Getter device for an X-ray tube

Abstract

Getter device (10) for an X-ray tube (20), comprising - an evacuated housing (11) with a high vacuum containing residual gases and - a first getter (12) for adsorbing the residual gases from the high vacuum and a second getter (13) for adsorbing the residual gases from the high vacuum, - wherein the second getter (13) is designed and arranged to adsorb hydrogen desorbed from the first getter (12), - wherein the housing (11) forms a vacuum-maintaining internal volume to which no vacuum pump can be connected after the evacuation of the housing (11), characterized in that the first getter (12) is a non-evaporable, block-shaped getter, that the second getter (13) is a non-evaporable, layer-shaped getter and that the second getter (14) is applied to a region of the inside of the housing (11) by means of vapor deposition before evacuation of the housing (11), in particular as a diffusion barrier for hydrogen.

Description

[0001] The invention relates to a getter device for an X-ray tube, the X-ray tube, an imaging system and a computed tomography system.

[0002] The invention is based on a getter device for an X-ray tube, which has an evacuated housing with a high vacuum containing residual gases and a first getter for adsorbing the residual gases and a second getter for adsorbing the residual gases, wherein the second getter is designed and arranged to adsorb hydrogen desorbed by the first getter and wherein the housing forms a vacuum-maintaining internal volume to which no vacuum pump can be connected after the evacuation of the housing.

[0003] Such getter devices with getters for absorbing residual gases are generally known from US Pat. No. 6,570,959 B1 or US Pat. No. 6,192,106 B1. Furthermore, it is known that molecules of noble gases, which may be components of the residual gases, cannot be adsorbed, i.e., pumped, with getter devices.

[0004] US Pat. No. 6,570,959 B1 describes an X-ray tube with a metal frame getter system. When activated, the getter system forms a layer of getter material on a grounded, conductive part of the metal frame or housing. This allows for larger quantities of getter material to be provided than in conventional getter systems, which are typically housed in the cathode assembly. Multiple independently controllable getter wires allow the getter material to be reactivated multiple times during operation by applying getter material in layers to cold surfaces on the inside of the housing, extending the service life of the X-ray tube.

[0005] US 6,192,106 B1 discloses an X-ray tube with an evacuated housing containing an electrode and a getter. The getter is electrically connected to conductors passing through the housing. Diodes are connected to the electrode and the getter to selectively conduct electrical energy through the conductors. This arrangement allows the getter to be activated and the performance of the X-ray tube to be improved by maintaining the vacuum in the housing.

[0006] DE 10 2012 212 409 A1 describes a vacuum housing containing a getter material and an oxidizing substance. The getter material is suitable for getterizing a chemisorbing compound, and the oxidizing substance converts a physisorbing substance into the chemisorbing compound at a temperature of at least 200°C. The chemisorbing compound can then be getterized by the getter material. The preferred oxidizing substances are copper(I) oxide (Cu2O) and copper(II) oxide (CuO). The vacuum housing is particularly advantageous for use in X-ray tubes, where it helps maintain a high vacuum by effectively binding residual gases.

[0007] DE 19 842 949 C1 discloses an electron beam tube, in particular an X-ray tube, with a getter device that is heated to its operating temperature by electron irradiation. The getter device is protected from direct irradiation by a solid shielding body that is thermally coupled to the getter device. This variant utilizes the unavoidable backscattered electrons in an electron beam tube to heat the getter to its operating temperature, thereby ensuring optimal gas binding and maintaining the vacuum in the tube. A specific feature of this variant is that the getter device must be positioned relatively close to the anode and / or shielded from excessive heating by means of the solid shielding body.

[0008] US 5,509,045 A discusses an X-ray tube with an evacuated housing containing an anode, a cathode, and a getter screen. The getter screen comprises a sleeve and a cap with an annular groove containing getter material. The getter material is activated during normal depletion of the X-ray tube and passively heated during operation to maintain its pumping temperature. This embodiment ensures efficient gas binding and maintains the vacuum condition in the X-ray tube. However, the activation of the getter is coupled to the operation of the electron emitter, so that the increased hydrogen content caused by the getter could impair the operation of the X-ray tube. Furthermore, the getter of this disclosure must necessarily be positioned relatively close to the heat source, such as the electron emitter or the anode.

[0009] US 6 044 129 A describes an X-ray tube comprising an anode, a cathode, and an electrode within an evacuated housing. The electrode is positioned so that arcs occur preferentially between the electrode and the anode rather than between the cathode and the anode. The electrode consists of an active metal that acts as a getter material, particularly when passively heated by the anode. This embodiment helps to reduce metal deposits detached from the electrode on the inner surface of the housing, which deposits can also act as getters, and improves the vacuum capability of the X-ray tube. What is specific about this embodiment is that the distribution of the metal deposit is uncontrolled and / or the activation occurs at undefined temperatures.

[0010] US 7 558 376 B2 discloses a rotating anode X-ray tube assembly comprising a vacuum housing having an anode target, a casing that accommodates and rotatably supports at least the vacuum housing, a circulation path that circulates a coolant in a closed state to the anode target of the vacuum housing, a cathode accommodated and arranged in the vacuum housing, a cathode support that supports the cathode, a bearing mechanism and a vacuum sealing mechanism arranged between the vacuum housing and the casing or a stationary part directly or indirectly attached to the casing, and a drive unit for rotating the vacuum housing.

[0011] DE 10 2012 223 569 A1 relates to an X-ray tube with a vacuum housing in which at least one cathode and one anode are insulated by insulating elements. When a high voltage is applied, the cathode emits electrons, which then strike the anode to generate X-rays. The invention includes a voltage diverter with an insulation path having a higher field strength than the insulation element, whereby voltage spikes are diverted by the voltage diverter and the insulation elements are protected from damage. This embodiment ensures reliable protection of the functional parts of the X-ray tube throughout its entire operating life.

[0012] In general, getter devices are essential for the operation of X-ray tubes. In an X-ray tube, X-ray radiation is generated primarily by bremsstrahlung. In the high vacuum of the evacuated housing, electrons strike an anode, which have previously been accelerated, for example, by the application of high voltage. The vacuum quality of the high vacuum is important, among other things, for the high voltage stability and / or X-ray beam quality of the X-ray tube. Therefore, the vacuum quality must ideally be maintained over the entire service life of the X-ray tube. In certain X-ray applications, e.g. in a computed tomography system for computed tomography, the X-ray tube cannot be connected to a vacuum pump, particularly after the X-ray tube has been manufactured, and preferably after the housing has been evacuated. In this case, the X-ray tube is a sealed vacuum recipient.In particular, the housing forms an internal volume that maintains a vacuum.

[0013] The high vacuum in the housing of the X-ray tube is affected by various, typically unavoidable, processes such as gas diffusion through the shell of the housing, outgassing of materials within the housing, vapor pressure from heated materials within the housing and / or leaks in the housing over time, so that the vacuum quality becomes increasingly poor.

[0014] The invention is based on the object of specifying a getter device for an X-ray tube, the X-ray tube, an imaging system and a computed tomography system, which are improved, in particular a vacuum quality in the evacuated housing is increased.

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

[0016] The getter device according to the invention for an X-ray tube has - an evacuated housing with a high vacuum containing residual gases and - a first getter for adsorbing the residual gases and a second getter for adsorbing the residual gases, - wherein the second getter is designed and arranged to adsorb hydrogen desorbed from the first getter, - wherein the housing forms a vacuum-maintaining internal volume to which no vacuum pump can be connected after the evacuation of the housing, characterized by that the first getter is a non-evaporable, block-shaped getter, that the second getter is a vapor-deposited, non-evaporable, layered getter and that the second getter is applied to a region of the inside of the housing by means of vapor deposition before evacuation of the housing, in particular as a diffusion barrier for hydrogen.

[0017] According to one embodiment, the greatest distance between the first getter and the second getter is smaller than the smallest distance between the first getter and a region of the housing not coated with the second getter.

[0018] According to one embodiment, the surface of the second getter facing the vacuum is larger than a surface of the first getter.

[0019] According to one embodiment, a surface of the second getter substantially corresponds to the area of the region.

[0020] According to one embodiment, the second getter is configured and arranged to be reactivated by the operation of the first getter.

[0021] An X-ray tube according to the invention has - the getter device, - an electron emitter within the housing for generating electrons and - an anode within the housing for generating X-rays depending on the electrons generated. Reference is made to all statements regarding the getter device according to the invention.

[0022] According to one embodiment, the first getter and the second getter are arranged as far away as possible from the electron emitter and the anode.

[0023] According to one embodiment, the first getter is spaced apart relative to the electron emitter and the anode such that during the emission of electrons the first getter is kept below its activation temperature.

[0024] According to one embodiment, the first getter can be heated to a temperature in the range between 280°C and 320°C in order to pump methane from the high vacuum, while the second getter acts to reduce the desorption.

[0025] An imaging system according to the invention comprises - the X-ray tube and - an X-ray detector for detecting the generated X-rays. Reference is made to all statements regarding the getter device and the X-ray tube according to the invention.

[0026] A computer tomography system according to the invention has - a gantry with a rotating part and a stationary part and - the imaging system, which is designed as a component of the rotating part of the gantry. Reference is made to all statements regarding the getter device according to the invention, the X-ray tube according to the invention, and the computed tomography system according to the invention.

[0027] One advantage of the invention is that the vacuum quality of the high vacuum of the X-ray tube is ensured, preferably kept constant. In particular, the high vacuum is not impaired by hydrogen molecules desorbed during heating of the first getter and / or the second getter. Particularly advantageously, the first getter can be operated at higher temperatures without significantly increasing thermal desorption in the regions of the interior of the housing adjacent to the first getter.

[0028] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

[0029] 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 generally designated by the same reference numerals as when the respective structure or unit first appeared.

[0030] They show: Fig. 1 a getter device according to the invention, Fig. 2 an X-ray tube according to the invention and Fig. 3 a computer tomography system according to the invention.

[0031] Fig. 1 shows a section of a getter device 10 according to the invention.

[0032] The getter device 10 for a non- Fig. The X-ray tube 20 shown in Figure 1 has an evacuated housing 11 with a high vacuum containing residual gases. The molecules of the residual gases limit the vacuum quality of the high vacuum. In other words, the vacuum quality of the high vacuum is higher the fewer residual gases are present within the housing 11.

[0033] The housing 11 forms a vacuum-maintaining internal volume to which no vacuum pump can be connected after evacuation. The housing 11 is vacuum-tight. The housing is a so-called closed vacuum recipient. During manufacture of the getter device 10, the high vacuum is created within the housing 11 during evacuation, in particular by means of a vacuum pump, preferably by removing the residual gases from the housing 11 as completely as possible. After the evacuation of the housing 11, no vacuum pump can be connected to the housing 11 because the housing 11 does not have a suitable vacuum valve. The residual gases present within the vacuum-tight housing 11 at this time can therefore preferably be adsorbed, or pumped, by the getter device.

[0034] The getter device 10 further comprises a first getter 12 for adsorbing the residual gases. The first getter 12 is arranged within the housing 11. The first getter 12 is a non-evaporable, block-shaped getter. The block-shaped getter is designed in a type of block. Non-evaporable means that no significant material loss occurs due to a temperature increase during operation of the getter, but rather (re-)activation of the non-evaporable getter is achievable. A getter design as a non-evaporable, block-shaped getter is generally known, for example from conventional X-ray tubes. In contrast, an evaporable getter, e.g., made of titanium, cannot be reactivated but evaporates at an activation temperature typical for reactivation.

[0035] The first getter 12 can, in particular, be an active getter, which can be (re-)activated by a heat source, which in particular is not an electron emitter of an X-ray tube. The heat source can, in particular, be a current-based heater. Alternatively, the first getter 12 can be a passive getter, which is configured to be directly or indirectly impacted, for example, by electrons generated by the electron emitter, in order to be passively heated and thus (re-)activated.

[0036] The getter device 10 further comprises a second getter 13 for adsorbing the residual gases. The second getter 13 is arranged within the housing 11. The second getter 13 is a vapor-deposited, non-evaporable, layered getter. The getter material of the second getter 13 was vapor-deposited, in particular, during the manufacture of the getter device before the housing was evacuated. A getter design as a vapor-deposited, non-evaporable, layered getter is generally known, for example, from particle accelerators. The second getter 13 is a passive getter.

[0037] The first getter 12 and the second getter 13 each comprise a conventional getter material, typically comprising metals or metal alloys. The getter material, in particular of the first getter 12, can be sintered. The getter material has a surface, preferably as porous as possible. The selection of the getter material and / or its surface quality and / or its volume define, in particular, the total amount that can be temporarily or permanently bound by the getter material, i.e., not just at the surface.

[0038] The residual gases comprise, in particular, CH molecules, in particular methane, hydrogen molecules, molecules of noble gases and of other gases, such as air. In this case, a pumping curve for each gas is typically temperature-dependent and different from one another. The inventive design of the getter device advantageously enables one getter to be at one operating point, i.e. at a first temperature, and the other getter to be at a different operating point, i.e. at a second temperature different from the first temperature, so that each getter can pump a gas particularly well out of the high vacuum. For example, one of the two getters can be at a temperature in the range between 280°C and 320°C, in particular of approximately 300°C, in order to be able to pump CH chains, in particular methane, particularly well, while the other getter pumps hydrogen.Such operation can occur during a maintenance state or during an operating state of the getter device or X-ray device. The getter that pumps methane can be made of a getter material such as ST170 or ST172, but can also be made of another getter material.

[0039] Some, or as many, or all, of the residual gas molecules are preferentially bound in a layer that forms on this porous surface during pumping. The getter material thus acts as a so-called pump for the residual gases. Noble gases can hardly or not at all be pumped using a getter device because they are chemically inert and cannot adhere to the surface of the getter material. Hydrogen molecules, on the other hand, can be pumped particularly well. However, bound hydrogen molecules can in principle leave the adsorbed molecular layer again, i.e., desorb, especially at higher temperatures. The solubility of the adsorbed hydrogen molecules correlates particularly with the temperature of the getter material.However, by heating the getter material, the surface can also be (re-)activated in a fundamentally repeatable manner, as the molecular layer, in particular the adsorbed molecules of the other gases, diffuse deeper into the getter material to become permanently bound there, while the adsorbed hydrogen molecules detach.

[0040] According to the invention, the second getter 13 is configured and arranged to adsorb hydrogen desorbed from the first getter 12, i.e., desorbed hydrogen molecules. The second getter 13 can be configured and arranged to be reactivated by the operation of the first getter 12. For this purpose, the second getter 13 is applied to a region of the interior of the housing 11 by vapor deposition before evacuating the housing 11. The vapor-deposited area of the interior is located in particular near the first getter 12. This advantageously allows the partial pressure of hydrogen to be kept low or reduced.

[0041] The second getter 13 is further preferably designed as a diffusion barrier for hydrogen. The second getter 13 particularly prevents increased desorption from the vapor-deposited area of the inner surface, thereby slowing down or preferably preventing, or particularly advantageously reversing, an increase in the partial pressure of hydrogen.

[0042] The fact that the second getter 13 was applied before the evacuation of the housing 11 means, in particular, that the second getter 13 is designed such that it already exists before the operation of the getter device 10, in particular the first getter 12. In other words, the second getter 13 is manufactured independently of the first getter 12 and does not depend on the presence of the first getter 12. The vapor deposition takes place, for example, during the manufacture of the getter device 10 and / or before the evacuation of the housing 11 by means of a vacuum pump.

[0043] A key aspect of the invention is the recognition that an increased partial pressure of hydrogen can impair the operation of a conventional getter device, in particular a conventional X-ray tube. Therefore, the first getter 12 is designed and arranged such that during operation of the getter device 10 or the X-ray tube 20, i.e., in particular during electron emission, the first getter 12 is not passively heated, and if so, at least only to below its activation temperature. In other words, during operation, the first getter 12 is at a defined operating temperature below its activation temperature. In this case, the reactivation of the first getter 12 occurs in a maintenance state after an electron emission has taken place, in particular after the getter device has cooled down, and before a subsequent electron emission.

[0044] Alternatively, the first getter 12 can be designed and arranged such that during operation of the getter device 10 or the X-ray tube 20, i.e., in particular during electron emission, the first getter 12 is passively heated to its activation temperature. In other words, the first getter 12 can be at its activation temperature for a period of time during operation. In this case, the reactivation of the first getter 12 occurs in an operating state.

[0045] As soon as the vacuum quality is impaired, which can be associated with and / or detected, for example, an increasing number of high-voltage arcings, the first getter 12 can advantageously be heated for an activation time at least to the activation temperature, e.g., to 900°C for 10 minutes, in order to carry out the previously described (re-)activation of the getter material of the first getter 12. The activation temperature can also be below 900°C, the longer the activation time. The activation temperature can be, for example, 500°C. Heating to an operating temperature, e.g., in order to be able to pump methane particularly well, and / or an activation temperature can be carried out actively or passively. During the activation time, the partial pressure of hydrogen typically increases, at least locally around the first getter 12, within the housing 11.Therefore, the (re)activation of the first getter 12 can preferably take place during a standby period and / or outside of routine operation. The arrows indicate the warming of the environment of the getter 12 due to its heating. After the first getter 12 cools down, especially after its (re)activation, the temporarily increased partial pressure typically decreases again.

[0046] The second getter 13 is preferably made of a getter material with an activation temperature below 500°C, preferably below 400°C, for example, approximately 250°C. The second getter 13, which is operated below the activation temperature of the second getter during the activation time of the first getter 12, can preferably reduce the partial pressure increased by the first getter 12 by the second getter 13 adsorbing the hydrogen molecules desorbed from the first getter. The second getter 13 advantageously also allows the first getter 12 to be heated for an extended activation time at least to the activation temperature for (re-)activation, which advantageously increases the pumping power of the second getter 13.

[0047] Fig. 1 further shows that the greatest distance between the first getter 12 and the second getter 13 is smaller than the smallest distance between the first getter 12 and a region of the housing 11 not coated with the second getter 13. The reference point for the distance between the first getter 12 and the second getter 13 is its geometric center. The reference point for the distance between the second getter 13 and the second getter 13 is any point on the surface of the second getter 13 facing the vacuum. In particular, the surface of the second getter 13 facing the vacuum can be larger than a surface area of the first getter 12. The surface area of the second getter 13 can essentially correspond to the surface area of the coated region of the housing 11. This embodiment is particularly advantageous because heating of parts of the housing 11 and thus an increase in outgassing can be reduced, preferably prevented. In particular, little to no gas can desorb from the shielded region.

[0048] Fig. 2 shows a section of an X-ray tube 20 according to the invention.

[0049] In addition to the getter device 10, the inventive X-ray tube 20 has an electron emitter 21 within the housing 11 for generating electrons. The electron emitter 21 is inserted into a cathode, which can act as a focus head for the generated electrons.

[0050] The electron emitter 21 can be a field effect emitter or a thermionic emitter. Shown in Fig. 2 is a helical emitter as a thermionic emitter. An alternative to the helical emitter is a sheet emitter. A field-effect emitter typically has a plurality of emitter needles, which can be segmented into pixel groups and / or preferably made of silicon.

[0051] The X-ray tube according to the invention further comprises an anode 22 within the housing 11 for generating X-rays in dependence on the generated electrons. The anode 22 of the Fig. 2 is a rotating anode. An alternative to a rotating anode is a stationary anode.

[0052] The first getter 12 and the second getter 13 are arranged as far away as possible from the electron emitter 21 and the anode 22. The first getter 12 is spaced apart from the electron emitter 21 and the anode 22 such that the first getter 12 is kept below its activation temperature during the emission of electrons.

[0053] The first getter 12 can be heated to a temperature between 280°C and 320°C to pump methane from the high vacuum, while the second getter 13 acts to reduce desorption. Typically, an X-ray tube with a helical emitter has a higher proportion of methane in the residual gases than an X-ray tube with a sheet emitter or a field-effect emitter.

[0054] Fig. 3 shows a section of a computer tomography system 40 according to the invention.

[0055] The computed tomography system 40 includes an imaging system 30. The imaging system 30 includes an X-ray tube 20 and an X-ray detector 31 for detecting the generated X-rays.

[0056] The computer tomography system 40 further comprises a gantry with a rotating part 41 and a non- Fig. 3. The imaging system 30 is formed as a component of the rotating part 41 of the gantry.

[0057] 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. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 6 570 959 B1 [0003, 0004] US 6 192 106 B1 [0003, 0005] DE 10 2012 212 409 A1

[0006] DE 19 842 949 C1

[0007] US 5 509 045 A

[0008] US 6 044 129 A

[0009] US 7 558 376 B2

[0010] DE 10 2012 223 569 A1

[0011]

Claims

[1] Getter device (10) for an X-ray tube (20), comprising - an evacuated housing (11) with a high vacuum containing residual gases and - a first getter (12) for adsorbing the residual gases from the high vacuum and a second getter (13) for adsorbing the residual gases from the high vacuum, - wherein the second getter (13) is designed and arranged to adsorb hydrogen desorbed from the first getter (12), - wherein the housing (11) forms a vacuum-maintaining internal volume to which no vacuum pump can be connected after the evacuation of the housing (11), characterized bythat the first getter (12) is a non-evaporable, block-shaped getter, that the second getter (13) is a non-evaporable, layer-shaped getter and that the second getter (14) is applied to a region of the inside of the housing (11) by means of vapor deposition before evacuation of the housing (11), in particular as a diffusion barrier for hydrogen. [2] Getter device (10) according to one of the preceding claims, wherein the greatest distance of the first getter (12) to the second getter (13) is smaller than the smallest distance of the first getter (12) to a region of the housing (11) not coated with the second getter (13). [3] Getter device (10) according to claim 2, wherein the surface of the second getter (13) facing the vacuum is larger than a surface of the first getter (12). [4] Getter device (10) according to one of claims 2 or 3, wherein a surface of the second getter (13) substantially corresponds to the area of the region. [5] Getter device (10) according to one of the preceding claims, wherein the second getter (13) is configured and arranged to be reactivated by the operation of the first getter (12). [6] X-ray tube (20), comprising - a getter device (10) according to one of the preceding claims, - an electron emitter (21) within the housing (11) for generating electrons and - an anode (22) within the housing (11) for generating X-rays depending on the electrons generated. [7] X-ray tube (20) according to claim 6, wherein the first getter (12) and the second getter (13) are arranged as far away as possible from the electron emitter (21) and the anode (22). [8] X-ray tube (20) according to claim 7, wherein the first getter (12) is spaced apart relative to the electron emitter (21) and to the anode (22) such that during the emission of electrons the first getter (12) is kept below its activation temperature. [9] X-ray tube (20) according to one of claims 6 to 8, wherein the first getter (12) can be heated to a temperature in the range between 280°C and 320°C in order to pump methane from the high vacuum, while the second getter (13) acts to reduce the desorption. [10] Imaging system (30) comprising - an X-ray tube (20) according to one of claims 6 to 9 and - an X-ray detector (31) for detecting the generated X-rays. [11] Computed tomography system (40), comprising - a gantry with a rotating part (41) and a stationary part and - an imaging system (30) according to claim 10, which is formed as a component of the rotating part (41) of the gantry.

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