Patent Application
20100002842
This invention relates to extending the useful lifetimes of cathodes used in X-ray sources. In conventional X-ray sources, X-rays are created by directing an electron beam onto a target anode. In accordance with the well-known process of creating and filling of holes in the electron structure of the anode material, specific monochromatic X-rays are thereby created.
A typical X-ray generator comprises a cathode and an anode. The anode can be rotating, such as those used in rotating anode generators or can be stationary such as those used in sealed tubes. The cathode and the anode are located in an evacuated chamber. The cathode comprises an electron source for electron emission and additionally, in some cases, a cup or focus cup that modifies the electric field lines to focus the electrons in a well defined spot on the anode. A power supply generates a heating current, if a filament is used as an electron emitter, or power for another type of electron emitter and a high voltage (typical 30-70 kilovolts) between the electron source and anode. Due to the electron source at the cathode and the high voltage between the cathode and the anode, electrons are generated at the cathode and accelerated to the anode. In rotating anode systems, the anode typically rotates with a speed of a few thousand revolutions per minute in order to spread the heat generated when the electrons strike the anode over a large surface of the anode.
The electron sources used in X-ray generators may be filaments made of tungsten wire or other emitters, such as LaB6 crystals. Such an electron source has a lifetime that depends on the conditions of operation and normally varies between two weeks and several months. If the electron source fails in a sealed X-ray tube, then the entire tube must be replaced. It is possible to replace electron sources in a rotating anode X-ray generator because the vacuum chamber can be opened. However, reducing the time that is required to change the electron source is important in order to reduce the downtime of the X-ray generator when the electron source must be replaced.
The time that is required to change an electron source depends not only on the time required to physically replace the source, but also on the time required to realign the X-ray generator and possibly the X-ray system that utilizes the generator. In particular, due to the focusing effect that is produced at the cathode by the focus cup, small changes in the position of the electron source relative to the anode produce movements of the electron spot on the anode. These movements in turn, may require the entire system to be re-aligned. In modern X-ray diffraction systems, X-ray optics are positioned between the X-ray generator and a sample, such as a crystal, a thin film or a powder. The X-ray optics select X-rays generated at the anode and direct the selected X-rays to the sample. If the spot position on the anode changes, the setting of the optics may also need to change thereby increasing the time required to replace the filament. Consequently, the cathode must be accurately mounted with respect to the anode in order to reduce the time required for alignment.
Some conventional X-ray sources require that the entire cathode assembly, which includes the electron source, a focus cup and a housing, be mechanically removed from the remainder of the X-ray source in order to replace a failed electron source. Once the cathode assembly is removed, the electron source can be replaced. Alternatively, the entire cathode assembly can be replaced. The cathode assembly must then be reattached to the remainder of the X-ray system.
Other conventional X-ray sources require that the cathode assembly be disassembled in order to access the electron source. Once the cathode assembly is disassembled, the electron source can be removed and replaced. For example, European patent EP0273162 B1 describes a two-piece cathode cup which allows the focusing portion of the cathode cup to be removed to facilitate access to a filament.
However, in these prior art sources, a problem arises in that it is difficult to maintain the precise alignment of the electron source to the rest of the X-ray source when the cathode assembly is mechanically reattached or the electron source is replaced. Consequently, an alignment of the X-ray source and system is inevitably necessary.
In accordance with the principles of the invention, the cathode assembly comprises two parts: a focusing part that is mechanically connected to the remainder of the X-ray source and permanently aligned with respect to the anode and a separate emission part that holds the electron source and is removably connected to the focusing part. The electron source is permanently mounted in the emission part and precisely aligned to the focusing part at the time of manufacture by specialists and the focusing part and the emission part are mechanically aligned relative to one another at the time of replacement. This arrangement allows the emission part, including the electron source to be quickly removed and replaced by an inexperienced user while maintaining the accuracy of the X-ray source alignment.
In one embodiment, the focusing part and the emission part are aligned by alignment structures on the emission part that mate with corresponding alignment structures on the focusing part. In a related embodiment, the alignment structures are rims and grooves.
In another embodiment, the focusing part and the emission part are mechanically connected together by a screw.
In still another embodiment, the electron source in the emission part can be positioned to act as a point or a line source for the electrons.
A typical rotating anode X-ray generation apparatus is illustrated in
In conventional focusing cups, a housing 200 with the permanently attached electron source 210 is mechanically connected by a fitting 208 to the apparatus on which the anode is mounted. This arrangement is shown in more detail in
In accordance with the principles of the invention, a focusing cup structure has a focusing part permanently mounted with respect to the anode and a removable emission part containing the electron source.
The focusing part 402 of the focusing cup structure is permanently connected to the X-ray source apparatus as schematically illustrated as part 406 in
Emission part 404 can be mounted on focusing part 402 by a single screw 422 or by another simple fastening system. While it is very important to position the electron beam spot accurately on the anode in order to maintain alignment of the X-ray optics, the accuracy of the relative position of the emission part 404 with respect to the focusing part 402 is less important since, in general, the image of the electron source on the anode is de-magnified (typically 20 times) by the action of the focusing part 402 on the electron beam and, accordingly, the positioning error is reduced with the same demagnification factor. This is illustrated in
If the electron source is shifted, for example, to the position 720, then the electron trajectories will also shift. Lines 722 and 724 show the new electron trajectories taken by electrons emitted from the end of the electron source 720. As indicated in the enlarged view 718, the new electron trajectories, 722 and 724, shift only a small distance due to the demagnification caused by the focusing part 402. Therefore, visual inspection of the alignment of part 404 with 402 (
While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
