of the cathode 10 on the anode 20 side and the surface of the anode cover 23A on the cathode 10 side. Here, the intersection (the focal point of the electron beam) between the tube axis TA and the surface of the anode target 21 is the same as the center of the surface of the anode target 21. Further, for convenience of explaining the trajectory of electrons, the incident electrons 100 and the recoil electrons 101 are schematically shown in FIG.

  As described above, the incident electrons 100 emitted from the filament 11 of the cathode 10 toward the anode 20 are focused by the focusing electrode 12 and enter the anode target 21. Hereinafter, as an example, the incident electron 100 is described as traveling straight along the tube axis TA and colliding with the anode target 21.

  In FIG. 2, Y1 indicates the protruding length of the anode cover 23A. When the incident electrons 100 collide with the anode target 21, a part of the incident electrons 100 can be scattered in all directions by elastic scattering. A part of the recoil electrons 101 scattered by the anode target 21 proceeds toward the cathode 10 through the opening of the anode cover 23A. Since a negative electric field is generated at the cathode 10, the recoil electrons 101 are decelerated by the electric field at the cathode 10. A part of the recoil electrons 101 reaches the surface of the cathode 10, but a part of the recoil electrons 101 pushed back is attracted toward the anode 20 again. The recoil electrons 101 attracted toward the anode 20 again collide with the surface of the anode cover 23A. The probability that the electrons colliding with the surface of the anode cover 23A will head toward the vacuum envelope 30 is considerably reduced. That is, the electrons directed to the vacuum envelope 30 are shielded by the anode cover 23A.

Hereinafter, the protruding length of the anode cover 23A will be described.
The recoil electrons 101 that have jumped out of the opening of the anode cover 23A collide with the surface of the anode cover 23A again in a parabolic orbit. Here, the recoil electrons 101 are electrons that are scattered by the anode base material 22 and travel toward the cathode 10 through the outermost side of the opening of the anode cover 23A. That is, the recoil electrons 101 are the electrons that travel most toward the outside of the anode hood 23 among the scattered electrons. In the following description, the recoil electrons 101 are described as the electrons that travel to the outside of the anode hood 23 among the scattered electrons, unless otherwise specified.

  As shown in FIG. 3A, when the protruding length of the anode cover 23A is minimized, the recoil electrons 101 collide with the outermost end of the surface of the anode cover 23A. The trajectory of the recoil electrons 101 can be approximated symmetrically with the vertex of the parabola as the base point. That is, the recoil electrons 101 are reflected so that the incident angle and the reflection angle are the same with respect to the vertex of the orbit. Therefore, it is considered that the recoil electrons 101 pass through the innermost end of the anode cover 23A and collide with the outermost end. By defining such a trajectory, the minimum protruding length of the anode cover 23A is required.

As shown in FIG. 3B, the trajectory of the recoiled electrons 101 that have reached the outermost periphery of the anode cover 23A can be derived geometrically. If the trajectory of the electrons reaching the outermost periphery can be derived, the minimum protrusion length X1 of the anode cover 23A necessary for shielding the recoil electrons 101 is obtained. Here, when the protruding length of the anode cover 23A is Y1, as described above, the protruding length Y1 of the anode cover 23A is expressed by the following equation.

As shown in FIG. 3B, the following expression is obtained from Expression 1 and the geometric similarity.

The minimum protrusion length X1 of the anode cover 23A from Expression 1 and Expression 2 is expressed by the following expression.

Therefore, in order for the anode cover 23A to shield the recoil electrons 101, the opening of the anode cover 23A is formed so as to have a length in the range of Equation 4 below.

Alternatively, the anode cover 23 </ b> A is formed to have a protruding length Y <b> 1 in the range of the following formula 5 in order to shield the recoil electrons 101.

By satisfying Equation 4 or Equation 5, an anode cover 23A that shields the recoil electrons 101 is formed.
According to this embodiment, the anode cover 23 </ b> A is formed on the anode hood 23. The anode cover 23 </ b> A is formed so as to shield the recoil electrons 101 that jump out of the opening and go to the vacuum envelope 30. The anode cover 23A is formed such that the recoil electrons 101 that jump out through the opening collide with the surface of the anode cover 23A again. Even when recoil electrons colliding with the surface of the anode cover 23A are elastically scattered again, the phenomenon of colliding again with the surface of the anode cover 23A is repeated in a multiple manner. As a result, the probability that the recoil electrons 101 reach the vacuum envelope 30 is much lower. Or, by repeating the scattering, the energy of recoil electrons is attenuated below the energy that damages the vacuum envelope 30. Therefore, damage to the vacuum envelope 30 due to recoil electrons is prevented. Since damage to the vacuum envelope 30 due to recoil electrons is prevented, an X-ray tube is provided in which voltage resistance is maintained during use. The actions and effects described above are higher as Y1 is larger, but the value of Y1 is smaller than the upper limit of the limit at which the electron beam can pass through the opening without colliding with the surface of the anode cover 23A. Must be set. This upper limit can be confirmed by electron orbit simulation or trial experiment.

  Hereinafter, some modified examples of the present embodiment will be described with reference to the drawings. Since the X-ray tube 1 of the modification has substantially the same configuration as the X-ray tube 1 of the first embodiment, the same reference numerals are given to the same components as those of the X-ray tube 1 of the first embodiment. A detailed description thereof will be omitted.

(First modification)
A first modification will be described. The X-ray tube 1 of the first modification differs from the X-ray tube 1 of the first embodiment in the shape of the opening of the anode cover 23A.

The shape of the opening of the anode cover 23A will be described below with reference to FIGS. 4A and 4B.
FIG. 4A is a cross-sectional view taken along the line AA showing the opening of the anode cover 23A according to a modification. FIG. 4B is an enlarged view of a part of FIG. 4A. As shown in FIGS. 4A and 4B, the opening of the modified anode cover 23A is formed in a quadrangular shape. In the first modification, it is assumed that the opening is formed in a square as shown in FIG. 4B.

Next, the size of the anode cover 23A will be described with reference to FIG. 4B.
In FIG. 4B, D ₃ indicates the length of the horizontal axis of the opening, and D ₄ indicates the length of the vertical axis. Here, the width of the diagonal opening, that is, the maximum width of the opening of the anode cover 23A is D1. D1 is equivalent to that of the first embodiment, and is expressed by Expression 2 and Expression 4. Since the opening of the anode cover 23A is square, D3 is equivalent to D4.

Assuming that D1 is the maximum value for shielding the recoil electrons 101 as shown in Equation 2, the maximum opening length that can be formed to shield the recoil electrons 101 (the maximum values of D3 and D4) is Desired. That is, the size of each part of the opening of the anode cover 23A when the recoil electrons 101 flying to the outermost land on the outermost end of the anode cover 23A is obtained.

From the geometrical relationship from FIG. 4B, D1 is shown below.

When Expression 2 is applied to Expression 7, D3 is expressed by the following expression.

  From the above formulas 6, 7, and 8, the maximum opening length that can be formed to shield the recoil electrons 101 is obtained. That is, the maximum values of D3 and D4 are obtained, respectively.

Next, the minimum protrusion length of the anode cover 23A in each direction for shielding the recoil electrons 101 is obtained.
The protruding length of the anode cover 23A in the horizontal axis direction is Y3, and the minimum value of Y3 is X3. Similarly, the protruding length of the anode cover 23A in the vertical axis direction is Y4, and the minimum value of Y4 is X4. In the modified example, if the protruding length of the anode cover 23A on the diagonal line of the opening is the same as X1 in Expression 3, the recoil electrons 101 that jump out from each part of the opening are shielded. That is, the recoil electrons 101 that have jumped out land on the surface of the anode cover 23A. Therefore, Y3 and Y4 are represented by the following equations, respectively.

Y3 is

Indicated by
Y4 is

  Indicated by

When Expression 8 is applied to Expression 9, X3 is expressed by the following expression.

  In the modification, the anode cover 23A opening is formed in a square shape, so X4 is the same as X3.

  Accordingly, in order to shield the recoil electrons 101, the widths D3 and D4 of the respective portions of the opening of the anode cover 23A are formed in the ranges of the following formulas 12 and 13, respectively.

D3 is

The filling,
D ₄ is,

It is formed to satisfy.

  Alternatively, in order to shield the recoil electrons 101, the protruding lengths Y3 and Y4 of each part of the anode cover 23A are formed within the ranges of the following formulas 14 and 15, respectively.

Y3 is

The filling,
Y4 is

It is formed to satisfy.
The anode cover 23A for shielding the recoil electrons 101 is formed by Expressions 12 to 15.

  According to this embodiment, the area of the opening of the anode cover 23A is smaller than the area of the recoil electrons 101 in which the recoil electrons 101 are directed to the vacuum envelope 30 compared to the anode cover 23A of the first embodiment. small. It is suppressed that the recoil electrons 101 pass through the opening. In other words, the modified anode cover 23A shields recoil electrons from the anode cover 23A of the first embodiment.

  In the modification, the opening of the anode cover 23A is square, but may be rectangular.

For example, D4 is formed smaller than D3. Therefore, it is formed so as to satisfy the following formula.

At this time, the protruding lengths Y3 and Y4 of the anode cover 23A are formed so as to satisfy the following expression.

  By forming the opening in a rectangular shape in this way, the recoil electrons 101 passing through the opening are shielded from the anode cover 23A of the first modification.

  Further, as shown in FIG. 5, the opening of the anode cover 23A may be elliptical.

At this time, for example, the major axis width of the opening is D1, and the minor axis width is D5. In FIG. 5, the protruding length of the anode cover 23A in the short axis direction is Y5. Here, D1 is equivalent to the first embodiment. The relationship between the major axis and the minor axis is such that the opening of the anode cover 23A satisfies the following formula.

Alternatively, in order to shield the recoil electrons 101, the protruding length Y5 of the anode cover 23A is formed in the range of the following Expression 19. Here, the minimum value X5 of the protruding length Y5 is equivalent to X1 in the first embodiment. Y1 is formed in the range of Formula 5 of the first embodiment.

  By thus forming the opening in an elliptical shape, the recoil electrons 101 that jump out of the opening are shielded from the anode cover 23A of the first embodiment.

  Furthermore, although the anode cover 23A is described as a part of the anode hood 23 in the above-described embodiment, the anode cover may be joined to the tip surface of the anode hood 23 on the cathode 10 side as a separate body. As shown in FIG. 6, the anode cover 26 (second shielding member) formed separately is joined to the anode hood 23. As described above, the anode hood 26 is formed separately from the anode hood 23, so that the processing of the anode hood 23 and the anode cover 26 becomes easier than when the anode hood 23 and the anode cover 26 are integrated. In addition, since the anode cover 26 is formed separately from the anode hood 23, the degree of freedom of installation of the anode cover 26 is improved.

  According to the above-described embodiment, by providing the anode covers 23A and 26, the recoil electrons that shield the recoil electrons toward the vacuum envelope are shielded. The anode covers 23A and 26 are formed so that recoil electrons passing through the openings collide with the surfaces of the anode covers 23A and 26 again. Even when the recoil electrons colliding with the surfaces of the anode covers 23A and 26 are elastically scattered again, the phenomenon that the recoil electrons collide again with the surfaces of the anode covers 23A and 26 is repeated multiple times. The envelope 30 is hardly reached. Or, by repeating the scattering, the energy of the recoil electrons is attenuated below the energy that damages the vacuum envelope. Therefore, damage to the vacuum envelope due to recoil electrons is prevented. As a result, an X-ray tube is provided in which voltage resistance is maintained. The actions and effects described above are higher as the sizes of the openings of the anode covers 23A and 26 are smaller, but the size of the openings passes through the openings without colliding with the surface of the anode cover 23A. It is necessary to set a size larger than the lower limit of the limit. This lower limit can be confirmed by electron orbit simulation or trial experiment.

  In addition, this invention is not limited to the said embodiment itself, In the stage of implementation, it can implement by modifying a component in the range which does not deviate from the summary. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

  DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 10 ... Cathode (anode), 11 ... Filament, 12 ... Focusing electrode, 20 ... Anode, 21 ... Anode target, 22 ... Anode base material, 23 ... Anode (anode) hood, 23A, 26 ... Anode (Anode) cover, 24 ... radiation window, 25 ... metal member, 30 ... vacuum envelope, 100 ... incident electrons, 101 ... reflected (recoil) electrons, D1 ... width (width) of the opening of the anode cover, D2: outside diameter of anode cover, D3: length of horizontal axis of opening of anode cover, D4: length of vertical axis of opening of anode cover, D5: length of short axis of opening of anode cover Y1... Projection length of the anode cover, Y3... Projection length of the anode cover in the horizontal axis direction, Y4... Projection length of the anode cover in the vertical axis direction, X1. Cover Small protrusion length, X4 ... minimum protrusion length of the anode cover in the vertical axis direction, L1 ... distance from the intersection of the tube axis and the surface of the anode target to the cathode end face on the anode cover, L2 ... anode anode side Between the tip surface of the anode and the tip surface on the cathode side of the anode cover

Claims (6)

A cathode having an electron emission source for emitting an electron beam;
An anode target installed opposite to the cathode and emitting X-rays upon incidence of the electron beam;
A cylindrical first shielding member that has the same potential as the anode target and surrounds the anode target;
A second shielding member, which is formed in a lid shape at the tip of the first shielding member facing the cathode and has an opening that is a through hole through which the electron beam emitted from the cathode to the anode target passes. When,
Possess said cathode, and said anode target, said first shielding member, and the vacuum envelope for holding said second shielding member within a vacuum airtight state, and
The second shielding member is formed in a disc shape,
In order to prevent recoil electrons scattered by the anode target from directly entering the vacuum envelope, a protruding length that is a difference between an inner radius and an outer radius is formed with a predetermined dimension,
The minimum protrusion length of the second shielding member is expressed by the following equation:

However, in the said formula, X1 shows the minimum value of the protrusion length of the said 2nd shielding member, D2 shows the outer diameter of the said 2nd shielding member, L1 is the said anode target side of the said cathode. The distance between the surface and the surface of the second shielding member on the cathode side is indicated, and L2 is the distance between the point where the electron beam collides with the anode target and the surface of the second shielding member on the cathode side A fixed anode X-ray tube comprising: The protruding length of the second shielding member is the following formula:

However, in the formula, Y1 is the second indicating the extension length of the shielding member, stationary anode X-ray tube according to claim 1 which is formed in the range of. The opening of the second shielding member is formed in a circular shape, fixed anode X-ray tube according to claim 2. Opening of the second shielding member is formed in an elliptical shape, stationary anode X-ray tube according to claim 2. The fixed anode X-ray tube according to claim 1 , wherein the opening of the second shielding member is formed in a square shape. The protruding length of the second shielding member on the diagonal line of the opening is expressed by the following equation:

The fixed anode X-ray tube according to claim 5 , which is formed in a range of

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