Patent Application
20180053624
The present invention relates to an X-ray generating tube generating an X-ray applicable to, for example, medical equipment and non-destructive testing apparatus, and an X-ray generating apparatus and a radiography system that use the X-ray generating tube.
X-ray generating tubes generate an X-ray by applying high voltage in a vacuum container, thus causing an electron source to emit an electron beam and creating a collision between electrons and a target made of a metal material that has a high atomic number, such as tungsten.
The voltage applied between a cathode, which includes the electron source, and an anode, which includes the target, is generally about 10 kV to about 150 kV, although varying depending on the use of the generated X-ray. The trunk of the vacuum container is built from an insulating tube that is made of an insulating material such as glass or a ceramic material in order to keep the interior under vacuum and to electrically insulate the cathode and the anode from each other.
When an X-ray generating tube is driven to cause the electron source to emit electrons, scattered electrons and secondary electrons are generated in the X-ray generating tube and are in some cases captured on the inner surface of the insulating tube to charge the inner surface. With the inner surface of the insulating tube charged, an electric field thereof disrupts the trajectory of an electron beam to change the irradiation points and focal spot size of the electron beam, and consequently changes the focal position and dose of an emitted X-ray. In addition, the location and amount of charge on the inner surface of the insulating tube vary depending on the distribution of points irradiated with the scattered electrons and secondary electrons, and the resultant difference in electric potential on the inner surface of the insulating tube may lead to a discharge that damages the insulating tube.
There is disclosed in Japanese Patent Application Laid-Open No. S58-44662 a technology of preventing the accumulation of electric charges by forming a conductive film that is made from fine metal particle groups and a glaze along the inner circumference of the insulating tube.
However, no particular consideration is given to the connection between a low conductivity film and the electrodes in Japanese Patent Application Laid-Open No. S58-44662. Accordingly, poor connection between the low conductivity film and the electrodes hinders the scattered electrons and the secondary electrons from freeing, thereby putting the conductive film itself into a charged state. The charged conductive film may disrupt the electron beam trajectory and change the X-ray output.
It is an object of the present invention to prevent charging of an inner surface of an insulating tube without fail by using a conductive film.
In order to achieve the above-mentioned object, according to a first embodiment of the present invention, there is provided an X-ray generating tube, including: an anode including: a target generating an X-ray upon an irradiation with an electron beam; and an anode member which is electrically connected to the target and which holds the target; a cathode including: an electron emitting source having an electron emitting portion configured to irradiate an electron beam to the target; and a cathode member electrically connected to the electron emitting source; and an insulating tube having a pair of ends in a tube axis direction, one end of which is connected to the anode member and the other end of which is connected to the cathode member so that the target and the electron emitting portion face each other, in which the anode further includes an inner circumferential conductive film, which is positioned on an inner surface of the insulating tube at a distance from the cathode, and an end surface conductive film formed on the one end of the insulating tube, and in which the inner circumferential conductive film is electrically connected to the anode member via the end surface conductive film.
According to a second embodiment of the present invention, there is provided an X-ray generating apparatus, including: the X-ray generating tube of the first embodiment of the present invention; and a drive circuit configured to apply a tube voltage between the anode and the cathode.
According to a third embodiment of the present invention, there is provided a radiography system, including: the X-ray generating apparatus of the second embodiment of the present invention; an X-ray detector configured to detect an X-ray that has been generated from the X-ray generating apparatus and transmitted through a subject; and a system control unit configured to control the X-ray generating apparatus and the X-ray detector in an integrated manner.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
An exemplary embodiment of the present invention is described in detail below with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangement, and the like of components described in this embodiment are not to limit the scope of the invention. The same reference symbols are used to denote the same components in the drawings described below.
<X-Ray Generating Tube>
A member which has airtightness for maintaining vacuum and which is solid enough to withstand atmospheric pressure is preferred for an envelope 111 of the X-ray generating tube 102. The envelope 111 of this embodiment includes an insulating tube 110, a cathode 51, which includes the electron emitting source 3 such as an electron gun, and an anode 52, which includes the target 9 held by a target holding portion 43a and an anode member 43. The cathode 51 and the anode 52 form a part of the envelope 111, with the anode member 43 joined to the insulating tube 110 at one end and a cathode member 41 joined to the insulating tube 110 at the other end. The target 9 has as a component a transmissive substrate 21 serving as a transmissive window through which an X-ray beam 11 generated by irradiating a target layer 22 with an electron beam is taken out of the X-ray generating tube 102, and which also forms a part of the envelope 111. It is preferred for the cathode member 41 and the anode member 43, which are joined to the insulating tube 110, to be made of a metal material that has a linear expansion coefficient close to that of the insulating tube 110. For example, Kovar (trademarked in the U.S. to CRS Holdings, Inc.) or Monel (trademarked in the U.S. to Special Metals Corporation) is used as the material. The insulating tube 110 and the joining of the anode member 43 to the insulating tube 110 are described later in detail.
The X-ray generating tube 102 generates the X-ray beam 11 by irradiating the target layer 22 of the target 9 with an electron beam 5, which is emitted from an electron emitting portion 2 included in the electron emitting source 3. A region 11a of the target layer 22 in which the X-ray is generated is called a focal spot of the X-ray beam 11. The target layer 22 is formed on the electron emitting source 3 side of the transmissive substrate 21 through which the X-ray is transmitted. The electron emitting portion 2 of the electron emitting source 3 is opposed to the target layer 22. For example, tungsten, tantalum, or molybdenum is used for the target layer 22.
The anode 52 of this embodiment includes the target 9, which generates an X-ray upon an irradiation with an electron beam, the target holding portion 43a, and the anode member 43, which defines the anode potential of the target 9. The anode member 43 includes the target holding portion 43a configured to hold the target 9, and an outer circumferential tubular portion 43b, which is provided in order to secure areal dimensions for joining the anode member 43 to the insulating tube 110. Metal such as Kovar, tungsten, molybdenum, or stainless steel is selected for the anode member 43, the outer circumferential tubular portion 43b, and the target holding portion 43a, which are included in the anode 52. Kovar, Monel, or the like is selected to give those components a linear expansion coefficient matching that of the insulating tube 110.
The outer circumferential tubular portion 43b is shaped like a sleeve that extends from the target holding portion 43a toward the cathode 51. The outer circumferential tubular portion 43b defines the anode potential of a cathode-side part of the anode 52. The distance from the target holding portion 43a to an end of the outer circumferential tubular portion 43b on the cathode 51 side is preferred to be constant in the circumferential direction from the viewpoint of in-plane symmetry of the anode-side electric potential distribution. The in-plane symmetry of the electric potential distribution means that the electric potential distribution in a plane parallel to the anode member 42 is continuous in a tube circumference direction, without finding a region that is locally high in electric field in the tube circumference direction.
The target holding portion 43a is joined to the target 9 to hold the target 9. The target holding portion 43a has a through-hole 42, and the opening of the through-hole 42 is closed to hold the target 9 at a point along the length of the through-hole 42. At least a part of the target holding portion 43a that extends outward from the target 9 to the outside of the envelope 111 is made of heavy metal such as tungsten or tantalum, or a material containing heavy metal, thereby enabling the target holding portion 43a to function as a collimator for controlling the emission angle of the X-ray beam 11. The target holding portion 43a and the outer circumferential tubular portion 43b may be formed as a seamless unitary member, or may be formed separately and subsequently joined together to form a unitary member.
The electron emitting source 3 is configured to irradiate the target 9 with an electron beam that is emitted from the electron beam emitting portion 2. For example, a hot cathode such as a tungsten filament or an impregnated cathode, or a cold cathode such as a carbon nanotube can be used for the electron emitting source 3. The electron emitting source 3 may include a grid electrode (not shown) and an electrostatic lens (not shown) for the purpose of controlling the beam diameter of the electron beam 5, the electron current density, on/off timing, and the like. Electrons contained in the electron beam 5 are accelerated to an energy level necessary to generate an X-ray in the target layer 22 by an accelerating electric field formed in an internal space 13 of the X-ray generating tube 102 which is sandwiched between the cathode 51 and the anode 52.
The internal space 13 of the X-ray generating tube 102 is vacuum in order to secure a mean free path for the electron beam 5. The degree of vacuum of the internal space 13 is preferably 10−8 Pa or more and 10−4 Pa or less, more preferably from the viewpoint of the lifetime of the electron emitting source 3, 10−8 Pa or more and 10−6 Pa or less. The internal space 13 of the X-ray generating tube 102 is put under vacuum by exhausting the internal space 13 with the use of an exhaust pipe (not shown) and a vacuum pump (not shown), and then sealing the exhaust pipe. A getter (not shown) may be formed in the internal space 13 of the X-ray generating tube 102 for the purpose of maintaining the vacuum.
The X-ray generating tube 102 has as its trunk the insulating tube 110 in order to electrically insulate the electron emitting source 3, which is set to the cathode potential, and the target layer 22, which is set to the anode potential, from each other. The insulating tube 110 is made of an insulating material such as a glass material or a ceramic material. The insulating tube 110 may have a function of defining the gap between the electron beam emitting portion 2 and the target layer 22.
Described next are the structure of the insulating tube 110, a joining structure for joining the anode 52 to the insulating tube 110, and a method of forming those structures.
As illustrated in
The inner circumferential conductive film 112 can be, for example, a film of metal such as silver, copper, tin, gold, zinc, titanium, molybdenum, manganese, chromium, aluminum, or magnesium, a conductive film that contains one of those metals, or a metal oxide film. The material of the inner circumferential conductive film 112 is selected by taking into consideration the adhesion to the inner surface of the insulating tube 110. The inner circumferential conductive film 112 can be formed by a method in which a paste that is a mixture of a conductive substance with an organic solvent, a binder, and the like is prepared and applied, an arbitrary deposition method such as vapor deposition or sputtering, or other methods.
A film preferred as the inner circumferential conductive film 112 has a thickness of from 100 nm to 500 μm, has sufficient conductivity, and is continuous in the tube circumference direction and a tube length direction so that the inner surface of the insulating tube 110 is not exposed within the extent of the inner circumferential conductive film 112. The preferred inner circumferential conductive film 112 is formed so as to stretch from the end of the insulating tube 110 on the target holding portion 43a side to a middle portion of the insulating tube 110 in the length direction as illustrated in
The end surface conductive film 113 can be formed from the same material, by the same method, and to the same thickness as the inner circumferential conductive film 112, and is formed so as to be continuous from the inner circumferential conductive film 112. In order to simplify the process and to facilitate the forming of a film that is continuous from the inner circumferential conductive film 112, it is preferred to form the end surface conductive film 113 at the same time as the inner circumferential conductive film 112.
The preferred end surface conductive film 113 is formed on a part of the end surface of the insulating tube 110 on the anode member 43 side in the tube circumference direction. For example, it is preferred to arrange a plurality of end surface conductive films 113 each having a width of from 100 μm to 5 mm discretely in two to ten places on the end surface of the insulating tube 110 as illustrated in
A peripheral portion of the anode member 43 is opposed to the end surface of the insulating tube 110 where the inner circumferential conductive film 112 and the end surface conductive film 113 are formed in the manner described above, to join the anode member 43 to the insulating tube 110. With the end surface conductive film 113 formed only on a part of the end surface of the insulating tube 110 in the tube circumference direction, a pressure at which the end surface of the insulating tube 110 is nipped by the anode member 43 in the joining of the anode member 43 to the insulating tube 110 concentrates on the end surface conductive film 113. The resultant effect is that the anode member 43 is pressed strongly against the end surface conductive film 113, which increases the chance of contact between the end surface conductive film 113 and the anode member 43. Forming a plurality of end surface conductive films 113 discretely improves the probability of contact between the end surface conductive films 113 and the anode member 43 while maintaining the pressure concentration effect.
The inner circumferential conductive film 112 is physically connected to the anode member 43 via the end surface conductive film 113 in this manner, which improves the reliability of electrical connection between the inner circumferential conductive film 112 and the anode member 43. The anode member 43 is further connected as the anode 52 to a drive circuit 103, and is accordingly capable of freeing electric charges caused by scattered electrons and secondary electrons in the X-ray generating tube 102 to the outside via the inner circumferential conductive film 112 and the end surface conductive film 113. The X-ray generating tube 102 that is capable of preventing the charging of the inner surface of the insulating tube 110 and is reduced in changes in X-ray output is thus provided.
A material that is smaller in Young's modulus than the anode member 43 and the insulating tube 110 is preferred for the end surface conductive film 113. This causes the end surface conductive film 113 to be deformed so as to fit closely to the anode member 43, thereby improving the reliability of electrical connection between the end surface conductive film 113 and the anode member 43 even more. The insulating tube 110 is usually made of a glass material, a ceramic material, or the like as described above, and is larger in Young's modulus than metal. It is therefore recommended to use copper, silver, titanium, zinc, aluminum, or the like as the material of the end surface conductive film 113 while using Kovar, nickel, molybdenum, tungsten, or the like as the material of the anode member 43.
The insulating tube 110 and the anode member 43 are joined hermetically in order to keep the interior of the X-ray generating tube 102 under vacuum. In the example of
Hermetic joining is accomplished by brazing that uses brazing filler metal as the joining member 115. The brazing filler metal that can be used is, for example, one that has Au—Cu as the main component, nickel brazing filler metal, brass brazing filler metal, or silver brazing filler metal.
Other examples of the structure of the insulating tube and a part of the anode member around the outer circumferential tubular portion are described with reference to
In the embodiment of
The insulating tube 110 is preferred to have a ring-shaped region where the outer tube diameter increases in a tube axis direction that runs from an end of the insulating tube 110 on the anode member 43 side toward the other end of the insulating tube 110 on the cathode member 41 side. The ring-shaped region in the embodiment of
<X-Ray Generating Apparatus>
The container 120, which houses the X-ray generating tube 102 and the drive circuit 103, desirably has strength sufficient as a container and excellent heat dissipating properties. The constituent material of the container 120 is, for example, a metal material such as brass, iron, or stainless steel.
An excess space in the container 120 which remains after the X-ray generating tube 102 and the drive circuit 103 housed in the container 120 take up spaces in the container 120 is filled with an insulating liquid 109. The insulating liquid 109 is a liquid having electrical insulation properties, maintains electrical insulation inside the container 120, and serves as a cooling medium for the X-ray generating tube 102. An electrical insulation oil such as a mineral oil, a silicone oil, or a perfluoro-based oil is preferred as the insulating liquid 109.
<Radiography System>
A structural example of a radiography system 60, which includes the X-ray generating tube 102 of the present invention, is described next with reference to
A system control unit 202 controls the X-ray generating apparatus 101 and an X-ray detector 206 in an integrated manner. The drive circuit 103 outputs, under control of the system control unit 202, various control signals to the X-ray generating tube 102. The control signals output by the drive circuit 103 are used to control the emission state of the X-ray beam 11 emitted from the X-ray generating apparatus 101.
The X-ray beam 11 emitted from the X-ray generating apparatus 101 is adjusted in irradiation range by a collimator unit (not shown) having a variable aperture, emitted to the outside of the X-ray generating apparatus 101, transmitted through a subject to be examined 204 (hereinafter referred to as simply “subject”), and detected by the detector 206. The detector 206 converts the detected X-ray into image signals, which are output to a signal processing portion 205. The signal processing portion 205 performs, under control of the system control unit 202, given signal processing on the image signals, and outputs the processed image signals to the system control unit 202. Based on the processed image signals, the system control unit 202 outputs display signals for displaying an image on a display apparatus 203. The display apparatus 203 displays on a screen an image based on the display signals as a photographed image of the subject 204.
The radiography system 60 of the present invention is applicable to non-destructive testing of an industrial product, and the diagnosis of human and animal pathology.
The X-ray generating tube 102 having the insulating tube 110 of
As illustrated in
A silver brazing filler metal paste containing Ti was next applied to a part of the circumference of the insulating tube 110 that was in contact with the outer circumferential tubular portion 43b, and dried. Thereafter, components were arranged so as to bring the anode member 43 into contact with the end surface conductive films 113 formed on the insulating tube 110, and to bring the outer circumferential tubular portion 43b into contact with the joining member 115 formed on the circumference of the insulating tube 110, and vacuum heat treatment was executed at 800° C. for brazing. In the heat treatment, a weight was put on the anode member 43 in order to help along press-fit between the end surface conductive films 113 and the anode member 43. The metallization of alumina and hermetical brazing were accomplished by using brazing filler metal that contained Ti. The material used for the anode member 43 and the outer circumferential tubular portion 43ba was Kovar.
Next, the X-ray generating tube 102 of Example 1 was mounted to the radiography system 60 of
For comparison with Example 1, the X-ray generating tube 102 that had no end surface conductive film 113 was manufactured. The rest of the structure and the method used to manufacture this X-ray generating tube 102 were the same as those in Example 1.
Changes in the position of the focal spot 11a of the X-ray beam 11 were evaluated next as in Example 1. It was found as a result that the center position of the focal spot 11a moved by 50 μm 30 minutes after the start of the driving. The cathode 51 was separated from the X-ray generating tube 102 after the completion of the evaluation, and a tester was used to measure an electric resistance value between the inner circumferential conductive film 112 on the anode 52 side and the anode member 43. The measured electric resistance value was 10 MΩ or more, which is equal to or more than a measurement limit.
It is therefore inferred that electrical connection was not established between the inner circumferential conductive film 112 and the anode member 43, and that gradual charging of the inner surface of the insulating tube 110 during the driving of the X-ray generating tube 102 bent the electron beam trajectory and caused the change in the position of the focal spot 11a.
The X-ray generating tube 102 having the insulating tube 110 of
As in Example 1, the inner circumferential conductive film 112 and the end surface conductive films 113 were formed from a Ti—Cu-based material, and the same material and the same method were used to form the outer circumferential conductive film 114 on the circumferential surface of the insulating tube 110 so as to be continuous from the end surface conductive films 113.
Next, a silver brazing filler metal wire was wound as the joining member 115 around the circumference of the insulating tube 110 in a part closer to the anode member 43 than the level difference 110a. Thereafter, components were arranged so as to bring the anode member 43 into contact with the end surface conductive films 113 formed on the insulating tube 110, and to bring the sleeve 43a into contact with the brazing filler metal 115 formed on the circumference of the insulating tube 110, and vacuum heat treatment was executed at 800° C. for brazing. In the heat treatment, a weight was put on the anode member 43 in order to help along press-fit between the end surface conductive films 113 and the anode member 43. The Ti—Cu-based outer circumferential conductive film 114 also acted to metallize alumina, and hermetical brazing was thus accomplished. Kovar was used as the material of the anode member 43. In the case where a brazing filler metal wire is used as the joining member 115, a groove portion (not shown) for holding the wire may be formed in the insulating tube 110 and the wire may be arranged in the groove portion.
Next, as in Example 1, the X-ray generating tube 102 of Example 2 was mounted to the radiography system 60, and changes in X-ray output were evaluated. A favorable result was obtained in which the change in the center position of the focal spot was 10 μm or less.
In Example 3, the X-ray generating apparatus 101 of Example 1 was used to construct the radiography system 60 of
According to the present invention, electrical connection is made between the inner circumferential conductive film and the anode member via the end surface conductive film, which is sandwiched between an end surface of the insulating tube at one end and the anode member and which is electrically connected to the anode member. This improves the reliability of electrical connection between the inner circumferential conductive film and the anode member, and prevents without fail the charging of the inner surface of the insulating tube, thereby providing an X-ray generating tube that is reduced in changes in X-ray output. In addition, an X-ray generating apparatus and a radiography system that include the highly reliable X-ray generating tube reduced in changes in X-ray output can be provided.
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. 2014-220083, filed Oct. 29, 2014, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-220083 | Oct 2014 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14882562 | Oct 2015 | US |
| Child | 15782186 | US |
