Improved target assembly for generating x-ray radiation

Abstract

The invention relates to a target assembly for generating x-ray radiation, to an electron linear accelerator, and to a fixed anode transmission x-ray tube. The target assembly according to the invention for generating x-ray radiation has: - a heat sink and - a target element for impinging with electrons, said target element being integrated into the heat sink in order to cool the target element and being structurally distinguishable from the heat sink, wherein the heat sink consists of a metal-diamond composite material, and the metal-diamond composite material consists of diamond grains between which metal is located. The invention is characterized in that the grain size of the metal-diamond composite material varies within the heat sink.

Description

[0001] Description

[0002] Improved target design for generating X-rays

[0003] The invention relates to a target structure for generating X-rays, an electron linear accelerator and a standing anode transmission X-ray tube.

[0004] A conventional target structure for generating X-rays comprises at least one target element for impinging on electrons. During the interaction of the electrons with the target element to generate X-rays, typically only a small portion of the kinetic energy can be utilized, while a relatively large portion is simply converted into heat.

[0005] Due to the comparatively high electron energy values ​​in the range between 20 and 150 keV for diagnostic imaging applications or between 1 and 18 MeV for radiation therapy or non-destructive material or safety testing, the bottleneck is ensuring sufficient cooling of the target element. Otherwise, the target element can age prematurely, be damaged, or even destroyed at such high beam powers and / or pulse energies.

[0006] For this reason, conventional target elements are embedded in a heat sink, for example, which enables the target element to be cooled and can generate comparatively little X-ray or scattered radiation compared to the target element itself. A conventional heat sink of this type is made of copper, for example. The challenge here is that with a target structure of this type, cracks can develop in the target element and / or a metallic connection between the target element and the heat sink can become detached. Alternatively or additionally, thermal stresses in particular occur in the target structure due to the intrinsic temperature gradient from the target element across the heat sink. The published patent application DE 29 26 823 A1 describes an electron linear accelerator with a device for generating bremsstrahlung.The device for generating bremsstrahlung has a target element made of a material with a comparatively high atomic number, for example tungsten (W, Z = 74). The target element is mounted in a carrier made of a material with a high thermal conductivity X, e.g. copper (Cu, Z = 29) with a thermal conductivity of XCu = 400 W / (m-K). In order to ensure the best possible thermal connection between the target element and the carrier, the target element is materially connected to the carrier at the jacket surfaces and / or the base surface. Furthermore, a cooling channel is arranged in the carrier which runs around the target element and in which a liquid cooling medium circulates (indirect cooling of the target). Since there is only one cooling channel, the cooling surfaces are relatively small and therefore only a correspondingly low level of heat dissipation.

[0007] Another possibility is to reduce the energy per X-ray pulse or to extend the cooling time between two consecutive X-ray pulses. Reducing the power typically requires increasing the pulse repetition rate. This usually results in a decrease in the efficiency of X-ray generation.

[0008] Alternatively or additionally, an expansion of the focal spot in the target structure, in particular in the target element in which the interaction with the electrons takes place to generate the X-radiation, can ensure that the energy deposited by the electrons in the focal spot does not lead to accelerated aging of the target element. The greater the expansion of the focal spot, the less sharp an image or irradiation generated by this X-radiation becomes. Therefore, ideally a focal spot is as small as possible. DE 10 2015 210 681 A1 relates to a device for generating bremsstrahlung with a bremsstrahlung target which is arranged in a carrier and can be cooled by a cooling device. The bremsstrahlung target is embedded in the carrier and the carrier is enclosed on the peripheral surface by a ring, wherein the ring has a smaller thermal expansion than the carrier.Such a device is highly thermally resilient and has a long service life.

[0009] Another known target element is designed to be rotatable, so that it can be rotated during X-ray generation and, for example, directly cooled in a liquid cooling medium. Depending on the design, this can broaden the electron beam directed onto the target element, increasing the blur. Furthermore, direct cooling can cause chemical wear through oxidation of the cooling medium and the target element.

[0010] A liquid metal target element is also known from the prior art. Due to its design, cooling is improved at the expense of increased blur, because a liquid metal target element typically has a lower density than a conventional tungsten (rhenium) target.

[0011] The inventors of this application have recently disclosed the prior art in the form of DE 20 2022 104 696 Ul. The target structure for generating X-rays of the DE utility model has

[0012] - a heat sink and

[0013] - a target element for applying electrons, which is embedded in the heat sink for cooling the target element, which is characterized in that the heat sink consists essentially of a metal-diamond composite material.

[0014] The present invention is based on the object of specifying a target structure for generating X-radiation, an electron linear accelerator and a standing anode transmission X-ray tube in which the thermomechanical stability of the target structure is further improved.

[0015] The problem is solved by the features of the independent claims. Advantageous embodiments are described in the subclaims.

[0016] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identity are included.

[0017] The target structure according to the invention for generating X-rays comprises:

[0018] - a heat sink and

[0019] - a target element for applying electrons, which target element is embedded in the heat sink for cooling the target element and is structurally distinguishable from the heat sink, wherein the heat sink consists of a metal-diamond composite material, wherein the metal-diamond composite material consists of diamond grains between which metal is located, characterized in that a diamond grain size of the metal-diamond composite material varies within the heat sink.

[0020] One advantage of the target structure is that a metal-diamond composite material is easier to construct than a conventional copper heat sink. The metal-diamond composite material advantageously allows for optimization of the cooling geometry, including an increase in the surface area of ​​the heat sink, for example, with cooling fins, due to the design freedom.

[0021] A further advantage may be that the lower density of the metal-diamond composite material compared to copper increases the photon yield when generating X-rays in the target element. Alternatively or additionally, the metal-diamond composite material is advantageous because, ideally, the thermal expansion coefficient of the metal-diamond composite material and the thermal expansion coefficient of the target material can be brought closer to one another. The approximation of the expansion coefficients particularly optimizes the thermomechanical connection between the target element and the heat sink. The thermomechanical connection is preferably low-stress, especially during operation of the target structure. Likewise, the metal-diamond composite material typically offers high thermal conductivity in order to be able to cool the target structure sufficiently when it is bombarded with electrons.Advantageously, the metal-diamond composite material enables an increase in the operating performance of the target assembly and / or an increase in the service life of the target assembly. The metal-diamond composite material typically exhibits a comparatively high specific thermal capacity.

[0022] The target structure for generating X-rays is, in particular, a device for generating bremsstrahlung radiation using a bremsstrahlung target. Depending on the application, the X-rays can, for example, have energy values ​​in the range between 20 and 150 keV for diagnostic imaging or non-destructive material testing and / or between 1 and 18 MeV for radiation therapy or non-destructive material or safety testing. At energy values ​​in the MeV range, the X-rays are typically called MeV X-rays.

[0023] The target element is the area of ​​the target structure which makes a significant contribution to the generation of X-radiation during operation. The target element preferably completely encompasses the focal spot. The X-radiation is generated in particular by the target element being exposed to electrons in the target structure. It follows that when the target element is exposed to electrons, the target element is hot. Before the target element is exposed to electrons, the target element is comparatively cold. After the target element is exposed to electrons, the target element cools in particular from a hot to a cold state. The target element can form a first area of ​​the target structure and the heat sink can form a second area of ​​the target structure. In an ideal target structure, only the target element contributes to the generation of X-radiation, while no electrons are scattered in the heat sink.The efficiency of X-ray generation and thus the proportion of useful X-ray radiation is determined in particular by the material of the target element and / or its thickness. The target element is, in particular, a bremsstrahlung target. The target element is particularly suitable for exposure to electrons with an energy greater than 20 keV, in particular greater than 1 MeV.

[0024] The target element is typically arranged in the heat sink. The target element is preferably embedded in the heat sink with a form-fitting fit. The heat sink in particular supports the target element. The target element is typically attached exclusively to the heat sink. The embedding of the target element in the heat sink means that the target element is in surface contact with the heat sink for heat dissipation. Typically, at least those sides of the target element which face away from the electron emitter and are therefore not directly exposed to the electron beam are at least partially in contact with the heat sink. In other words, one side of the target element is typically in contact with a vacuum at least in sections.

[0025] The target element is typically in the shape of a disc. The disc has a cylindrical shape. The disc is a cylindrical body, usually with a small height in relation to its diameter. The target element consists of a material with a high atomic number (atomic number, Z) and / or high density, e.g. silver, copper, gold, rhodium, tungsten, molybdenum, rhenium, chromium, cobalt, iron, manganese, vanadium, tantalum or iridium or an alloy of the conventional target materials described above. The target material of the target element can in particular be tungsten. Advantageously, the target element contains, for example, rhenium in addition to tungsten, so that the target element is more ductile and therefore more robust. Such a tungsten-rhenium target element typically cracks later under comparable loads than a pure tungsten target.

[0026] The mean free electron path depends on the energy of the incoming electrons. The target element is typically dimensioned such that its extent in any spatial direction is at least half, preferably a whole, of the mean free electron path. This advantageously increases the proportion of electrons that interact in the target element. If the target element is designed as a disc, the small height is usually at least 50% or 100% of the mean free electron path, while the radius of the disc is typically much larger, for example from 2 mm and / or up to 10 mm or 25 mm.

[0027] The target structure is fundamentally suitable for reflection and / or transmission with regard to the generated X-rays. Typically, the efficiency of a reflection target structure is higher at electron energies below 1 MeV, whereas the efficiency of a transmission target structure is higher at electron energies above 1 MeV. The efficiency relates to the proportion of useful radiation. Useful radiation is the X-ray radiation which exits the housing as intended and can be used in the application. In a reflection target structure, a large proportion of the X-ray radiation generated by the incoming electrons radiates back from the target structure and / or laterally. In a transmission target structure, a large proportion of the X-ray radiation generated by the incoming electrons preferably penetrates the target structure. The heat sink is a heat-dissipating element of the target structure.The heat sink preferably comprises exclusively the metal-diamond composite material. In principle, it is advantageous if the heat sink consists exclusively of the metal-diamond composite material. The heat sink is therefore preferably made of the metal-diamond composite material. The heat sink advantageously only has an insignificant proportion of further materials, which preferably can hardly and / or not at all reduce the thermal properties of the metal-diamond composite material. The further materials can be provided in particular to stabilize the heat sink and / or to increase the cooling performance, for example by means of a heat pipe. Depending on the geometry of the heat sink and / or depending on the performance range of the target structure, a further heat sink can be provided for cooling the target element and / or the heat sink in the target structure.

[0028] The heat sink and / or the target element can be in contact with a cooling medium on a side facing away from the electron beam. This allows the heat sink to be cooled, preferably during operation. Advantageously, the cooling medium is essentially transparent to the generated X-rays.

[0029] The metal-diamond composite material forms, in particular, a metal-diamond matrix. The metal-diamond composite material is characterized by its components of metal and diamond. The metal-diamond composite material is a mixture of pure metal and diamond. The diamond consists, in particular, of diamond powder. The diamond powder, in particular, has a large number of diamond grains. The diamond is a carbon-based diamond. In principle, other minerals and / or crystalline substances are also conceivable as replacements for diamond in combination with the metal, provided they have corresponding thermal properties.

[0030] According to the invention, a diamond grain size of the metal-diamond composite material varies within the heat sink. The variation in the diamond grain size is, in particular, a directional variation. The directional variation, in particular, goes beyond a purely statistical variation.

[0031] The fact that the diamond grain size varies means, in particular, that at least two diamond powders with different diamond grain sizes were used in the production of the metal-diamond composite. In other words, the metal-diamond composite preferably consists of at least two diamond powders that differ in diamond grain size.

[0032] The variation in diamond grain size may mean that the metal-diamond composite material has a first region with a first diamond grain size and a second region with a second diamond grain size, wherein the first diamond grain size and the second diamond grain size differ. The variation in diamond grain size particularly includes a variation in diamond grain size between regions of the metal-diamond composite material.

[0033] The diamond grain size can vary in more than two dimensions. For example, the diamond grain size can vary according to a diamond grain size gradient. The diamond grain size gradient typically exhibits linearly increasing or decreasing diamond grain sizes. Alternatively, it is conceivable that the diamond grain size gradient exhibits non-linear, for example, exponential, increasing or decreasing diamond grain sizes.

[0034] The variation of diamond grain size generally involves a repeated, for example, regular, increase and decrease in the diamond grain size. The variation of diamond grain size can be sinusoidal or sawtooth-shaped.

[0035] The variation in the diamond grain size can occur with respect to an average diamond grain size, wherein the average diamond grain size represents an average value of a plurality of diamond grains. The average value of the plurality of diamond grains can relate to a region of the metal-diamond composite material, in particular if this region comprises the plurality of diamond grains. The diamond grain size, in particular the average diamond grain size, regularly varies by at least 2%, for example 10%, preferably 15%, between the smallest and the largest (average) diamond grain size. It is particularly conceivable that the diamond grain size, in particular the average diamond grain size, varies in particular by less than 1000%, preferably less than 200%, between the smallest and the largest (average) diamond grain size. The smallest of the (average) diamond grain sizes is in particular 50 to 98% of the largest (average) diamond grain size.The diamond grain size varies in particular in such a way that the expansion varies by a factor between 0.1 and 10 , in particular between 2 and 3 .

[0036] One embodiment provides that the diamond grain size increases with increasing distance from the target element to compensate for an expansion dependent on a temperature gradient dependent on the exposure of the target element to electrons. The distance corresponds in particular to a distance. Expansion is defined in particular as the location-dependent expansion in the heat sink and not as an overall expansion of the heat sink. The temperature gradient arises in particular from the exposure of the target element to electrons. The expansion of the metal-diamond composite material is fundamentally temperature-dependent. The expansion varies in particular as a function of the radial and / or axial temperature distribution. The expansion depends in particular on the distance, as long as the diamond grain size is constant. In this embodiment, the diamond grain size advantageously depends on the distance of the respective diamond grain orof the respective section with the plurality of diamond grains from the target element. The diamond grain size in particular correlates directly with this distance. By increasing the diamond grain size, the expansion of the target element can advantageously be at least partially compensated. In principle, it is conceivable that the expansion is completely compensated, in particular equalized. The fact that the diamond grain size increases means that the diamond grain size increases with increasing distance. With decreasing distance, the diamond grain size decreases in particular.

[0037] An advantageous development of the previous embodiment is that the diamond grain size increases in such a way that in a region of the heat sink away from the target element, an expansion of the heat sink is the same as an expansion of the heat sink in a region of the heat sink adjacent to the target element. The region of the heat sink away from the target element is in particular a distant region. The region of the heat sink adjacent to the target element is in particular a close region. The close region can border on the target element or be delimited from the target element by another region.

[0038] The near region lies, for example, between the far region and the target element, for example along a line and / or at least in relation to the respective distances from the target element. The far region can border on the near region or be separated from the near region by another different region. The far region is typically at a greater distance from the target element than the near region. The fact that the diamond grain size increases in this way means, in particular, that the diamond grain size in the far region is larger than the diamond grain size in the near region. The fact that the dimensions are equal means, in particular, that the dimension in the far region is equal to, preferably corresponds to, the dimension in the near region.This embodiment is particularly advantageous since thermomechanical stress due to the different expansions in the heat sink can be at least reduced and advantageously prevented.

[0039] An alternative to the previous development is that the diamond grain size increases in such a way that in the region of the heat sink away from the target element the expansion of the heat sink is greater than the expansion of the heat sink in the region of the heat sink adjacent to the target element. This embodiment can in particular lead to thermomechanical stresses being shifted away from the interface between the target element and the heat sink to the remote region of the heat sink. This can in particular increase the stability of the target structure. If the expansion is greater in the remote region, a deformation, for example a certain curvature, of the target structure can occur, particularly in the remote region, while at the same time the expansion in the close region is less.Illustratively, in this embodiment it can be approximately assumed that the target structure curves due to the greater expansion in the distant area and / or forms a circular segment.

[0040] One embodiment provides that the diamond grain size is selected such that when the target element is subjected to electrons in a region of the heat sink adjacent to the target element, the expansion is smaller than the expansion of the target element. In this case, the nearby region directly borders the heat sink. Advantageously, this embodiment ensures that the metal-diamond composite material embedding the target element experiences less expansion than the target element. Thus, advantageously, a pressure directed particularly radially inwards can be exerted on the target element.

[0041] One embodiment provides for the diamond grain size to vary in planar layers. An advantage of this embodiment is that the diamond grain size can be varied comparatively easily in planar layers, thus making manufacturing less complex. For example, a first diamond powder can be applied first and a second diamond powder can be applied afterward. In this embodiment, for example, a first layer comprises the near region and a second layer the far region.

[0042] An alternative embodiment to the previous embodiment provides for the diamond grain size to vary in curved layers. An advantage of this embodiment is that radial pressure can be exerted relative to the target element. In this embodiment, for example, a first layer comprises the near region and a second layer the far region.

[0043] One embodiment provides that the target element has a division that deviates from a cylindrical shape. The fact that the target element has a division means in particular that the target element is divided into at least two target sub-elements. In other words, at least two target sub-elements form the target element. The target element is in particular made up of multiple pieces and / or not in one piece. The number of target sub-elements is at least two, for example more than three. It is conceivable that the number of target sub-elements is greater than five. The number of target sub-elements is regularly less than 1000, in particular 100, preferably 50. The fact that the target element has a division that deviates from a cylindrical shape means in particular that at least one of the target sub-elements does not have a cylindrical shape.In other words, the at least one target sub-element has a shape that deviates from a cylinder. The target sub-elements are preferably divided such that interfaces between target sub-elements have the largest possible angle to the incoming electron beam in order to prevent electrons from passing the target along these interfaces. The interfaces are therefore typically not aligned parallel to the electron beam. In principle, it is conceivable that several target sub-elements or all target sub-elements do not have a cylinder shape and / or have a shape that deviates from a cylinder. At least two target sub-elements can in principle have the same shape and / or the same dimensions. It is conceivable that each target sub-element has a different shape and / or different dimensions.For example, it is conceivable for two target sub-elements to have a cylindrical shape, in particular with the same dimensions. The two cylindrical target sub-elements can be stacked one on top of the other to form the target element. This embodiment is particularly advantageous if the shape of the target sub-elements preferably improves heat dissipation of the target element, ensuring at least equivalent heat dissipation to the unsegmented target element. Advantageously, the thermomechanical load, in particular, is reduced.

[0044] One embodiment provides that the target element is divided into conical target sub-elements, wherein at least some of the conical target sub-elements have such a conical cavity, so that stacked conical target sub-elements form a cylinder. The conical shape of the target sub-elements typically relates to the outer or positive shape of the target sub-elements. If a target sub-element has a conical cavity, the conical shape of the target sub-element can relate to the conical cavity and thus to the inner or negative shape. It is conceivable for a target sub-element to have a negative conical shape, but no positive conical shape. In this case, the target sub-element has in particular a conical cavity which is defined by a non-conical part of the target sub-element.One example is when the target sub-element has a cylindrical shape according to the positive shape of the target sub-element, which has a conical recess on one side. In this embodiment, the conical target sub-elements are advantageously designed such that a positive conical shape of one target sub-element fits preferably seamlessly into a conical cavity of another target sub-element. The distance between the conical surfaces of one target sub-element and the other target sub-element is preferably less than 1 mm, in particular less than 50 μm, preferably less than 10 μm. This distance is in particular 1 to 4 μm. The conical surface of the conical target sub-elements, in particular the conical surface of the innermost target sub-element, is in particular inclined to such an extent that propagation of the electrons along the conical surface is suppressed.

[0045] A further development which is particularly advantageous compared to the previous embodiment is when the shape of the target sub-elements is adapted to the temperature gradient which is dependent on the action of the electrons on the target element in such a way that heat conduction between conical surfaces of two adjacent target sub-elements is reduced. In other words, the shape of the target sub-elements follows the temperature gradient. Advantageously, the shape of the target sub-elements corresponds to the temperature gradient in such a way that heat conduction perpendicular to the conical surface is reduced. Following the shape or adapting the shape or corresponding to the shape concerns in particular an inclination of the conical target sub-elements. This embodiment is particularly advantageous because it means that heat conduction and thus the cooling of the target element is not impaired by the division of the target element.

[0046] One embodiment provides for the heat sink to have a shape that deviates from a truncated cone. A truncated cone shape is particularly characterized as a cone shape whose tip is truncated in a plane parallel to the base of the cone. Deviating from the truncated cone shape is particularly advantageous because it can improve the cooling of the heat sink, for example, because the shape of the heat sink causes less turbulence and / or impedes the flow of the cooling medium.

[0047] One embodiment provides for the heat sink to have a hemispherical shape. The term "hemispherical" also includes, in particular, a curvature with a height that does not equal the radius of the base of the hemispherical shape. This embodiment offers the advantage of simple manufacturing and increases the flow of the cooling medium in the area of ​​the curvature.

[0048] An alternative embodiment to the previous embodiment provides for the heat sink to have a bell shape, wherein the shape of the bell is designed to taper to a point such that a central beam of the X-ray radiation generated by the electrons impinging on the target element is attenuated in the center. The bell shape is characterized in particular by the fact that the cross section of the bell shape has an S-shape and is axially symmetrical. The tip of the bell shape particularly comprises a concave or convex line in the cross section.

[0049] One embodiment provides that the shape of the heat sink is adapted to the temperature gradient dependent on the exposure of the target element to electrons. In particular, the proportion of those surfaces of the heat sink that have a comparable distance from the target element can be increased. Alternatively or additionally, a distribution of distances, which indicates the respective distance from a point on the surface to the target element, can be narrowed. This can advantageously improve the cooling of the target structure.

[0050] One embodiment provides that the target element consists of silver, copper, gold, rhodium, tungsten, molybdenum, rhenium, chromium, cobalt, iron, manganese, vanadium, tantalum or iridium or an alloy of the previously described target materials and, in particular, the target element is structurally distinguishable from the heat sink. In this embodiment, a conventional structurally distinguishable target is advantageously combined with the heat sink made of the metal-diamond composite material. One embodiment provides that the metal-diamond composite material has silver, copper, gold, aluminum, rhodium, tungsten, molybdenum, rhenium, zirconium, chromium, cobalt, iron, manganese, vanadium, titanium, iridium or beryllium as the metal. The aforementioned metals can be provided as metal in the composite material in pure form or in mixed form, e.g. as an alloy, in particular tungsten-rhenium or titanium-zirconium-molybdenum.In principle, all conventional target materials can be used as metal in the metal-diamond composite.

[0051] A preferred embodiment provides that the metal-diamond composite material is a gold-diamond composite material or a silver-copper-aluminum-diamond composite material. This embodiment is particularly advantageous when the target element and the heat sink are made of the same metal-diamond composite material.

[0052] A further preferred embodiment provides that the metal-diamond composite material is a silver-diamond composite material. The selection of silver as the metal (in pure form) is particularly advantageous because with such a composite material, for example, in a temperature range between 0 and 200 °C, a thermal conductivity between 700 and 1000 W / (mK) , e.g. at 20°C room temperature, more than 800 W / (mK) , can be present. The silver-diamond composite material typically has a thermal expansion coefficient in the range around 6 ppm / K and / or a specific thermal capacity in the range around 310 J / (kg-K). This embodiment is particularly advantageous in combination with a conventional target element that is structurally different from the heat sink.

[0053] One embodiment provides that the metal-diamond composite material consists of diamond grains with metal between them. The diamond grains originate in particular from the diamond powder. Depending on the size of the diamond grains, there is a gap between the diamond grains in which the metal is provided. The metal is bonded to the diamond grains in particular in a form-fitting and / or material-fitting manner. The metal-diamond composite material is in particular produced using an infiltration process in which the metal is preferably introduced between the diamond grains. The larger the diamond grains, the smaller the contact area is typically between the individual grains and the introduced metal and / or the smaller the proportional volume of the diamond. In this embodiment, the metal-diamond composite material is typically produced using a preferred manufacturing process.This embodiment can be produced, for example, by incorporating the target element during the production of the metal-diamond composite material, for example by covering it with diamond powder.

[0054] One embodiment provides that the diamond grains are dimensioned such that the thermal conductivity of the metal-diamond composite material at room temperature is at least 400 W / (m-K), preferably at least 600 W / (m-K), particularly advantageously at least 800 W / (m-K). This embodiment is particularly advantageous because it allows for effective cooling of the target element by means of the heat sink.

[0055] One embodiment provides that the thermal conductivity of the metal-diamond composite material is at least twice, preferably at least three times, particularly advantageously at least four times the thermal conductivity of the target element. The thermal conductivity of the metal-diamond composite material therefore preferably significantly exceeds that of a conventional target element, which is particularly advantageous because typical target materials such as tungsten have a comparatively low thermal conductivity in the range around 150 W / (m-K). One embodiment provides that the diamond grains are dimensioned such that a thermal expansion coefficient of the metal-diamond composite material is less than 12 ppm / K, preferably less than 9 ppm / K, particularly advantageously less than 6 ppm / K.The thermal expansion coefficient of tungsten, which is regularly used as a target material, is in the range of 4.3 ppm / K. The smaller the difference between the thermal expansion coefficient of the metal-diamond composite material and the thermal expansion coefficient of the target element, the less stress can be exerted when joined.

[0056] One embodiment provides that the value of the thermal expansion coefficient of the metal-diamond composite material is at most 3 times, preferably at most 2 times, particularly advantageously at most 1.5 times the value of the thermal expansion coefficient of the target element. Preferably, the value of the thermal expansion coefficient of the metal-diamond composite material corresponds to the value of the thermal expansion coefficient of the target element. An advantage of this embodiment is that the heat sink and the target element are advantageously connected to one another with low stress, thus ensuring the thermomechanical stability of the target structure due to the improved cooling. In other words, the relative expansion during heating of the heat sink and the target element is in a range in which the lifetime connection of the target element to the heat sink is guaranteed.

[0057] One embodiment provides for the target element to be held in the heat sink by a material-to-material connection. This preferably ensures good thermal connection between the target element and the heat sink. This embodiment can be achieved, for example, by incorporating the target element during the production of the metal-diamond composite material, for example, by preserving the diamond powder, or by forming the target element and the heat sink in one piece and / or from a single casting.

[0058] One embodiment provides that the heat sink completely surrounds the target element. In other words, the heat sink encloses the target element such that, in particular, each side of the target element, preferably additionally the side facing the vacuum, is surrounded by the heat sink. On the side facing the vacuum, the heat sink preferably has a thickness which is less than 0.3 times, preferably 0.05 times, the mean free electron path in the material of the metal-diamond composite.

[0059] Such a heat sink is particularly suitable for a transmission target structure. An advantage of this embodiment is that bending stress and / or temperature and / or the number of backscattered electrons are reduced.

[0060] In the following embodiments, the respective electron emitter, the respective housing and the respective target structure can be constructed essentially the same and / or have the essentially same properties, so that individual adaptation to the respective intended use should be generally known and is therefore not repeated separately for each embodiment.

[0061] The electron emitter and target assembly are typically arranged in a housing. The housing is evacuated, preferably with a high vacuum.

[0062] The electron emitter is typically arranged on a side of the evacuated housing opposite the target structure. The electron emitter in particular has a thermionic emitter, for example a helical emitter or a spherical emitter, or a cold emitter, for example with carbon tubes, silicon tubes or molybdenum tubes. The electron emitter can have a grid for regulating the electron beam. Depending on the embodiment, an X-ray exit window can be characterized in that it is formed by a recess in a shielding device which is arranged around the housing. The recess can be closed, for example, with the target structure or with a metal or glass that is quasi-transparent to X-rays. Depending on the embodiment, the X-ray exit window encloses in particular the vacuum and thus usually forms part of the housing.

[0063] An electron linear accelerator according to the invention has

[0064] - linearly arranged cavities for accelerating electrons as part of an evacuated housing,

[0065] - an electron emitter for emitting the electrons in the housing and

[0066] - a target structure in the housing, wherein the target element is arranged between an X-ray exit window of the housing and the electron emitter.

[0067] For radiation therapy or non-destructive material or safety testing, the electron linear accelerator can advantageously combine a high dose rate with a small focal spot. Typical energy values ​​are, for example, 3 J per pulse with an electron beam diameter of 1.3 mm and an average power of 1 kW.

[0068] The electron linear accelerator is designed in particular to generate MeV X-rays. The electron linear accelerator enables the acceleration of electrons along a substantially straight line within the linearly arranged cavities. The electron linear accelerator is in particular multi-energy capable. The cavities are linear accelerator cavities and / or form at least part of the housing. The housing can further comprise a drift tube following the cavities. The electron linear accelerator has the evacuated housing. The target structure is in particular a transmission target structure.

[0069] In this embodiment, the electrons are typically emitted in pulsed form from the electron emitter into the evacuated enclosure. The electrons form an electron beam with a current of typically up to 1 A.

[0070] Depending on the type of electron linear accelerator, the electrons are accelerated to energies above 1 MeV, particularly by means of a radio-frequency source in the cavities. The kinetic energy in the MeV range is typically adjustable and can be up to 18 MeV, or in the range of 3 to 9 MeV.

[0071] The high-frequency source is designed to accelerate the electrons within the linearly arranged cavities and typically comprises a magnetron or a klystron for this purpose. A reflection phase shifter combined with a circulator for rapidly varying the high-frequency power can also be provided between the high-frequency source and the cavities.

[0072] The electron linear accelerator can be used in a stationary or mobile configuration. A stationary application, for example, is in a medical radiation device. Particularly for non-destructive safety testing or materials testing, the electron linear accelerator can be mounted on a truck. In principle, any application can be carried out using the electron linear accelerator in a stationary or mobile configuration.

[0073] One embodiment provides for the target structure to be vacuum-tightly soldered to the evacuated housing, forming the X-ray exit window. An advantage of the metal-diamond composite material is that it can generally be soldered. This allows the target structure to be used as an X-ray exit window. Such a target structure is particularly suitable for use in a standing anode transmission X-ray tube.

[0074] In contrast to the electron linear accelerator, the X-ray tubes described below do not use a radio-frequency source to accelerate the electrons. Instead, X-ray tubes typically have a high-voltage source that accelerates the electrons from a cathode toward the anode. The acceleration voltage is, for example, between 20 and 150 keV. The housing is typically made of metal and / or glass and / or ceramic. The housing usually has an insulator.

[0075] A standing anode transmission X-ray tube according to the invention has

[0076] - an evacuated housing,

[0077] - an electron emitter for emitting electrons in the housing and

[0078] - a target structure as an anode in the housing, wherein the target element is arranged between an X-ray exit window of the housing and the electron emitter.

[0079] The target structure of the vertical anode transmission X-ray tube and the electron linear accelerator are both transmission target structures. The anode used in this X-ray tube is a vertical anode and typically has an anode angle of 0°, so that the electron beam impinges perpendicularly on the anode surface.

[0080] A standing anode reflection X-ray tube has

[0081] - an evacuated housing,

[0082] - an electron emitter for emitting electrons in the housing and

[0083] - a target structure as a standing anode in the housing, wherein an X-ray exit window of the housing is arranged to the side of or opposite the target structure. The standing anode reflection X-ray tube with the X-ray exit window arranged to the side of the target structure is typically a so-called side-window tube. The standing anode reflection X-ray tube with the X-ray exit window arranged opposite the target structure is regularly a so-called end-window tube. In the latter case, the electron emitter is typically arranged to the side of the target structure so that the generated X-rays can exit the housing from the front (axially).

[0084] The anode used in this X-ray tube is a vertical anode and can have an anode angle greater than or equal to 0 ° so that the X-ray radiation can exit through the X-ray exit window facing the particularly tilted surface.

[0085] A rotating anode reflection X-ray tube has

[0086] - an evacuated housing,

[0087] - an electron emitter for emitting electrons in the housing and

[0088] - a target structure as a rotating anode in the housing, wherein an X-ray exit window of the housing is arranged laterally or opposite the target structure.

[0089] The vertical anode transmission X-ray tube, the vertical anode reflection X-ray tube, and / or the rotating anode reflection X-ray tube feature a heat sink made of a metal-diamond composite material and thus share the previously described advantages of using a metal-diamond composite material. The vertical anode transmission X-ray tube, the vertical anode reflection X-ray tube, and / or the rotating anode reflection X-ray tube are specifically designed to generate X-rays up to 150 keV.

[0090] The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures. In the following description of the figures, essentially identical structures and units are designated by the same reference numerals as when the respective structure or unit first appeared.

[0091] They show:

[0092] Fig. 1 a target structure,

[0093] Fig. 2 shows the target structure in another embodiment,

[0094] Fig. 3 shows the target structure in a further embodiment,

[0095] Fig. 4 shows a conventional target structure,

[0096] Fig. 5 the spatial thermal expansion of the conventional target structure,

[0097] Fig. 6 the spatial thermal expansion of the target structure according to the invention,

[0098] Fig. 7 the spatial thermal expansion of the other target structure,

[0099] Fig. 8 a standing anode transmission X-ray tube,

[0100] Fig. 9 an electron linear accelerator,

[0101] Fig. 10 a target structure and the electron emitter,

[0102] Fig. 11 the target structure of Fig. 2 in a further view,

[0103] Fig. 12 an illustration of the properties of metal-diamond composites,

[0104] Fig. 13 shows a relationship between the properties of metal-diamond composite materials,

[0105] Fig. 14 a target structure with a hemispherical heat sink,

[0106] Fig. 15 a target structure with a bell-shaped heat sink,

[0107] Fig. 16 a target element in a special embodiment and

[0108] Fig. 17 Target sub-elements in an exploded view.

[0109] Fig. 1 shows a target structure 10 for generating X-rays. The target structure 10 has a heat sink 11 and a target element 12 for the application of electrons. The target element 12 is embedded in the heat sink 11 for cooling the target element 12 and is structurally distinguishable from the heat sink 11. The heat sink 11 consists of a metal-diamond composite material. The metal-diamond composite material consists of diamond grains with metal sandwiched between them. The diamond grain size of the metal-diamond composite material varies within the heat sink 11, typically in flat or curved layers. The heat sink 11 has a truncated cone shape.

[0110] In this exemplary embodiment, the connection of the target structure 10 to an upper section of a housing 13 is shown. The intermediate space enclosed by the target structure 10 or by this section of the housing 13 is evacuated during operation and tapers towards the target structure 10. The target element 12 has a lower side exposed to the vacuum. The upper side of the target element 12 borders flatly on the heat sink 11 and is thus in contact with it for cooling. The upper side of the heat sink 11 can be designed with a cooling medium for additional cooling.

[0111] Fig. 2 shows the target structure 10 of Fig. 1 in another embodiment, which differs in its geometric configuration. In this embodiment, the space enclosed by the target structure 10 or the housing 13 has a T-shape. The heat sink 11 has a truncated cone shape.

[0112] Furthermore, it is shown in Fig. 2 with the arrows that the diamond grain size increases with increasing distance from the target element 12 in order to compensate for an expansion which is dependent on a temperature gradient which is dependent on the action of the electrons on the target element 12. The diamond grain size preferably increases in such a way that in a region of the heat sink 11 away from the target element 12, an expansion of the heat sink 11 is the same or greater than an expansion of the heat sink 11 in a region of the heat sink 11 adjoining the target element 12. Alternatively or additionally, the diamond grain size can be selected in such a way that when the target element 12 is acted upon by electrons in a region of the heat sink 11 adjoining the target element 12, an expansion is smaller than an expansion of the target element 12.

[0113] Fig. 3 shows the target structure 10 of Fig. 1 or Fig. 2 in a further embodiment, which has a further geometric configuration. In this embodiment, the heat sink 11 completely surrounds the target element 12. The target element 12 is thus separated from the vacuum by a portion of the heat sink 11. The heat sink 11 has a truncated cone shape.

[0114] The target structure 10 is arranged in an arrangement with the housing 13, an insulator 16 and a current measuring unit 17. The insulator 16 electrically insulates the housing 13 from the target structure 10, in particular from the heat sink 11. The insulator 16 can be made of ceramic, for example. Advantageously, the insulator 16 is a disk with a central recess through which the electrons can fly towards the target element 12. The current measuring unit 17 is connected to the heat sink 11 and the housing 13. By means of the current measuring unit 17, a target current, which indicates the amount of incoming electrons, can advantageously be measured. Advantageously, the expansion coefficient of the heat sink 11 is adapted to the insulator in a stress-relieved manner by means of an appropriate diamond grain size.

[0115] The target structures 10 shown in Fig. 1 to 3 can, despite the geometric differences, be constructed in principle in the same way with individual or in a combination of the individual developments:

[0116] In a further development, the target element 12 consists of silver, copper, gold, rhodium, tungsten, molybdenum, rhenium, chromium, cobalt, iron, manganese, vanadium, tantalum or iridium or an alloy of the previously described target materials.

[0117] In a further development, the metal-diamond composite material can contain silver, copper, gold, aluminum, rhodium, tungsten, molybdenum, rhenium, zirconium, chromium, cobalt, iron, manganese, vanadium, titanium, iridium or beryllium as the metal.

[0118] In a particularly advantageous further development, the metal-diamond composite material is a silver-diamond composite material.

[0119] The metal-diamond composite material consists primarily of diamond grains with metal sandwiched between them.

[0120] The diamond grains are preferably dimensioned such that a thermal conductivity of the metal-diamond composite material at room temperature is at least 400 W / (m-K), preferably at least 600 W / (m-K), particularly advantageously at least 800 W / (m-K).

[0121] The thermal conductivity of the metal-diamond composite material is advantageously at least 2 times, preferably at least 3 times, particularly advantageously at least 4 times the thermal conductivity of the target element 12. The diamond grains can in particular be dimensioned such that a thermal expansion coefficient of the metal-diamond composite material is less than 12 ppm / K, preferably less than 9 ppm / K, particularly advantageously less than 6 ppm / K.

[0122] The value of the thermal expansion coefficient of the metal-diamond composite material is advantageously at most 3 times, preferably at most 2 times, particularly advantageously at most 1.5 times the value of the thermal expansion coefficient of the target element 12.

[0123] The target element 12 is preferably held in the heat sink 11 by a material connection.

[0124] Fig. 4 shows a conventional target structure 20 according to the prior art for generating X-rays. The target structure 20 comprises a conventional heat sink 14 and a target element 12 for applying electrons. The target element 12 is embedded in the heat sink 14 for cooling the target element 12. The heat sink 14 is made of copper and has a truncated cone shape.

[0125] Fig. 5 shows the spatial thermal expansion of the conventional target structure 20 with copper as an example.

[0126] Fig. 6 shows the spatial thermal expansion of the target structure 10 according to the invention with the metal-diamond composite material as an example.

[0127] Fig. 7 shows the spatial thermal expansion of the other target structure 10 as an example.

[0128] Figs. 5 to 7 are based on the heat distribution that typically arises during operation of such target assemblies 10, 20. The heat distribution includes, in particular, temperatures of over 1000°C at the focal spot of the target element 12, temperatures in the range between 200 and 800°C at the interface between the target element 12 and the heat sink 11, depending on the size and design of the target, as well as significantly lower temperatures on the sides of the heat sink 11 facing away from the target element 12.

[0129] Based on this operational heat distribution, the arrows indicate a sideways, i.e. radial, expansion, which is relatively scaled for illustration purposes.

[0130] In the target structure 10 according to the invention in Fig. 6, in comparison to the conventional target structure 20, it can be seen that at the interface between the heat sink 11 and the target element 12 there is a significantly smaller expansion than between the conventional heat sink 14 and the target element 12.

[0131] In the exemplary embodiment of the other target structure 10 in Fig. 7, the arrows represent the expansion when the diamond grain size increases with increasing distance from the target element 12 to compensate for an expansion dependent on a temperature gradient dependent on the exposure of the target element 12 to electrons. The diamond grain size is selected such that when the target element 12 is exposed to electrons in a region of the heat sink 11 adjacent to the target element 12, the expansion is smaller than the expansion of the target element 12.

[0132] Fig. 8 shows a vertical anode transmission X-ray tube 30 in a cross-section. The vertical anode transmission X-ray tube 30 has an evacuated housing 13, an electron emitter 43 for emitting electrons in the housing 13 and a target structure 10 as a vertical anode in the housing 13. The target structure 10 is soldered to the evacuated housing 13 in a vacuum-tight manner. Fig. 9 shows an electron linear accelerator 40. The electron linear accelerator 40 has linearly arranged cavities for accelerating the electrons as part of an evacuated housing 13, an electron emitter (not shown) for emitting the electrons in the housing 13 and a target structure 10 in the housing 13. The target element 12 is arranged between an X-ray exit window 31 of the housing 13 and the electron emitter.

[0133] As a further development, it is conceivable that the target structure 10 is soldered to the evacuated housing 13 in a vacuum-tight manner in order to form the X-ray exit window 31.

[0134] A shielding device 41 is arranged around the housing 13 and is shown in sections. The shielding device 41 is designed in particular to reduce scattered radiation and, for this purpose, comprises, in particular, lead and / or tungsten. In this exemplary embodiment, a further X-ray exit window 42 is formed by a recess in the shielding device 41.

[0135] Another part of the housing 13 is the cylindrical drift tube 15 made of a nickel-iron-cobalt alloy, which connects the target structure 10 to the linearly arranged cavities. The cavities are typically made of copper.

[0136] The electron linear accelerator 40, in particular the target structure 10, can be cooled with a cooling medium, in particular a liquid one, which is preferably guided through the X-ray exit window 31 and along the housing 13 as indicated by the arrows in Fig. 9. The shielding device 41 and / or the housing 13 can in particular be cylindrical in order to enable the flow of coolant in the tubes formed thereby. Typically, the cooling channels for cooling the target structure 10 are part of the drift tube 15. Fig. 10 shows the target structure 10 and the electron emitter 43 within the evacuated housing 13. The schematic representation of Fig. 10 applies essentially to all electron accelerators such as the electron linear accelerator 40 or the X-ray tube 30. The electrons are generated by means of the electron emitter 43 and accelerated towards the target structure 10 within the high vacuum.

[0137] Fig. 11 shows the target structure 10 of Fig. 2. In Fig. 11, an exemplary and exaggerated illustration shows the relationship between the increasing diamond grain size and the greater expansion in the distant region. The vertical arrow represents the increase in diamond grain size. The horizontal arrow indicates the expansion in the distant region. The dashed outline shows the deformation of the heat sink 11 resulting from the hot state of the target structure 10.

[0138] Fig. 12 shows an illustration of the properties of metal-diamond composite materials. The falling line alpha shows the thermal expansion coefficient alpha in the unit ppm / K. The S-shaped line shows the thermal conductivity TC in the unit W / (m-K). These properties form the basis of the invention, according to which the different thermal expansion coefficients depending on the diamond grain size can be used to reduce mechanical stress. Varying diamond grain size allows locally adapted expansion coefficients to adapt and / or compensate for the internal mechanical stress. In this way, for example, that aspect of the invention can be realized wherein a more distant and therefore typically cooler region of the heat sink has a higher expansion coefficient due to larger diamond grains.

[0139] Fig. 13 shows a relationship between the properties of metal-diamond composite materials in four diagrams over the distance of the respective diamond grain to the target element.

[0140] The temperature gradient is shown at the top left, with the temperature decreasing with increasing distance from the target element. At the top right, the diamond grain size of an advantageous embodiment is plotted against the distance. The solid line indicates an ideal curve, which corresponds to a continuous variation. The dashed line shows a step curve, in which the transition between the diamond grain sizes is more significant than in the ideal curve.

[0141] The bottom row contains two different representations in which the expansion alpha * T is plotted against the distance. On the bottom left, the diamond grain size increases in such a way that in an area of ​​the heat sink away from the target element, the expansion of the heat sink is the same as in an area of ​​the heat sink adjacent to the target element. On the bottom right, the diamond grain size increases in such a way that in an area of ​​the heat sink away from the target element, the expansion of the heat sink is greater than in an area of ​​the heat sink adjacent to the target element.

[0142] Fig. 14 shows a target structure 10 with a hemispherical heat sink. The heat sink 11 has a shape other than a truncated cone. The heat sink 11 has a hemispherical shape.

[0143] The shape of the heat sink 11 is adapted to a temperature gradient dependent on the electron impingement on the target element 12, which means that the shape of the heat sink follows a temperature isoline that is perpendicular to the temperature gradient. Furthermore, the shape of the heat sink 11 is optimized with respect to the flow behavior of a cooling medium directed toward the heat sink 11.

[0144] Fig. 15 shows a target structure 10 with a bell-shaped heat sink. The heat sink 11 has a shape other than a truncated cone. The heat sink 11 has a bell shape.

[0145] The shape of the heat sink 11 is adapted to a temperature gradient dependent on the exposure of the target element 12 to electrons. Furthermore, the shape of the heat sink 11 is optimized with respect to the flow behavior of a cooling medium directed toward the heat sink 11. The shape of the bell is tapered in such a way that a central beam of the X-ray radiation generated by the exposure of the target element 12 to electrons is attenuated in the center. The heat sink 11 acts, in particular, as a so-called flattening filter.

[0146] Fig. 16 shows the target element 12 in a special embodiment.

[0147] The target element 12 has a division that deviates from a cylindrical shape. The target element 12 is divided into conical target sub-elements. At least some of the conical target sub-elements, in Fig. 16 all except the central target sub-element, have such a conical cavity that stacked conical target sub-elements form a cylinder. The shape of the target sub-elements is adapted to a temperature gradient that is dependent on the exposure of the target element 12 to the electrons in such a way that heat conduction between conical surfaces of two adjacent target sub-elements is reduced.

[0148] Fig. 17 shows one half of each conical target sub-element in an exploded view.

[0149] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is nevertheless not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Patent claims 1. Target structure (10) for generating X-ray radiation, comprising: - a heat sink (11) and - a target element (12) for the application of electrons, which is inserted into the heat sink (11) for cooling the target element (12) and is structurally distinguishable from the heat sink (11), wherein the heat sink (11) consists of a metal-diamond composite material, wherein the metal-diamond composite material consists of diamond grains between which metal is located, characterized in that a diamond grain size of the metal-diamond composite material varies within the heat sink (11).

2. Target structure (10) according to claim 1, wherein the diamond grain size increases with increasing distance from the target element (12) to compensate for an expansion dependent on a temperature gradient dependent on the exposure of the target element (12) to the electrons.

3. Target structure (10) according to claim 2, wherein the diamond grain size increases such that in a region of the heat sink (11) away from the target element (12) an expansion of the heat sink (11) is equal to or greater than an expansion of the heat sink (11) in a region of the heat sink (11) adjacent to the target element (12).

4. Target structure (10) according to one of the preceding claims, wherein the diamond grain size is selected such that when the target element (12) is subjected to electrons in a region of the heat sink (11) adjacent to the target element (12), an expansion is smaller compared to an expansion of the target element (12).

5. Target structure (10) according to one of the preceding claims, wherein the diamond grain size varies in planar layers.

6. Target structure (10) according to one of claims 1 to 4, wherein the diamond grain size varies in curved layers.

7. Target structure (10) according to one of the preceding claims, wherein the target element (12) has a division other than a cylindrical shape.

8. Target structure (10) according to claim 7, wherein the heat sink (11) has a shape other than a truncated cone.

9. Target structure (10) according to claim 8, wherein the heat sink (11) has a hemispherical shape.

10. Target structure (10) according to claim 8, wherein the heat sink (11) has a bell shape, wherein the shape of the bell is tapered in such a way that a central beam of the X-ray radiation generated by the impingement of the target element (12) with the electrons is attenuated centrally.

11. Target structure (10) according to one of claims 8 to 10, wherein the shape of the heat sink (11) is adapted to a temperature gradient dependent on the application of the electrons to the target element (12).

12. Target assembly (10) according to one of claims 7 to 11, wherein the target element (12) is divided into conical target sub-elements, wherein at least a portion of the conical target sub-elements has such a conical cavity, so that stacked conical target sub-elements form a cylinder.

13. Target assembly (10) according to one of claims 7 to 12, wherein the shape of the target sub-elements is adapted to a Exposure of the target element (12) to the electron-dependent temperature gradients is adapted such that heat conduction between conical surfaces of two adjacent target sub-elements is reduced.

14. Electron linear accelerator (40) comprising - linearly arranged cavities for accelerating electrons as part of an evacuated housing (13), - an electron emitter for emitting the electrons in the housing (13) and - a target structure (10) according to one of the preceding claims in the housing (13).

15. Standing anode transmission X-ray tube (30) comprising - an evacuated housing (13) , - an electron emitter for emitting electrons in the Housing (13) and - a target structure (10) according to one of claims 1 to 13 as a standing anode in the housing (13).

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