Radiation generating tube, radiation generating apparatus, radiography system and manufacturing method thereof

The present invention relates to a radiation generating tube. The radiation generating tube includes an envelope including an insulating tubular member having at least two openings, a cathode connected to one of the openings of the insulating tubular member, and an anode connected to the other of the openings of the insulating tubular member. At least one of the cathode and the anode and the insulating tubular member are bonded at a bonded portion with an electrically conductive bonding member; and the bonded portion bonded with the electrically conductive bonding member is coated with a dielectric layer.

TECHNICAL FIELD

The present invention relates to a radiation generating tube and a radiation generating apparatus equipped with the same.

BACKGROUND ART

As radiation generating apparatuses have reduced in size and the energy of radiation emitted has increased, enhancing the withstand voltage characteristics (hereinafter referred to as withstand voltage) of radiation generating tubes has been required. One of portions of radiation generating tubes that require withstand voltage is a bonded portion of an insulating tubular member and a cathode. Particularly when the bonded portion is exposed from the radiation generating tube as viewed from the outside thereof, electric field concentration tends to occur in the vicinity of the exposed portion, where discharge is prone to occur. PTL 1 discloses providing a corona ring so as to cover a fusing portion of a glass vacuum envelope and a cathode to relax electric field concentration in the vicinity of the fusing portion, thereby preventing damage to the vacuum envelope due to a local discharge impact.

PTL 2 discloses a structure in which an electrically conducting portion located at the side of a radiation emission window and an electrically insulating portion located at the side of a voltage apply portion are fixed, and an electrically resistive film is disposed outside the electrically insulating portion to prevent discharge due to the disturbance of the electric field between a corona ring and a vacuum envelope.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Even if the conventional measures for the high withstand voltage are taken, changes in the output intensity of radiation or minute discharge sometimes occur. Such minute discharge causes changes in the current density and the energy of electrons emitted to the target during operation, resulting in changes in radiation output intensity. If the frequency of the minute discharge increases, accelerating voltage cannot be continuously applied between the target and the electron emitting devices.

The present invention provides a radiation generating tube and a radiation generating apparatus having high reliability and high radiation-output stability in which electric field concentration generated at the bonded portion of a cathode or an anode and an insulating tubular member is made difficult to occur, thereby preventing discharge.

Solution to Problem

A radiation generating tube according to an aspect of the present invention includes an envelope including an insulating tubular member having at least two openings, a cathode connected to one of the openings of the insulating tubular member, and an anode connected to the other of the openings of the insulating tubular member; an electron emitting source connected to the cathode; and a target connected to the anode, wherein the internal space of the envelope is under negative pressure relative to the external space, wherein at least one of the cathode and the anode an the insulating tubular member are bonded at a bonded portion with an electrically conductive bonding member; and the bonded portion bonded with the electrically conductive bonding member is coated with a dielectric layer.

Advantageous Effects of Invention

The present invention can provide a high-reliability radiation generating apparatus in which high withstand voltage characteristics are maintained over a long period.

DESCRIPTION OF EMBODIMENTS

An example of the configurations of a radiation generating tube and a radiation generating apparatus according to an embodiment of the present invention will be described with reference toFIG. 1andFIGS. 2A to 2C.

FIGS. 2A to 2Care cross-sectional views of a radiation generating tube according to an embodiment of the present invention, andFIG. 1is a cross-sectional view of a radiation generating apparatus that accommodates the radiation generating tube according to the embodiment of the present invention.

A radiation generating tube1is equipped with a cathode19, an electron emitting source3connected to the cathode19, an anode20, a target8connected to the anode20, and an insulating tubular member21. The cathode19and the anode20are individually connected to two openings of the insulating tubular member21at separate locations. A container6formed of the cathode19, the anode20, and the insulating tubular member21to define an internal space12is referred to as an envelope6in the present invention. The internal space12of the envelope6is reduced in pressure (evacuated) so that electrons emitted from an electron emitting portion2of the electron emitting source3disposed in the internal space12of the envelope6can be emitted to the target8as an electron beam5. The degree of vacuum in the internal space12can be selected as appropriate in consideration of the kind, the driving conditions, and so on of the electron emitting source3used; for example, a vacuum level of 1 E-4 to 1 E-8 Pa can be selected. If a cold-cathode electron emitting source of Spindt type, metal-insulator-metal (MIM) type, or the like is used, a vacuum of 1 E-6 Pa or less may be selected in view of the stability of electron emission characteristics. To maintain the degree of vacuum, a getter (not shown) may be disposed in the internal space12or an auxiliary space (not shown) that communicates with the internal space12.

The electron emitting source3may be any electron emitting source whose electron emission amount can be controlled from the outside of the envelope6; for example; in addition to the above-described cold-cathode electron emitting source, a hot-cathode electron emitting source can be applied as appropriate. An impregnated hot-cathode electron emitting source can be employed because it can stably emit a large-current electron beam5.

The electron emitting source3is electrically connected to a driving circuit14disposed outside the envelope6so that the electron emission amount and the on/off timing of electron emission can be controlled via a current lead-in terminal4provided on the cathode19. The location of the driving circuit14is not limited thereto; it may be disposed in the envelope6.

The cathode19according to an embodiment of the present invention defines an electrostatic field around the mounting portion of the electron emitting source3to the envelope6so as to relax the spatial asymmetry of the electrostatic field around the electron emitting source3and to prevent local electric field concentration. The electron emitting source3has the electron emitting portion2. The electron emitting portion2has two electrodes that supply emission electron currents as an emitter electrode pair (not shown). If electrooptical functions, such as electron beam convergence and astigmatism correction, are to be added, additional auxiliary electrodes (not, shown) are provided. The electrode group composed of the above-described emitter electrode pair and the auxiliary electrodes can be connected to the driving circuit14outside the radiation generating tube1from the cathode19side via the current lead-in terminal4. The cathode19may be set at a constant potential sufficiently lower than the electrode potential of the anode20in view of the relaxation of the asymmetry of the electrostatic field, described above. The potential may be set to the same potential as that of one of the emitter electrode pair that supplies potential to the electron emitting portion2or may be set to an intermediate potential of the potentials of the emitter electrode pair.

The anode20has functions of defining the potential of the target8with a voltage source (not shown) and supplying an anode current flowing in the target8to a grounding terminal16via the voltage source. The anode20is an electrode having the function of defining an electrostatic field around the target8of the radiation generating tube1, like the cathode19. Accordingly, to prevent local electric field concentration in the electrostatic fields around the electron emitting source3and the target8and to bring the electric field distribution between the cathode19and the anode20close to a parallel electric field as much as possible, the cathode19and the anode20may individually define the potentials of predetermined areas, it may also be the case that the areas are equal to the opening cross-sectional area of the insulating tubular member21. The anode20may separately have a shielding member (not shown) that can define the radiation range of radiation15. The anode20and the target8may also be connected via the shielding member.

The materials of the cathode19and the anode20may be determined depending on electrical conductivity, airtightness, strength, and Coefficient of linear Thermal Expansion matching with the insulating tubular member21; for example, Kovar and tungsten can be employed.

The target8is disposed in the radiation generating tube1so as to be irradiated with electrons emitted from the electron emitting portion2. The target8may be opposed to the electron emitting portion2in view of the symmetry of the electric field between the cathode19and the anode20.

For the target8, a positive potential of 10 kV to 200 kV is applied to the electron emitting portion2, so that electrons having an energy of 10 keV to 200 keV are emitted from the electron emitting portion2to the target8as the electron beam5to cause the target8to generate radiation. Accordingly, substantially the same positive potential as that of the target8may be applied to anode20in view of suppression of asymmetry of the electric field distribution between the cathode19and the anode20. The target8has a target component containing a heavy element that generates radiation due to a collision of electrons. The target8may be of an independent type composed of only the target component. An example of the independent type has a configuration in which a diaphragm-type metal thin film is connected to the anode20. The target8may be of a distributed type in which a target material is distributed in a material that allows radiation to pass therethrough and a layered type in which a metal thin film that contains a target material is stacked on a substrate formed of a material that allows radiation to pass there through. Examples of the substrate that allows radiation to pass therethrough include substrates made of a material having a low atomic number, such beryllium and diamond. The target8may be of a layered type in which a target layer and a support substrate that supports the target layer are layered. A metal thin film having a thickness of several micrometers may be supported by the substrate in view of prevention of attenuation of radiation and prevention of defocusing due to the thermal deformation of the target8. The metal thin film may be formed of a heavy metal material having an atomic number 26 or larger in view of radiation/incoming electrons conversion efficiency. Specifically, the metal thin film may be made of tungsten, molybdenum, chrome, copper, cobalt, iron, rhodium, rhenium, or an alloy thereof. If such the metal thin film is formed on the support substrate as a target material of the target8, any method that ensures the contact between it and the support substrate may be employed; various deposition methods, for example, sputtering, CVD, and vapor deposition, may be employed.

The insulating tubular member21has a dielectric property and has at least two openings that connect to the cathode19and the anode20, respectively. The insulating tubular member21is configured such that the two openings communicate with each other so that electrons emitted from the electron emitting portion2irradiate the target8in the envelope6. That is, the radiation generating tube1may have not only the configuration in which the cathode19and the anode20are exposed and opposed, as shown inFIG. 1, but also a configuration in which the internal space of the insulating tubular member21is separated by a partition and the electron emitting source3passes through the partition, as shown inFIG. 9D. As shown inFIG. 9Aand a cross-sectional view9B of the radiation generating tube1inFIG. 9A, taken along IXB-IXB, the anode20(or the cathode19) may be connected to the side of the insulating tubular member21. As an alternative, as shown inFIG. 9C, the cathode19and the anode20need not be opposed and may be in a nonparallel positional relationship. The insulating tubular member21need not have a cylindrical shape in cross section, as inFIG. 1, but may have a polygonal outer shape or inner shape in cross section. The material of the insulating tubular member21is selected in view of the electrical insulation performance, airtightness, low gas emission performance, heat resistance, and Coefficient of linear Thermal Expansion matching between the cathode19and the anode20; for example, insulating ceramics, such as boron nitride and alumina, and insulating inorganic glass, such as borosilicate glass.

The bonded portion of the cathode19or the anode20and the insulating tubular member21becomes an electric field concentration area when the radiation generating tube1is operated, where discharge occurs at high probability, and the withstand voltage characteristics of the radiation generating tube1are restricted. Particularly the bonded portion of the cathode19and the insulating tubular member21is referred to as a triple point, where field electron emission from the cathode19side tends to occur. Accordingly, one of measures to reduce discharge is reducing ununiformity of electric field distribution particularly in the vicinity of the bonded portion23adjacent to the cathode19. Disposing the bonded portion23via the electrically conductive bonding member22as a bonded portion allows the electric field distribution at the triple point to be uniformized around the circumference of the bonded portion23(hereinafter referred to as a circumferential direction). the electrically conductive bonding member22may be hard solder (brazing alloy), such as silver alloy brazing filler and copper solder, having electrical conductivity and high bondability between different kinds of material, metal and insulator. As shown inFIGS. 2A and 2C, the electrically conductive bonding member22may be shaped like a ring and may hermetically seal the bonded portion23in a ring shape in view of the uniformity of the electric field in the circumferential direction of the bonded portion23of the insulating tubular member21and the cathode19or the anode20.FIGS. 2A and 2Care cross-sectional views of the radiation generating tube1shown inFIG. 2B, taken along IIA-IIA and IIC-IIC, respectively.

At least one of the cathode19and the anode20of the radiation generating tube1is bonded together with the electrically conductive bonding member22. The bonded portion23having the electrically conductive bonding member22interposed therebetween is coated with a dielectric layer24made of a dielectric material, such as epoxy resin, silicone resin, aluminum oxide, silicon oxide, and boron nitride.

The dielectric layer24exhibits a plurality of actions that improve the withstand voltage characteristics of the radiation generating tube1according to an embodiment of the radiation generating tube1.

The dielectric layer24has the action of making the electric field concentration generated at the electrically conductive bonding member22difficult to extend directly to the electrostatic field in a space that the electrically conductive bonding member22faces. This action will be specifically described hereinbelow.

The electrically conductive bonding member22is set to substantially the same potential as that of the cathode19. Note, however, that the boundary between the electrically conductive bonding member22having electrical conductivity, the insulating tubular member21having a dielectric property, and the vacuum internal space12is a triple point in a microscopic scale, where electric field concentration occurs. On the other hand, in a radiation generating tube manufactured through an actual manufacturing process, sometimes the surface of the electrically conductive bonding member22is not formed into a completely smooth surface, and the boundary between the electrically conductive bonding member22and the insulating tubular member21is not formed into a completely smooth ring shape. For example, the electrically conductive bonding member22is a material that comes into close contact with the members which are to be bonded to mutually to ensure a bonded surface in a bonding process, owing to a property of softening to be deformed more easily than the cathode19and the insulating tubular member21and a property of wettability to members which are to be bonded to mutually. As a result of bonding, local deformation, such as protrusions, and locally spreading wetting occurred in the electrically conductive bonding member22to cause the electrically conductive bonding member22to be varied in shape. Such variations in the shape of the electrically conductive bonding member22further accelerate electric field concentration at the triple point. In the bonded portions23shown inFIGS. 10A to 10F, the dielectric layer24coats at least the boundary between the electrically conductive bonding member22and the insulating tubular member21. The coating of the bonded portion23with the dielectric layer24can relax electric field concentration generated at the surface of the electrically conductive bonding member22and the boundary between the surface of the electrically conductive bonding member22and the insulating tubular member21. The effect of relaxing the electric field concentration on the space in the vicinity of the bonded portion23depends on the relative dielectric constant, the shape, and the coating range of the dielectric layer24. The relative dielectric constant of the dielectric layer24may be smaller than the relative dielectric constant of the insulating tubular member21. For the configuration of the dielectric layer24, a thickness of 100 micrometers or more or a thickness of 10% or higher than the thickness of the side wall of the insulating tubular member21allows the electric field concentration to be suppressed more effectively. Setting the thickness of the dielectric layer24to 100% or less of the thickness of the side wall of the insulating tubular member21prevents the electric field concentration area from coming excessively close to the current lead-in terminal4or the electrode group of the cathode19, thus allowing a drop in withstand voltage to be prevented. The coating range of the dielectric layer24may be set to at least continuous part of the insulating tubular member21next to the boundary between the electrically conductive bonding member22and the insulating tubular member21in consideration of the alignment tolerance of the coating. The bonded portion23including the thickwise direction of the electrically conductive bonding member22(the distance between the cathode19and the insulating tubular member21) may be coated. That is, the dielectric layer24inFIG. 10Coffers a higher field-concentration relaxing effect than that inFIG. 10B, and the dielectric layer24inFIG. 10Aoffers a higher field-concentration relaxing effect than that inFIG. 10C. As shown inFIGS. 10D to 10F, the electrically conductive bonding member22and the dielectric layer24need not necessarily be in close contact; according to some embodiments of the present invention, there may be a space between the dielectric layer24and the electrically conductive bonding member22. As shown inFIG. 3andFIGS. 10A to 10D and 10F, a configuration in which one of the side of the electrically conductive bonding member22facing the internal space12of the envelope6and the side of the electrically conductive bonding member22facing the external space of the envelope6is coated with the dielectric layer24is also included as some embodiments of the present invention. Furthermore, as shown inFIG. 10E, a configuration in which both of the surface of the electrically conductive bonding member22facing the internal space12of the envelope6and the surface of the electrically conductive bonding member22facing the external space of the envelope6are coated so as to sandwich the electrically conductive bonding member22therebetween is also included as an embodiment of the present invention. As shown inFIGS. 2A and 2C, the dielectric layer24may be shaped like a ring and may coat the bonded portion23in a ring shape in view of the uniformity of the electric field in the circumferential direction of the bonded portion23.

As described above, the dielectric layer24has the operational advantage of directly suppressing electric field concentration in the vicinity of the bonded portion23. Furthermore, as shown inFIGS. 10A, 10D, 10E, and 10F, the dielectric layer24has the operational advantage of indirectly suppressing electric field concentration in the vicinity of the bonded portion23, depending on the configuration of coating. This will be described hereinbelow.

The target8generates the radiation15when irradiated with the electron beam5. The conversion efficiency thereof is extremely smaller than 1. Most of the kinetic energy of the electron beam5input to the target8is converted to heat and does not contribute to generation of radiation. Accordingly, the bonded portion23of the radiation generating tube1is subjected to a temperature change history from a storage temperature (environmental temperature or room temperature) in an inoperative state to an operating temperature (the order of several hundred Celsius degrees). Furthermore, continuous compressive stress occurs in the bonded portion23due to the pressure difference (atmospheric pressure) between the interior and the exterior of the radiation generating tube1. Furthermore, in the radiation generating tube1, a linear expansion amount difference due to a Coefficient of linear Thermal Expansion difference between the cathode19or the anode20and the insulating tubular member21and the temperature distribution occurs among the components due to the temperature changes, described above. The mismatch of the Coefficient of linear Thermal Expansion and the liner expansion amount causes an intermittent and changing stress in the bonded portion23.

As mentioned above, there are some probabilities of cracks due to the stresses within the radiation generating tube. One of the stresses concerned with the linear expansion difference due to the operating temperature changes and the other of the stresses concerned with the pressure difference between the interior and exterior of the radiation generating tube. The bonded portion23with the electrically conductive bonding member22that contains metal, such as silver alloy brazing filler, has a faculty to prevent cracks, breaking, and so on in the other components of the radiation generating tube1due to distortion (deformation) of the electrically conductive bonding member22, such as viscoelastic deformation. However, the stress that is generated in the bonded portion23for a long period, described above, and the repletion thereof serve as driving force to grow needle crystal28called whisker from the electrically conductive bonding member22. Specifically, the electrically conductive bonding member22is an alloy made of hard solder, such as silver alloy brazing filler, and has the property of relaxing compressive stress generated in the alloy when subjected to continuous stress, particularly, compressive stress by generating needle crystal28of a selected component thereof, such as silver, copper, gold, zinc, and tin, outwards from the surface of the alloy composition. The inventors have found that the withstand voltage characteristics sometimes deteriorate due to the needle crystal28protruding from the surface of the electrically conductive bonding member22.FIG. 7Ashows the initial state of the radiation generating tube1in which the bonded portions23of the insulating tubular member21and the cathode19and the anode20are not coated with dielectric layers.FIG. 7Bschematically shows the same radiation generating tube1after being operated for 1,000 hours. Since the needle crystal28has electrical conductivity and has a shape with a high aspect ratio (length in a growing direction/cross-sectional width), it caused further acceleration of the electric field concentration in the vicinity of the bonded portion23.

The dielectric layer24has the action of physically preventing the needle crystal28from protruding from the surface of the electrically conductive bonding member22into a space in the vicinity of the bonded portion23by covering the bonded portion23. Accordingly, the coating of the bonded portion23with the dielectric layer24is effective particularly when the electrically conductive bonding member22contains a metal element selected from silver, tin, zinc, and gold as simple metal, or metal elements selected therefrom as components of an alloy or components of a metal mixture.

Furthermore, as shown inFIGS. 10A, 10D, and 10F, the dielectric layer24may be configured to coat both of part of the cathode19and part of the insulating tubular member21in such a manner so as to connect the cathode19and the insulating tubular member21. In other words, the dielectric layer24may be configured to coat part of the members which are to be bonded to mutually to be bonded by the bonded portion23in such a manner as to bridge the members which are to be bonded to mutually. Such a configuration allows part of the stress generated in the electrically conductive bonding member23to be shared by the dielectric layer24to reduce the stress generated in the electrically conductive bonding member22, thereby preventing generation of the needle crystal28. Furthermore, as shown inFIG. 10E, providing both of a configuration in which both of the side of the electrically conductive bonding member22facing the internal space12of the envelope6and the side of the electrically conductive bonding member22facing the external space of the envelope6are coated, with the electrically conductive bonding member22interposed therebetween, and the configuration in which the cathode19and the insulating tubular member21, which are target members, are coated so as to be bridged allows the action of sharing part of the stress generated in the bonded portion23, described above, to be further enhanced. AlthoughFIGS. 10A to 10Fillustrate the bonded portion23at the cathode19side, the bonded portion23at the anode20side offers the same operational advantages as those of the bonded portion23at the cathode19side by employing the same configurations as those of the bonded portion23at the cathode19side.

Furthermore, the process of manufacturing the radiation generating apparatus1includes the step of bonding at least one of the cathode19and the anode20and an opening of the insulating tubular member21with the electrically conductive bonding member22; the step of coating the bonded portion23with the dielectric layer24so as to bridge part of at least one of the cathode19and the anode20and part of the insulating tubular member21; the step of forming the envelope6by defining the internal space12by hermetically sealing the cathode19, the anode20, and the insulating tubular member21; and the step of reducing the pressure of the internal space12of the envelope6into negative pressure relative to an external space. By performing the pressure-reducing step after the step of coating the bonded portion23with the dielectric layer24, compressive stress generated in the bonded portion23due to the negative pressure in the internal space of the envelope6can be reduced more effectively by forming the dielectric layer24before compressive stress is generated in the bonded portion23, particularly in the electrically conductive bonding member22.

The radiation generating tube1may be accommodated in the container11to configure the radiation generating apparatus13. The internal space17between the radiation generating tube1and the container11may accommodate insulating fluid (insulating liquid18) in view of stabilization of the withstand voltage characteristics and the performance characteristics of the radiation generating apparatus13in operation. The introduction of the insulating liquid18can enhance the heat radiation performance of the radiation generating tube1in operation while ensuring the insulation between the cathode19and the anode20. The insulating liquid18may have high electrical insulation performance, high cooling performance and is less prone to degradation due to heat; for example, electrically insulating oil, such as silicone oil, transformer oil, and fluorine oil and fluorine insulating liquid, such as hydrofluoroether, can be used. However, in the case where the insulating liquid18is disposed around the radiation generating tube1, sometime a foreign substance31gets mixed or a foreign substance30is generated in the insulating liquid18, as shown inFIG. 8A. Possible causes of the contaminant foreign substance31are dropping-off of part of some of the components that constitute the radiation generating apparatus13resulting from deterioration due to heat generated during operation or vibration during operation and inevitable intrusion thereof into the insulating liquid18during manufacture. Possible causes of the generated foreign substance30are deterioration of the insulating liquid18itself into a solid due to an increase in the temperature of the insulating liquid18, absorption of electromagnetic waves, or the like with the operation of the radiation generating apparatus (hereinafter the contaminant foreign substance31and the generated foreign substance30are collectively referred to as foreign substances).

As shown inFIG. 8B, these foreign substances sometimes come into contact with the bonded portion23of the cathode19or anode20and the insulating tubular member21as the insulating liquid18flows. The contact between the bonded portion23and the foreign substances, irrespective of whether the foreign substances have electrical conductivity or dielectricity, has a possibility that the electric field distribution in the vicinity of the bonded portion23is locally disturbed to cause a new electric field concentration area.

The dielectric layer24also has the effect of suppressing the generation of an electric field concentration area due to the contact between the foreign substances in the insulating liquid18and the bonded portion23by obstructing such contact itself between the foreign substances and the bonded portion23in the insulating liquid18. Accordingly, as shown inFIG. 1, at least part of the outer surface6of the insulating tubular member21and the electrically conductive bonding member22are continuously coated so as to separate the insulating liquid18and the bonded portion23from each other using the dielectric layer24.

Furthermore, by setting the relative dielectric constant of the insulating liquid18smaller than that of the dielectric layer24, the electric field concentration in the vicinity of the bonded portion23can be further relaxed.

The driving circuit14for driving the radiation generating tube1may be disposed either inside or outside the container11.

The container11may be set at a predetermined potential in view of operational stability and safety of the radiation generating apparatus13. The predetermined potential may be a grounding potential set via the grounding terminal16. The material of the container11may be various kinds of material; for example, metal, such as iron, stainless steel, lead, brass, and copper, may be employed in view of radiation shielding performance, strength, and surface-potential setting performance.

The auxiliary electrode may be connected to a collection circuit (not shown) disposed outside the radiation generating tube1. Both of the correction circuit and the voltage source may be provided in the driving circuit14.

EXAMPLES

Example 1 is an example of the configuration shown in the foregoing embodiment and will be described in detail usingFIGS. 4 and 5.FIG. 4shows a cross-section of the radiation generating tube1of Example 1.FIG. 5is a block diagram of an experimental device for examining the performance characteristics of the radiation generating tube1of Example 1.

The radiation generating tube1of Example 1 was formed as follows. First, a high-pressure synthetic diamond made by Sumitomo electric industries, Ltd., was prepared as a support substrate. The support substrate has a disc shape (cylindrical shape) having a diameter of 5 mm and a thickness of 1 mm. The prepared support substrate was subjected to UV-ozone ashing to remove organic matter on the surface thereof.

A titanium contact layer was formed by sputtering at a thickness of 10 nm on one of the two circular surfaces of the support substrate having a diameter of 1 mm by using argon as a carrier gas. The support substrate during deposition of titanium was heated to 260 Celsius degrees with a heating stage. Next, a tungsten target layer was formed by sputtering at a thickness of 7 micrometers on the contact layer by continuous deposition using argon as a carrier gas without venting an atmosphere around the deposition unit. The diamond support substrate during deposition of tungsten was heated to 260 Celsius degrees with a heating stage, as in the deposition of titanium.

The thicknesses of the titanium contact layer and the tungsten target layer were adjusted to designated film thicknesses depending on the deposition times before the deposition by acquiring calibration curve data about the thicknesses of the individual films and deposition times in advance. Measurement of the film thicknesses for acquiring the calibration curve data was performed using a spectroscopic ellipsometer, UVISEL ER, made by Horiba, Ltd. Thus, the target8in which the diamond support substrate, the titanium contact layer, and the tungsten target layer were deposited in this order was obtained.

Next, a cylindrical opening having a diameter of 1.1 mm was formed in the center of a disc-shaped metal plate formed of Kovar and having a diameter of 60 mm and a thickness of 3 mm to form the anode20. The anode20was subjected to organic solvent cleaning, rinse, and UV-ozone ashing to remove organic matter on the surface of the anode20.

Next, silver alloy brazing filler was applied between the opening of the anode20and the outer circumference of the disc-shaped target8as an electrically conductive bonding member to solder them, and thus the anode20connected to the target8was obtained.

Next, the current lead-in terminal4was provided at the center of a disc-shaped metal plate made of Kovar and having a diameter of 60 mm and a thickness of 3 mm to form the cathode19. The cathode19was subjected to cleaning similar to that for the anode20, such as organic solvent cleaning, rinse, and UV-ozone ashing, to remove organic matter on the surface of the cathode19.

Next, the current lead-in terminal4and an impregnated electron gun were electrically and mechanically connected to obtain the cathode19connected to the electron emitting source3.

Next, the insulating tubular member21made of alumina and having a length of 70 mm, an outside diameter of 60 mm, and an inside diameter of 50 mm was prepared. The insulating tubular member21was also subjected to cleaning similar to that for the cathode19and the anode20to remove organic matter on the surface thereof.

Next, ring-shaped silver alloy brazing filler, Japanese Industrial Standard, BAg-8 (Ag72-Cu28, melting point: 780 Celsius degrees), was inserted between the surface of the cathode19on which the electron emitting source3is provided and one of the openings of the insulating tubular member21and is subjected to brazing at 820 Celsius degrees to form the bonded portion23having the ring-shaped hermetically bonded bonding member22. Thus, the insulating tubular member21bonded to the cathode19was obtained.

Next, as shown inFIG. 4, a two-component epoxy adhesive is applied to the side of the bonded portion23of the cathode19and the insulating tubular member21exposed to the interior of the insulating tubular member21and the side of the bonded portion23exposed to the outside of the insulating tubular member21and is hardened. Thus, the bonded portion23is coated with the dielectric layers24formed of epoxy resin obtained by hardening the epoxy adhesive so as to sandwich the bonded portion23therebetween. The coating ranges of the dielectric layers24were set to a range of 1 mm from the boundary between the cathode19and the electrically conductive bonding member22toward the cathode19to 5 mm from the boundary between the insulating tubular member21and the electrically conductive bonding member22toward the anode20on both of inside and outside of the insulating tubular member21. The thickness of the dielectric layers24used was set to 1 mm. The relative dielectric constant of the alumina in the insulating tubular member21was 9.5 (at room temperature, 1 MHz). The relative dielectric constant of the epoxy resin used was 4.0 (at room temperature, 1 MHz). The melting point of the alumina in the insulating tubular member21was 2,020 Celsius degrees.

Next, the other of the openings of the insulating tubular member21and the same exposed surface of the anode20as the surface of the target8from which tungsten is exposed was soldered by inserting ring-shaped silver alloy brazing filler, Japanese Industrial Standard, BAg-8 (Ag72-Cu28, melting point: 780 Celsius degrees), in the same manner as the bonding of the cathode19side to form the bonded portion23having the ring-shaped hermetically bonded bonding member22.

Thus, the cathode19and the anode20and the insulating tubular member21are individually connected at the two openings of the insulating tubular member21by airtight bonding to form the envelope6.

Next, the interior of the envelope6was evacuated to a vacuum of 1E-5 Pa with an exhaust pipe and an exhauster (not shown), and thereafter the exhaust pipe is sealed to form the radiation generating tube1.

Five radiation generating tubes1, shown inFIG. 4, were formed by the method described above.

As shown inFIG. 5, the formed radiation generating tubes1were disposed in the atmosphere, and the cathode19, the anode20, and the current lead-in terminal4of each of the radiation generating tubes1were connected to a cathode terminal that outputs −½ Va, an anode terminal that outputs +½ Va, and a terminal group that controls the amount of electrons of the electron beam5to be emitted from the electron gun3, which are provided in the driving circuit14in advance, where Va is an acceleration voltage between the electron emitting portion2and the target8.

Next, a radiation-intensity detector26equipped with a semiconductor detector was disposed at a position 100 cm away from the target8on the central vertical axis of the target8of the radiation generating apparatus1, that is, a position on the radiation emission center axis. Output-stability evaluation with the radiation-intensity detector26was performed in such a manner that radiation was emitted for five seconds every time the electron emitting source3repeats 100 times of a one-second emission and a three-second idle period, with the acceleration voltage Va set at 60 kV, the output intensity of radiation was measured for three seconds except the preceding and following one seconds, and thus, temporal changes in the output intensity of the radiation generating tube1were measured. The electron emission was performed in such a manner that the emission axis of the electron beam5was aligned so that the focus on the target8is well within the target8, and the spot radius of the electron beam5is 0.5 mm, and the density of a current flowing in the anode20was controlled to a variable value within 1% while monitoring the current flowing through the path between the anode20and the grounding electrode with a negative feedback circuit (not shown).

The discharge counter25observed whether discharge has occurred in a connection wire from the cathode19to the driving circuit14, a connection wire from the anode20to the driving circuit14, and a connection wire group from the current lead-in terminal4to the driving circuit14with inductive probes. A discharge-withstand-voltage characteristic test was performed by gradually increasing the acceleration voltage Va while stopping the current supplied to the electron emitting portion2.

The average value of the output changes of the radiation generating tubes1of Example 1 was 1.9%, which was a good result.

Furthermore, the average of voltages that the radiation generating tubes1of Example 1 discharged first was 91 kV, and the average of the accumulated numbers of discharge to an application of 100 kV was 1.3, which were good results.

As shown inFIG. 5, in the foregoing experiment, the discharge counter25, the driving circuit14, and the radiation-intensity detector26were grounded with the grounding terminal16.

Another five radiation generating tubes1of Example 1 were formed, and when the individual bonded portions23were observed by conducting a test to give a 1,000-hour temperature history in which temperatures from a room temperature to 300 Celsius degrees are repeated 100 times using environment testing equipment, as in Example 1, no needle crystal was observed at the cathode-side bonded portions23and the anode-side bonded portions23.

Comparative Example 1

The process of forming the radiation generating tubes1was performed as in Example 1, except the step of coating with the dielectric layers24, to form five radiation generating tubes1shown inFIG. 7A.

A radiation-intensity-output-stability test and a discharge-withstand-voltage characteristic test were performed on the radiation generating tubes1formed in Comparative Example 1 for the experimental device shown inFIG. 5, as in Example 1.

The average value of the output changes of the radiation generating tubes1of Comparative Example 1 was 3.9%, the performance of which was poorer than that of Example 1.

Furthermore, the average of voltages that the radiation generating tubes1of Comparative Example 1 discharged first was 65 kV, and the average of the accumulated numbers of discharge to an application of 100 kV was 12.3, the performance of which was poorer than that of Example 1.

Another five radiation generating tubes1of Comparative Example 1 were formed, and when the individual bonded portions23were Observed by conducting a test to give a 1,000-hour temperature history in which temperatures from a room temperature to 300 Celsius degrees are repeated 100 times using environment testing equipment, as in Example 1, three pieces of needle crystal28were observed at two locations on the cathode19side and one location on the anode20side.

In the step of forming the dielectric layers24on the cathode-side bonded portion23of the process of forming the radiation generating tubes1in Example 1, the dielectric layer24was formed only on the inside of the insulating tubular member21, and the other forming steps were performed as in Example 1 to form five radiation generating tubes1shown inFIG. 7A.

A radiation-intensity-output-stability test and a discharge-withstand-voltage characteristic test were performed on the radiation generating tubes1formed in Example 2 for the experimental device shown inFIG. 5, as in Example 1.

The average value of the output changes of the radiation generating tubes1of Example 2 was 2.3%, which was a good result.

Furthermore, the average of voltages that the radiation generating tubes1of Example 2 discharged first was 84 kV, and the average of the accumulated numbers of discharge to an application of 100 kV was 1.6, which was a good result.

Another five radiation generating tubes1of Example 2 were formed, and when the individual bonded portions23were observed by conducting a test to give a 1,000-hour temperature history in which temperatures from a room temperature to 300 Celsius degrees are repeated 100 times using environment testing equipment, as in Example 1, no needle crystal was observed at the cathode-side bonded portions23and the anode-side bonded portions23.

In Example 3, in the step of forming the dielectric layers24on the bonded portion23of the same forming method as in Example 1, the dielectric layer24was formed only on the outside of the envelope6as in Example 1, except that both of bonded portion23at the cathode19side and the bonded portion23at the anode20side were coated with dielectric layers24, to form five radiation generating tubes1shown inFIGS. 2A, 2B, and 2C. Next, as shown inFIG. 6, the thus-formed radiation generating tubes1were each accommodated in the brass container11together with the driving circuit14. Next, as in Example 1, the driving circuit14and the radiation generating tube1were electrically connected. Next, as in Example 1, the dielectric probes of the discharge counter25disposed outside the container11were disposed on connection wires between the driving circuit14and the radiation generating tube1, and the container11was filled with silicone oil having a relative dielectric constant of 2.8 (at room temperature, 1 MHz), and thereafter, the container11was closed with a brass cover. Thus, the radiation generating apparatus13whose output changes can be measured was formed.

Next, for the thus-formed radiation generating apparatus13, the radiation-intensity detector26equipped with the semiconductor detector was disposed at a position facing a radiation extracting portion10of the container11, on the radiation irradiation center axis27, and 100 cm distant from the target8, as in Example 1.

The average value of the output changes of the radiation generating tubes3of Example 3 was 2.0%, which was a good result.

Furthermore, the average of voltages that the radiation generating tubes1of this example discharged first was 94 kV, and the average of the accumulated numbers of discharge to an application of 100 kV was 1.3, which was a good result.

When the five radiation generating apparatuses13of Example 3 subjected to the output-stability estimation test were disassembled, and the bonded portions23of the individual radiation generating tubes1were observed, 3.3 foreign substances were observed on the dielectric layer24at the cathode19side, and 7.2 foreign substances were observed on the dielectric layer24at the anode side on average,

Comparative Example 2

The process of forming the radiation generating tubes1in Example 3 was performed as in Example 1, except the step of coating with the dielectric layers24, to form five radiation generating tubes21shown inFIG. 7A. The thus-formed radiation generating apparatuses13were each accommodated in the container11, as in Example 3, and were filled with the insulating liquid18formed of silicone oil, and the radiation generating tube5was connected to the driving circuit14, the discharge counter25, and the radiation-intensity detector26.

A radiation-intensity-output-stability test and a discharge-withstand-voltage characteristic test were performed on the radiation generating tubes1formed in Comparative Example 2 for the experimental device shown inFIG. 6, as in Example 3.

The average value of the output changes of the radiation generating tubes1of Comparative Example 2 was 3.8%, the performance of which was poorer than that in Example 3.

Furthermore, the average of voltages that the radiation generating tubes1of Comparative Example 2 discharged first was 62 kV, and the average of the accumulated numbers of discharge to an application of 100 kV was 11.1, the performance of which was poorer than that of Example 3.

When the five radiation generating apparatuses13subjected to the output-stability estimation test of Comparative Example 2 were disassembled, and the bonded portions23of the individual radiation generating tubes1were observed, 3.2 foreign substances were observed on the bonded portion23at the cathode19side, and 7.6 foreign substances were observed on the bonded portion23at the anode side on average.

This application claims the benefit of Japanese Patent Application No. 2011-245793, filed Nov. 9, 2011, which is hereby incorporated by reference herein in its entirety.