Patent Application
20170287668
The present invention relates to an X-ray generating tube including an electron gun, which is applicable to medical equipment, a nondestructive inspection apparatus, and other such apparatus, an X-ray generating apparatus and a radiography system using the X-ray generating tube.
Imaging systems utilizing material permeability of an X-ray are widely used for medical, industrial, and other such purposes. In a radiography system, an X-ray generating tube, which is configured to generate an X-ray, has the structure in which an anode and a cathode, which are used to apply a tube voltage, are opposed to each other via an insulating tube. The inside of the insulating tube is in a vacuum state. The cathode includes an electron gun configured to radiate an electron ray, and the anode includes a target configured to generate an X-ray by being irradiated with the electron ray. The electron gun includes an electron emitting portion and a grid electrode, and the target is irradiated with an electron beam, which is formed of electrons emitted from the electron emitting portion, in a spot shape through control of a locus by the grid electrode and a tube voltage applied between the cathode and the anode. The grid electrode is joined to one end of a support member, which extends in a tube axis direction of the X-ray generating tube, and another end of the support member is arranged in the cathode, which forms a part of an envelope of the X-ray generating tube.
As disclosed in Japanese Patent Application Laid-Open No. S58-123643, in a related-art X-ray generating tube, a grid electrode and a support member are joined to each other by welding.
In the X-ray generating tube, the electron emitting portion is heated when being driven. Therefore, heat is transferred also to the grid electrode and the support member near the electron emitting portion to cause thermal expansion, and thermal stress resulting from a difference in coefficient of thermal expansion between the grid electrode and the support member is applied to a joining portion between the grid electrode and the support member. Therefore, when the X-ray generating tube is used for a long time, the thermal stress is repeatedly applied to the joining portion between the grid electrode and the support member, and there has been a fear that the joining portion may be disconnected. When the joining portion is disconnected, there is a fear that a position at which the grid electrode is mounted to the support member may vary to shift the locus of the electron beam from a desired position, to thereby shift the position of the electron beam on the target irradiated with the electron beam.
It is an object of the present invention to reduce, in an X-ray generating tube including an electron gun, which includes a grid electrode secured to a support member, thermal stress generated at a joining portion between the support member and the grid electrode, to thereby maintain a position of an electron beam on a target irradiated with the electron beam accurately for a long time. It is another object of the present invention to provide a radiography system, which includes the X-ray generating tube and has excellent durability.
According to a first embodiment of the present invention, there is provided an X-ray generating tube including an electron gun, the electron gun including: a grid electrode; a support member configured to support the grid electrode; and a buffer member having an elastic coefficient that is lower than each elastic coefficient of the grid electrode and the support member, wherein the grid electrode and the buffer member are joined to each other to form a first joining portion, and the support member and the buffer member are joined to each other to form a second joining portion, and wherein the grid electrode and the support member are secured to each other via the first joining portion and the second joining portion.
According to a second embodiment of the present invention, there is provided a radiography system, including the above-mentioned X-ray generating tube according to the first embodiment of the present invention.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail below in accordance with the accompanying drawings. However, the present invention is not limited to the embodiments to be described below. In the present invention, embodiments obtained by, for example, appropriately changing or modifying the embodiments to be described below based on the ordinary knowledge of a person skilled in the art so that such change or modification may not deviate from the gist thereof are also included in the scope of the present invention.
In the X-ray generating tube 1, an anode 2 and a cathode 6 are arranged to be opposed to each other via an insulating tube 5. The anode 2 at least includes a target and an anode member 4, and the cathode 6 at least includes a cylindrical electron gun 7 and a cathode member 8. The X-ray generating tube 1 is configured to irradiate the target 3 with an electron beam 11 emitted from the electron gun 7 to generate an X-ray. In other words, the target 3 is provided in the X-ray generating tube 1 and is configured to generate an X-ray in response to electron irradiation. Therefore, the target 3 and the electron gun are arranged to be opposed to each other. Electrons contained in the electron beam 11 are accelerated to incident energy required to generate the X-ray at the target 3 by an acceleration electric field formed in an inner space 12, which is sandwiched by the anode 2 and the cathode 6, of the X-ray generating tube 1.
The inner space 12 of the X-ray generating tube 1 is vacuum for securing a mean free path of the electron beam 11. A pressure inside the X-ray generating tube 1 is preferably 1×10−4 Pa or less, and more preferably 1×10−6 Pa or less from the viewpoint of life of an electron emitting portion 9. In order to achieve the pressure, there may be used a method involving using an exhaust pipe and a vacuum pump (not shown) to evacuate the inside of the X-ray generating tube 1 through the exhaust pipe in advance, and then sealing the exhaust pipe. In order to maintain a degree of vacuum inside the X-ray generating tube 1, a getter (not shown) may be arranged inside the X-ray generating tube 1. As the getter, there may be applied, for example, a getter of a non-evaporable type, which adsorbs gas components inside the X-ray generating tube 1 after activation through heating, or a getter of an evaporable type, which adsorbs gas to an active metal deposition surface formed by heating and evaporating a metal, for example, titanium.
The anode 2 of the X-ray generating tube 1 functions as an electrode, which defines an anode potential. The anode member 4 is made of a conductive material, and is electrically connected to the target 3. Moreover, as the anode member 4, a metal, such as copper, iron, tungsten, or kovar (trade name: Kovar, Westinghouse Electric Corporation), may be used, and the anode member 4 is joined to the insulating tube 5 with a brazing filler metal or the like. When the insulating tube 5 is made of a ceramic, kovar, which has a coefficient of thermal expansion that is close to that of the ceramic, may be suitably used as the anode member 4.
The target 3 made of a heavy metal, for example, tungsten, includes a target layer (not shown) and a support substrate (not shown), and is arranged so that the target layer faces the electron gun 7 side. The target layer is configured to generate an X-ray when being irradiated with the electron beam 11, and the support substrate is configured to hold the target layer. The target 3 serves as a transmissive window for extracting the X-ray, which is generated from the target layer, out of the X-ray generating tube 1, and forms a part of a circumferential wall of the X-ray generating tube 1 for maintaining the vacuum of the inner space 12 of the X-ray generating tube 1.
The cathode 6 of the X-ray generating tube 1 functions as an electrode, which defines a cathode potential. As the cathode member 8, a metal, such as copper, iron, tungsten, or kovar, may be used, and the cathode member 8 is joined to the insulating tube 5 with a brazing filler metal or the like. When the insulating tube 5 is made of a ceramic, kovar, which has a coefficient of thermal expansion that is close to that of the ceramic, may be suitably used as the cathode member 8.
The insulating tube 5 is arranged to electrically insulate the cathode 6, which is defined as the cathode potential of the X-ray generating tube 1, and the anode 2, which is defined as the anode potential. The insulating tube 5 is made of an insulating material, for example, a glass material or a ceramic material, and alumina is preferably used for processability, the cost, and other such factors.
The electron gun 7 includes the electron emitting portion 9, a cathode heater 10, a cylindrical grid electrode 13, and a cylindrical support member 14. The cathode heater 10 is a member configured to heat the electron emitting portion 9, and from the electron emitting portion 9 heated by the cathode heater 10, electrons are extracted with a predetermined voltage applied to the grid electrode 13 from a power supply (not shown). The extracted electrons are accelerated to the incident energy required to generate the X-ray at the target 3 by a voltage applied between the cathode 6 and the anode 2. As the electron emitting portion 9, there is suitably used, for example, an indirectly heated electron source, in which an impregnated electron source, which is formed by impregnating tungsten with barium, is heated with the cathode heater 10 to extract the electrons, or a directly heated electron source, in which a tungsten filament itself serves as an electron emitting portion.
The grid electrode 13 in the present invention is a cylindrical member including a passage hole 13a, through which electrons pass, at its center, has a function of extracting the electrons from the electron emitting portion 9, and hence may be used for on/off control of the irradiation with the X-ray by the X-ray generating tube 1, for example. The grid electrode 13 also has a function of converging the electrons, which are emitted from the electron emitting portion 9, as the electron beam 11. Without limiting to this embodiment, the grid electrode 13 may be formed of an electrode for extracting the electrons, and an electrode for converging the electrons.
The grid electrode 13 in the electron gun 7 is supported by the cylindrical support member 14, and is configured to control positional accuracy of the electron beam 11 with which to irradiate the target 3. In using the X-ray generating tube 1 according to the present invention, when the cathode heater 10 is turned on, heat radiation from the cathode heater 10 increases temperatures of the surrounding grid electrode 13 and support member 14, and when the cathode heater 10 is turned off, the temperatures are decreased. Therefore, with the cathode heater 10 being driven to be turned on and off, thermal stress resulting from a difference in coefficient of thermal expansion between the grid electrode 13 and the support member 14 acts repeatedly on a joining portion between the grid electrode 13 and the support member 14. When the grid electrode 13 and the support member 14 are directly joined to each other, and thermal stress applied to the joining portion is high, problems, such as the joining portion is disconnected, may occur.
In the present invention, the grid electrode 13 and the support member 14 are joined to each other via a buffer member 17, which has an elastic coefficient that is lower than each elastic coefficient of the grid electrode 13 and the support member 14, a first joining portion 16, and a second joining portion 19. The first joining portion is a joining portion between the grid electrode 13 and the buffer member 17, and the second joining portion 19 is a joining portion between the support member 14 and the buffer member 17. In other words, the grid electrode and the buffer member are joined to each other to form a first joining portion, while the support member and the buffer member are joined to each other to form a second joining portion. Thus, in the present invention, the grid electrode and the support member are secured to each other via the first joining portion and the second joining portion. Therefore, a part of the thermal stress applied to the joining portion between the grid electrode 13 and the support member 14 is absorbed by deformation of the buffer member 17 having the low elastic coefficient. Moreover, thermal stress that is left without being absorbed by the buffer member 17 is dispersed to the first joining portion 16 on the grid electrode side of the buffer member 17, and to the second joining portion 19 on the support member side. Therefore, thermal stress applied to each of the first joining portion 16 and the second joining portion 19 is lower than thermal stress applied to a joining portion obtained when the grid electrode 13 and the support member 14 are directly joined to each other, with the result that damage to the first joining portion 16 and the second joining portion 19 is suppressed.
The buffer member 17 used in the present invention has the elastic coefficient that is lower than the elastic coefficients of the grid electrode 13 and the support member 14. A normal operating temperature range of the X-ray generating tube 1 is a temperature range of from ambient temperature (27° C.) to 200° C. In order that the elastic coefficient of the buffer member 17 maintain a value that is lower than the elastic coefficients of the grid electrode 13 and the support member 14 in this temperature range, it is preferred that the elastic coefficient of the buffer member 17 be 10% or more lower than the elastic coefficients of the grid electrode 13 and the support member 14 at the ambient temperature. Examples of a material that satisfies the relationship among the elastic coefficients include a combination of using kovar as the buffer member 17 and using molybdenum or stainless steel (SUS) as the grid electrode 13 and the support member 14. The elastic coefficients at the ambient temperature (27° C.) are 159 GPa for kovar, 327 GPa for molybdenum, and 200 GPa for SUS.
In reducing the thermal stress applied to each of the first joining portion 16 and the second joining portion to suppress the damage to the joining portions more effectively, it is preferred to set a magnitude relationship among coefficient of thermal expansions of the grid electrode 13, the buffer member 17, and the support member 14. Specifically, it is preferred to set the coefficient of thermal expansions as follows: the grid electrode 13<the buffer member 17<the support member 14, or the grid electrode 13>the buffer member 17>the support member 14.
The X-ray generating tube 1 is increased in temperature when in use. Therefore, thermal stress is generated in the first joining portion 16 and the second joining portion 19 due to a difference (Δα) in coefficient of thermal expansion between adjacent members at each joining portion, and a strain (Δε) is produced in each member. In a metal material, a permanent set at the time when steel yields is about 0.002 (0.2%), and hence stress that produces the permanent set of 0.2% when the load is removed is called “0.2% offset yield strength”, and is used as a substitute for yield strength. When the members are selected so as to satisfy Δε=Δα×ΔT<0.002 when thermal stress is generated, where ΔT is a temperature difference generated in the X-ray generating tube 1, the members are used without yielding, and the possibility of damaging the joining portions is low even with repeated application of thermal stress. Assuming that the temperature difference ΔT is 150° C., from Δα=0.002/150° C.=1.33×10−5/° C., it is preferred that the difference in coefficient of thermal expansions between adjacent members be 1.33×10−5/° C. or less.
Examples of a combination that satisfies the above-mentioned preferred condition of the coefficient of thermal expansions include a combination of using kovar as the buffer member 17, and using SUS and molybdenum as the support member 14 and the grid electrode 13, respectively, or molybdenum and SUS as the support member 14 and the grid electrode 13, respectively. The coefficient of thermal expansions are 5.2×10−6/° C. for molybdenum, 7.0×10−6/° C. for kovar, and 18×10−6/° C. for SUS.
In this example, the first joining portion 16 is a joining portion with a joining member 18 having a solidus temperature that is lower than solidus temperatures of the grid electrode 13 and the buffer member 17. The solidus temperature is measured with a method defined in JIS Z 3198 “Test methods for lead-free solders”. At the first joining portion 16, the joining member 18 having the solidus temperature that is lower than the solidus temperatures of the grid electrode 13 and the buffer member 17 may be used to join the grid electrode 13 and the buffer member 17 without melting the grid electrode 13 and the buffer member 17. Therefore, even when the grid electrode 13 is made of a material having a high melting point, the buffer member and the grid electrode 13 may be satisfactorily connected to each other, and a positional shift therebetween may also be suppressed to a small amount.
As the joining member 18, it is preferred to use a brazing filler metal. As the brazing filler metal, a brazing filler metal made of an alloy containing gold, silver, copper, tin, or other such metals may be used, and the brazing filler metal may be selected as appropriate depending on compositions of members to be joined. Brazing with the brazing filler metal is a method involving placing a solid brazing filler metal at an area to be brazed, heating the brazing filler metal to a predetermined temperature to melt the brazing filler metal once, and then allowing the brazing filler metal to reach the ambient temperature again and solidify, to thereby join surfaces of the materials. It is desired that the joining member 18 have a thickness of from 50 μm or more to 500 μm or less, and more preferably, a thickness of from 80 μm or more to 200 μm or less. The joining member 18 is placed between the grid electrode 13 and the buffer member 17 while being in a solid state so as to have desired thickness and area, and is used for joining by being melted under inert atmosphere.
In this example, the grid electrode 13 and the buffer member 17 are first joined to each other via the joining member 18. Then, the grid electrode 13 and the support member 14 are aligned to each other at a positioning portion 15, which is to be described later, and the buffer member 17 and the support member 14 are joined to each other under a state in which the positions are maintained.
In this example, it is preferred that the joining at the second joining portion 19 be performed by welding. As types of welding, TIG welding, spot welding, and laser beam welding are applicable to the present invention, for example. In general, the welding may join members even when there is a gap between the members to be joined. In this example, in order to absorb a thickness error of the joining member 18 of the first joining portion 16, it is preferred that a gap of from 10 μm to 100 μm be formed between the buffer member 17 and the support member 14. Therefore, a height (length in the tube axis direction) of the buffer member 17 may be set so that the above-mentioned gap is formed between the buffer member 17 and the support member 14 under a state in which the grid electrode 13 and the support member 14 are securely in contact with each other at the positioning portion 15.
In this example, there has been described the example in which the first joining portion 16 is the joining portion with the joining member 18, but the second joining portion 19 may be joining with the joining member 18. In this case, as the joining member 18, a material having a solidus temperature that is lower than solidus temperatures of the support member 14 and the buffer member 17 is used. Then, the support member 14 and the buffer member 17 may be joined to each other with the joining member 18. Thereafter, the grid electrode 13 and the support member 14 may be aligned to each other at the positioning portion 15, and the buffer member 17 and the grid electrode 13 may be joined to each other by welding under a state in which the positions are maintained.
The joining member 18 has the solidus temperature that is lower than the members to be joined, and hence it is preferred that the joining member 18 be located farther from a heat source. As illustrated in
In this example, the positioning portion 15, at which the grid electrode 13 and the support member 14 are brought into direct contact with each other, is provided on an inner side of the second joining portion 19 in a direction perpendicular to the tube axis. At the positioning portion 15, an end surface on the cathode side of the grid electrode 13 and an end surface on the anode surface of the support member 14 are brought into contact with each other as positioning surfaces to restrict positional relationship between the grid electrode 13 and the support member 14 in the tube axis direction.
Electrons extracted from the electron emitting portion 9 are converged when passing through the passage hole 13a of the grid electrode 13, and a locus of the electrons is controlled. Therefore, accuracy of a position of the electron beam 11 on the target 3 irradiated with the electron beam 11 depends on accuracy of a position at which the grid electrode 13 is mounted to the support member 14. When alignment accuracy in joining the grid electrode 13 to the support member 14 is high, the position of the electron beam 11 on the target 3 irradiated with the electron beam 11 may be controlled with high accuracy. In this example, the grid electrode 13 and the support member 14 are joined to each other under a state in which the positional relationship between the grid electrode 13 and the support member 14 is restricted by the positioning portion 15, with the result that a position of the grid electrode 13 in the tube axis direction may be controlled with high accuracy.
Moreover, in this example, as illustrated in
As described above with reference to
The positioning portion 15 may be provided in a continuous annular shape in a circumferential direction with the tube axis being the center, but a plurality of positioning portions 15 may be provided separately in the circumferential direction. When the plurality of positioning portions 15 are provided separately, it is preferred to provide three or more positioning portions 15, and desirably, three positioning portions 15 for stable positioning. In
In
It is desired that the container 38 containing the X-ray generating tube 1 and the tube voltage circuit 41 have sufficient strength as a container, and be excellent in heat radiation property. A metal material, such as brass, iron, or stainless steel may be used as a component of the container 38. An insulating liquid 42 is filled in a remaining space other than the X-ray generating tube 1 and the tube voltage circuit 41 in the container 38. The insulating liquid 42 is a liquid having an electrical insulating property, and has a role of maintaining an electrical insulating property inside the container 38 and a role of a cooling medium of the X-ray generating tube 1. As the insulating liquid 42, it is preferred to use electrical insulating oil, such as mineral oil, silicone oil, or perfluoro oil.
The radiography system 31 according to the present invention includes, as illustrated in
The system control unit 35 is configured to control the irradiation with the X-ray by actuating the X-ray generating apparatus 32 through the tube voltage circuit 41, and to process a signal from the X-ray detector 33 through the signal processing unit 34. In other words, the system control unit 35 is configured to control the X-ray generating apparatus 32 and the X-ray detector 33 in conjunction. The X-ray emitted from the X-ray generating apparatus 32 is detected by the X-ray detector 33 via the object 37 so that an X-ray transmission image of the object 37 is taken. The obtained X-ray transmission image is displayed on the display unit 36. Moreover, in driving the X-ray generating apparatus 32, the system control unit 35 may control a voltage signal applied to the X-ray generating tube 1 through the tube voltage circuit 41 to set an appropriate imaging condition.
The electron gun 7 having the structure illustrated in
In joining at the second joining portion 19, the grid electrode 13 and the support member 14 were aligned to each other at the annular positioning portion 15 with the tube axis being the center, and then a boundary portion between the buffer member 17 and the support member 14 was laser-beam welded from the outer circumference. At this time, a gap between the buffer member 17 and the support member 14 was 50 μm. It was confirmed that the joining portion position between the grid electrode 13 and the support member 14 of the electron gun 7 produced as described above had no shift or inclination, and that the grid electrode 13 and the support member 14 were firmly joined to each other.
Next, the X-ray generating tube 1 equipped with the electron gun 7 was produced. In the X-ray generating tube 1, kovar was used as the anode member 4 and the cathode member 8, alumina ceramic was used as the insulating tube 5, and a tungsten film was formed as a target film (not shown) of the target 3.
Finally, the radiography system 31 of
The X-ray generating tube 1 having the structure similar to Example 1 was produced except that the grid electrode 13 was made of SUS 304, the support member 14 was made of molybdenum, and the joining member 18 was arranged between the buffer member 17 and the support member 14. In this example, first, the buffer member 17 was joined to the support member 14 through the joining member 18. Thereafter, the grid electrode 13 and the support member 14 were aligned to each other at the positioning portion 15, and then the buffer member 17 was joined to the grid electrode 13 by welding.
Also in this example, it was confirmed that the joining portion position between the grid electrode 13 and the support member 14 of the produced electron gun 7 had no shift or inclination, and that the grid electrode 13 and the support member 14 were firmly joined to each other. Further, when an X-ray imaging experiment similar to that in Example 1 was performed, the taken image was satisfactory, and there was no variation in irradiation position of the electron beam 11 on the target 3. In this Example, the support member 14 made of a molybdenum material, which has a high elastic coefficient, is arranged at a position close to the cathode member 8, and hence the electron gun 7 has high rigidity. Therefore, even when oscillation is externally applied to the radiography system 31, oscillation of the electron gun 7 is suppressed to cause no fluctuation in the electron beam 11, and there is obtained the effect of reducing a variation in position of the electron beam 11 with which to irradiate the target 3.
The X-ray generating tube 1 was produced similarly to Example 1 except that a shape of the end surface on the cathode side of the grid electrode 13 was changed to the shape illustrated in
Also in this example, it was confirmed that the joining portion position between the grid electrode 13 and the support member 14 of the produced electron gun 7 had no shift or inclination, and that the grid electrode 13 and the support member 14 were firmly joined to each other. Further, when an X-ray imaging experiment similar to that in Example 1 was performed, the taken image was satisfactory, and there was no variation in irradiation position of the electron beam 11 on the target 3.
In the present invention, similar effects are obtained even when there are portions at which members are substantially in contact with each other and portions at which members are not in contact with each other at the annular positioning portion 15 with the tube axis being the center due to accuracy of finishing the members.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2016-067013, filed Mar. 30, 2016, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-067013 | Mar 2016 | JP | national |
