JP2004146158A - Field emission type x-ray source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray source in which such difficulty that operation is necessary in the ultra-high vacuum is overcome in the X-ray source, which has a simple structure without filaments and has a field emission type electron emission source having advantages such as that high quality X-ray emission can be made. <P>SOLUTION: A negative electrode 12 selects gas adsorption materials of Pd or the like as a substrate to become a growth parent of a carbon nanofiber 12b and it constitutes a grouping having a structure that a microcrystal 12c of the gas adsorption materials is added or built in the tip part of the carbon nanofiber 12b on the substrate. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a field emission (FE) type X-ray source that emits high-energy electrons emitted by an electric field to an anode to generate controlled X-rays. This field emission type X-ray source is applicable to an X-ray imaging (X-ray) apparatus, a high-speed imaging CT (computer tomography) apparatus, an image exclusion apparatus, and the like in a medical field, and an internal invasion in a general industrial field. The present invention is applicable to a type industrial inspection device, a flat type X-ray device, and the like.
[0002]
[Prior art]
An X-ray source utilizing the phenomenon of emitting X-rays by a braking action when high-energy electrons enter a metal target is widely used in the medical field, such as an X-ray imaging apparatus and a high-speed imaging CT. An X-ray source that emits high-energy X-rays of 100 keV or more is utilized in industrial nondestructive inspection devices by utilizing its excellent material permeation characteristics. Furthermore, an X-ray source that emits X-rays of low and medium energy up to several tens of keV is used in an elemental analyzer by X-ray fluorescence analysis. Utilizing the effects of X-ray irradiation on the material structure and biological function, the X-ray source is also used in process equipment such as killing and sterilizing equipment for medical equipment and curing equipment for polymer materials. Not only that, in the latest medical field, it has been attempted to use the device for treating a malignant tumor using X-ray irradiation. As described above, X-ray sources have become widespread in a wide range of fields such as medical diagnosis and treatment, industrial inspection, analysis, and process processing. Creation is also expected.
[0003]
In a conventional X-ray source, as an electron emission source (electron emitter) for emitting the first electrons, a tungsten filament is heated to a high temperature of 2000 ° C. by an electric current, and the electrons are emitted by this. It is common to use a release type (Thermionic Emission, TE). FIG. 12 shows a configuration of a conventional X-ray source using the thermionic electron emission source (for example, see Non-Patent Document 1). This thermionic emission type X-ray source includes an X-ray tube 80, a high voltage power supply 91, and a filament power supply 92. The X-ray tube 80 has a vacuum vessel 81 whose inside is maintained at a high vacuum and in which a part of the X-ray extraction window 81a is formed. In a vacuum vessel 81, a filament 82 as an electron emission source and an anode 83 electrically insulated from the filament 82 are provided, and a solid cathode 84 is provided between the filament 82 and the anode 83. Have been. The filament 82 is connected to a filament power supply 92 for supplying a heating current. A high-voltage power supply 91 for applying a high-voltage power supply current for accelerating electrons e between the filament 82 and the solid cathode 84 and the anode 83 is connected between the filament 82 and the solid cathode 84 and the anode 83.
[0004]
In this thermoelectron emission type X-ray source, the emitted electrons e are controlled by the amount of heating current supplied to the filament 82 by the filament power supply 92. The electrons e emitted from the filament 82 are accelerated and focused by a voltage applied between the solid cathode 84 for electric field control and the anode 83 at the same potential as the filament 82, and are incident on the anode 83. . Thus, the X-rays x are emitted by the braking action on the surface of the anode 83, and the X-rays x are emitted from the X-ray extraction window 81a.
[0005]
On the other hand, a configuration of a general X-ray source using a field emission type electron emission source is shown in FIG. 13 (for example, see Non-Patent Document 2). This field emission type X-ray source includes an X-ray tube 85, a high voltage power supply 93, and a grid power supply 94. The X-ray tube 85 also has a vacuum vessel 86 in which the inside is maintained at a high vacuum and a part of which is formed with an X-ray extraction window 86a. A cathode 87 as an electron emission source and an anode 88 electrically insulated from the cathode 87 are provided in the vacuum vessel 86, and a cathode grid 89 is provided between the cathode 87 and the anode 88. Have been. The cathode 87 has a very sharp tip in order to obtain a high electric field. The cathode 87 and the cathode grid 89 are connected to a grid power supply 94, and the cathode grid 89 and the anode 88 are connected to a high-voltage power supply 93 that applies a high-voltage power supply current for accelerating electrons.
[0006]
In this field emission type X-ray source, emitted electrons e are controlled by a voltage applied between the cathode 87 and the cathode grid 89. Then, since the electrons e are emitted only from a limited area at the tip of the cathode 87, an electron beam having a very good convergence and an X-ray x having a good convergence can be obtained.
[0007]
[Non-patent document 1]
"X-ray imaging", edited by Takeshi Iinuma and Yukio Tateno, Corona Co., March 2001, p. Fifteen
[Non-patent document 2]
Hiroshi Sugie, Masayuki Okumura, Valerieux Philip, Kazuhisa Iwata, Kazumi Takahashi, Fumio Okuyama, "Applied Physics Letters, Vol. 78," American Physical Society, April 23, 2001, P.S. 2578-2580
[0008]
[Problems to be solved by the invention]
However, in the above-mentioned conventional thermionic emission type X-ray source, as shown in FIG. 12, a large filament power supply 92 for supplying a large current to the filament 82 is required, and the filament 82 is thermally worn. (1 if time), the need to adjust the optimal amount of current for heating with the deterioration of the filament 82, and the local electric field due to the geometrical shape of the filament 82. Non-uniformity occurs, and it is difficult to obtain a spatially uniform emission of the electron beam. As a result, the point of incidence of the electron beam on the surface of the anode 83 cannot be completely focused, and as a result, an ideal point light source There are problems such as difficulty in using it as an X-ray source. For this reason, it is impossible to place such a thermionic X-ray source in a narrow space between fan blades of, for example, a jet engine (this is also necessary to check the pain of the fan). . Although there is an urgent need for a small, lightweight and portable X-ray source regardless of medical or industrial use, a thermionic emission type X-ray source cannot meet this demand.
[0009]
In this regard, in the field emission type X-ray source, since the field emission type electron emission source is provided, the filament power supply 92 required for the thermionic emission type X-ray source becomes unnecessary, and thus the electron emitting electrons are emitted. By choosing the geometry of the surface of the emission source, electron beams can be emitted with good convergence. For this reason, the field emission type X-ray source can obtain high quality X-rays close to those emitted from an ideal point light source, and can solve the problems of the thermionic emission type X-ray source. In order to reduce the size of the X-ray source, it is essential to make the electron emission source much smaller. The basic principle that can solve this is field emission.
[0010]
However, in the field emission type X-ray source, as shown in FIG. 13, since a thermal and physical load is concentrated on the sharp portion of the cathode 87, it is extremely difficult to perform a stable operation for a long time. I have.
[0011]
That is, in the X-ray source including the general thermionic emission type electron emission source shown in FIG. ^-5 At a vacuum degree of the order of Pa, at this degree of vacuum, the surface shape of the filament 82 is microscopically changed by sputtering caused by the incidence of a large amount of ions. In the thermionic emission type electron source, which depends on heat emission, stable operation can be maintained almost without being affected by the operation time of about several tens of hours. If so, it is easy to adopt a sealed tube structure, which is the reason why many sealed X-ray tubes are constituted by hot cathode type electron emission sources.
[0012]
On the other hand, in the field emission type electron emission source shown in FIG. 13, since the surface geometry of the electron emission source greatly affects the electron emission characteristics, at least 10 ^-6 Since it is necessary to operate at a vacuum degree of the order of Pa or less, it is very difficult to adopt a sealed structure, so that the field emission type electron emission source is an X-ray source, especially a sealed X-ray tube 85. There are almost no examples of application to electron microscopes, and many are limited to application to electron microscopes equipped with a vacuum pump for maintaining an ultra-high vacuum. Field emission is simply an electron tunnel, which is a quantum mechanical phenomenon. Since it is a typical surface phenomenon, it depends exponentially on the chemical state of the surface of the electron emission source (absence and degree of adsorption). I do. That is, the field emission type electron emission source should originally be operated in an ultra-high vacuum (UHV) which can keep the chemical state of the surface of the electron emission source unchanged. An enormous amount of gas molecules remains, and thus normal vacuum (<10) where adsorption and desorption of gas molecules are repeated on the surface of the electron emission source. ^-5 It is extremely difficult to operate a field emission type electron emission source stably for a long time in Pa). However, it is technically impossible to seal off the X-ray tube 85 with UHV, and it is only possible to realize a field emission type X-ray source for a long time (for example, 100 hours or more) in a normal vacuum. The permissible fluctuation range of the electron flow in the X-ray tube 85 required for the radiation source depends on whether or not a field emission type electron emission source capable of operating with a stability of ± 0.5% can be developed. In other words, in the field emission type electron emission source, ions generated by the ionization of the residual gas component enter the tip of the cathode 87 sharply sharpened in order to obtain a high electric field, and the sputtering at the tip of the cathode 87 is performed. It is difficult to cause local heating and evaporation at the same time. In order to suppress this effect, it has been necessary to make the density of the residual gas extremely low, that is, to install the electron emission source in an environment with an ultra-high vacuum. To reiterate, the crying point of the field emission type electron emission source is the life and stability. To avoid breaking these two barriers, the realization of a practical field emission type X-ray source is a dream. It becomes.
[0013]
The present invention requires an operation in an ultra-high vacuum in an X-ray source having a field emission type electron emission source having a simple structure without a filament and having advantages such as enabling high-quality X-ray emission. It is an object of the present invention to provide an X-ray source that overcomes the essential difficulty.
[0014]
[Means for Solving the Problems]
The present invention examines what the structure of a miniature X-ray tube should be as a part of "Development of a miniature X-ray source", which is an innovative technology development research (currently, industry-academia-government-government innovation creation project) adopted in 2001. It was inspired at the stage.
[0015]
We took an X-ray image using field emission electrons from carbon nanotubes (CNT) in 2001 (Appl. Phys. Lett. 78 (2001) 2778). This is the world's first successful example of field emission X-ray imaging. However, even in the case of an electron emission source made of carbon nanotubes, the electron current always fluctuates in a short cycle (fluctuation), and the current value gradually decreases with time as shown in FIG. In addition, the lifetime of the electron emission source made of carbon nanotubes was at most 2 hours, but this was an epoch-making result at that time. The downward sloping phenomenon of the electron current shown in FIG. 1 is caused by the chemical interaction between the surface of the electron emission source and the residual gas. To avoid this, the surrounding gas is insensitive to the gas, that is, it is chemically stable. There is no alternative but to make the electron emission source from the material.
[0016]
We have been conducting experimental research on the interaction between Pd (palladium) and hydrogen for more than a decade, and in that regard, have been of extraordinary interest in methods for producing spatially dispersed Pd nanoparticles. Was. In recent years, as shown in FIG. 2, we have focused on carbon nanofibers (CNFs), which are carbon-based nanostructures whose structure is fundamentally different from that of carbon nanotubes (NM Rodriguez “A review of catalytically grown”). Carbon nanofibers, J. Mater. Res., Vol. 8, No. 12, Dec. 1993 "1993 Materials Research Science). The carbon nanotube is composed entirely of carbon atoms, has a hemispherical tip and a cylindrical side wall. On the other hand, carbon nanofibers have carbon atoms grown on a substrate that is a metal catalyst (base), and the side wall is formed of carbon atoms like a carbon nanotube. Always occupied by substrate crystallites. Then, a method of supporting a Pd nanoparticle group on a carbon nanofiber was established. This is because Pd is selected as a substrate serving as a growth base for the carbon-based nanostructure, and an aggregate having a structure in which Pd microcrystals are added or incorporated at the tip of the carbon-based nanostructure is placed on the substrate. It is what was constituted. As a precautionary measure, when the carbon nanofiber having the group of Pd nanoparticles was used for a field emission type electron emission source, it was found that the electron current did not decrease to the right as shown in FIG. There is no other proof that carbon nanofibers having Pd nanoparticles can be a long-life field emission type electron emission source. Pd is regarded as a stable catalyst after Pt, and is extremely chemically stable. The reason why carbon nanofibers having a group of Pd nanoparticles (hereinafter, referred to as “Pd-emitter”) are not easily robust even in a non-ultra-high vacuum is that Pd has excellent chemical stability.
[0017]
Even with a Pd-emitter, a short-term variation of ± 8% is inevitable. The short-period fluctuation occurs because the residual gas molecules hit the surface of the electron emission source, and is an unavoidable phenomenon in a non-ultra-high vacuum regardless of the material of the electron emission source. The short cycle variation shown in FIG. 3 is about ± 8%. How can this be reduced to ± 0.5%? Since the short-period fluctuations decrease in proportion to the degree of vacuum, there is no problem if the X-ray tube is evacuated to UHV, but there is no technique for that as described above. Furthermore, as shown in FIG. 4, our conclusion is that "short-period fluctuations will be reduced by electronic circuit technology", which is one pillar of the present invention. The present invention is intended for practical use of field emission X-ray radiography and is a typical example of applied research.
[0018]
The field emission type X-ray source of the present invention is provided with a vacuum vessel maintained at a high vacuum, a cathode provided in the vacuum vessel, and a cathode provided in the vacuum vessel in an electrically insulated state. An X-ray tube having an anode on which electrons emitted from a cathode by an electric field are incident to emit X-rays;
[0019]
A field emission type X-ray source comprising a high voltage power supply for applying a high voltage power supply current for accelerating electrons between the cathode and the anode,
[0020]
The cathode has a structure in which a gas adsorbent is selected as a substrate serving as a growth base of the carbon-based nanostructure, and microcrystals of the gas adsorbent are added or incorporated at the tip of the carbon-based nanostructure. It is characterized in that an aggregate is formed on the substrate.
[0021]
In the field emission type X-ray source of the present invention, microcrystals made of a gas adsorbing material such as palladium (Pd) or titanium (Ti) having excellent impurity gas adsorption characteristics provided at the tip of the carbon-based nanostructure are provided. By taking in impurities, it is possible to reduce the cause of sputtering. The carbon-based nanostructure is a fiber-like aggregate having a diameter of several tens of nm or less, as represented by carbon nanofibers and carbon nanotubes. Since the number of the aggregates is spatially widely distributed, the total surface area of the adsorbed substance becomes very large. The structure described above is ideal because the adsorption of impurities depends on this surface area.
[0022]
Therefore, according to the field emission type X-ray source of the present invention, operation in an ultra-high vacuum has been indispensable, and it has been difficult to stably emit electrons due to abrasion of the tip portion due to sputtering. 10 for source ^-5 Electrons can be stably emitted for a long time even at a degree of vacuum of about Pa. For this reason, the field emission type X-ray tube of the present invention can be applied to a miniature X-ray source. The most significant feature of the field emission electron source is that strong convergence of the electron beam can be realized. This means that a field emission miniature X-ray source can provide an ultra-high resolution X-ray image, and the present invention is a path to the next generation X-ray technology, “Field-emission x-ray technology”. Could be opened.
[0023]
In the field emission type X-ray source of the present invention, the X-ray tube has a cathode grid for controlling the electron current emitted from the cathode, and the electron current emitted from the cathode is provided between the cathode and the cathode grid. A grid power supply for controlling may be provided. In this case, the field emission type X-ray source has a triode configuration, and an effect of suppressing ions incident on the cathode can be expected.
[0024]
In the field emission type X-ray source of the present invention, a current detector for detecting a short-period fluctuation of a high-voltage power supply current and a feedback circuit for controlling an output voltage of a grid power supply so as to suppress the current fluctuation of the current detector are provided. Preferably, it is provided. In this case, the amount of electron emission can be feedback-controlled by capturing a short-period change in the electron current. In an X-ray source provided with a field emission type electron emission source, a short-period variation is generally observed in an electron current as shown in FIG. This is because, as shown in FIG. 5A, the impurity atoms ia are ionized into the ions i by the electrons e field-emitted from the cathode C, and the ions C cause the cathode C to be sputtered. As shown in FIG. 5), when the tip of the cathode C has a finer pointed surface shape due to sputtering, the electric field increases and the electron current increases, and as shown in FIG. It is presumed that when the fine structure returns to the original surface shape, the electric field decreases and the electron current also decreases. As described above, the short-period increase in the electron current serves as an index indicating that the surface shape has begun to change due to sputtering. Wear can be suppressed.
[0025]
It is preferable that the feedback circuit can be applied to a general field emission type electron tube. The feedback circuit that ensures the life of the electron emission source and the stability of the emission current contributes to the practical use of the field emission display, which has been recently developed in Japan, the United States and Europe. As a result, the feedback circuit becomes versatile, and the manufacturing cost of the field emission X-ray source can be reduced.
[0026]
In the field emission type X-ray source of the present invention, it is preferable that the grid power supply is provided with a high-frequency filter for controlling short-period fluctuations of the high-voltage power supply current. It is also preferable that the high-voltage power supply is provided with a high-frequency filter for controlling short-period fluctuations of the high-voltage power supply current. These high-frequency filters can be used in place of the feedback circuit. The introduction of the high-frequency filter suppresses a rapid increase in the electron current. Therefore, by the same operation as in the feedback circuit, it is possible to prevent the progress of sputtering and to suppress the wear of the tip of the cathode. The high frequency filter can be composed of an inductor, a resistor, and the like.
[0027]
In the field emission type X-ray source of the present invention, it is also preferable to provide a cooling means for maintaining good gas adsorption characteristics of the cathode. The gas-adsorptive metal medium added to the base of the cathode material and the tip of the carbon-based nanostructure is effective in suppressing the temperature to improve the adsorption characteristics. The cooling means works effectively to enhance the performance of the adsorbent. As the cooling means, a radiator plate, a cooling pipe, an electronic cooling element, or the like can be employed.
[0028]
We have confirmed the effect of the present invention when the carbon-based nanostructure is a carbon nanofiber. In addition, we have confirmed the effect of the present invention when the gas adsorbent is palladium. It is considered that at least one of titanium and zirconium can be employed as the gas adsorbent, in addition to palladium.
[0029]
Japanese Patent Application Laid-Open No. 2001-35427 describes that polycrystalline diamond, diamond-like material, amorphous diamond, and diamond-like carbon are used as an electron emission source. The structure does not include these. Further, the X-ray source of the present invention and the X-ray source described in the publication are the same in that the latter is merely an X-ray generator, and no consideration is given to the life and stability which are the lifelines of the electron emission source. , Basically different from the former. Incidentally, the stability of the electron current emitted from the diamond electron source described in the publication is 10 ^-6 Even under an ultra-high vacuum of Pa, the fluctuation range reaches ± 3 to 30%, far from the allowable fluctuation range of ± 0.5% or less.
[0030]
Japanese Patent Application Laid-Open No. 2001-250496 describes that a carbon nanotube, which is a carbon-based nanostructure, is used as an electron emission source. The structure is fundamentally different from carbon nanofibers with microcrystals. The electron emission source according to the present invention extends the life by adding or incorporating microcrystals of a gas adsorbent such as Pd at the tip of a carbon nanofiber that is a carbon-based nanostructure. Has a much longer life than the electron emission source of carbon nanotubes.
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, first to fifth embodiments of the field emission type X-ray source according to the present invention will be described with reference to the drawings.
[0032]
(Embodiment 1)
This field emission type X-ray source includes an X-ray tube 10 and a high voltage power supply 20, as shown in FIG. The X-ray tube 10 has a vacuum vessel 11 whose inside is maintained at a high vacuum and in which a part of an X-ray extraction window 11a is formed. A cathode 12 as an electron emission source and an anode 13 electrically insulated from the cathode 12 are provided in the vacuum vessel 11. The cathode 12 and the anode 13 are connected to a high-voltage power supply 20 for applying a high-voltage power supply current for accelerating electrons.
[0033]
As shown in FIG. 7, the cathode 12 is based on a base material 12a made of a gas adsorbent having gas adsorption characteristics such as palladium (Pd) and titanium (Ti), and further has a carbon material formed thereon by a method such as a plasma chemical reaction. The configuration is such that an aggregate of the nanofibers 12b is formed. At the tips of the carbon nanofibers 12b, microcrystals 12c of a gas adsorbent serving as a substrate are added or incorporated.
[0034]
When a voltage is applied between the anode 13 and the cathode 12 by the high-voltage power supply 20, a high electric field is concentrated at the tip of each element of the carbon nanofibers 12b constituting the cathode 12. As a result, electrons e are emitted from the surface of the cathode 12, accelerated, and incident on the anode 13. Therefore, the X-rays x are emitted by the braking action of the anode 13, and the X-rays x are emitted to the outside from the X-ray extraction window 11a.
[0035]
In the field emission type X-ray source of the first embodiment configured as described above, the microcrystal 12c made of the gas adsorbent at the tip of the carbon nanofiber 12b takes in the impurity gas near the tip, thereby reducing the vicinity of the tip. This has the effect of keeping the concentration of impurities low. Although the effect of adsorbing the impurity gas can be expected for the base material 12a of the cathode 12, the latter effect is dominant in view of the total surface area of the microcrystals 12c at the tips of the myriad carbon nanofibers 12b. You can think. For this reason, the structure of the electron emission source has a peculiar shape in which the microcrystals 12c made of a gas adsorbent are widely distributed in a spatial manner, thereby exhibiting an impurity suppression effect that cannot be achieved by other structures. It becomes.
[0036]
Generally, in a field emission type X-ray source, 10 ^-7 Although an ultra-high vacuum of about Pa is required, this electron emission source has a function of taking impurities into the gas adsorbent at the tip portion, so that it is sufficiently stable even at a degree of vacuum of 1 to 2 orders of magnitude lower. Electrons can be emitted, and the ionization of the impurity gas to suppress the action of causing ions to enter the tip of the cathode 12 to cause sputtering, thereby preventing the tip of the cathode 12 from being worn. And a long life can be achieved.
[0037]
(Embodiment 2)
This field emission type X-ray source includes an X-ray tube 10, a high voltage power supply 20, and a grid power supply 30, as shown in FIG. A cathode grid 14 is provided in the X-ray tube 10 near the cathode 12. The cathode 12 and the cathode grid 14 are connected to a grid power supply 30, and the cathode grid 14 and the anode 13 are connected to a high-voltage power supply 20. Other configurations are the same as those of the field emission type X-ray source of the first embodiment, and thus the same configurations are denoted by the same reference numerals and detailed description thereof will be omitted.
[0038]
In this field emission type X-ray source, by applying a voltage between the cathode grid 14 and the cathode 12 by the grid power supply 30, electrons e are emitted from the tip of each element of the carbon nanofibers 12b constituting the cathode 12. The component of the electrons e that has passed through the cathode grid 14 is accelerated by the high voltage applied between the cathode grid 14 and the anode 13 by the high-voltage power supply 20 and is incident on the anode 13. Therefore, the X-rays x are emitted by the braking action of the anode 13, and the X-rays x are emitted to the outside from the X-ray extraction window 11a. Further, in this field emission type X-ray source, the addition of the cathode grid 14 makes it possible to control the amount of electron emission irrespective of the high voltage applied by the high voltage power supply 20, and to control the incidence of impurity ions on the cathode 12. The amount can be reduced. Other functions and effects are the same as those of the first embodiment.
[0039]
(Embodiment 3)
In this field emission type X-ray source, as shown in FIG. 9, a current detector 40 capable of detecting an electron current at high speed is provided in a circuit loop of a grid power supply 30 and a high-voltage power supply 20. A feedback circuit 41 for controlling the output voltage of the grid power supply 30 is provided between the grid power supply 30 and the grid power supply 30 so as to suppress the current fluctuation of the current detector 40. Other configurations are the same as those of the field emission type X-ray source according to the second embodiment. Therefore, the same configurations are denoted by the same reference numerals, and detailed description is omitted.
[0040]
In this field emission type X-ray source, the output voltage of the grid power supply 30 is controlled at high speed by the feedback circuit 41 in response to the signal from the current detector 40, and the short-term fluctuation of the electron current is suppressed. As described above, the rapid increase in the electron current is considered to be due to the fact that the shape of the tip of the cathode 12 becomes a fine structure by sputtering and the amount of emitted electrons increases. And the effect of increasing wear due to sputtering is suppressed, and wear of the tip of the cathode 12 can be prevented beforehand. When the electron current is controlled to be constant using the feedback circuit 41, the electron current changes as shown in FIG. Thus, by suppressing the short-period fluctuation of the electron current, the life of the cathode 12 can be extended. Other functions and effects are the same as those of the first embodiment.
[0041]
(Embodiment 4)
As shown in FIG. 10, the field emission type X-ray source has a high-frequency filter 50 in a circuit loop between the cathode 12 and the grid power supply 30 in order to form a triode structure. A high frequency filter 51 is provided in a circuit loop between the power supply and the high voltage power supply 20. These high frequency filters 51 are composed of inductors, resistors, and the like. Other configurations are the same as those of the field emission type X-ray source according to the second embodiment. Therefore, the same configurations are denoted by the same reference numerals, and detailed description is omitted.
[0042]
In this field emission type X-ray source, when a phenomenon occurs in which the electron current sharply increases, the amount of voltage applied to the high frequency filters 50 and 51 increases, and the increase in the electron current decreases. As a result, this field emission type X-ray source can suppress short-period fluctuations of the electron current as a result, and can obtain the same effect as the field emission type X-ray source of the third embodiment. Other functions and effects are the same as those of the first embodiment.
[0043]
When the field emission type X-ray source has a triode structure, the high frequency filters 50 and 51 can be provided in either the circuit loop of the grid power supply 30 or the circuit loop of the high voltage power supply 20.
[0044]
(Embodiment 5)
As shown in FIG. 11, the field emission type X-ray source includes an electron cooling element 60 on the cathode 12. Other configurations are the same as those of the field emission type X-ray source according to the second embodiment. Therefore, the same configurations are denoted by the same reference numerals, and detailed description is omitted.
[0045]
In this field emission type X-ray source, it is possible to suppress a rise in the temperature of the cathode 12 during operation of the electron cooling element 60 and maintain the effect of adsorbing impurity gas. Due to this cooling action, the adsorption characteristics of the base material 12a of the cathode 12 using the gas adsorbent and the microcrystals 12c added to the tips of the carbon nanofibers 12b are favorably maintained, thereby reducing the impurity concentration causing spatter. It is possible to keep it low and extend the life of the cathode 12. Other functions and effects are the same as those of the first embodiment.
[0046]
In the case where it is difficult to provide the electronic cooling element 60 due to the structure of the field emission type X-ray source, the cooling pipe through which the cooling liquid is passed is provided in the cathode 12 instead, or a radiation fin is provided near the cathode 12 instead. , The temperature of the cathode 12 is suppressed from rising.
[Brief description of the drawings]
FIG. 1 is a graph showing a time variation of an electron current emitted from an electron emission source made of carbon nanotubes.
FIG. 2 is a micrograph showing a carbon nanofiber supporting a Pd nanoparticle group.
FIG. 3 is a graph showing a time variation of an electron current emitted from an electron emission source made of carbon nanofibers carrying Pd nanoparticles.
FIG. 4 is a graph when a stabilizing circuit is activated with respect to FIG. 3;
FIG. 5 is an explanatory diagram showing a change in the shape of the tip of the cathode due to a sputtering phenomenon.
FIG. 6 is a schematic configuration diagram of a field emission X-ray source according to the first embodiment.
FIG. 7 is a schematic diagram of a main part of an electron emission source of the field emission type X-ray source according to the first embodiment.
FIG. 8 is a schematic configuration diagram of a field emission X-ray source according to a second embodiment.
FIG. 9 is a schematic configuration diagram of a field emission X-ray source according to a third embodiment.
FIG. 10 is a schematic configuration diagram of a field emission X-ray source according to a fourth embodiment.
FIG. 11 is a schematic configuration diagram of a field emission X-ray source according to a fifth embodiment.
FIG. 12 is a schematic configuration diagram of a conventional thermionic emission type X-ray source.
FIG. 13 is a schematic configuration diagram of a conventional general field emission type X-ray source.
[Explanation of symbols]
10 X-ray tube
11 ... Vacuum container
20 ... High voltage power supply
12 ... Cathode
12b: carbon nanofiber (carbon-based nanostructure)
12c: Microcrystals of gas adsorbent
13… Anode
14 ... Cathode grid
30 ... Grid power supply
40 ... Current detector
41 ... Feedback circuit
50, 51 ... High frequency filter
60 ... Electronic cooling element (cooling means)
e ... Electronic
x ... X-ray

Claims (9)

A vacuum vessel maintained in a high vacuum, a cathode provided in the vacuum vessel, and the cathode is provided in an electrically insulated state in the vacuum vessel, and electrons emitted by the electric field from the cathode enter. An X-ray tube having an anode for emitting X-rays;
A field emission type X-ray source comprising a high voltage power supply for applying a high voltage power supply current for accelerating electrons between the cathode and the anode,
The cathode has a structure in which a gas adsorbent is selected as a substrate serving as a growth base of the carbon-based nanostructure, and microcrystals of the gas adsorbent are added or built in at the tip of the carbon-based nanostructure. A field emission type X-ray source, wherein an aggregate is formed on the substrate. The X-ray tube has a cathode grid for controlling an electron current emitted from the cathode in the vacuum vessel, and controls an electron current emitted from the cathode between the cathode and the cathode grid. 2. The field emission type X-ray source according to claim 1, further comprising a grid power source for the field emission. A current detector for detecting a short-period fluctuation of the high-voltage power supply current, and a feedback circuit for controlling an output voltage of the grid power supply so as to suppress the current fluctuation of the current detector. 3. The field emission type X-ray source according to 2. 4. The field emission type X-ray source according to claim 3, wherein the feedback circuit is applicable to a general field emission type electron tube. The field emission type X-ray source according to any one of claims 1 to 4, wherein the grid power supply is provided with a high-frequency filter for controlling short-period fluctuations of the high-voltage power supply current. The field emission type X-ray source according to any one of claims 1 to 5, wherein the high-voltage power supply is provided with a high-frequency filter for controlling short-period fluctuations of the high-voltage power supply current. 7. The field emission type X-ray source according to claim 1, further comprising a cooling unit for maintaining good gas adsorption characteristics of the cathode. The field emission type X-ray source according to any one of claims 1 to 7, wherein the carbon-based nanostructure is a carbon nanofiber. 9. The field emission type X-ray source according to claim 1, wherein the gas adsorbent is at least one of palladium, titanium and zirconium.

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