JP3561664B2 - X-ray tube hot cathode and method of manufacturing the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hot cathode of an X-ray tube, and more particularly to a hot cathode having a structure in which a thermionic emitter is supported by a heating element.
[0002]
[Prior art]
It is known to use lanthanum hexaboride (LaB ₆ ) as a material for a hot electron emitter of a hot cathode of an X-ray tube (for example, Japanese Patent Application Laid-Open No. Hei 10-32119). This lanthanum hexaboride may constitute a hot cathode only with this material (see FIGS. 1 and 14 of JP-A-10-32119), or is attached to a heating element such as carbon to form a hot cathode. In some cases (see FIGS. 9 and 10 of JP-A-10-32119). The present invention can be applied to the latter usage (where a thermionic emitter is attached to a heating element).
[0003]
[Problems to be solved by the invention]
In the examples of FIGS. 9 and 10 of JP-A-10-32119, a thin plate made of lanthanum hexaboride is attached to a heating element made of carbon. In order to attach the thin plate in this way, boron carbide is added to a commercially available carbon adhesive, and this is applied to the bonding surface of the carbon heating element and the thin plate made of lanthanum hexaboride and its surroundings, and then baked and hardened. I was In this case, carbon is softer than lanthanum hexaboride and can be deformed by external force, whereas lanthanum hexaboride is hard, so when carbon is deformed, lanthanum hexaboride cannot follow this, Lanthanum hexaboride falls off. Also, since lanthanum hexaboride has an extremely low electrical resistance compared to carbon, it is necessary to reduce the cross-sectional area through which electricity passes, and in order to do so, it is necessary to make lanthanum hexaboride thin. It is hard to do. Furthermore, since lanthanum hexaboride made of a thin plate protrudes from the surface of carbon by the thickness of the lanthanum hexaboride, thermions are also emitted from the side surface (thickness portion) of the lanthanum hexaboride and this is an X-ray tube. Causes uneven brightness of the focus on the target.
[0004]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to easily form a thermionic emitter into an arbitrary shape in a hot cathode of an X-ray tube having a structure in which the thermionic emitter is supported by a heating element. Another object of the present invention is to provide a hot cathode which can be manufactured in such a manner that the hot electron emitter does not fall off. Another object of the present invention is to provide a hot cathode capable of reducing the uneven brightness of a focus on a target by eliminating thermionic emission in a lateral direction. Still another object of the present invention is to provide a simple method for manufacturing such a hot cathode.
[0005]
[Means for Solving the Problems]
The hot cathode of the X-ray tube according to the present invention is obtained by forming a concave portion in a heating element, filling the inside of the concave portion with a powder of a thermoelectron emitting material, and sintering and fixing the powder. The sintered thermionic emission material becomes a thermionic emitter. In this way, a relatively thin thermionic emitter can be easily manufactured into an arbitrary shape. Then, the thermoelectron emitter is firmly fixed to the heating element and hardly falls off. Furthermore, since only the front surface (the surface facing the target) of the thermionic emitter is exposed and the side surface is not exposed, thermionic emission from the side surface of the thermionic emitter is eliminated, and uneven brightness of the focal point of the X-ray tube is reduced. Less.
[0006]
Carbon is suitable as the material of the heating element, and glassy carbon is particularly preferable, but graphite can also be used. The material of the powder of thermionic emission material is not particularly limited, but for example, tungsten, molybdenum, tantalum, various borides, and the like can be used. Among them, lanthanum hexaboride (LaB ₆ ) is particularly suitable. The material of the powder of thermionic emission material can be selected according to the material of the target of the X-ray tube. For example, as a countermeasure for contamination in EXAFS measurement, as a combination of a target and a powder of thermionic emission material, tungsten or TaB ₂ can be used as a powder of thermionic emission material for a tungsten target. For the target, molybdenum or lanthanum hexaboride can be used as the powder of thermionic emission material.
[0007]
The particle diameter of the thermionic emission material powder can be in the range of 0.1 μm to 50 μm, and among them, relatively fine particles having a particle size of 10 μm or less are preferable. The above-mentioned minimum value of the particle size of 0.1 μm is near the lower limit value that can be currently manufactured.
[0008]
This hot cathode can be made as follows. First, a concave portion is formed in the heating element. Next, the inside of the recess is filled with a powder of thermionic emission material. Then, the powder is sintered by energizing the heating element filled with the powder to raise the temperature of the powder. As described above, since the powder can be sintered only by supplying a current to the heating element, a heating furnace or the like for sintering is not required.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a perspective view of a heating element according to a first embodiment of the hot cathode of the present invention. The heating element 10 is made of glassy carbon. Glassy carbon has a much higher electrical resistance than graphite, and without having to lengthen the current path (without complicating its shape), it is sufficient to use thermionic emission material with the same power as when using a tungsten coil. Can be heated to temperature. In addition, glass-like carbon has a higher processing accuracy than graphite, because edges can be sharpened when processing grooves. The heating element 10 includes a pair of terminals 12 and 14 for conducting electricity, a pair of legs 16 and 18, and an emitter support 20. A groove 22 (recess) is formed on the upper surface of the emitter support 20. The length L of the groove 22 is 12 mm, the width W is 2 mm, and the depth D is 0.2 mm. The inside of the groove 22 is filled with lanthanum hexaboride powder.
[0010]
Lanthanum hexaboride powder having a particle size in the range of 0.1 μm to 50 μm can be used, and preferably one having a particle size in the range of 0.1 μm to 10 μm. The following three methods were tried to fill the inside of the groove 22 with the powder. Either method can be used in the present invention.
[0011]
In the first filling method, the powder of lanthanum hexaboride was directly filled into the groove 22 and pressed with a flat plate from above. At this time, the surface of the powder was made to be at the same height as the upper surface of the emitter support 20.
[0012]
In the second filling method, lanthanum hexaboride powder was suspended in butyl acetate (liquid) to form a clay, which was then applied to the inside of the groove 22 using a spatula. At this time, the clay-like mixture was made to be at the same height as the upper surface of the emitter support 20. Butyl acetate will evaporate when lanthanum hexaboride is heated.
[0013]
In the third filling method, a commercially available carbon adhesive was applied to the inner surface of the groove 22, and then lanthanum hexaboride powder was filled thereon. At this time, the surface of the powder was made to be at the same height as the upper surface of the emitter support 20. Boron carbide (B ₄ C) may be mixed in the carbon adhesive in order to improve the adhesion with lanthanum hexaboride.
[0014]
When the lanthanum hexaboride powder is filled into the groove 22 as described above, the heating element 10 is energized to sinter the powder. Specifically, the heating element 10 filled with lanthanum hexaboride powder is attached to an open X-ray tube. Then, while evacuating the inside of the X-ray tube to a vacuum, the heating element is energized. At this time, the voltage applied to both ends of the heating element is gradually increased, so that the temperature rise is moderated. For example, the temperature is raised to about 1000 ° C. over 1 to 2 hours. Thus, first, degassing of the lanthanum hexaboride powder is performed. At the same time, the powder sinters due to the temperature rise due to the heating by the heating element and the self-generated heat. Here, sintering means that the powder material is heat-treated at a temperature equal to or lower than its melting point, and the powders are combined with each other to be integrated. Since the melting point of lanthanum hexaboride is as high as 2720 ° C., the powder is heated to a considerably lower temperature to sinter the powder.
[0015]
When filling the powder, it is common that the surface of the powder is flush with the upper surface of the emitter support as described above, but the surface of the powder is slightly lower than the upper surface of the emitter support. No problem. That is, the surface of the thermionic emitter does not have to protrude from the upper surface of the emitter support, and only needs to be at the same height as or slightly lower than the emitter support.
[0016]
FIG. 2 is a perspective view of the completed hot cathode. The powder filled in the groove 22 of FIG. 1 is in a state of being sintered and fixed, and this forms the thermionic emitter 24. The thermionic emitter 24 has a rectangular thermionic emission surface with a size of 2 mm x 12 mm and can be used to create a 0.5 mm x 12 mm focus (normal focus) on the target of the X-ray tube. it can. Thermionic electrons will be emitted upward from FIG. When an electron current is generated using the hot cathode, a current is supplied to the heating element 10 so that the temperature of the thermionic emitter is 1400 to 1500 ° C. For example, when a voltage of about 9 V is applied between the pair of terminals 12 and 14 of the hot cathode, a current of about 6 A flows through the hot cathode, and the thermionic emitter 24 is heated to about 1500 ° C.
[0017]
Generally, to sinter a powder material, it is necessary to press-mold the powder material and then heat it to a desired temperature using a heating means such as a heating furnace. On the other hand, in the hot cathode of the present invention, since the heating element is filled with the powder, the powder can be heated by energizing the heating element, and no separate heating means is required. Furthermore, if this heating element is actually attached to the hot cathode terminal of the open type X-ray tube, the hot cathode current can be controlled using the hot cathode heating control device of the X-ray tube as it is, so that the heating element is heated. Temperature control becomes possible. Therefore, in sintering the powder, neither a dedicated heating means nor a dedicated temperature control device is required.
[0018]
By the way, if not limited to the X-ray tube, in the technical field of the hot cathode, there is a known example in which a recess formed in a heating element is filled with powder and sintered. For example, in the example of FIG. 6 of JP-A-58-66235, a high-melting metal powder is filled in a hole formed in a belt-shaped heater and sintered. The sintered body is impregnated with a thermionic emission material. In Japanese Patent Application Laid-Open No. 56-35347, a nickel powder is filled in a hole formed in a nickel support and sintered. The sintered body is impregnated with a thermionic emission material. In these known techniques, a recess formed in a heating element is filled with "powder for producing a porous body serving as a substrate impregnated with a thermionic emission material" and sintered. On the other hand, the hot cathode of the present invention is characterized in that the powder of the thermionic emission material is directly filled in the recess of the heating element and sintered. For a hot cathode of an X-ray tube, a hot cathode impregnated with a thermionic emission material is not preferable. This is because elements constituting the porous body (sintered body) serving as the base body impregnated with the thermionic emission material may cause contamination of the target of the X-ray tube. Therefore, it is important for X-ray tubes to make a thermionic emitter from only thermionic emitting materials. Furthermore, in an X-ray tube, it is necessary to obtain a uniform and large amount of electron emission from a relatively small surface of thermionic emission. An electron emitter is required. It is difficult to obtain a large amount of electron emission with the impregnated type.
[0019]
3 to 6 are perspective views showing another embodiment of the hot cathode in which the size and shape of the thermionic emitter are changed. Both are made in the same manner as the hot cathode of FIG. A hot cathode (second embodiment) shown in FIG. 3 has a hole (concave portion) having an inner diameter of 0.5 mm and a depth of 0.2 mm on the upper surface of an emitter supporting portion 20a (a square having an upper surface size of, for example, 1 mm × 1 mm) having a small dimension. ), Lanthanum hexaboride powder is filled into the hole, and sintered. This thermionic emitter 24a has a circular thermionic emission surface with a diameter of 0.5 mm and can be used to create a focus of 0.01 mm × 0.01 mm on the target of the X-ray tube.
[0020]
The hot cathode (third embodiment) shown in FIG. 4 has a thermoelectron emission surface on the side surface of the emitter support portion 20b and emits thermoelectrons in a direction toward the front of the figure. The hot cathode is configured such that a thermionic emission surface does not exist at a position facing the focal point 30b of the target. This type of hot cathode (type in which a thermionic emission surface does not exist at a position facing the focal point of the target) is disclosed in JP-A-10-302688. In this hot cathode, grooves are formed on the upper and lower side surfaces of the generally annular emitter support portion 20b, and the grooves are filled with lanthanum hexaboride powder and sintered to obtain thermionic electron emitters 24b. Was. The size of the upper and lower thermionic emission surfaces is 1 mm × 12 mm, respectively, and the interval between these thermionic emission surfaces is about 0.5 to 2 mm. Thermions emitted from the thermionic emitter 24b in the forward direction in the figure are focused by an opening 28b formed in the Wehnelt electrode 26b and focused on a target as a focal point 30b of (0.5 mm to 1 mm) × 12 mm. .
[0021]
The hot cathode of FIG. 5 (fourth embodiment) is almost the same as that of FIG. 4, but the shape of the thermionic emitter 24c is substantially annular. The thermoelectron emitter 24c can also be made by filling a groove formed in the emitter support 20c with lanthanum hexaboride powder and sintering it. The thermoelectron emission surface of the thermoelectron emitter 24c has an inner diameter of 1 to 1.5 mm and a radial width of 0.5 mm. The thermoelectrons emitted from the substantially annular thermoelectron emitter 24c in the forward direction of the figure are converged by an opening 28c formed in the Wehnelt electrode 26c to form a point-like focal point 30c having a diameter of 0.3 mm on the target. Form.
[0022]
The hot cathode of FIG. 6 (fifth embodiment) is similar to the hot cathode of FIG. 2, and the shape of the thermionic emitter 24d is rectangular. However, the size of the thermionic emission surface is 0.5 mm × 1 mm, and the focal point size on the target is 0.1 mm × 1 mm.
[0023]
FIG. 7 is a plan view of the hot electron emission surface (the upper surface in FIG. 2) of the hot cathode of FIG. 2 viewed through a Wehnelt electrode. An elongated opening 28 is formed in the Wehnelt electrode 26. From this opening 28, the emitter supporting portion 20 of the hot cathode and the thermionic emitter 24 thereon can be seen.
[0024]
FIG. 8 is a sectional end view taken along line AA of FIG. Thermionic electrons emitted from the thermionic emitter 24 become an electron stream 32, which is converged at the opening 28 of the Wehnelt electrode 26 and forms a focal point 30 on the target 34. In this embodiment, the potential of the Wehnelt electrode 26 can be the same as the potential of the hot cathode (for example, minus 60 kV). Alternatively, a bias voltage (for example, up to about 1 kV on the negative side with respect to the potential of the hot cathode) can be applied to the Wehnelt electrode 26. The width W of the thermionic emitter 24 is 2 mm, while the width FW of the focal point 30 is 0.5 mm. The length L in the direction perpendicular to the paper is approximately 12 mm for both thermionic emitter 24 and the focal point 30. It is preferable that the size of the thermionic emission surface of the thermionic emitter 24 is made larger than the focal size in this way, and the electron flow from the thermionic emitter is narrowed to form the focal point. In this case, the current of the electron current (the current of the X-ray tube) can be increased, and high-output X-rays can be generated. Note that the size of the thermionic emission surface of the thermionic emitter 24 can be reduced to about the same size as the focal size.
[0025]
In this hot cathode, the thermoelectron emitter 24 is embedded in the recess of the emitter support 20 and does not protrude from the surface of the emitter support 20. Therefore, the side surface 36 outside the width W of the thermoelectron emitter 24 is not exposed, and no thermoelectrons are emitted from the side surface 36. When the thermoelectrons are emitted from the side surface 36, when the electron flow is reduced by the action of the electron lens by the Wehnelt electrode 26, the aberration of the electron lens increases, and the electron flow is reduced to a desired size on the target 34. It becomes very difficult to stop down, resulting in uneven brightness. On the other hand, in this embodiment, since the thermoelectrons are not emitted from the side surface 36, the uneven brightness of the focal point 30 is reduced. In addition, even without applying a bias voltage to the Wehnelt electrode 26 (even if the potential is the same as that of the hot cathode), it is possible to obtain the focus 30 with less uneven brightness.
[0026]
By the way, as can be seen from FIG. 2, the side surfaces 44 of the thermionic emitter 24 are exposed at both longitudinal ends of the thermionic emitter 24. Ideally, it is preferable that the side surfaces 44 of the thermionic emitter 24 are not exposed at both ends in the longitudinal direction. However, in this longitudinal direction, since the electron flow is not restricted by the Wehnelt electrode (ie, the longitudinal size of the thermionic emitter 24 and the longitudinal size of the focal point 30 on the target are both 12 mm), There is no need to consider the aberration of the lens, and the focus 30 is not blurred or the brightness is not uneven due to the effect of thermions emitted from the side surfaces 44 at both ends in the longitudinal direction. After all, it is important that at least the side surface of the thermionic emitter in the direction in which the electron lens is narrowed is not exposed.
[0027]
FIG. 9 is a perspective view of a hot cathode according to a sixth embodiment of the present invention. The difference from the hot cathode of FIG. 2 is that a step 38 for electron current focusing is formed on the upper surface of the emitter supporting portion 20e. This hot cathode can be made as follows. First, a first groove 40 (first concave portion) is formed on the upper surface of the emitter supporting portion 20e, and a second groove 42 (second concave portion) is formed at the bottom of the first groove 40. The inside of the second groove 42 is filled with lanthanum hexaboride powder, and the power is supplied to the heating element to sinter the powder, thereby completing the thermionic emitter 24e. Side walls on both sides of the first groove 40 are steps 38. The step 38 serves to focus the electron flow. This will be described below with reference to FIG.
[0028]
FIG. 10 is a cut end view similar to FIG. 8 for the hot cathode of FIG. Thermions emitted from the thermionic emitter 24e become an electron stream 32e, which is narrowed by a step 38 formed on the emitter support 20e to form a focal point 30e on the target 34. The opening 28e of the Wehnelt electrode 26e is located outside the emitter support 20e. In this embodiment, the step 38 plays the role of an electron lens that focuses the electron flow 32e (the role played by the opening 28 of the Wehnelt electrode 26 in FIG. 8). The distance between the step 38 and the thermionic emitter 24e is determined by the processing accuracy when the first groove and the second groove are formed in the emitter support 20e, and the accuracy is properly obtained. Therefore, the size of the focal point 30e focused on the target 34 is accurately determined as designed. On the other hand, as shown in FIG. 8, when the electron flow 32 is focused by the opening 28 of the Wehnelt electrode 26, the center of the opening 28 in the width direction and the center of the thermionic emitter 24 in the width direction are not necessarily required. If they do not match, the error depends on the assembly accuracy when assembling the Wehnelt electrode 26 and the hot cathode. Therefore, in the case of FIG. 8, the size of the focal point 30 may deviate from the design value.
[0029]
【The invention's effect】
In the hot cathode of the X-ray tube according to the present invention, the recesses of the heating element are filled with the powder of thermionic emission material and sintered, so that a relatively thin thermoelectron emitter can be easily manufactured. The emitter is firmly fixed to the heating element and hardly falls off. Further, since only the front surface of the thermionic emitter is exposed and the side surface is not exposed, thermionic emission from the side surface is eliminated, and uneven brightness at the focal point of the X-ray tube is reduced.
[Brief description of the drawings]
FIG. 1 is a perspective view of a heating element in a first embodiment of a hot cathode according to the present invention.
FIG. 2 is a perspective view of a hot cathode completed using the heating element of FIG.
FIG. 3 is a perspective view of a second embodiment of the hot cathode according to the present invention.
FIG. 4 is a perspective view of a third embodiment of the hot cathode according to the present invention.
FIG. 5 is a perspective view of a fourth embodiment of the hot cathode according to the present invention.
FIG. 6 is a perspective view of a hot cathode according to a fifth embodiment of the present invention.
FIG. 7 is a plan view of a hot electron emission surface of the hot cathode of FIG. 2 viewed through a Wehnelt electrode.
FIG. 8 is a sectional end view taken along line AA of FIG. 7;
FIG. 9 is a perspective view of a hot cathode according to a sixth embodiment of the present invention.
FIG. 10 is a cut end view similar to FIG. 8 for the hot cathode of FIG. 9;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Heating element 12, 14 Terminal 16, 18 Leg 20 Emitter support 22 Groove 24 Thermoelectron emitter 26 Wehnelt electrode 28 Opening 30 Focus 34 Target

Claims (4)

In a hot cathode of an X-ray tube having a structure in which a thermoelectron emitter is supported by a heating element,
A hot cathode, wherein a recess is formed in the heating element, and a powder of thermionic emission material is fixed in the recess in a sintered state. 2. The hot cathode according to claim 1, wherein the material of the heating element is glassy carbon, and the thermionic emission material is lanthanum hexaboride. 3. The hot cathode according to claim 1, wherein a first concave portion is formed in the heating element, and a second concave portion is further formed at a bottom of the first concave portion. A hot-electron emitting material powder fixed in a sintered state to the hot cathode. A method for manufacturing a hot cathode of an X-ray tube having a structure in which a thermoelectron emitter is supported by a heating element, comprising the following steps (a) to (c).
(B) forming a recess in the heating element;
(B) filling the inside of the recess with powder of thermionic emission material;
(C) sintering the powder by energizing a heating element filled with the powder to increase the temperature of the powder.

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