Patent Application
20210020398
The present application relates generally to systems and methods for generating x-rays.
Conventional x-ray sources generate x-rays by bombarding a target with an electron beam, however, the target can be degraded (e.g., damaged) by the heat generated by being bombarded by an electron beam with a high current density. As a result, such conventional x-ray sources suffer from x-ray brightness limitations resulting from keeping the electron current density below a predetermined level to avoid thermal damage.
Several approaches have previously been used to overcome the x-ray brightness limitations. For rotating anode x-ray sources (e.g., marketed by Rigaku Corp. of Tokyo, Japan), an anode disk rapidly rotates while under vacuum and different regions of the anode disk along a circular track are sequentially irradiated by the electron beam, thereby distributing the heat load over the circular track. In addition, the anode disk is cooled by coolant (e.g., water) flowing through cooling channels in the anode disk. A challenge in such rotating anode x-ray sources is to provide a rotating seal around the rapidly rotating shaft which maintains the vacuum in which the anode disk resides while also coupling the coolant lines through the rotating seal. An additional challenge is that ball bearings in such rotating anodes cannot be lubricated through conventional means, such as organic lubricants, because such lubricants will volatize in vacuum. Moreover, due to minimum requirements for the air gaps (e.g., at least 3 mm) for the vacuum envelope motors, the magnetic driving induction utilizes higher powers to overcome a large magnetic resistance.
For liquid metal jet x-ray sources (e.g., marketed by Excillum AB of Kista, Sweden), instead of a solid anode, a jet of liquid metal (e.g., alloy of Ga, In, and in some cases, Sn) is bombarded by the electron beam. Such x-ray sources have limitations resulting from the evaporation of the metal (e.g., contamination of the vacuum chamber), and from the limited choice of target materials and their spectral characteristics.
For microstructural target anode x-ray sources (e.g., marketed by Sigray, Inc. of Concord Calif.), x-ray generating microstructures are formed on high thermal conductivity substrates (e.g., diamond) and these microstructures are bombarded by the electron beam. While such x-ray sources provide a wide choice of anode materials, and in many cases higher x-ray brightness than do other x-ray sources, thermal damage to the anode target caused by high heat loads still limits the x-ray brightness.
In one aspect disclosed herein, an x-ray source comprises an anode assembly comprising at least one surface configured to rotate about an axis, the at least one surface in a first region. The x-ray source further comprises an electron-beam source configured to emit at least one electron beam configured to bombard the at least one surface of the anode assembly. The electron-beam source comprises a housing, a cathode assembly, and a window. The housing at least partially bounds a second region and comprises an aperture. The cathode assembly is configured to generate the at least one electron beam within the second region. The window is configured to hermetically seal the aperture, to maintain a pressure differential between the first region and the second region, and to allow the at least one electron beam to propagate from the second region to the first region.
In certain embodiments, the first region 10 comprises air, nitrogen, and/or helium at or near atmospheric pressure (e.g., in a range of 0.8 atmosphere to 1 atmosphere) or low vacuum (e.g., less than atmospheric pressure and greater than 10 Torr) and the second region 20 is at a pressure (e.g., less than 10−6 Torr; less than 10−8 Torr; less than 10−9 Torr) lower than the pressure of the first region 10. As schematically illustrated by
In certain embodiments, as schematically illustrated by
In certain embodiments, the rotating anode 180 comprises the at least one surface 112. In certain embodiments, as schematically illustrated by
In certain embodiments, the at least one surface 112 comprises at least one material configured to emit x-rays having a predetermined spectrum in response to being bombarded by the at least one electron beam 122. For example, the at least one surface 112 can comprise at least one layer (e.g., coating) having a ring-like shape around the axis 114, a thickness in a range of 3 microns to 100 microns (e.g., in a range of 10 microns to 100 microns; in a range of 5 microns to 25 microns), a ring width (e.g., in a direction parallel to the at least one surface 112) in a range of 1 millimeter to 250 millimeters (e.g., a range of 1 millimeter to 10 millimeters; in a range of 10 millimeters to 55 millimeters; in a range of 1 millimeter to 100 millimeters; in a range of 60 millimeters to 250 millimeters), and comprising one or more of: aluminum, chromium, copper, gold, molybdenum, tungsten, tantalum, titanium, platinum, rhenium, rhodium, silicon carbide, tantalum carbide, titanium carbide, boron carbide, or a combination thereof. For another example, the at least one surface 112 of the rotating anode 180 can comprise a plurality of discrete microstructures distributed on or within the at least one surface 112. Example rotating anodes 180 compatible with certain embodiments described herein are described more fully in U.S. Pat. Nos. 9,390,881, 9,543,109, 9,823,203, 10,269,528, and 10,297,359, each of which is incorporated in its entirety by reference herein.
In certain embodiments, the at least one surface 112 comprises at least one coating or at least one strip (e.g., multiple thin strips) of the x-ray generating material on a second high thermal conductivity material, such as diamond or copper. The at least one coating or at least one strip can further comprise one or more additional interface layers between the x-ray generating material and the second material (e.g., titanium nitride; titanium carbide; boron carbide; silicon carbide; or any combination thereof) and having a thickness in a range of 1 nanometer to 5 nanometers. These interface layer materials can serve one or more purposes, such as improved adhesion, anti-diffusion, and/or improved thermal performance. The second material can comprise the substrate or can be layered on a supporting substrate, such as copper or graphite. Such substrates can have thicknesses in a range of 5 millimeters to 20 millimeters.
In certain embodiments, as schematically illustrated by
The anode assembly 110 of certain embodiments can further comprise a plurality of bearing assemblies 196 (e.g., mechanically coupled to the enclosure 160; comprising portions of the enclosure 160) configured to support the shaft 170. For example, as schematically illustrated in
In certain embodiments, convective cooling of the anode 180 by the gas within the first region 10 is sufficient to prevent thermal damage to the anode 180. For example, the anode 180 can comprise cooling structures (e.g., fins; protrusions separated by grooves) configured to convectively transmit heat away from the anode 180 into the first region 10. In certain other embodiments, the x-ray source 100 further comprises a cooling subsystem (not shown) in thermal communication with the anode 180, the cooling subsystem configured to remove heat from the at least one surface 112 (e.g., at a rate in a range of 100 watts to 5 kilowatts; at a rate in a range of 50 watts to 2 kilowatts). For example, the cooling subsystem can comprise a nozzle (e.g., liquid jet cooling) configured to spray coolant (e.g., water; ethylene glycol; air; helium) onto the at least one surface 112 (e.g., onto a portion of the at least one surface 112 away from the portion 112a of the at least one surface 112 currently being bombarded by the at least one electron beam 122 so as to avoid the coolant from interfering with the east one electron beam 122). For another example, the cooling subsystem can comprise one or more channels extending along the shaft 170 and within the anode 180, the one or more channels configured to allow coolant (e.g., water; ethylene glycol; air; helium) to flow through the channels in thermal communication with the anode 180 and to remove heat from the anode 180. In certain such embodiments, the coolant flowing through the one or more channels is recirculated (e.g., in a closed-loop cooling subsystem in which the coolant heated by the anode 180 is subsequently cooled by a chiller and returned to flow through the one or more channels). In certain embodiments, the cooling subsystem is configured to also cool at least a portion of the electron-beam source 120. For other examples, the cooling subsystem can comprise one or more heat pipes or other structures configured to remove heat from the anode 180.
In certain embodiments, as schematically illustrated by
In certain embodiments, the aperture 132 of the housing 130 of the electron-beam source 120 has an area in a range of 1 mm2 to 900 mm2 or in a range of 9 mm2 to 900 mm2 (e.g., having a square, rectangular, circular, or oval shape; having a width in a range of 3 mm to 30 mm). The window 150 of certain embodiments comprises a frame 152 (e.g., silicon; metal; copper; steel) configured to be mechanically coupled (e.g., brazed; soldered; epoxied) to a portion of the housing 130 surrounding the aperture 132 to form a vacuum seal between the housing 130 and the window 150 (e.g., hermetic seal between the first region 10 and the second region 20). The material of the frame 152 can have a coefficient of thermal expansion that is substantially equal to a coefficient of thermal expansion of the window 150.
The window 150 of certain embodiments further comprises an electron-transmissive portion 154 configured to allow at least a portion of the electrons generated by the cathode assembly 140 to be transmitted from the electron-beam source 120 in the second region 20 to bombard the anode 180 in the first region 10. For example, the electron-transmissive portion 154 can comprise at least one material in the group consisting of: diamond, silicon, silicon oxide, silicon nitride, quartz, boron nitride, boron carbide, beryllium, titanium, aluminum, and a combination of two or more thereof. For materials that are susceptible to electron charging, the materials can be doped to provide electrical conductivity and/or the window 150 can further comprise a thin conductive coating. The electron-transmissive portion 154 can have a thickness in a range of 0.1 micron to 10 microns or a range of 0.3 micron to 10 microns, an area in a range of 100 square microns to 4×106 square microns (e.g., having a square, rectangular, circular, or oval shape; having a width in a range of 10 microns to 2000 microns or a range of 10 microns to 200 microns). Certain other embodiments utilize thinner windows (e.g., thickness in a range of 1 nanometer to 5 nanometers) supported by grids that form a support layer (see, e.g., U.S. Pat. No. 6,803,570). Commercial suppliers of windows 150 compatible with certain embodiments described herein include, but are not limited to, Silson Ltd. of Warwickshire, United Kingdom, Diamond Materials GmbH of Freiburg, Germany, and Materion Corp. of Mayfield Heights, Ohio.
In certain embodiments, as schematically illustrated by
In certain other embodiments, as schematically illustrated by
Various configurations have been described above. Although this invention has been described with reference to these specific configurations, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention. Thus, for example, in any method or process disclosed herein, the acts or operations making up the method/process may be performed in any suitable sequence and are not necessarily limited to any particular disclosed sequence. Features or elements from various embodiments and examples discussed above may be combined with one another to produce alternative configurations compatible with embodiments disclosed herein. Various aspects and advantages of the embodiments have been described where appropriate. It is to be understood that not necessarily all such aspects or advantages may be achieved in accordance with any particular embodiment. Thus, for example, it should be recognized that the various embodiments may be carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may be taught or suggested herein.
The present application claims the benefit of priority to U.S. Provisional Appl. No. 62/874,298, filed Jul. 15, 2019, which is incorporated in its entirety by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62874298 | Jul 2019 | US |
