WO2024170816A1

WO2024170816A1 - X-ray tube and method of manufacturing a field emission cathode for an x-ray tube

Info

Publication number: WO2024170816A1
Application number: PCT/FI2023/050092
Authority: WO
Inventors: Alexander Obraztsov; Petr OBRAZTSOV; Yuri SVIRKO; Sergei Malykhin
Original assignee: University Of Eastern Finland
Priority date: 2023-02-14
Filing date: 2023-02-14
Publication date: 2024-08-22

Abstract

An X-ray tube and a method for manufacturing a field emission cathode for an X-ray tube are disclosed. The X-ray tube comprises: a field emission cathode comprising a conductive substrate comprising an emitting area with a non-flat emitting surface, and a carbon layer5 deposited on the non-flat emitting surface of the substrate, the carbon layer comprising a plurality of graphitic flakes protruding from the emitting surface of the substrate, wherein each graphitic flake comprises a plurality of mutually parallel graphene sheets; an anode; and a sealed housing containing the anode and the field emission cathode in a vacuum, wherein the field emission cathode is configured to emit an electron beam to the anode0 upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode.

Description

X-RAY TUBE AND METHOD OF MANUFACTURING A FIELD EMISSION CATHODE FOR AN X-RAY TUBE

Technical Field

The present solution generally relates to an X-ray tube, and a method of manufacturing a field emission cathode for an X-ray tube.

Background

X-ray tubes are a type of electronic device for generating X-ray radiation. X-ray tubes are widely used in medicine, security applications, product flaw detection, analytical scientific equipment, and other application areas. Some issues that X-ray devices may struggle with are low life time, high voltage and temperature requirements, large dimensions and weight, dangerous malfunctions, and complexity of the devices and their operation.

Summary of the Invention

The scope of protection sought for various embodiments of the invention is set out by the independent claims. Various embodiments are disclosed in the dependent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

According to an aspect, an X-ray tube comprises: a field emission cathode comprising a conductive substrate comprising an emitting area with a non-flat emitting surface, and a carbon layer deposited on the non-flat emitting surface of the substrate, the carbon layer comprising a plurality of graphitic flakes protruding from the emitting surface of the substrate, wherein each graphitic flake comprises a plurality of mutually parallel graphene sheets; an anode; and a sealed housing containing the anode and the field emission cathode in a vacuum, wherein the field emission cathode is configured to emit an electron beam to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode. The anode and the field emission cathode may be arranged in a diode configuration.

The carbon layer may be deposited only on the non-flat emitting surface of the substrate.

The emitting area may be a circular emitting area.

The emitting surface of the field emission cathode may be a curved surface.

The emitting surface may be a concave surface.

The emitting surface may be a spherically concave surface.

A radius of the emitting area may be smaller than or equal to a radius of curvature of the emitting surface.

The emitting area may have a radius of 3 mm to 4 mm, and the emitting surface may have a radius of curvature of 9 mm to 11 mm.

An aspect ratio of the graphitic flakes may be in the range of 5 to 5000, wherein the aspect ratio represents a ratio of a length and a thickness of the graphitic flakes at their top ends facing the anode.

The aspect ratio may be in the range of 900 to 1100.

The graphitic flakes may have a thickness of 1 nm to 100 nm and a width of 0.5 pm to 5 pm.

A distance between adjacent graphitic flakes may be 1 to 2 times the length of the graphitic flakes.

The distance between adjacent graphitic flakes may be 0.5 pm to 5 pm.

A thickness of the carbon layer may be 0.5 pm to 5 pm.

The thickness of the carbon layer may be 3 pm to 5 pm.

The graphitic flakes may be substantially perpendicular to the emitting surface.

The anode may comprise an anode target; and an anode holder comprising a window, wherein the anode target is configured to output the X-ray radiation through the window of the anode holder upon collision of the electron beam with the anode target.

The X-ray tube may further comprise an intermediate electrode positioned in the sealed housing between the field emission cathode and the anode, and the field emission cathode may be further configured to emit the electron beam upon application of an intermediate voltage to the intermediate electrode, wherein the intermediate voltage is between the voltages applied to the anode and the field emission cathode.

The intermediate electrode may be ring-shaped and positioned such that the electron beam emitted from the field emission cathode to the anode passes through the ring-shaped intermediate electrode.

The field emission cathode, the anode and the intermediate electrode may be arranged in a triode configuration.

A distance between the cathode and the intermediate electrode may be 250 pm to 350 pm.

According to an aspect, a method of manufacturing a field emission cathode for an X-ray tube comprises: placing a conductive substrate in a holder dimensioned to cover surfaces of the conductive substrate other than a non-flat emitting surface of the conductive substrate, leaving only the non-flat emitting surface of the conductive substrate exposed; and depositing a carbon layer on the emitting surface of the substrate by a chemical vapor deposition process such that the carbon layer comprises a plurality of graphitic flakes protruding from the emitting surface of the substrate, and such that each graphitic flake comprises a plurality of mutually parallel graphene sheets.

The method may further comprise installing the field emission cathode opposite an anode in a sealed housing of an X-ray tube such that the field emission cathode is configured to emit an electron beam to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode.

Brief Description of the Drawings

FIG. 1 is a cross-sectional view illustrating embodiments of an X-ray tube with a diode configuration;

FIG. 2 is a cross-sectional view illustrating embodiments of an X-ray tube with a triode configuration;

FIG. 3 illustrates embodiments related to a carbon layer;

FIG. 4 illustrates an example simulation of an X-ray tube; FIG. 5 illustrates example measurements performed on an X-ray tube;

FIG. 6 illustrates embodiments of a method; and

FIG. 7 is a flow chart illustrating embodiments of the method.

Detailed Description of the Invention

The following description and drawings are illustrative and are not to be construed as unnecessarily limiting. The specific details are provided for a thorough understanding of the disclosure. However, in certain instances, well-known or conventional details are not described in order to avoid obscuring the description. In this specification, reference to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. References to an embodiment can be, but are not necessarily, references to the same embodiment in the present disclosure.

X-ray tubes represent a type of electronic device for generating X-ray radiation in response to accelerated electrons impacting a metal target. X-ray tubes with different intensities and photon energies are widely used in medicine, security applications, product flaw detection, analytical scientific equipment, and other applications. Typically X-ray tubes comprise a source of electrons (cathode); a metal target (anode); a hermetic envelope, providing a vacuum necessary for electrons traveling from the cathode (electrons emitter) to the anode (target); the envelope provides also electrical isolation between the cathode and the anode; a getter for long time support of the vacuum level; and an output window for X-ray radiation. Constructions with two electrodes realize a vacuum diode principle while with constructions with three electrodes realize a vacuum triode principle. While X-ray radiation intensity is dependent on electron beam current (I), X-ray photon energy is determined by accelerating voltage (V), materials of the anode target, and the radiation output window.

Thermionic cathodes are a type of an electron source for X-ray tubes. To achieve required level (current) of electron emission, the thermionic cathodes are made preferably of materials with low work function and are heated during operation up to a high temperature. The simple construction of the X-ray tubes with thermionic cathodes is combined with serious drawbacks, including but not limited: sensitivity of cathode material to an air environment and its low life time; presence of construction elements heated up to high temperature; necessity special power supply and circuitry with high voltage isolation for providing cathode heating; which increase weight, dimensions, complexity and cost of the device production and exploitation.

Alternatively, electron emission may be obtained using field emission effect without heating (e.g. at room temperature) but requiring a strong electric field strength applied to the cathode surface. Practical limitations in providing a high enough voltage to obtain the strong electric field may be overcome by electric field focusing, which may be achieved with emitters shaped as sharp tips or blades. Robust materials are desirable for the construction of field emission cathodes to sustain the action of a strong electric field. A cathode surface comprising a massive of tips and/or blades may be used to achieve total currents suitable for X-ray tube operation.

High electrical conductivity and strength of interatomic bonding make materials belonging to graphene family suitable for the field emission cathode. A candidate of those materials is carbon nanotubes which are single (for single wall nanotubes) or few (for multiwall nanotubes) cylindrically rolled graphene sheets. Surprisingly, another possibility is to use separate graphene sheets or tiny crystallites made of a few parallel graphene sheets. These crystallites may also be called graphite flakes, graphite nanocrystallites, or carbon nanowalls. Small transversal dimensions (e.g. thickness of the crystallites) with relatively large length provides the electric field focusing for the crystallites attached to the conductive substrate.

Efficient electrical, thermal, and mechanical connections of the graphene elements with the substrate surface prevent voltage drop, overheating at the contact area, or mechanical detachment under strong ponderomotive forces, which provides stability for the electron emission of the field emission cathode. Usage of conductive binders and glues is not desirable particularly because of pollution of the emitting area by structurally and chemically foreign materials. At the same time, direct growth of carbon nanotubes on the cathode substrate surface is not desirable because of the presence of metallic particles, which are necessary at the stage of nanotube growth as a catalyst.

There are some general contradictory requirements in the design of an X-ray tube with a field emission cathode, such as: the total emission area and the averaged current density of the cathode are limited due to the separation of the individual emitters, and enlargement of the cathode dimensions is necessary to provide an increased total current with an achievable current density, but the cathode dimension enlargement may create difficulties with providing homogeneous electric potential distribution. With reference to Fig. 1 and Fig. 2, the present disclosure relates to an X-ray tube comprising a field emission cathode 2, an anode 3, and a sealed housing containing the anode and the field emission cathode in a vacuum. The field emission cathode is configured to emit an electron beam 8 to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation 10 upon collision of the electron beam with the anode. An example of the voltage applied between the anode and the field emission cathode is 70 kV, for example such that a voltage applied to the cathode is 0 kV, and a voltage applied to the anode is 70 kV.

As shown in Fig. 1 and Fig. 2, the field emission cathode is placed in a cathode holder 4. However, any arrangement suitable for holding the cathode in place relative to the anode may be used. The field emission cathode may have a cylindrical shape. The housing may comprise a dielectric tube 1 made of a non-conductive material (e.g. glass or ceramic), and two caps 6, 7 made of an electrically conductive material. The two caps may be hermetically coupled to the dielectric tube 1. As shown in Fig. 2, the housing may comprise a plurality of dielectric sections made of a non-conductive material (e.g. glass or ceramic), such as sections 13 and 14. The sections 13, 14 may be hermetically coupled to each other (possibly via other components of the X-ray tube, as seen in Fig. 2) and to the caps 6, 7. The generated X-ray radiation may be output from the X-ray tube via a window (made of e.g. beryllium or aluminum) hermetically arranged in the non-conductive material of the tube 1 , or directly through a thin enough tube wall as shown in Fig. 1 and Fig. 2. Other constructions of the X-ray tube are also possible, and embodiments are not limited to the example illustrations of Fig. 1 and Fig. 2.

The field emission cathode 2 comprises a conductive substrate comprising an emitting area with a non-flat emitting surface, and a carbon layer deposited on the non-flat emitting surface of the substrate. The carbon layer comprises a plurality of graphitic flakes protruding from the emitting surface of the substrate. Each graphitic flake of the plurality of graphitic flakes comprises a plurality of (e.g. 5 to 50) mutually parallel graphene sheets. The arrangement of the graphene sheets may be similar or the same as in graphite. Interatomic bonding on the edges of the flakes may improve their stability. The carbon layer may be considered a nano-carbon film. The graphitic flakes may form nano-carbon walls on the emitting surface.

Technical effects of the invention include improved stability of electron emission, reduced need for polluting the emitting area by binders or glues, reduced size (dimensions, weight) of the cathode, prevention of overheating, increased emissive surface area, improved homogeneity of electric field distribution over the cathode surface, and a reduced need for heating.

In an embodiment, the anode comprises an anode target. The anode target may be made of a conductive material which generates X-ray radiation 10 with desired photon energies. The surface of the anode target that is bombarded by electrons 8 may be flat and at an angle with respect to the trajectory of the electron beam 8 to direct the generated X-ray radiation. The anode may further comprise an anode holder 5 comprising a hole 9. The anode target may be configured to output the X-ray radiation through the hole 9 of the anode holder 5 upon collision of the electron beam 8 with the anode target.

In an embodiment, the anode and the field emission cathode are arranged in a diode configuration. The embodiments of Fig. 1 illustrate such a configuration.

With reference to Fig. 2, in an embodiment, the X-ray tube further comprises an intermediate electrode 11 positioned in the sealed housing between the field emission cathode 4 and the anode 3. In this case, the field emission cathode may be further configured to emit the electron beam 8 upon application of an intermediate voltage to the intermediate electrode 11 , wherein the intermediate voltage is between the voltages applied to the anode 3 and the field emission cathode 4. The X-ray tube may comprise an intermediate electrode holder 12 that is configured to support the intermediate electrode in its position. The voltage applied to the cathode may be 0 kV, the voltage applied to the intermediate electrode may be 20 kV, and the voltage applied to the anode may be 70 kV, for example.

In an embodiment, the field emission cathode 4, the anode 3 and the intermediate electrode 11 are arranged in a triode configuration. The embodiments of Fig. 2 illustrate such a configuration. The triode configuration provides additional options for controlling the shape and current of the electron beam.

In an embodiment, the intermediate electrode 11 is ring-shaped and positioned such that the electron beam 8 emitted from the field emission cathode 4 to the anode passes through the ring-shaped intermediate electrode. Specifically, the electron beam 8 may pass through the hole of the ring-shaped electrode as shown in Fig. 2. When compared to e.g. a mesh intermediate electrode, overheating of the intermediate electrode may be prevented.

In an embodiment, the carbon layer is deposited only on the non-flat emitting surface of the substrate. For example, the carbon layer is not deposited on lateral sides of the cathode that do not form a part of the emitting surface, or on other surfaces inside the X-ray tube. Limiting the deposition of the carbon layer only to the non-flat emitting surface prevents unwanted cathode enlargement and reduce the amount of carbon dust in the vacuum of the X-ray tube, which may be a reason for arcing and device malfunctions.

In an embodiment, the emitting area is a circular emitting area. The emitting area is circular when viewed from the anode. This allows for focusing the electron beam 8 into a spot (with a desired diameter) on the anode.

In an embodiment, the emitting surface of the field emission cathode is a curved surface.

In an embodiment, the emitting surface is a concave surface. This increases the area of the emitting surface, and may improve control of the electron beam 8. In an embodiment, the emitting surface is a spherically concave surface.

The increase the total emitting surface helps with achieving a total current l_totai= S x |_cur.dens, where lcur.dens is the achievable level of the current density and S is the surface area of the emitting surface. The achievable current density (lcur.dens.) may be a property of the emissive material, i.e. it may be a non-adjustable constant. In contrast l_totai may be determined by technical requirements and may be, for example, 10 mA. It determines necessary value of S. Taking into account that for graphite lcur.dens. = 25 mA/cm² we arrive at l_totai = 10 mA at S = 0.4 cm². For a spherical concave surface with the radius r and the radius of curvature R the surface area S = 2TTR[R - (R² - r²)^1/2].

Continuing the above example, other circumstances which may be taken into account in determination of the geometry of the emitting surface include possibilities for creating an electric field with the desired strength, and homogeneity of electric field distribution on the surface. For example, the electric field strength may be set to about 8 V/um, the conditions for field distribution homogeneity may be determined and optimized using computer simulations taking into account electrostatic laws. For example, parameters such as r=3.5 mm, R=10 mm, and S-0.4 cm² may be obtained. Details of such simulations will be described in more detail below.

In an embodiment, a radius of the emitting area is smaller than or equal to a radius of curvature of the emitting surface. In an embodiment, the radius of the emitting area is smaller than or equal to half of the radius of curvature of the emitting surface. By limiting the radius of curvature with respect to the emitting area, the graphitic flakes on the emitting area are better exposed to and directed towards the anode for electron emission. The radius of curvature may be e.g. about 10 mm. In an embodiment, the emitting area has a radius of 3 mm to 4 mm, and the emitting surface has a radius of curvature of 9 mm to 11 mm.

Dimensions and locations of the components of the X-ray tube may be determined with the help of the following tasks:

1 ) Calculation of electric potential distribution and electric field stress, which determine the value of current produced by the field emission cathode. More details of the calculation methodology are be found in V. I. Kleshch, A. N. Obraztsov, and E. D. Obraztsova, Modeling of field emission from nano-carbons, Fullerenes, Nanotubes and Carbon Nanostructures, 2008. V. 16. P. 384. DOI: 10.1080/15363830802269356. For this purpose, a Laplace task is solved for electrostatic potential A<p = 0 (Formula 1 ). The cathode potential, (pi, is taken equal to 0 V, the anode potential (pu = V varies during the calculations.

2) Calculation of emission current density j(E) of an individual emitter in accordance with the Fowler-Nordheim equation (Formula 2) taking into account particular characteristics of the cathode, e.g. using methodology presented in Alexander N. Obraztsov, Victor I. Kleshch, Elena A. Smolnikova, A nano-graphite cold cathode for an energy-efficient cathodoluminescent light source, Beilstein J. Nanotechnology 2013, V. 4, P. 493. DOI: 10.3762/bjnano.4.58.

where E is the electric field strength at the cathode surface, p is the electron work function, and constants /^ = 1 .5414x1 O’⁶ A eV V’²; B = 6.4894x10⁷ eV³^ V-srrT¹; and C = 10.1 eV

The averaged emission current density of the cathode containing a plurality of the individual emitters is calculated using Formula (3):

J(E,L) = j(J (L)x E)sn (3) where j is the emission current density of an individual emitter, characterized by an emitting area s and determined by Formula (2), n is the distribution density of the individual emitters, L is a distance between the individual emitters which is supposed to be in the same order as their height or length h, E is the average electric field strength on the cathode surface, fi(L) is a field enhancement factor which may be estimated as an aspect ratio r/h , where r is the radius of the emission area of an individual emitter (i.e. s = Trr²).

3) Calculation of electron trajectories by solving Newton equations for electrons traveling from the cathode to the anode under the action of an electrostatic field. The shape of the beam of electrons, distribution of electron current density on the anode surface, and dimensions of the focal spot on the anode may be determined from the calculated trajectories.

Fig. 3 illustrates graphitic flakes 30-33 deposited on the emitting surface 34. It is appreciated by the skilled person that the illustration of the graphitic flakes in Fig. 3 is only for the purpose of illustrating certain properties of the flakes, and it is not intended as a realistic reproduction of the flakes. It is further appreciated that the flakes need not be flat and may contain bends and/or folds, and/or that they may have different shapes and/or sizes with respect to each other.

In an embodiment, the graphitic flakes 30-33 have a thickness 35 of 1 nm to 100 nm and a width 36 of 0.5 pm to 5 pm. The width of the graphitic flakes may be defined as their largest measurement or dimension along the emitting surface. The thickness of the graphitic flakes may be defined as their smallest measurement or dimension along the emitting surface. Additionally or alternatively, the thickness of a flake 30 may be measured at the tip 39 of the flake, as shown in Fig. 3. The tip may refer to the end of the flake furthest away from the emitting surface 34, and/or or closest to the anode (anode not shown in Fig. 3).

In an embodiment, an aspect ratio of the graphitic flakes is in the range of 5 to 5000. The aspect ratio represents a ratio of a length 37 and a thickness 35 of the graphitic flakes (at their tips facing, or closest to, the anode). The length of the graphitic flakes may be defined as a measurement along a normal of the emitting surface. Alternatively or additionally the length may be defined as a distance from the tip of a flake to the emitting surface, the distance measured along the flake.

In an embodiment, the aspect ratio is approximately 1000. In an embodiment, the aspect ratio is in the range of 900 to 1100. At least a part of the graphitic flakes may have the specified aspect ratio. Flakes with such an aspect ratio are the most productive in providing electron emission. Flakes with a smaller aspect ratio may not emit electrons, and flakes with a larger aspect ratio may be destroyed during the first seconds of operating the cathode of the X-ray tube.

In an embodiment, a distance 38 between adjacent graphitic flakes is 1 to 2 times the length of the graphitic flakes. The distance 38 may be measured along the emitting surface as shown in Fig. 3, or between tips of the graphitic flakes facing the anode. In an embodiment, a distance between adjacent graphitic flakes is 0.5 pm to 5 pm. Separation of the individual emission sites located at the tips of the flakes provides improved electric field focusing.

In an embodiment, a thickness of the carbon layer is 0.5 pm to 5 pm, preferably 3 pm to 5 pm. The thickness may be determined as a distance from the emitting surface to the tips of the graphitic flakes.

In an embodiment, the graphitic flakes are substantially perpendicular to the emitting surface. As shown in Fig. 3, the graphitic flakes, such as flake 31 , may still be at a small angle with respect to the normal of the emitting surface 34. The tips of the flakes may be considered to point away from the emitting surface 34.

Fig. 4 illustrates a computer simulation of electric potential distribution and electron trajectories within the X-ray tube. A cathode holder and cathode 40, an intermediate electrode 42, and an electron beam 44 emitted from the cathode are shown. Electric potential is indicated by shades of grey ranging from black (corresponding to 0 kV) to white (corresponding to 70 kV). Electric field lines are also shown. Electron trajectories are indicated by the shape of the electron beam 44, i.e. by lines starting from cathode surface and focusing onto the anode target surface, in this case into a spot of 0.6 mm in diameter.

Tasks 1-3 and formulas 1 -3 explained above were used in the computer simulations. The simulation was performed with variation of the shapes and distances between the electrodes to find conditions allowing achievement of the electron beam current value equal to 1 mA at average electric field strength on the cathode surface of 7V/pm, which may be considered a limitation for the long time stable operation of the cathode made with carbon nanowalls. As a further condition, maximal electric field strength was established at a level of 35 V/pm, which is half of the value igniting vacuum arcing at 10'⁶ Torr pressure, which is assumed to be the worst case in sealed vacuum electronic device.

Fig. 4 further illustrates example dimensions of the X-ray tube along two axes: a z axis extending from the cathode towards the anode (not shown), and an r axis perpendicular to the z axis. In an embodiment, the distance between the cathode and the intermediate electrode is 250 pm to 350 pm.

Fig. 5 illustrates example measurements performed on the field emission cathode of the X- ray tube. For the example measurements, a cathode of 7 mm in diameter with a spherical concave surface having a radius of curvature of 10 mm, the spherical concave surfaces centered at 9.4 mm from the top edge of the cathode, was constructed. The face (emitting) surface of the cathode was covered by a carbon layer of about 5 pm. An anode was made as a flat glass plate covered by a conductive, optically transparent layer of ITO (indium-tin oxide) and a luminescent material layer over the ITO exhibiting cathode luminescence under the action of electrons. The distance between the cathode and the anode was about 300 pm.

Images 500-503 of the anode illustrate emission site distributions on the emitting surface of the cathode for different voltages applied to the anode. The bright spots in the images 500- 503 indicate locations of emission sites on the surface of the cathode. Homogeneity of the electron emission is shown to improve with increasing voltage, with the highest homogeneity shown for 5.5 kV. Diagram 505 illustrates the relationship between the current I of the electron beam and the voltage II applied to the anode. As shown, the total electric current corresponding to the voltage of 5.5 kV causing the highest homogeneity, was about 9 mA.

With reference to Fig. 6 and Fig. 7, a method of manufacturing a field emission cathode for an X-ray tube comprises: placing 700 a conductive substrate in a holder 15 dimensioned to cover surfaces of the conductive substrate 4 other than a non-flat emitting surface of the conductive substrate, leaving only the non-flat emitting surface of the conductive substrate exposed; and depositing 702 a carbon layer on the emitting surface of the substrate by a chemical vapor deposition process such that the carbon layer comprises a plurality of graphitic flakes protruding from the emitting surface of the substrate, and such that each graphitic flake comprises a plurality of mutually parallel graphene sheets.

The carbon layer or film may be deposited by condensation of carbon atoms from a gaseous mixture 16 comprising hydrogen, methane or other types of hydrocarbons or a solid source of carbon atoms. The gas mixture may be activated by a hot temperature inducing pyrolysis of the hydrocarbons, or by gas discharge ignition under the action of a constant (i.e. direct current (DC)) radio-frequency (RF), microwave frequency (MWF), and/or optical frequency (OF) electric field, that induces chemically active hydrocarbon radical formation. The carbon material condensation occurs on the (spherically concave) working surface of the substrate.

An example gas mixture used with direct current gas discharge activation comprises 90 % to 95 % hydrogen and the remaining 5 % to 10 % methane, e.g., 90 % hydrogen and 10 % methane. During the deposition, the total pressure of the example gas mixture is in the range of 8 to 12 kPa, such as 10 kPa, and the substrate temperature is in the range of 940 to 980 °C, for example 970 °C. The duration of the deposition process may be 60 to 120 minutes, for example 90 minutes.

To ensure carbon layer deposition exclusively on the working surface the cathode, the substrate may be placed into the holder 15 for the carbon film deposition as shown in Fig. 6. The holder 15 may be constructed as a cavity in a holder plate for placing the cathode substrate 4. The dimensions (e.g. diameter, depth) of the cavity are selected to minimize penetration of the activated gas mixture 16, and subsequent carbon material deposition on the lateral surfaces of the substrate. The holder may comprise a hole at the bottom of the cavity to facilitate removal of the substrate 4 from the holder 15 after the deposition process, e.g. by pushing a suitable tool through the hole to push the substrate out of the cavity. After the carbon film deposition, the cathode 4 may be ready for use in the X-ray tube without any additional treatments.

In an embodiment, the method further comprises installing the field emission cathode opposite an anode in a sealed housing of an X-ray tube, such that the field emission cathode is configured to emit an electron beam to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode. Thus, a complete X-ray tube as described herein may be constructed.

If desired, the different functions discussed herein may be performed in a different order and/or concurrently with other. Furthermore, if desired, one or more of the above-described functions and embodiments may be optional or may be combined.

Although various aspects of the embodiments are set out in the independent claims, other aspects comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. It is also noted herein that while the above describes example embodiments, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications, which may be made without departing from the scope of the present disclosure as defined in the appended claims.

Claims

1 . An X-ray tube comprising: a field emission cathode comprising: a conductive substrate comprising an emitting area with a non-flat emitting surface; and a carbon layer deposited on the non-flat emitting surface of the substrate, the carbon layer comprising a plurality of graphitic flakes protruding from the emitting surface of the substrate, wherein each graphitic flake comprises a plurality of mutually parallel graphene sheets, an anode; and a sealed housing containing the anode and the field emission cathode in a vacuum, wherein the field emission cathode is configured to emit an electron beam to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode.

2. The X-ray tube of claim 1 , wherein the anode and the field emission cathode are arranged in a diode configuration.

3. The X-ray tube of any preceding claim, wherein the carbon layer is deposited only on the non-flat emitting surface of the substrate.

4. The X-ray tube of any preceding claim, wherein the emitting area is a circular emitting area.

5. The X-ray tube of any preceding claim, wherein the emitting surface of the field emission cathode is a curved surface.

6. The X-ray tube of claim 5, wherein the emitting surface is a concave surface.

7. The X-ray tube of claim 6, wherein the emitting surface is a spherically concave surface.

8. The X-ray tube of claim 7, wherein a radius of the emitting area is smaller than or equal to a radius of curvature of the emitting surface.

9. The X-ray tube of claim 8, wherein the emitting area has a radius of 3 mm to 4 mm, and the emitting surface has a radius of curvature of 9 mm to 11 mm.

10. The X-ray tube of any preceding claim, wherein an aspect ratio of the graphitic flakes is in the range of 5 to 5000, wherein the aspect ratio represents a ratio of a length and a thickness of the graphitic flakes at their tips facing the anode.

11 . The X-ray tube of claim 10, wherein the aspect ratio is in the range of 900 to 1100.

12. The X-ray tube of any preceding claim 10-11 , wherein the graphitic flakes have a thickness of 1 nm to 100 nm and a width of 0.5 pm to 5 pm.

13. The X-ray tube of any preceding claim, wherein a distance between adjacent graphitic flakes is 1 to 2 times the length of the graphitic flakes.

14. The X-ray tube of any preceding claim, wherein a distance between adjacent graphitic flakes is 0.5 pm to 5 pm.

15. The X-ray tube of any preceding claim, wherein a thickness of the carbon layer is 0.5 pm to 5 pm.

16. The X-ray tube of claim 15, wherein the thickness of the carbon layer is 3 pm to 5 pm.

17. The X-ray tube of any preceding claim, wherein the graphitic flakes are substantially perpendicular to the emitting surface.

18. The X-ray tube of any preceding claim, wherein the anode comprises: an anode target; and an anode holder comprising a hole, wherein the anode target is configured to output the X-ray radiation through the hole of the anode holder upon collision of the electron beam with the anode target.

19. The X-ray tube of any preceding claim, further comprising an intermediate electrode positioned in the sealed housing between the field emission cathode and the anode, wherein the field emission cathode is further configured to emit the electron beam upon application of an intermediate voltage to the intermediate electrode, wherein the intermediate voltage is between the voltages applied to the anode and the field emission cathode.

20. The X-ray tube of claim 19, wherein the intermediate electrode is ring-shaped and positioned such that the electron beam emitted from the field emission cathode to the anode passes through the ring-shaped intermediate electrode.

21. The X-ray tube of any preceding claim 19-20, wherein the field emission cathode, the anode and the intermediate electrode are arranged in a triode configuration.

22. The X-ray tube of any preceding claim 19-21 , wherein a distance between the cathode and the intermediate electrode is 250 pm to 350 pm.

23. A method of manufacturing a field emission cathode for an X-ray tube, the method comprising: placing a conductive substrate in a holder dimensioned to cover surfaces of the conductive substrate other than a non-flat emitting surface of the conductive substrate, leaving only the non-flat emitting surface of the conductive substrate exposed; and depositing a carbon layer on the emitting surface of the substrate by a chemical vapor deposition process such that the carbon layer comprises a plurality of graphitic flakes protruding from the emitting surface of the substrate, and such that each graphitic flake comprises a plurality of mutually parallel graphene sheets.

24. The method of claim 23, further comprising installing the field emission cathode opposite an anode in a sealed housing of an X-ray tube such that the field emission cathode is configured to emit an electron beam to the anode upon application of a voltage between the anode and the field emission cathode, and the anode is configured to output X-ray radiation upon collision of the electron beam with the anode.