Patent Description:
The present invention particularly relates to a transmission X-ray target that includes a target layer and a diamond substrate supporting the target layer, a radiation generating tube that includes the transmission X-ray target, a radiation generating apparatus that includes the radiation generating tube, and a radiography system that includes the radiation generating apparatus.

Radiation generating apparatus that generate X-rays used in medical diagnosis desirably have enhanced durability and require less maintenance so as to raise the generators' operating rate and serve as medical modality that can be used in home medical care and emergency medicine in the event of disasters and accidents.

One of the main factors that determine the durability of radiation generating apparatus is the heat-resistance of targets used as sources for generating radiation.

In a radiation generating apparatus configured to generate radiation by irradiating a target with an electron beam, the radiation generation efficiency at the target is less than <NUM>% and almost all of the energy applied to the target is converted to heat. If heat generated at the target is not sufficiently released, adhesion of the target may decrease due to thermal stress and the heat-resistance of the target becomes limited.

One known way of improving the radiation generation efficiency of a target is to use a transmission type target that includes a target layer, which is in a form of a thin film that contains a heavy metal, and a substrate that transmits the radiation and supports the target layer. <CIT> discloses a rotating anode-type transmission type target offering a radiation generation efficiency at least <NUM> times higher than that of known rotating anode-type reflection targets.

One known way of accelerating the release of heat from the target to outside is to use diamond as a substrate that supports the target layer of a multilayered target. <CIT> discloses use of diamond as the substrate that supports a target layer composed of tungsten, by which the heat releasing property is enhanced and microfocusing is realized. Diamond has not only high heat-resistance and high heat conductivity but also a high radiation transmitting property, and is thus suitable as a material for the substrate that supports a transmission type target on one hand.

On the other hand, diamond has low wettability to molten metal and linear expansion coefficient mismatch occurs between diamond and solid metal. Thus, diamond has low affinity to a target metal. Ensuring the adhesion between a target layer and a diamond substrate has been the issue that needs to be addressed in improving the reliability of transmission type targets.

<CIT>discloses a transmission type target in which an intermediate layer whose material is not disclosed is provided as an adhesion promoting layer and interposed between a diamond substrate and a target layer.

<CIT> discloses that thermal stress occurs between a target layer and a diamond substrate due to linear expansion coefficient mismatch in a radiation generating tube equipped with a transmission type target and that separation and cracks occur in the target layer due to the thermal stress. <CIT> discloses a structure in which a target layer is warped toward a diamond substrate so that during operation of a radiation generating tube, the target layer is pressed toward the diamond substrate and separation of the target layer is suppressed thereby.

Even the transmission type targets in which adhesion between a target layer and a diamond substrate is enhanced and which are disclosed in PCT <CIT>and <CIT> sometimes suffer microcracks in the target layer, resulting in fluctuation in radiation output.

<FIG> are respectively a plan view and a cross-sectional view of a transmission type target unit <NUM> that has come to cause fluctuations in radiation output. <FIG> are obtained by microscopic observation and are provided as a reference example. The transmission type target unit <NUM> shown in <FIG> is removed from a radiation generating apparatus not shown in the drawing after a cumulative total of <NUM> times of exposure. <FIG> is a cross-sectional view of the transmission type target unit <NUM> taken along line XB-XB in the plan view of <FIG>.

In the transmission type target unit <NUM> of the reference example, a microcrack <NUM> has branches extending at random within a region that corresponds to the irradiation spot of an electron beam not shown in the drawing, and forms a damaged region <NUM>.

More detailed observation of the microcrack <NUM> within the damaged region <NUM> revealed that, as shown in <FIG>, the microcrack <NUM> propagates from the upper surface to the lower surface of a target layer <NUM>. As shown in <FIG>, in a plane parallel to a target layer <NUM>, a microcrack <NUM> that has closed loops and island regions <NUM> and <NUM>' divided by the closed loops of the microcrack <NUM> are observed. The island regions <NUM> and <NUM>' are regions where electrical connection to an anode member <NUM> is not established.

No microcracks <NUM> were observed in the target layer <NUM> before the transmission type target unit <NUM> is assembled into a radiation generating apparatus not shown in the drawings. In the initial stage after the transmission type target unit <NUM> was assembled into the radiation generating apparatus, no fluctuation in radiation output was observed. Accordingly, generation of the microcrack <NUM> observed in the target layer <NUM> and fluctuation in radiation output are presumably caused by driving of the radiation generating apparatus.

In this specification, a "microcrack" refers to a crack that disrupts the continuity of a target layer when observed with a microscope. Such a crack is observed as a local region where higher light scattering is observed if an optical microscope is used, or as a difference in contrast indicating presence of microscopic and discrete voids if a scanning electron microscope such as a scanning electron microscope (SEM), a scanning transmission electron microscope (STEM), or a scanning transmission microscope (STM) is used.

In the observation results shown in <FIG>, an interlayer is not provided between the target layer <NUM> and a diamond substrate <NUM>. However, in samples in which a titanium interlayer was formed between the target layer <NUM> and the diamond substrate <NUM> also, a similar microcrack <NUM> and a damaged region <NUM> that included an island region <NUM> were sometimes observed.

As described above, the inventors of the present invention have found that as the radiation generating apparatus is driven, microcracks occur in a target layer, performance of the target in maintaining the anode potential at the target layer is degraded, the tube current (anode current) flowing in the target layer <NUM> becomes unstable, and radiation output from the target layer <NUM> fluctuates as the driving history of the radiation generating apparatus accumulates. Further cited prior art documents are <CIT> and <CIT> showing a x-ray generator and x-ray imaging apparatus, and <CIT> showing high efficiency x-ray anode sources.

The present invention provides a highly reliable transmission type target that has advantages of a transmission type target including a diamond substrate and that suffers less from microcracks in a target layer resulting from operation of a radiation generating tube.

The present invention also provides a highly reliable transmission type target with which an anode potential at the target layer is stabilized and radiation output fluctuations are suppressed. The present invention also provides highly reliable radiation generating tube, radiation generating apparatus, and radiography system with which output fluctuations are suppressed.

The present invention in its first aspect provides a transmission type target as specified in the respective claims.

The present invention in its second aspect provides a transmission type target unit as specified in the respective claims.

The present invention in its third aspect provides a radiation generating tube as specified in the respective claims.

The present invention in its fourth aspect provides a radiation generating apparatus as specified in the respective claims.

The present invention in its fifth aspect provides a radiography system as specified in the respective claims.

Preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. The dimensions, materials, shapes, relative positions, and other attributes of the constitutional members described in these embodiments do not limit the scope of the present invention.

<FIG> is a cross-sectional view of an example of a radiation generating tube equipped with a transmission type target according to an embodiment of the invention and <FIG> is a cross-sectional view of an example of a radiation generating apparatus.

<FIG> shows an embodiment of a transmission-type radiation generating tube <NUM> that includes an electron emitting source <NUM> and a transmission type target <NUM> (hereinafter a transmission type target is simply referred to as a "target" in this specification) that faces the electron emitting source <NUM> with a space therebetween.

In this embodiment, an electron beam <NUM> emitted from an electron emitting portion <NUM> of the electron emitting source <NUM> hits a target layer <NUM> of the target <NUM> and a radiation flux <NUM> is generated as a result.

Electrons in the electron beam <NUM> are accelerated by an acceleration electric field between the electron emitting source <NUM> and the target layer <NUM> up to incident energy needed to generate radiation. The acceleration electric field is generated in an inner space <NUM> of the radiation generating tube <NUM> due to a drive circuit <NUM> that outputs a tube voltage Va and a cathode and an anode electrically connected to the drive circuit <NUM>. In other words, the tube voltage Va output from the drive circuit <NUM> is applied between the target layer <NUM> and the electron emitting portion <NUM>.

In this embodiment, as shown in <FIG>, the target <NUM> is constituted by the target layer <NUM> and a diamond substrate <NUM> that supports the target layer <NUM>. A target unit <NUM> at least includes the target <NUM> and an anode member <NUM> and functions as an anode of the radiation generating tube <NUM>.

The details of the target <NUM> and the target unit <NUM> are described later.

A vacuum atmosphere is created in the inner space <NUM> of the radiation generating tube <NUM> in order to guarantee the mean free path of the electron beam <NUM>. The degree of vacuum in the radiation generating tube <NUM> is preferably <NUM>-<NUM> Pa or more and <NUM>-<NUM> Pa or less and, from the viewpoint of the lifetime of the electron emitting source <NUM>, more preferably <NUM>-<NUM> Pa or more and <NUM>-<NUM> Pa or less.

The interior of the radiation generating tube <NUM> can be evacuated by using a vacuum pump not shown in the drawings via an exhaust duct not shown in the drawing and then sealing the exhaust duct. A getter not shown in the drawings may be installed inside the radiation generating tube <NUM> to maintain the degree of vacuum.

The radiation generating tube <NUM> includes an insulation tube <NUM> that serves as a body that electrically insulates between the electron emitting source <NUM> at a cathode potential and the target layer <NUM> at an anode potential. The insulation tube <NUM> is composed of an insulating material such as a glass material or a ceramic material. In this embodiment, the insulation tube <NUM> defines the distance between the electron emitting source <NUM> and the target layer <NUM>.

The radiation generating tube <NUM> may be constituted by an envelope that is airtight and has an atmospheric pressure resisting strength in order to maintain the degree of vacuum. In this embodiment, the envelope is constituted by the insulation tube <NUM>, a cathode equipped with the electron emitting source <NUM>, and an anode equipped with the target unit <NUM>. The electron emitting portion <NUM> and the target layer <NUM> are disposed in the inner space <NUM> or on an inner wall of the envelope.

In this embodiment, the diamond substrate <NUM> serves as a transmission window for allowing radiation generated at the target layer <NUM> to go out of the radiation generating tube <NUM> and also as a part of the envelope.

The electron emitting source <NUM> is positioned to face the target layer <NUM> of the target <NUM>. A hot cathode such as a tungsten filament or an impregnated type cathode or a cold cathode such as a carbon nanotube may be used as the electron emitting source <NUM>. The electron emitting source <NUM> may be equipped with a grid electrode or an electrostatic lens electrode (not shown in the drawings) so as to control the beam diameter and electron current density of the electron beam <NUM>, the ON/OFF timing, and the like.

<FIG> shows an embodiment of a radiation generating apparatus <NUM> that emits a radiation flux <NUM> from a radiation transmitting window <NUM>. In the radiation generating apparatus <NUM> of this embodiment, the radiation generating tube <NUM> that serves as a radiation source and a drive circuit <NUM> configured to drive the radiation generating tube <NUM> are housed in a container <NUM> that has the radiation transmitting window <NUM>.

A tube voltage Va is applied between the target layer <NUM> and the electron emitting portion <NUM> from the drive circuit <NUM> illustrated in <FIG>. An appropriate tube voltage Va may be selected in accordance with the thickness of the target layer <NUM> and the metal type of the target so as to form a radiation generating apparatus <NUM> that generates a desired type of line.

The container <NUM> housing the radiation generating tube <NUM> and the drive circuit <NUM> desirably has a sufficient strength as a housing and a good heat releasing property. For example, the container <NUM> may be composed of a metal material such as brass, iron, or stainless steel.

In this embodiment, an insulating liquid <NUM> fills an extra space <NUM> inside the container <NUM> and surrounding the radiation generating tube <NUM> and the drive circuit <NUM>. The insulating liquid <NUM> has an electrical insulating property, maintains the electrical insulation inside the container <NUM>, and serves as a cooling medium for the radiation generating tube <NUM>. An electrically insulating oil such as a mineral oil, a silicone oil, or a perfluoro-based oil may be used as the insulating liquid <NUM>.

Next, an example of a radiography system <NUM> equipped with a target according to an embodiment of the invention is described with reference to <FIG>.

A system control unit <NUM> is configured to integrally control the radiation generating apparatus <NUM> and a radiation detector <NUM>. The drive circuit <NUM> under control of the system control unit <NUM> outputs various control signals to the radiation generating tube <NUM>. Although the drive circuit <NUM> is housed in the container <NUM> of the radiation generating apparatus <NUM> together with the radiation generating tube <NUM> in this embodiment, the drive circuit <NUM> may alternately be disposed outside the container <NUM>. In response to the control signals output from the drive circuit <NUM>, the state of emission of the radiation flux <NUM> emitted from the radiation generating apparatus <NUM> is controlled.

The radiation flux <NUM> emitted from the radiation generating apparatus <NUM> has its irradiation range adjusted by a collimator unit (not shown) equipped with a movable aperture, is emitted outside the radiation generating apparatus <NUM>, passes through a specimen <NUM>, and detected with the radiation detector <NUM>. The radiation detector <NUM> converts the detected radiation into an image signal and outputs the image signal to a signal processor <NUM>.

The signal processor <NUM> processes the image signal under control of the system control unit <NUM> and outputs the processed image signal to the system control unit <NUM>.

Based on the processed image signal, the system control unit <NUM> outputs to a display device <NUM> a display signal for displaying an image in the display device <NUM>.

The display device <NUM> displays the image based on the display signal so as to show a captured image of the test object <NUM>.

A representative example of the radiation discussed in this specification is an X-ray. The radiation generating apparatus <NUM> and the radiography system according to embodiments of the invention can be used as an X-ray generating unit and an X-ray imaging system. X-ray imaging systems can be used in non-destructive inspection of industrial products and medical diagnosis of human and animals.

Next, a structure and operation state of a basic embodiment of a target of the invention are described with reference to <FIG>.

According to an embodiment shown in <FIG>, a target <NUM> at least includes a target layer <NUM> containing a target metal and a diamond substrate <NUM> that supports the target layer <NUM>. The diamond substrate <NUM> has a region <NUM> constituted by sp3 bonds. The target layer <NUM> has a diamond-substrate-<NUM>-side portion connected to a carbon-containing region <NUM> having sp2 bonds. In this specification, the target <NUM> illustrated in <FIG> is a first embodiment of the invention.

<FIG> shows an operation state of the target <NUM> illustrated in <FIG>. One of the surfaces of the target layer <NUM> is irradiated with an electron beam <NUM> and thus radiation is emitted in a radial manner. The target <NUM> is a transmission type target with which some of components are selected with a collimator or the like (not shown in the drawing) from the components of the radiation emitted from the target layer <NUM> and passed through the diamond substrate <NUM> in the substrate thickness direction, and output as the radiation flux <NUM>.

Either natural diamond or a synthetic diamond prepared by chemical vapor deposition (CVD), a high-temperature high-pressure synthetic process, or the like can be used as the diamond substrate <NUM>. From the viewpoint of controlling the operation properties of the target, synthetic diamond having uniform physical property values such as heat-resistance, heat conductivity, and the like is favored. In particular, from the viewpoint of heat-resistance, synthetic diamond prepared by a high-temperature high-pressure synthetic process is favored.

The thickness of the diamond substrate <NUM> is <NUM> to <NUM> so that both heat conductivity and a radiation transmission property in the substrate thickness direction can be achieved. The diamond substrate <NUM> may be composed of single crystal diamond or polycrystal diamond. From the viewpoint of heat conductivity, single crystal diamond is preferred. The diamond substrate <NUM> may contain <NUM> ppm to <NUM> ppm of nitrogen since impact resistance is improved and thus the portability of a radiation generating apparatus to which the target <NUM> can be applied can be improved.

The target layer <NUM> contains a metal element having a large atomic number, a high melting point, and a high specific gravity as a target metal. From the viewpoint of affinity to the diamond substrate <NUM>, the target metal may be at least one metal selected from the group consisting of tantalum, molybdenum, and tungsten whose carbides exhibit negative standard free energy of formation. The target metal may be a single metal or an alloy, or a metal compound such as a carbide, nitride, or oxynitride of the metal.

The thickness of the target layer <NUM> is within the range of <NUM> or more and <NUM> or less. The lower limit and upper limit of the range of the thickness of the target layer <NUM> are determined from the viewpoints of ensuring radiation output intensity and decreasing interfacial stress. The thickness of the target layer <NUM> may be in the range of <NUM> or more and <NUM> or less.

The thickness of the carbon-containing region <NUM> will now be described. The thickness of the carbon-containing region <NUM> in the thickness direction of the target layer <NUM> is <NUM> times to <NUM> times the thickness of the target layer <NUM>. The lower limit of the thickness of the carbon-containing region <NUM> is determined based on the carbon's stress relaxing effect and is more preferably <NUM> or more. The upper limit of the thickness of the carbon-containing region <NUM> is determined from the viewpoint of heat-resistance of the target <NUM> and is more preferably <NUM> or less.

In this embodiment, the carbon-containing region <NUM> is a region of the diamond substrate <NUM> near the surfaces of the diamond substrate <NUM>. In this region, thermal structural changes occurred so that sp3 bonds are converted into sp2 bonds. In other words, in this embodiment, the carbon-containing region <NUM> is part of the diamond substrate <NUM>.

In this specification, the carbon-containing region <NUM> refers to a region where carbon atoms are bonded to one another through sp2 hybrid orbitals resulting from σ bonds and π bonds and where carbon-carbon double bonds are present. Accordingly, so-called one-and-a-half bonds found in π electron conjugated system and aromatic carbon compounds indicate the state in which <NUM>% of the bonds are double bonds.

Diamond solely constituted by sp3 bonds has high elastic modulus, high hardness, and high thermal conductivity due to its covalently bonded cubic structure. In contrast, graphite, which is an allotrope of diamond, has a layered hexagonal structure and carbon-carbon bonds in each layer form sp2 hybrid orbitals. Within each layer of graphite, carbon atoms are covalently bonded and the bonding strength of between carbon atoms is relatively high. However, layers are bonded to each other through Van Der Waals force and thus bonding strength between carbon atoms of adjacent layers is relatively weak.

Due to such a difference in structure, the Young's modulus of diamond and the Young's modulus of graphite are <NUM> GPa and <NUM> GPa, respectively, and differ from each other by two orders of magnitude. Accordingly, it is possible to decrease the Young's modulus of a portion of the diamond substrate <NUM> for a control width of several to several hundred GPa by controlling the percentage of sp3 bonds to be converted into sp2 bonds in the diamond substrate <NUM>.

The linear expansion coefficient can also be increased by several times from about <NUM> × <NUM>-<NUM> °C-<NUM> of diamond to about <NUM> × <NUM>-<NUM> °C-<NUM> of graphite when the percentage of the sp3 bonds of the diamond substrate <NUM> to be converted to sp2 bonds is controlled. As a result, the carbon-containing region <NUM> exhibits a linear expansion coefficient close to the target metal (for example, <NUM> × <NUM>-<NUM> °C-<NUM> of tungsten) than the region <NUM> constituted by sp3 bonds.

Accordingly, the carbon-containing region <NUM> in this embodiment can be regarded as a stress relaxing region against thermal stress and also as a matching region that reduces the mismatch of linear expansion coefficient.

Next, the relationship between the issue the invention is directed to and the structure of the target layer is described in detail with reference to <FIG>. The "degradation of performance of a target in maintaining the anode potential at the target layer" is strongly correlated with the layer structure of the target.

<FIG> are respectively a schematic diagram of a typical cross-sectional layer structure of a transmission type target <NUM> and a schematic diagram of a typical cross-sectional layer structure of a reflection target <NUM>. <FIG> respectively show a distribution of microcracks <NUM> in the transmission type target <NUM> and a distribution of microcracks <NUM> in the reflection target <NUM>.

As shown in <FIG>, which does not relate to an embodiment of the claimed invention, in the reflection target <NUM> that includes a target layer <NUM> and a substrate <NUM>, radiation generated at the electron penetration depth dp is not output from the opposite side of the target layer <NUM> and is output in a rear direction <NUM> from the surface on which the electron beam <NUM> is incident. Accordingly, the thickness t of the target layer <NUM> of the reflection target <NUM> can be set to a sufficiently large value relative to the electron penetration depth dp without considering the radiation transmittance in the layer thickness direction.

To be more specific, the electron penetration depth dp, which is dependent on tube voltage, is typically within the range of several micrometers to less than <NUM> micrometers. The thickness t of the target layer <NUM> is typically within the range of several millimeters to less than <NUM> millimeters due to thermal capacity design and strength design requirements, etc. The thickness of a heat-generating portion of the target layer <NUM> substantially coincides with the electron penetration depth dp of the electron beam <NUM> relative to the target layer <NUM>. Accordingly, the thickness of the heat-generating portion of the reflection target <NUM> is sufficiently small relative to the thickness t of the target layer <NUM>.

Accordingly, the thermal stress generated in the reflection target <NUM> is concentrated near the surface layer of the target layer <NUM>. The possibility that a microcrack <NUM> would penetrate entirely through the target layer <NUM> in the thickness direction is low. Furthermore, due to design flexibility of the reflection target <NUM>, a supporting member having electrical conductivity such as copper can be disposed on the rear surface of the target layer <NUM>. Accordingly, degradation of the performance of the target in maintaining the anode potential at the target layer <NUM> rarely occurs in the reflection target <NUM>.

In contrast, as shown in <FIG>, radiation generated at an electron penetration depth dp in the target layer <NUM> of the transmission type target <NUM> passes through the target layer <NUM> and the diamond substrate <NUM> and is output in a front direction <NUM>. Accordingly, the thickness t of the target layer <NUM> of the transmission type target <NUM> is set to a thickness substantially equal to the electron penetration depth dp (<NUM> × dp or more and <NUM> × dp or less) while taking into consideration attenuation in the target layer <NUM>.

Accordingly, thermal stress generated in the transmission type target <NUM> is distributed over the entire thickness t of the target layer <NUM>. Thus, as shown in <FIG>, a microcrack <NUM> that penetrates entirely through the layer of the target layer <NUM> in the thickness direction may occur. In such a case, unlike in the case of the reflection target <NUM>, the performance of the target in maintaining the anode potential at the target layer <NUM> is degraded due to the microcrack <NUM> since it is difficult to supply the anode potential from the rear side of the target layer <NUM>.

As discussed above, the phenomenon of "degradation of performance of the target in maintaining the anode potential at the target layer" due to occurrence of the microcrack <NUM> is an issue that is deeply related to the structure of a transmission type target.

Next, thoughts of the inventors regarding the mechanism by which a microcrack <NUM> propagates inside the target layer <NUM> and comes to cause formation of island regions <NUM> and <NUM>' are discussed.

A first factor responsible for generation of a microcrack <NUM> in the target layer <NUM> is thermal stress. Thermal stress is defined by mismatch in linear expansion coefficient between the target layer <NUM> and the diamond substrate <NUM> (Δα = α62 - α61, <NUM> to <NUM> × <NUM>-<NUM> °C-<NUM>) and the temperature increase (<NUM> to <NUM>) during operation of the transmission type target.

The thermal stress has a vector in a direction parallel to the layer surface of the target layer <NUM> and serves as a driving force responsible for causing separation of and generating cracks in the target layer. This is qualitatively disclosed in <CIT>.

A second factor responsible for generation of the microcrack <NUM> in the target layer <NUM> is that the diamond substrate <NUM> has a high elastic modulus.

Diamond has a particularly high elastic modulus (<NUM> GPa at room temperature (<NUM>)) due to its unique crystal structure. The elastic modulus of a high-melting-point metal element selected as the target metal is not as high as that of diamond. Thus, thermal stress in the target tends to be concentrated in the target layer <NUM>.

The elastic modulus (Young's modulus) of representative examples of metal elements that can be used as the target metal is <NUM> for tungsten, <NUM> for molybdenum, and <NUM> for tantalum in terms of GPa at room temperature, <NUM>.

A third factor responsible for generation of a microcrack <NUM> in the target layer <NUM> is that the target layer <NUM> has a polycrystal structure.

A gas-phase deposition process (dry film-forming process) such as sputtering, vapor deposition, or chemical vapor deposition (CVD) is generally employed to form the target layer <NUM>. A target layer <NUM> formed by a gas-phase deposition process frequently takes a columnar structure with columnar crystal grains extending in the layer thickness direction. Crystal grain boundaries contained in the columnar structure intersect the direction in which thermal stress acts. Accordingly, the crystal grains contained in the columnar structure are a factor that limits the mechanical stress of a shear failure mode of the target layer <NUM>.

A fourth factor responsible for generation of the microcrack <NUM> in the target layer <NUM> is that the crystal grains constituting the target layer <NUM> become more coarse as they undergo an increase in temperature.

<FIG> is a conceptual rendering of a grain boundary energy distribution of a polycrystal structure constituting a target layer immediately after completion of deposition. The vertical axis indicates grain boundary energy per unit grain boundary area and the horizontal axis indicates the positions of crystal grains G1 to G9 in the polycrystal structure of the target layer.

The positions of ten peaks found in this grain boundary energy distribution correspond to crystal grain boundaries and the peak-to-peak distance Δx of the energy distribution indicates a crystal grain diameter of each of the crystal grains G1 to G9. At this initial stage, no significant variation is observed in terms of crystal grain diameter and grain boundary energy and no microcracks are present in the target layer.

<FIG> is a conceptual rendering of a grain boundary energy distribution of the target layer that underwent heat history accompanying radiation generating operation. The distribution corresponds to crystal grains G1 to G4 and G6 to G9. <FIG> shows that both crystal grains that are growing and crystal grains that are shrinking are present at this intermediate stage. The crystal grain G5 which was present in the target layer in the initial stage merges into the crystal grain G4 that is growing, and disappears. The grain boundary energy at the grain boundary defined by a growing crystal grain whose grain diameter is increasing is high compared to the initial stage. At this stage, no microcracks are found in the target layer.

<FIG> is a conceptual rendering of the grain boundary energy distribution corresponding to the crystal grains G2, G4, and G8 that have become more coarse as a result of merging with adjacent crystal grains. <FIG> shows that selective coarsening of crystal grains and the increase in grain boundary energy are more notable in this later stage than in the intermediate stage. The increase in grain boundary energy is presumably caused by grains taking in the energy of dislocations, microdefects, etc., at grain boundaries of fine crystal grains that have been merged and disappeared.

Presumably, a microcrack occurs and the grain boundary energy is released as the grain boundary energy is increased and comes to exceed the upper limit Eth up to which a continuous film structure can be maintained.

As discussed above, the mechanism of radiation output fluctuation observed in the aforementioned study results have all of the first to fourth factors correlated to one another although the detailed mechanism is not clear. It is presumed that because of this mechanism, a microcrack <NUM> that extends in the in-plane direction and the thickness direction of the target layer <NUM> occurred as shown by the transmission type target unit <NUM> in <FIG>.

The stress relaxing effect exhibited by the carbon-containing region of this embodiment presumably suppresses generation of a microcrack in the target layer that has undergone heat history because the carbon-containing region apparently increases the upper limit Eth of the grain boundary energy as shown in <FIG>.

The carbon-containing region not only has a stress relaxing effect but also has electrical conductivity attributable to the π electron conjugated system since a particular concentration of sp2 bonds are contained. Thus, the carbon-containing region also has an effect of ensuring the electrical connection to the target layer. Note that from the viewpoint of the electrical conductivity attributable to the π electron conjugated system, <NUM>% or more of the carbon-carbon bonds are preferably sp2 bonds and more preferably <NUM>% or more of the carbon-carbon bonds are sp2 bonds.

Since the carbon-containing region <NUM> has electrical conductivity, the electrical connection between the island region <NUM> and the anode member <NUM> can be ensured in the layer surface direction in the event of occurrence of a microcrack <NUM> in the target layer <NUM> shown in <FIG>.

Since the diamond substrate <NUM>-side of the target layer <NUM> has a portion that is connected to the carbon-containing region <NUM> containing sp2 bonds, a target <NUM> in which the target layer <NUM> is stably maintained at the anode potential can be offered. In other words, since the carbon-containing region <NUM> is positioned between the target layer <NUM> and the region <NUM> of the diamond substrate <NUM>, a target <NUM> in which the target layer <NUM> is stably maintained at the anode potential can be offered.

The material constituting the carbon-containing region <NUM> may be a carbon compound that has sp2 bonds in a cyclic main chain, a linear main chain, or a three-dimensional network main chain or a diamond allotrope having sp2 bonds.

Typical materials used as the allotrope of diamond are graphite, graphene, and glassy carbon solely constituted by sp2 bonds. These materials can be used to form the carbon-containing region <NUM>. However, the material for the carbon-containing region <NUM> need not be solely formed of sp2 bonds.

Examples of the material for the carbon-containing region <NUM> include amorphous carbon having dangling bonds, carbon nanotubes mainly constituted by six-membered rings, and fullerene constituted by five-membered rings and six-membered rings. Other examples of the material constituting the carbon-containing region <NUM> include graphite nanofibers in which fibrous graphene sheets are stacked and diamond-like carbon having three dimensional carbon-carbon atomic network containing sp3 bonds and sp2 bonds.

The carbon compound used as the material for the carbon-containing region <NUM> may be a hydrocarbon compound having functional groups introduced into an allotrope of diamond, a polymer complex having coordinate bonds with a particular metal ion, or an electrically conductive polymer having π electron conjugated system developed on a main chain as long as sp2 bonds are in the main chain.

According to the claimed invention the diamond substrate includes a carbon-containing region containing sp2 bonds and a crystalline carbon region composed of diamond mainly composed of sp3 bonds.

Next, a basic production method of the first embodiment for producing the target <NUM> shown in <FIG> is described with reference to <FIG>.

The production method of the first embodiment includes the following steps.

First, as shown in <FIG>, a diamond substrate <NUM> is prepared. The diamond substrate <NUM> is a polyhedron. Next, as shown in <FIG>, the diamond substrate <NUM> is heat-treated in a deoxidizing atmosphere to convert part of the diamond substrate <NUM> into graphite or the like. In other words, in the heating step of this embodiment, some of sp3 bonds contained in the diamond substrate <NUM> are structurally changed by applying heat and converted into sp2 bonds.

Next, as shown in <FIG>, a target layer <NUM> containing a target metal is formed on the diamond substrate <NUM> having a carbon-containing region <NUM> so as to form a target <NUM>.

The deoxidizing atmosphere employed in this embodiment has a technical significance of suppressing a volume decrease and loss of the diamond substrate <NUM> resulting from combustion of the diamond substrate <NUM>. The deoxidizing atmosphere in the heating step is created by filling the interior of a process chamber with an inert gas such as nitrogen or rare gas or by purging oxygen from the process chamber through evacuation. The heating step may be performed in a vacuum atmosphere or an inert gas atmosphere.

The heating step in this embodiment may be performed at a temperature in the range of <NUM> or more and <NUM> or less from the viewpoints of process time and retention of strength of the diamond substrate. The heating step may be performed while bringing the diamond substrate <NUM> in contact with a metal stage having high heat conductivity or while thermally insulating and supporting the diamond substrate <NUM> with a jig composed of a porous ceramic having low heat conductivity.

The carbon-containing region <NUM> may be distributed over the entire diamond substrate <NUM> but is preferably at least present at high concentration at the surface on which the target layer <NUM> is to be formed.

The inventors have conducted investigations and found that a carbon-containing region <NUM> is preferentially formed at the surfaces of the diamond substrate <NUM> as shown in <FIG> even when the diamond substrate <NUM> is uniformly heated in a deoxidizing atmosphere. This is presumably because the number of defects is larger at the surfaces of the diamond substrate <NUM> than in the inner side of the substrate and thus sp3 bonds in the surface-side portions are preferentially converted into sp2 bonds compared to the inner side.

Modification examples of the production method of the first embodiment will now be described with reference to <FIG>.

The production method illustrated in <FIG> differs from the production method illustrated in <FIG> in that a metal-containing layer forming step (<FIG>) of forming a metal-containing layer <NUM> containing a target metal on the diamond substrate <NUM> is performed before the heating step (<FIG>). This production method has two advantages over the production method shown in <FIG>.

The first advantage is that the metal-containing layer <NUM> has a larger absorbance index in an infrared wavelength region than the diamond substrate <NUM> and heat conductivity lower than that of the diamond substrate. Accordingly, the metal-containing layer <NUM> is selectively heated during the heat treatment. As shown in <FIG>, the carbon-containing region <NUM> is preferentially formed at the target layer <NUM>-side surface of the diamond substrate <NUM> compared to the interior and other surfaces of the diamond substrate <NUM>. As a result, the quantity of heat needed to form the carbon-containing region <NUM> can be decreased, contributing to energy conservation.

The second advantage is that the metal material constituting the metal-containing layer <NUM> is supplied with carbon as carbon atoms contained in the diamond substrate <NUM> diffuse during the heating step shown in <FIG>. The carbon atoms contained in the diamond substrate <NUM> diffuse into the metal-containing layer <NUM> since there is a carbon concentration gradient from the diamond substrate <NUM> toward the metal-containing layer <NUM>. Diffusion of the carbon atoms into the metal-containing layer <NUM> continues until a concentration gradient in a thermal equilibrium state is reached.

During the elementary process of carbon diffusion, sp3 bonds constituting the diamond substrate <NUM> and being present at the surface in contact with the metal-containing layer <NUM> are unbonded and carbon atoms are supplied to the metal-containing layer <NUM>. Since the diamond substrate <NUM> supplies carbon atoms to the metal-containing layer <NUM>, the dangling bonds generated by consumption of carbon atoms form sp2 bonds.

The production method illustrated in <FIG> differs from the production method illustrated in <FIG> in that the diamond substrate <NUM> is subjected to a heating step while performing the step of forming a metal-containing target layer <NUM> on the diamond substrate <NUM>. In this embodiment also, it is possible to preferentially form the carbon-containing region <NUM> at the target layer <NUM>-side of the diamond substrate <NUM>. This production process has the first and second advantages described above over the production method illustrated in <FIG> and also a third advantage of requiring fewer steps.

Modification examples of the target of the first embodiment will now be described with reference to <FIG>.

The modification example shown in <FIG> differs from the first embodiment in that the carbon-containing region <NUM> is discretely formed between the target layer <NUM> and the region <NUM> constituted by sp3 bonds. The carbon-containing region <NUM> may be a continuous layer or a discontinuous layer or may be even dispersed in the diamond substrate <NUM> without forming a particular layer as long as sp2 bonds are contained in the target layer <NUM>-side portion of the substrate.

The target <NUM> shown in <FIG> can be formed by irradiating the metal-containing layer <NUM> with an infrared laser beam in the steps shown in <FIG>, for example.

In the modification example illustrated in <FIG>, which is not part of the claimed invention, a carbon-containing region is formed as a carbon-containing layer <NUM> between the target layer <NUM> and the diamond substrate <NUM>. In this modification example also, the carbon-containing layer <NUM> may be a continuous layer or a discontinuous layer as long as sp2 bonds are contained on the target layer <NUM>-side. In this specification, the modification example illustrated in <FIG> is referred to as a target of a second embodiment.

An example of a method for producing the target <NUM> according to the second embodiment illustrated in <FIG> is illustrated in <FIG>.

First, as shown in <FIG>, a polyhedral diamond substrate <NUM> is prepared. Then as shown in <FIG>, a carbon-containing layer <NUM> containing sp2 bonds such as graphite or glassy carbon is formed on one surface of the diamond substrate <NUM>. Then as shown in <FIG>, a target layer <NUM> is formed on the carbon-containing layer <NUM>. Thus, a target <NUM> of the second embodiment having the carbon-containing layer <NUM> as an intermediate layer can be produced.

Next, a first modification example of the method for producing the target <NUM> according to the second embodiment is described with reference to <FIG>.

The production method of the first modification example differs from the production method illustrated in <FIG> in that a carbon-containing film <NUM> which may contain sp2 bonds or no sp2 bonds is formed and used as a starting material, and a heat treatment is performed to increase the concentration of the sp2 bonds so as to form a carbon-containing layer <NUM>.

In particular, first, as illustrated in <FIG>, a polyhedral diamond substrate <NUM> is prepared. Then in a step shown in <FIG>, a carbon-containing film <NUM> is formed on the diamond substrate <NUM>. In a step shown in <FIG>, at least the carbon-containing film <NUM> is subjected to a heat treatment. Then in a step shown in <FIG>, a target layer <NUM> is formed on the carbon-containing layer <NUM>. As a result, a target <NUM> of the second embodiment that includes the carbon-containing layer <NUM> as an intermediate layer is obtained.

A second modification example of the method for producing the target <NUM> of the second embodiment will now be described with reference to <FIG>.

The production method of the second modification example differs from the production method illustrated in <FIG> in that a step of converting a carbon-containing film <NUM> into a carbon-containing layer <NUM> illustrated in <FIG> is performed after a step of forming a target layer <NUM> on the carbon-containing film <NUM> illustrated in <FIG>.

A third modification example of the method for producing the target <NUM> of the second embodiment will now be described with reference to <FIG>.

The production method of the third modification example differs from the production method illustrated in <FIG> in that a metal-containing layer <NUM> containing a target metal is formed as shown in <FIG> and then a step of converting a carbon-containing film <NUM> into a carbon-containing layer <NUM> by a heat treatment and a step of converting the metal-containing layer <NUM> into a target layer <NUM> are performed simultaneously as shown in <FIG>.

The basic method for producing the target <NUM> of the second embodiment illustrated in <FIG> is advantageous over the first to third modification examples in that the method involves fewer steps. The second and third modification examples are advantageous over the basic method and the first modification example in that the second and third advantages described above are achieved because the carbon-containing layer <NUM> containing sp2 bonds is formed by a heat treatment after formation of the target layer <NUM> or the metal-containing layer <NUM>.

The target of the second embodiment is disadvantageous in terms of production compared to the target of the first embodiment in that a step of forming an independent intermediate layer is necessary. However, the target of the second embodiment is advantageous in terms of production compared to the target of the first embodiment in that there is no need to perform a high-temperature treatment in a deoxidizing atmosphere needed to convert the diamond substrate <NUM>. Whether to choose the first embodiment or the second embodiment can be appropriately determined by considering the consistency with other production steps.

As shown in <FIG> and <FIG>, in both the first embodiment and the second embodiment, the target production method includes a target layer formation step of forming a target layer <NUM> on one surface of a diamond substrate <NUM>. The target production method also includes a sp2 bond formation step of forming a carbon-containing region <NUM> having sp2 bonds and being in contact with a diamond substrate <NUM>-side of the target layer <NUM>.

As shown in <FIG>, the target layer formation step of the target production method of the first embodiment includes a step of forming a metal layer (the target layer <NUM> or metal-containing layer <NUM>) containing a target metal on one surface of a diamond substrate <NUM>. The steps illustrated in <FIG> each correspond to the step of forming a metal layer (the target layer <NUM> or metal-containing layer <NUM>).

Moreover, as shown in <FIG>, the sp2 bond formation step of the target production method of the first embodiment includes a heating step of heating the diamond substrate and converting at least some of sp3 bonds contained in the surface of the diamond substrate into sp2 bonds. The steps illustrated in <FIG> each correspond to the heating step.

As shown in <FIG>, the sp2 bond formation step of the target production method of the second embodiment includes forming a carbon-containing layer <NUM> containing sp2 bonds on one surface of the diamond substrate <NUM>. The step of forming the carbon-containing layer <NUM> containing sp2 bonds corresponds to the step illustrated in <FIG>.

As shown in <FIG>, the sp2 bond formation step of the target production method of the second embodiment includes a step of forming a carbon-containing film <NUM> having sp3 bonds on one surface of a diamond substrate <NUM> and a step of heating at least the carbon-containing film <NUM> so as to convert the carbon-containing film <NUM> into a carbon-containing layer <NUM> having sp2 bonds.

Steps illustrated in <FIG> each correspond to the step of forming the carbon-containing film <NUM> having sp3 bonds. Steps illustrated in <FIG> each correspond to the step of forming the carbon-containing film <NUM> into the carbon-containing layer <NUM> having sp2 bonds.

Next, an embodiment in which a target <NUM> is used in a target unit <NUM> mounted as an anode into a radiation generating apparatus <NUM> shown in <FIG> is described with reference to <FIG>.

<FIG> shows a transmission type target unit <NUM> (hereinafter simply referred to as a target unit <NUM>) equipped with the target <NUM> shown in <FIG> in a hollow portion of a cylindrical anode member <NUM>. The inner peripheral portion of the hollow portion of the target unit <NUM> is connected to the outer peripheral portion of the target <NUM> via brazing <NUM>. The brazing <NUM> may be an alloy having a low melting point such as an alloy containing tin, silver, or the like. In this embodiment, in the outer peripheral portion of the target <NUM>, the periphery of the diamond substrate <NUM> overlaps the periphery of the target layer <NUM>.

In the target unit <NUM>, the brazing <NUM> serves as a bonding material that holds the target <NUM> and is responsible for establishing an electrical connection between the anode member <NUM> and the target layer <NUM>.

<FIG> is a cross-sectional view of the target unit <NUM> shown in the plan view of <FIG> taken along line IVE-IVE in <FIG>. According to the structure of this embodiment, the sp2 bonds exhibit electric conductivity and the reliability of the electrical connection between the target <NUM> and the anode member <NUM> is enhanced.

The anode member <NUM> is composed of a material having a large specific gravity. Thus the anode member <NUM> can define radiation output angle (radiation angle) in a required direction and can block the radiation from going out in undesirable directions as shown in <FIG>.

From the viewpoint of further size reduction, a metal element having a specific absorption edge energy may be appropriately selected as the material for forming the anode member <NUM> on the basis of a characteristic X-ray energy of the radiation generated from the target layer <NUM>.

Specific examples of such a metal element for forming the anode member <NUM> include copper, silver, molybdenum, tantalum, tungsten, KOVAR (US registered trademark, Ni-Co-Fe alloy produced by CRS Holdings Inc. ), MONEL (Ni-Cu-Fe alloy, US registered trademark co-owned by Special Metals Corporation and Huntington Alloys Corporation), and stainless steel. The same metal element as the target metal contained in the target layer <NUM> may also be contained in the anode member <NUM>.

The scope of the invention disclosed in this specification also encompasses a multilayered target layer <NUM> formed of two or more layers and an embodiment in which the carbon-containing region has sp1 bonds (carbon-carbon triple bonds) as long as the effect brought by the carbon-containing region having sp2 bonds is obtained.

A radiation generating apparatus equipped with a target according to an embodiment of the invention was prepared by the procedure described below, and operated to evaluate output stability.

<FIG> is a schematic diagram of a target <NUM> prepared in Example <NUM>. The procedure of preparing the target <NUM> of Example <NUM> is shown in <FIG>. A section specimen <NUM> of the target <NUM> of this example is shown in <FIG> and analyzed regions <NUM> and <NUM> in the section specimen <NUM> are shown in <FIG> shows a profile of the results of electron energy loss spectroscopy. <FIG> shows a calibration graph used to identify the normalized sp2 bond concentration Csp2.

<FIG> shows a schematic structure of a radiation generating tube <NUM> loaded with the target <NUM> of this example. <FIG> shows a radiation generating apparatus <NUM> loaded with the radiation generating tube <NUM>. The evaluation system used to evaluate stability of radiation output of the radiation generating apparatus <NUM> of this example is shown in <FIG>.

First, as shown in <FIG>, a diamond substrate <NUM> having a diameter of <NUM> and a thickness of <NUM> and composed of single crystal diamond was prepared. The diamond substrate <NUM> was cleaned with a UV ozone ashing device to remove residual organic matter on the surfaces of the diamond substrate <NUM>.

Next, as shown in <FIG>, a metal-containing layer <NUM> having a thickness of <NUM> and composed of tungsten was sputter-deposited on one of the clean surfaces of the diamond substrate <NUM> by using an argon gas as a carrier gas and a sintered tungsten target as a sputtering target.

Then a stack of the metal-containing layer <NUM> and the diamond substrate <NUM> was placed in an image furnace (not shown) by using a ceramic holder jig (not shown) composed of alumina. Then a vacuum atmosphere was created in the interior of the image furnace. The stack was irradiated with an infrared ray for <NUM> hours so that the temperature of the stack was <NUM> so as to conduct a heating step. As a result, a target <NUM> of Example <NUM> was obtained.

The thickness of the target layer <NUM> of the target <NUM> came out to be <NUM>.

Visual observation of the target <NUM> of this example revealed that brown to black shaded regions were distributed mainly near the surfaces of the diamond substrate <NUM>, as shown in <FIG>.

Next, as shown in <FIG>, a section specimen <NUM> was cut out from the target <NUM> by mechanical polishing and focused ion beam (FIB) processing. The section specimen <NUM> was taken to include a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the target layer <NUM> side and a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the diamond substrate side.

A region near the border between the target layer <NUM> and the diamond substrate in the section specimen <NUM> was observed with a scanning transmission electron microscope (STEM). From the image contrast, a region constituted by elements having a specific gravity smaller than that of the target layer <NUM> was found near the target layer <NUM> but within the diamond substrate <NUM>. This region was assumed to be the carbon-containing region <NUM>.

The assumed carbon-containing region <NUM> exhibited a halo pattern when measured with an electron beam diffractometer (STEM-ED) attached to STEM and no lattice fringes were observed in a high-resolution observation mode, thereby revealing that the carbon-containing region <NUM> was an amorphous phase. EDX analysis attached to STEM found that the carbon-containing region <NUM> is a region mainly composed of carbon.

In order to identify the carbon-containing region, which is a feature of the invention, STEM-EELS evaluation was performed by using an electron loss energy spectrometer attached to STEM.

<FIG> shows the EELS profile obtained by the STEM-EELS analysis. The horizontal axis indicates the electron loss energy and the vertical axis indicates the intensity I of an EELS signal. The signal intensity I<NUM> at an electron loss energy of <NUM> eV corresponds to the concentration of π bonds. The signal intensity I<NUM> at an electron loss energy of <NUM> eV corresponds to the concentration of σ bonds.

In EELS analysis, the sp2 bonds which are carbon-carbon double bonds are detected as the EELS signal at <NUM> eV and the EELS signal at <NUM> eV (I<NUM> and I<NUM>) attributable to σ bonds and π bonds. The sp3 bonds which are carbon-carbon single bonds are detected as the EELS signal (I<NUM>) at <NUM> eV attributable to σ bonds.

It can be qualitatively understood from the profile shown in <FIG> that in the analyzed region <NUM>, the concentration of sp2 bonds constituted by π bonds and σ bonds is significantly high and, in the analyzed region <NUM>, the concentration of sp3 bonds constituted by σ bonds is significantly high.

An EELS signal was observed at <NUM> eV in the analyzed region <NUM>. However this EELS signal is presumably attributable to unavoidable hydrocarbons that had been physically adsorbed from the analytic atmosphere or polymeric carbon resulting from irradiation with the electron beam. In other words, it is assumed that the EELS signal detected at <NUM> eV possibly indicates real carbon bonds of the specimen superimposed with background signals irrelevant to the specimen.

In order to confirm the influence of the background signals, synthetic diamond not subjected to a heat treatment was analyzed as a reference specimen through EELS. The EELS signal having about the same intensity as that detected in the analyzed region <NUM> was detected at <NUM> eV. It was confirmed that background signals (noise component) inherent to the measurement system are dominant in the EELS signals detected at <NUM> eV from the analytical position <NUM> and the diamond reference specimen.

In general, the detection sensitivity differs for every characteristic energy of electron loss energy. In particular, depending on conditions inherent to the measurement system and conditions of measurement, π bond concentration/I<NUM> signal intensity (= π bond detection sensitivity) does not match σ bond concentration/I<NUM> signal intensity (= σ bond detection sensitivity). The EELS profile shown in <FIG> is raw data and at least affected by this.

In order to eliminate the influence of these errors related to identification of the sp2 bond concentration, the inventors of the present application performed correction as below so as to eliminate the influence of background signals and the dependency of the detection sensitivity on the characteristic energy so as to identify the value of sp2 bond concentration.

To be more specific, single crystal synthetic diamond not subjected to a heat treatment and single crystal graphite (highly ordered pyrolytic graphite or HOPG) not subjected to a heat treatment were used as the reference specimens and a calibration line shown in <FIG> was drawn by using the two reference specimens to eliminate the error caused by background signals.

The error attributable to the dependency of the detection sensitivity on characteristic energy was eliminated by using the EELS signal intensity ratio I<NUM>/I<NUM>, which is the ratio obtained by dividing the EELS signal intensity I<NUM> detected at <NUM> eV by the EELS signal intensity I<NUM> detected at <NUM> eV.

Then normalized sp2 bond concentration Csp2 obtained by eliminating the aforementioned two types of errors was defined by using formula (<NUM>) shown below. <NUM>] <MAT>.

As a result, it was found that the normalized sp2 bond concentrations in the analyzed region <NUM> and the analyzed region <NUM> were <NUM>% and <NUM>%, respectively.

As discussed above, based on the results of the EELS analysis of the diamond substrate <NUM> of the target <NUM> of this example and other composition-structure analysis, the analyzed region <NUM> was identified to be composed of amorphous carbon mainly containing sp2 bonds and the analyzed region <NUM> was identified to be composed of diamond mainly composed of sp3 bonds.

The analyzed regions <NUM> and <NUM> were set so that they respectively lay in the assumed carbon-containing region <NUM> and the region <NUM> from which lattice fringes caused by crystallinity of diamond were observed in a high resolution observation mode as shown in <FIG>. STEM-EELS line analysis conducted in the thickness direction of the target layer <NUM> revealed that the assumed carbon-containing region <NUM> was distributed by having a thickness of <NUM> to <NUM>. The STEM-EELS line analysis was performed at <NUM> intervals and the detection intervals were appropriately decreased to several nanometers in the area near the border between the assumed carbon-containing region <NUM> and the other structure.

Next, a radiation generating tube <NUM> equipped with the target <NUM> prepared in Example <NUM> was produced by the following procedure. First, as shown in <FIG>, the target <NUM> was brazed onto an anode member <NUM> composed of copper to form a target unit <NUM> that served as an anode. Then an electron emitting source <NUM> including an impregnated type electron gun equipped with an electron emitting portion <NUM> composed of lanthanum boride (LaB<NUM>) was brazed onto a cathode member (not shown) composed of Kovar so as to prepare a cathode.

Then the cathode and the anode were respectively brazed onto two openings of an insulation tube <NUM> composed of alumina (<FIG>) so as to form an envelope. An inner space <NUM> of the envelope was evacuated with an evacuator (not shown) until a degree of vacuum of <NUM> × <NUM>-<NUM> Pa was reached. As a result, a radiation generating tube <NUM> shown in <FIG> was produced.

A drive circuit <NUM> was electrically connected to the cathode and the anode of the radiation generating tube <NUM>. The radiation generating tube <NUM> and the drive circuit <NUM> were housed in an extra space <NUM> of a container <NUM>. As a result, a radiation generating apparatus <NUM> shown in <FIG> was obtained.

An evaluation system <NUM> shown in <FIG> was prepared to evaluate the drive stability of the radiation generating apparatus <NUM>. In the evaluation system <NUM>, a dosimeter <NUM> was placed in front of a radiation transmitting window <NUM> of the radiation generating apparatus <NUM> and disposed at a position <NUM> from the radiation transmitting window <NUM>. The dosimeter <NUM> was connected to the drive circuit <NUM> via a measurement control unit <NUM> and was capable of measuring the intensity of the output radiation from the radiation generating apparatus <NUM>.

The drive conditions for evaluation of drive stability were as follows. The tube voltage of the radiation generating tube <NUM> was +<NUM> kV, the current density of the electron beam irradiating the target layer <NUM> was <NUM> mA/mm<NUM>, and pulsed drive of repeating an electron irradiation period of <NUM> seconds and a non-irradiation period of <NUM> seconds was performed. An average value of output intensity was determined from the intensities detected during <NUM> second at the middle of the electron irradiation period and the obtained average value was assumed to be the detected radiation output intensity.

The stability evaluation of the radiation output intensity was performed in terms of a retention rate determined by normalizing the radiation output intensity after elapse of <NUM> hours from the start of the radiation output by using the initial radiation output intensity.

In evaluating the stability of the radiation output intensity, the tube current flowing from the target layer <NUM> to a ground electrode <NUM> was measured and constant-current control was performed by using a negative feedback circuit (not shown) so that the fluctuation in the electron current density irradiating the target layer <NUM> was within <NUM>%. During evaluation of drive stability of the radiation generating apparatus <NUM>, a discharge counter <NUM> monitored that the radiation generating apparatus <NUM> was driven stably without causing discharge.

The retention rate of the radiation output of the radiation generating apparatus <NUM> of this example was <NUM>. This confirmed that the radiation generating apparatus <NUM> equipped with the target <NUM> of this example does not show significant fluctuations in radiation output even after long hours of driving history and stable radiation output intensity is obtained. The radiation generating apparatus <NUM> of this example that underwent the test of evaluating stability of radiation output intensity was disassembled to take out the target unit <NUM>. No microcrack were observed in the target layer <NUM> of the target unit <NUM>.

In Example <NUM>, a radiation generating apparatus <NUM> was produced as in Example <NUM> except that the steps shown in <FIG> were performed to prepare a target <NUM>. The stability of radiation output of the radiation generating apparatus <NUM> was evaluated.

First, as in Example <NUM>, surfaces of a diamond substrate <NUM> were washed as shown in <FIG>. Then as shown in <FIG>, the diamond substrate <NUM> was placed in an image furnace (not shown) by using a ceramic holder jig (not shown) composed of alumina. Then a vacuum atmosphere was created in the interior of the image furnace. The diamond substrate <NUM> was irradiated with an infrared ray for <NUM> hours so that the temperature of the diamond substrate <NUM> was <NUM> so as to conduct a heating step.

Next, a target layer <NUM> composed of tungsten and having a thickness of <NUM> was sputter-deposited on one of the surfaces of the diamond substrate <NUM> that underwent the heat treatment as shown in <FIG>. As a result, a target <NUM> of Example <NUM> was obtained.

Visual observation of this example revealed that grown to black shaded regions were distributed mainly near the surfaces of the diamond substrate <NUM>, as shown in <FIG>.

As in Example <NUM>, a section specimen <NUM> was cut out from the target <NUM> of Example <NUM> by mechanical polishing and FIB processing. The section specimen <NUM> was taken to include a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the target layer <NUM> side and a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the diamond substrate side as shown in <FIG>.

A region near the border between the target layer <NUM> and the diamond substrate in the section specimen <NUM> was observed with a scanning transmission electron microscope (STEM). From the image contrast, a region constituted by elements having a specific gravity smaller than that of the target layer <NUM> was found near the target layer <NUM> but within the diamond substrate <NUM>. This region was assumed to be a carbon-containing region <NUM>. STEM-EELS line analysis of the target layer <NUM> was performed in the thickness direction and it was confirmed that the assumed carbon-containing region <NUM> was distributed by having a thickness of <NUM> to <NUM>.

Next, STEM-EELS evaluation was performed by using an electron loss energy spectrometer attached to STEM in order to identify the carbon-containing region featured in the present invention.

As a result, it was found that the normalized sp2 bond concentration was <NUM>% in the detection region corresponding to the carbon-containing region and that the normalized sp2 bond concentration was <NUM>% in the detection region corresponding to the diamond substrate <NUM>.

A radiation generating tube <NUM> and a radiation generating apparatus <NUM> were produced as in Example <NUM> but by using the target <NUM> prepared in Example <NUM>. The radiation generating apparatus <NUM> was loaded into the evaluation system <NUM> for measuring the drive stability shown in <FIG>.

The radiation output retention rate of the radiation generating apparatus <NUM> of this example was <NUM>. This confirmed that the radiation generating apparatus <NUM> equipped with the target <NUM> of this example does not show significant fluctuations in radiation output even after long hours of driving history and stable radiation output intensity is obtained. The radiation generating apparatus <NUM> of this example that underwent the test of evaluating stability of radiation output intensity was disassembled to take out the target unit <NUM>. No microcrack were observed in the target layer <NUM> of the target unit <NUM>.

A radiation generating apparatus <NUM> was produced as in Example <NUM> except that the steps illustrated in <FIG> were performed to prepare a target <NUM> in Example <NUM>. The radiation output stability of the radiation generating apparatus <NUM> was evaluated.

First, as in Example <NUM>, surfaces of a diamond substrate <NUM> were washed as shown in <FIG>. Then as shown in <FIG>, a target layer <NUM> composed of tungsten and having a thickness of <NUM> was sputter-deposited on one of the surfaces of the diamond substrate <NUM> so as to form a stack.

Next, the stack was placed in a chamber (not shown) and the interior of the chamber was purged with nitrogen gas. The stack's surface on which the target layer <NUM> was formed was irradiated with an infrared ray having a wavelength of <NUM> via a quartz window of the chamber by using a semiconductor laser beam source. Irradiation with the laser beam was performed <NUM> times by pulsed driving using a Q switch. As a result, a target <NUM> in which a portion of the diamond substrate <NUM> near the interface with the target layer <NUM> turned brown to black in color was obtained as shown in <FIG>.

As shown in <FIG>, a section specimen <NUM> was cut out from the target <NUM> of Example <NUM> by mechanical polishing and FIB processing. The section specimen <NUM> was taken to include a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the target layer <NUM> side and a region that extended from the lower edge of the target layer <NUM> by <NUM> toward the diamond substrate side.

The radiation output retention rate of the radiation generating apparatus <NUM> of this example was <NUM>. This confirmed that the radiation generating apparatus <NUM> equipped with the target <NUM> of this example does not show significant fluctuations in radiation output even after long hours of driving history and stable radiation output intensity is obtained. The radiation generating apparatus <NUM> of this example that underwent the test of evaluating stability of radiation output intensity was disassembled to take out the target unit <NUM>. No microcrack were observed in the target layer <NUM> of the target unit <NUM>. EXAMPLE <NUM> not being part of the claimed invention.

A radiation generating apparatus <NUM> was produced as in Example <NUM> except that the steps illustrated in <FIG> were performed to prepare a target <NUM> of Example <NUM>. The radiation output stability of the radiation generating apparatus <NUM> was evaluated.

First, as in Example <NUM>, surfaces of a diamond substrate <NUM> were washed as shown in <FIG>. Then as shown in <FIG>, a carbon-containing film <NUM> composed of diamond-like carbon and having a thickness of <NUM> was formed by a CVD device on one of the surfaces of the diamond substrate <NUM> so as to form a stack.

Next, the stack was placed in an image furnace (not shown) by using a ceramic holder jig (not shown) composed of alumina. Then a vacuum atmosphere was created in the interior of the image furnace. The stack was irradiated with an infrared ray for <NUM> hours so that the temperature of the stack was <NUM> so as to conduct a heating step. As a result, the carbon-containing film <NUM> was transformed into a carbon-containing layer <NUM> including a sp2 bond. Next, as shown in <FIG>, a target layer <NUM> composed of tungsten and having a thickness of <NUM> was sputter-deposited on the carbon-containing layer <NUM>. As a result, a target <NUM> of Example <NUM> was obtained.

A region near the border between the target layer <NUM> and the diamond substrate in the section specimen <NUM> was observed with a scanning transmission electron microscope (STEM). As a result, formation of a carbon-containing layer <NUM> constituted by elements having a smaller specific gravity than the target layer <NUM> was confirmed between the target layer <NUM> and the diamond substrate <NUM>. STEM-EELS line analysis of the target layer <NUM> was performed in the thickness direction and it was confirmed that the carbon-containing layer <NUM> was distributed by having a thickness of <NUM> to <NUM>.

As a result, it was found that the normalized sp2 bond concentration was <NUM>% in the detection region corresponding to the carbon-containing region and that the normalized sp2 bond concentration was <NUM>% in the detection region corresponding to the diamond substrate.

In Example <NUM>, the radiation generating apparatus <NUM> prepared in Example <NUM> was used to produce a radiography system <NUM> shown in <FIG>.

An X-ray image having a high S/N ratio was obtained from the imaging apparatus <NUM> of Example <NUM> since the radiation generating apparatus <NUM> in which fluctuation of radiation output was suppressed was used therein.

It should be noted here that in Examples <NUM> to <NUM>, identification of the carbon-containing region and identification of the normalized sp2 concentration were performed by EELS. However, the identification technique is not limited to EELS and any other analysis technique such as Raman spectrometry or X-ray photoelectron spectrometry capable of separating carbon-carbon bonds can be employed.

According to the present invention, it becomes possible to provide a highly reliable transmission type target with which stress occurring at the interface between a diamond substrate and a target layer is relaxed and generation of microcracks is suppressed even when the target is operated at high temperature.

Furthermore, it becomes possible to assuredly provide an anode potential to the target layer even when operation at high temperature is conducted and thus a highly reliable radiation generating tube with which radiation output fluctuations are suppressed can be obtained. Moreover, a radiation generating apparatus and a radiography system each having a highly reliable radiation emission tube can be provided.

Claim 1:
A transmission type target (<NUM>) comprising:
a target layer (<NUM>) containing a target metal configured to generate an X-ray upon irradiation with electrons and having a thickness within a range of <NUM> or more and <NUM> or less; and
a diamond substrate (<NUM>, <NUM>) including a carbon-containing region (<NUM>, <NUM>) containing sp2 bonds and configured to support the target layer and allow the X-ray generated in the target layer being to be passed through and a crystalline carbon region (<NUM>, <NUM>) composed of diamond mainly composed of sp3 bonds, wherein an amount of the sp2 bonds contained in the carbon-containing region (<NUM>, <NUM>) corresponds to at least <NUM>% of an amount of carbon-carbon bonds included in the carbon-containing region,
wherein the carbon-containing region has a thickness <NUM> times to <NUM> times a thickness of the target layer in the thickness direction of the target layer; wherein a thickness of the diamond substrate is <NUM> to <NUM>, and
wherein the carbon containing region is located on a target layer side of the diamond substrate configured to be in contact with the target layer.