X-ray anode

Description

[0001] The present invention relates to an X-ray anode according to the preamble of claim 1.

[0002] X-ray anodes are required in X-ray devices such as computer tomography scanners used in medical diagnostics or luggage X-ray machines. During operation of an X-ray device, electrons emitted by a cathode are accelerated to the X-ray anode by high voltage and penetrate the anode material, generating X-ray radiation. A large portion of the electron beam's energy is dissipated as heat in the X-ray anode, resulting in very high thermal loads in the focal area of ​​the X-ray anode. X-ray anodes are typically designed as stationary components in the form of a standing anode with a focal spot or as rotating components in the form of a rotary anode with an annular focal path. Linear extensions for X-ray anodes, also known as linear anodes, with an elongated focal path are also known.Linear anodes are non-rotating and are usually designed as static anodes, but can be moved, for example, for successive X-ray images during a computed tomography scan.

[0003] X-ray anodes are usually constructed as a composite of at least one carrier body, which provides mechanical stability and is preferably made of a high-melting material with high thermal conductivity, and at least one emission layer, also referred to as focal coating or focal track coating, in which X-ray radiation is generated when high-energy electrons strike.

[0004] In the case of standing anodes, this carrier body usually has a bevelled, cylinder-like basic shape, on whose usually bevelled end face a comparatively thin, disc-shaped burning coating made of an X-ray generating material, for example tungsten or a tungsten alloy, is arranged.

[0005] Rotating anodes usually have a ring-shaped burning coating made of a

[0006] X-ray-generating material. Due to the rotating motion of the X-ray anode, the electron beam scans the burning deposit in a circular path, allowing for a better distribution of the thermal load within the rotating anode.

[0007] Linear anodes have an elongated orientation, for example, a bar-shaped basic shape; an example of this is described in WO 2013 / 020151 A1. In linear anodes, an elongated fuel coating is typically arranged on a side surface, not the end face, of the elongated support body.

[0008] The service life of X-ray anodes is severely limited due to the interaction with the high-energy electron beam and the high thermomechanical loads, which occur cyclically, especially in rotating anodes. With increasing use of the X-ray anode, fatigue of the firing layer occurs. Microcracks can form in the firing layer, which, with further stress, can spread into the body of the X-ray anode in a network-like manner. Damage to the firing layer has adverse consequences for the X-ray dose yield and negatively impacts the image quality of the X-ray images. If the X-ray dose yield falls below a critical threshold, either the entire X-ray anode must be replaced or at least the damaged firing layer must be reconditioned or replaced.To improve the quality of the used fuel deposit, it is known to remove the used fuel deposit until it has a crack-free surface. This is not possible indefinitely due to the limited thickness of the fuel deposit. The service life of the X-ray anode can also be extended by applying a new fuel deposit to the previously removed, used fuel deposit. JP 2003 142000 A is cited as an example. However, this process for reconditioning the fuel deposit is extremely complex and associated with high costs. Therefore, industry requires X-ray anodes with a long service life.

[0009] DE 10 2005 049 519 A1 shows an X-ray anode with an X-ray active layer, which has a high thermomechanical load capacity by introducing at least one intermediate layer and a cover layer in order to achieve a comparatively long service life of the X-ray anode.

[0010] An X-ray anode having the features of the preamble of claim 1 is shown in DE 2 415 578 A1.

[0011] The object of the present invention is to provide an X-ray anode with an extended service life. This object is achieved by an X-ray anode according to claim 1. Advantageous developments of the invention are specified in the subclaims.

[0012] According to the present invention, an X-ray anode for generating X-rays is proposed, comprising a carrier body and a first and at least one second emission layer made of X-ray emitting material, wherein the emission layers are separated on one side of the carrier body by an intermediate layer and arranged at a distance from each other in a central direction of the X-ray anode. The intermediate layer between the emission layers is formed from the same material as the carrier body. The first and at least one second emission layers are arranged on the side of the X-ray anode, also referred to below as the focal path side, which, during operation of the X-ray anode, faces the electron beam generating X-rays. The central direction generally refers to a direction oriented perpendicular to a plane that is essentially spanned by the extent of the first emission layer.In the case of a rotating anode, the central direction corresponds to the rotational axis of the rotating anode. In the case of a stationary anode, in which the emission layer is typically formed as a compact disk, the central direction refers to the axial direction of the stationary anode. In the case of a linear anode, the plane defining the central direction corresponds to the focal path side of the linear anode, i.e., the usually flat side surface with the elongated emission layer that, when the X-ray anode is installed, faces the X-ray-generating electron beam.

[0013] The central idea of ​​the present invention is that the X-ray anode, in addition to a first, active emission layer arranged on an outer surface of the X-ray anode, has at least one further, second emission layer which is initially hidden inside the carrier body and protected by an intermediate layer. As long as the first emission layer is used to generate X-rays through interaction with high-energy electrons, the at least one second emission layer is protected from the impact of electrons by an appropriately dimensioned intermediate layer and is therefore inactive. When the previously active first emission layer is used up and the necessary X-ray dose yield can no longer be achieved, this and the intermediate layer can be removed down to the surface of the previously inactive second emission layer.The exposed second emission layer now becomes the active emission layer, which the electrons impinge upon during further operation of the X-ray tube and in which X-ray radiation is generated through interaction with the X-ray anode material. The X-ray anode according to the invention therefore only needs to be replaced when all emission layers are worn out. Through machining (grinding, turning), the service life of the X-ray anode according to the invention can be significantly extended, approximately double that of a conventionally constructed X-ray anode. The fact that the intermediate layer between the emission layers is made of the same material as the carrier body achieves, among other manufacturing advantages.

[0014] In addition to a single second emission layer, the X-ray anode can contain additional second emission layers, each spaced apart in the central direction by an intermediate layer and successively activated, i.e., gradually exposed after the overlying emission layer has worn away and used to generate X-rays. Depending on the number of emission layers, the average lifetime of such an X-ray anode can be many times longer than that of a conventional X-ray anode with only one emission layer.

[0015] For improved heat dissipation, the emission layers, the intermediate layer, and the carrier body of the X-ray anode are preferably bonded to one another. To protect the initially inactive second emission layer, the distance between the inactive second emission layer and the surface of the X-ray anode in the central direction should be greater than the average penetration depth of the electrons into the X-ray anode. The thickness of the intermediate layer, i.e., the distance in the central direction between two adjacent emission layers, is at least 0.5 mm, preferably at least 2 mm. This ensures that the interaction of the electron beam with the initially inactive second emission layer, and thus the risk of premature damage, is as low as possible.In order to keep the weight of the X-ray anode, especially the moment of inertia in the case of rotating anodes, low, care should be taken to ensure that the intermediate layer is not dimensioned significantly thicker than necessary; the thickness is advantageously less than 10 mm, in particular less than 5 mm.

[0016] According to an advantageous development, the first and at least one second emission layer are arranged in such a way that the geometry and position of the respectively active emission layer does not change significantly when the active emission layer is changed. Preferably, the first and second emission layers are congruent in the central direction in an impact region of the electrons when viewed from the focal path side. The impact region of the electrons refers to the region on the surface of the X-ray anode which is swept over by the electron beam during operation of the X-ray anode. Advantageously, the distance between the first and second emission layers is essentially constant in the impact region of the electrons. Due to the congruent orThe parallel arrangement of the first and second emission layers ensures that the geometry of the active emission layer, and thus the functionality of the X-ray anode, remains unchanged when an unused emission layer is exposed and becomes the active emission layer. Therefore, a change in operating parameters such as complex repositioning of the X-ray anode or altering the electron beam path is not necessary. To compensate for the slightly reduced thickness of the X-ray anode after removal of the used emission layer, the X-ray device can be designed to allow the X-ray anode to be displaced in the central direction.

[0017] Materials known for X-ray generation, such as tungsten or tungsten alloys, especially tungsten-rhenium alloys, are suitable for the emission layers. To ensure that the radiation characteristics of the X-ray anode do not change when changing the emission layers, the same material is preferably chosen for the first and at least one second emission layer. The thickness of the emission layer is typically in the range of 0.2 to 2 mm.

[0018] Suitable materials for the carrier body include, in particular, molybdenum and molybdenum-based alloys (e.g., TZM, MHC), tungsten or tungsten-based alloys, and a copper-based alloy. A molybdenum-based, tungsten-based, or copper-based alloy refers to an alloy containing at least 50 at.% molybdenum, tungsten, or copper, in particular at least 90 at.% molybdenum, tungsten, or copper. TZM refers to a molybdenum alloy with a titanium content of 0.5 wt.%, an irconium content of 0.08 wt.%, a carbon content of 0.01-0.04 wt.%, and the remaining content (excluding impurities) being molybdenum. In this context, MHC is understood to mean a molybdenum alloy which has a hafnium content of 1.0 to 1.3 wt.%, a carbon content of 0.05 to 0.12 wt.%, an oxygen content of less than 0.06 wt.% and the remaining portion (excluding impurities) molybdenum. The carrier body may also comprise a tungsten-copper composite, a copper composite, a particle-reinforced copper alloy, a particle-reinforced aluminum alloy, or graphite.

[0019] As already mentioned, the intermediate layer separating the individual emission layers is made of the same material as the carrier body. This offers, among other things, manufacturing advantages.

[0020] In one embodiment, it also proves advantageous if the thermal expansion coefficient of the intermediate layer material differs by no more than 35% from the thermal expansion coefficient of the first or second emission layer. In the X-ray anode, the active emission layer and the areas immediately adjacent to it are the areas of the X-ray anode subject to the greatest thermal stress. Excessive differences in the thermal expansion coefficient would cause high mechanical stresses during operation, which could impair the service life of the X-ray anode.

[0021] The intermediate layer can be constructed homogeneously in a simple variant, but it can also be structured into different functional intermediate layers, in variants that are not part of the claimed invention.

[0022] The intermediate layer can, for example, have at least one barrier layer. Such a barrier layer can be designed as a diffusion barrier layer to suppress diffusion, for example, unwanted carbon diffusion into the emission layer, and for this purpose can be formed from rhenium, molybdenum, tantalum, niobium, zirconium, titanium, or compounds or alloys of these metals, or combinations of these metals.

[0023] The barrier layer can be designed as a barrier against the propagation of cracks that occur in the active emission layer during operation of the X-ray anode due to interaction with high-energy electrons. In particular, such a barrier layer helps prevent the propagation of these cracks or the formation of a crack network into the still unused second emission layer. A barrier layer designed to suppress cracks can be made of tantalum, niobium, or rhenium, for example. Generally, when selecting the material for the intermediate layer or individual intermediate layers directly adjacent to the emission layer, care should be taken to ensure that no disruptive diffusion of the intermediate layer material into the first or second emission layer occurs.

[0024] The intermediate layer may have at least one bonding layer that improves the bonding of the emission layer. Such a bonding layer may preferably be enriched with the components of the emission layer, such as tungsten or rhenium or a compound thereof.

[0025] The idea of ​​providing the X-ray anode with additional initially inactive and gradually activatable emission layers can be applied to X-ray anodes of various designs. In particular, the X-ray anode according to the invention can be designed as a vertical anode or a linear anode.

[0026] The X-ray anode can particularly preferably be designed as a rotating anode. Advantageously, the first and second emission layers can be ring-shaped and arranged one above the other in the central direction. Rotating anodes have a relatively high base price, and therefore, especially with rotating anodes, it is economically worthwhile to repair the emission layer when it becomes worn rather than replacing the entire rotating anode. The rotating anode according to the invention has the advantage over a repaired rotating anode in that the second emission layer is still untouched when it is finally used.

[0027] To produce the X-ray anode according to the invention, use can be made of the manufacturing processes for X-ray anodes that have been proven in the prior art. The emission layers and intermediate layer can be produced as a layered composite using powder metallurgy processes by pressing, sintering and forging appropriately layered powders or powder mixtures. In the case of metallic materials for the carrier body with a high melting point, such as TZM or MHC, the emission layers and the intermediate layer are preferably produced together with the carrier body. For this purpose, in a first step, a compact is produced from appropriately layered powder or powder mixture by introducing the powder or powder mixture for the carrier body into a fit and pressing it, then the powder or powder mixture for the second emission layer is applied and pressed, and in a next step the powder or powder mixture isThe powder mixture for the intermediate layer is applied and pressed, and finally, the powder or powder mixture for the first emission layer is applied and pressed. The resulting compact is then sintered, forged, and mechanically finished in a conventional manner. For metallic materials for the carrier body, the comparatively lower

[0028] For materials with a high melting point, such as copper, a powder-metallurgically produced composite layer consisting of emission layers and an intermediate layer can be back-cast with a melt of the carrier body material. If graphite is used as the material for the carrier body, a reliable bond between the carrier body and an independently powder-metallurgically produced composite layer containing the emission layers is difficult. Particularly in this case, the emission layers and the intermediate layer can be applied to the carrier body using known coating processes such as chemical vapor deposition, physical vapor deposition, or thermal coating processes, particularly plasma spraying.

[0029] The invention is described in more detail in the following description of three embodiments with reference to the attached figures.

[0030] They show in a not to scale representation: Fig. 1: a sectional view of a standing anode; Fig. 2: a perspective view of a linear anode; Fig. 3: a sectional view of the linear anode of Fig. 2 along section plane II; Fig. 4: a top view of a rotating anode; Fig. 5: a sectional view of the rotating anode from Fig. 4 along the section plane II-II; Fig. 6: an axonometric view of the rotating anode from Fig. 4 ; Fig. 7: a flow diagram for a powder metallurgical manufacturing process of a rotating anode; Fig. 8a: a side view of a sintered component; Fig. 8b: a section through the sintered component of Fig. 8a ; Fig. 9a: a top view of a forging blank; Fig. 9b: a section through the forging blank of Fig. 9a .

[0031] Fig. 1 shows a schematic sectional view of a vertical anode 10, the basic structure of which is known in the prior art. On a substantially cylindrical carrier body 13, a first emission layer 14 is arranged in a known manner on its beveled end face, which faces the electron beam during operation. High-energy electrons are accelerated onto this first emission layer during operation, generating X-rays through interaction with the material of the emission layer. The vertical anode according to the invention differs from the prior art in that it has a second emission layer 15 located inside the vertical anode and spaced from the first emission layer in a central direction 17. The central direction 17 corresponds to the axial direction of the vertical anode 10.The intermediate layer 16 separates the two emission layers 14, 15 and protects the initially inactive second emission layer 15 from the electrons impinging on the first emission layer. When the first emission layer 14 is no longer suitable for further operation, the second emission layer 15 is exposed and used to generate X-rays. The standing anode according to the invention can therefore be used again after the first emission layer has worn out and only needs to be reworked or replaced when both emission layers are unusable. The standing anode 10 according to the invention therefore has a service life approximately twice as long as a standing anode of the prior art. The geometry and position of the second emission layer 15 is preferably adapted to that of the first emission layer 14, so that when the emission layer is replaced, the standing anode does not require complex readjustment, apart from a shift in the central direction.The first and second emission layers 14,15 are parallel to each other and congruent when viewed along the central direction 17.

[0032] Fig. 2 und Fig. 3 show the application of the inventive idea to a linear anode 11. An example of a linear anode from the prior art is described in WO 2013 / 020151 A1. Linear anodes have an elongated extension along an extension direction, in the present exemplary embodiment a bar-shaped basic shape, wherein the main extension direction of the anode does not necessarily have to run along a straight line, but can also run along a curved line. This means that a cuboid which has a curvature over at least part of its extent is also to be understood as a linear anode in the context of the present invention. The first and second emission layers 14, 15 are arranged on the side surface of the cuboid-shaped carrier body which faces the electron beam during operation. The first emission layer 14 is elongated and spans a plane which is perpendicular to the central direction 17.The first and second emission layers 14, 15 are arranged spaced apart in a central direction 17, separated by an intermediate layer 16. The distance between the two emission layers 14, 15 is constant across the planar extent of the emission layer. Preferably, the first and second emission layers 14, 15 are congruent when viewed along the central direction 17. Analogous to the vertical anode, the second emission layer 15 is only used when the required X-ray dose yield can no longer be achieved with the first emission layer 14 and the first emission layer 14 and the intermediate layer 16 have been ground away for further use of the linear anode.

[0033] In Fig. 4 bis 6 1 schematically shows a rotating anode 12 according to the invention with a plate-shaped, rotationally symmetrical support body 13. As with rotating anodes known from the prior art, a first emission layer 14 is arranged in an annular region on the beveled shoulders of the support body on the side facing the electron beam during operation. This region corresponds to the impact region 50 of the electrons during operation of the rotating anode. Analogous to the previous embodiments, the rotating anode 12 according to the invention has, in addition to a first emission layer 14, a second emission layer 15 which is arranged at a distance in the central direction 17, separated by an intermediate layer 16. The central direction 17 is defined by the direction of the rotation axis of the rotating anode. In the present exemplary embodiment, the second emission layer 15 extends beyond the impact region 50 of the electrons into an inner region.As explained below, this is due to manufacturing reasons in powder metallurgy production, but is clearly not absolutely necessary. If the first emission layer 14 is no longer suitable for further use, it and the intermediate layer 16 are removed.

[0034] Preferably, when grinding or turning the rotating anode, that part of the second emission layer 15 which is located in the inner region, i.e. outside the region over which the first emission layer extends, is also removed. The second emission layer 15 then, as the active emission layer, has the same extent as the first emission layer 14. The first and second emission layers 14, 15 are arranged parallel to one another, and in the impact area 50 of the electrons, in the present rotating anode, the two emission layers 14, 15 are congruent in a viewing direction along the central direction 17. The geometry of the two emission layers 14, 15 is thus coordinated with one another so that when changing the active emission layer, apart from a shift in the central direction, no further adjustments to the rotating anode are necessary.Preferably, the material of the first and second emission layers 14, 15 is also matched to one another and the same material is used for the two emission layers 14, 15, so that the emitted radiation spectrum of the X-ray anode does not change when the active emission layer is changed.

[0035] During operation of the rotating anode, the intermediate layer 16 protects the initially inactive second emission layer 15 from the impinging electrons and should be dimensioned with sufficient thickness to prevent premature damage due to interaction with impinging electrons. A distance (in the central direction) between the two emission layers of between 2 and 5 mm has proven advantageous because, on the one hand, this ensures sufficient protection under common loads, and on the other hand, the moment of inertia of the rotating anode is not significantly increased by the additional mass. To ensure good heat transfer, the first emission layer is materially bonded to the intermediate layer, and the second emission layer is materially bonded to the intermediate layer and to the carrier body.It is also advantageous if the intermediate layer acts as a barrier against the further propagation of cracks, such as those that develop in the active emission layer. The intermediate layer 16 can also form a barrier against the diffusion of harmful substances into the emission layers (for example, carbon from the common carrier material TZM or MHC). It is also advantageous if the intermediate layer 16 improves the bonding of the emission layer to the carrier body.

[0036] For these purposes, the intermediate layer 16 can be constructed from several different layers with different functionalities, in particular from a barrier layer and / or bonding layer. Such an intermediate layer is not part of the claimed invention.

[0037] Fig. 7 The left shows the flow diagram for a powder metallurgical manufacturing process for an X-ray anode according to the invention, in particular a rotating anode. The manufacturing process according to the invention is primarily suitable for the production of a metallic carrier body from a

[0038] Refractory metal or an alloy based on a refractory metal such as TZM or MHC and comprises at least the following steps: Filling a fit with a powder or a powder mixture for the carrier body Pressing the powder or the powder mixture to form a shaped body Applying the powder or the powder mixture for the second emission layer to the shaped body obtained in the previous step Pressing the component obtained in the previous step to form a shaped body Applying the powder or the powder mixture for the material of the intermediate layer to the shaped body obtained in the previous step Pressing the component obtained in the previous step to form a shaped body Applying the powder or the powder mixture for the first emission layer to the shaped body obtained in the previous step Pressing the component obtained in the previous step to form a shaped body Sintering the shaped body at temperatures greater than 2000°C Forging the sintered shaped body at temperatures greater than 1300°C Optionally, mechanical finishing to form an X-ray anode, in particular a rotating anode.

[0039] In the Fig. 7 On the right, the intermediate products resulting from individual process steps and the final product are shown schematically using the example of a rotating anode. 18, 18', 18", 18‴ denotes the compact, 19 the sintered molded body, 20 the forged blank, and 12 the finished rotating anode.

[0040] Fig. 8a, Fig. 8b , Fig. 9a und Fig. 9b show some of these intermediate products based on a specific embodiment, in which a TZM powder is used as the starting powder for the carrier body and a W95Re5 powder for the two emission layers. The powders were layered according to the previously described process steps, pressed at pressures of up to 50 kN / cm 2 , and then sintered at a temperature of approximately 2000°C to 2400°C. The resulting sintered component 19 is shown in Fig. 8a in a side view and in Fig. 8b shown in a sectional side view. The sintered component was then forged in a screw press at temperatures above 1300°C to form a component with hanging shoulders. The forging blank 20 is shown in Fig. 9a in a top view and in Fig. 9b shown in a sectional view. In these prototypes, the emission layers 14, 15 extend over the entire length of the component for manufacturing reasons. For large-scale production, it can of course be provided that the powder for the emission layers is only applied in the ultimately required area on the hanging shoulders. The forged blank is then mechanically reworked, including grinding off the first emission layer in the unused inner area. To increase heat radiation capacity, a radiation body can be arranged (in a known manner) on the side of the rotating anode opposite the focal path side.

Claims

1. An x-ray anode (10, 11, 12) for generating x-radiation, comprising a carrier body (13) and a first and at least one second emission layer (14,15), which generate x-radiation when they are impinged by electrons, wherein the x-ray anode (10, 11, 12) has a central direction (17) which is oriented perpendicularly to a plane that is defined substantially by the extent of the first emission layer (14), or, in case the x-ray anode is a rotary anode, corresponds to the axis of rotation of the rotary anode, and wherein the first emission layer (14) is arranged at an outer surface of the x-ray anode (10, 11, 12) and can be used as active emission layer (14) for generating x-radiation by interacting with high-energy electrons, and the distance between the at least two emission layers (14, 15) separated by an intermediate layer (16) in the central direction (17) of the x-ray anode (10, 11, 12) is at least 0.5 mm so that the at least second emission layer (15) is protected from the impingement of electrons, and is therefore inactive, characterized in that the emission layers (14, 15) are arranged at one side of the carrier body (13) so that the first emission layer (14) can be removed along with the intermediate layer (16) down to the surface of the until then inactive second emission layer (15), and the exposed second emission layer (15) then becomes the active emission layer, wherein the intermediate layer (16) between the emission layers (14, 15) is formed from the same material as the carrier body (13).

2. The x-ray anode as claimed in claim 1, characterized in that the first and the second emission layer (14, 15) are congruent in a viewing direction along the central direction (17) in a region of impingement of the electrons (50).

3. The x-ray anode as claimed in one of the preceding claims, characterized in that at least in certain portions, the distance between the first and the second emission layer (14, 15) is substantially constant.

4. The x-ray anode as claimed in one of the preceding claims, characterized in that the first and / or the second emission layer (14,15) is / are produced from tungsten, rhenium or a tungsten alloy, such as in particular a tungsten-rhenium alloy.

5. The x-ray anode as claimed in one of the preceding claims, characterized in that the first and the at least one second emission layer (14, 15) are produced from the same material.

6. The x-ray anode as claimed in one of the preceding claims, characterized in that the carrier body (13) comprises at least one material from the group molybdenum, tungsten, copper, an alloy on the basis of tungsten, molybdenum or copper, a tungsten-copper composite material, a copper composite material, a particle-reinforced copper alloy, a particle-reinforced aluminum alloy or graphite.

7. The x-ray anode as claimed in one of the preceding claims, characterized in that the x-ray anode is configured as a stationary anode (10) or a linear anode (11).

8. The x-ray anode as claimed in one of claims 1 to 6, characterized in that the x-ray anode is configured as a rotary anode (12).

9. The x-ray anode as claimed in claim 8, characterized in that the first and the second emission layer (14, 15) are formed in an annular manner and are arranged one above the other in the central direction (17).