Patent Grant
7657003
The invention relates generally to x-ray tubes and, more particularly, to x-ray tubes having cathodes configured to produce small electron beam spots on targets, without producing halos surrounding these spots.
A typical miniature x-ray tube includes an evacuated ceramic tube with a cathode structure at one end of the tube and an anode structure at or near an opposite end of the tube. Traditionally, the cathode is heated to facilitate releasing electrons, and a high-voltage electric field is established between the cathode and the anode to accelerate the released electrons toward, and possibly beyond, the anode. There may be a short focusing element within the tube. If present, the focusing element includes electrically conducting material for creating an electric field. The focusing element is formed such that the electric field tends to concentrate the flow of electrons into a compact stream. The effectiveness of any focusing depends to a significant degree on the size of the area on the cathode from which the electrons are emitted. The smaller the area, the easier it is to develop a well defined small spot on the target.
The electron beam strikes a target at the far end of the ceramic tube, resulting in the production of x-rays. The target may be the anode or another structure. The target usually includes a thin, heavy metal coating, such as gold (Au) or tungsten (W), on the surface of a material that allows the x-rays to pass through with little attenuation. In some cases, the x-ray beam may be taken off of a more conventional solid, x-ray opaque target at an angle as scattered x-rays. In either case, the x-rays are produced from a spot on the target where the electron beam strikes the target.
To improve electron emissions from cathodes, most cathodes are now made from thoriated tungsten using a process described by Langmuir. In that process, about 2% thorium oxide is mixed with tungsten. Cathodes made of this material are then “activated” by heating them to about 2800 degrees Kelvin (K), which reduces any thorium oxide to a mono layer of metallic thorium on the surface of the tungsten. Carbon is added to the surface to carbonize some of the tungsten to tungsten carbide, which limits the rate of evaporation of the thorium from the surface. The result is a cathode that has several orders of magnitude more emission than pure tungsten. Other details regarding construction of prior-art miniature x-ray tubes are disclosed in U.S. Pat. No. 7,236,568.
In many applications, it is important that the area or a dimension of the spot on the target is as small as possible. However, it has been found that a conventional x-ray tube produces a spot of x-ray emissions surrounded by an undesirable “halo” of x-ray emissions, as a result of heat spreading in the cathode, as described in the following paragraphs.
Traditionally, the cathode is either a directly heated filamentary cathode or a planar cathode. U.S. Pat. No. 6,320,932 discloses heating a cathode by a laser light source. The use of a laser heat source makes planar cathodes easier to implement. In addition, heating a small area in the center of a thin metal cathode gives a more intense emission from the heated area than from unheated areas. An electron beam spot on the order of a few hundred microns in diameter is achievable using a laser-heated planar cathode.
However, heat is conducted from the central, directly heated area of the cathode to adjacent portions of the cathode, which causes the cooler portion of the cathode to emit electrons, albeit at a much lower rate, such as about 1/20 to about 1/100 that of the central, directly heated part of the activated cathode area. This results in a “halo” around the x-ray spot on the target. In a miniature x-ray tube, an exemplary cathode is on the order of 2-3 mm in diameter, and the central electron beam is about 0.2 mm in diameter. Thus, the area of the halo may be approximately 100 times the area of the central spot. Such a halo forms an undesirable background in a measurement.
An embodiment of the present invention provides an x-ray source with an enhanced small spot cathode. Such an x-ray source includes a housing, a cathode disposed within the housing and an anode spaced apart from the cathode. The cathode has an area and a passivation layer over only a portion of the area. The anode is adapted for a voltage bias with respect to the cathode for accelerating electrons emitted from the cathode. The x-ray source also includes an x-ray emitter target disposed within the housing. The x-ray emitter target is spaced apart from the cathode for impact by the accelerated electrons. The passivation layer may include a pyrolytic material, such as platinum or tantalum. The cathode may also include a thoriated tungsten layer. The portion of the cathode that is not covered by the passivation layer may be activated, such as with carbon.
Another embodiment of the present invention provides a method for manufacturing a cathode for an x-ray source. The method includes providing a base layer that has an area and passivating only a portion of the area of the base layer, thereby defining an emission portion of the base layer.
Passivating the portion of the base layer may include applying a pyrolytic material, such as platinum or tantalum, to the portion of the base layer. Providing the base layer may include providing a thoriated tungsten layer. The method may also include activating at least an emission portion of the thoriated tungsten layer, such as by activating the emission portion with carbon.
The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which:
In accordance with preferred embodiments of the present invention, an x-ray source with an enhanced small spot cathode is disclosed, as well as methods for manufacturing such an x-ray source. Such an x-ray source overcomes the halo problem, and corresponding undesirable background, of prior art x-ray tubes, while retaining the high emissivity, and well-defined central beam, of an activated thoriated tungsten cathode with a small activated area.
As noted, in many applications, it is important that the area or a dimension of the x-ray spot of an x-ray source is as small as possible. The size of the x-ray spot on the target depends largely on the size of the area from which electrons are emitted from the cathode and any focusing or dispersion that takes place as the electrons transit to the target. In the case of miniature x-ray tubes, such as x-ray tubes produced by North Star Imaging, Inc., Rogers, Minn., Moxtek, Inc. Orem, Utah and twX, LLC, West Concord, Mass., the electric field structure is such that the electron beam spreads very little in transit to the target. The electron beam spot on the target is, therefore, a relatively faithful image of the cathode emission area, with a very slight size change.
In the illustrated embodiment, the cathode 110 is passivated by an apx. 10-30 μm thick layer 210 of pyrolytic material, such as platinum or tantalum, except for a small (apx. 150 μm diameter) area 215, from which desired emissions take place. Considerations for selecting an appropriate passivation material are discussed below. The emission area 215 is activated, as discussed below, and may be circular or any other desired shape. The passivation 210 eliminates or substantially reduces the halo effect described above, while precisely defining the area 215 of emission.
Platinum and tantalum are well-suited passivators, because both materials have work functions greater than that of thoriated tungsten. Platinum has a work function of approximately 6.3 eV, and tantalum has a work function of approximately 4.1 eV, whereas thoriated tungsten has a work function of approximately 2.6 eV. Thus, emissions from the platinum-passivated or tantalum-passivated area 405 are several orders of magnitude less than emissions from the activated thoriated tungsten portion 215. The emissivity of various material can be estimated using the Richardson-Dushman equation (1):
I=AT2e−b
where:
The passivation material 210 may be selectively deposited on the thoriated tungsten disk 205 using any appropriate technique, such as vacuum deposition using a small mask in the area 215 of the emission portion of the cathode 110, masking and electrodeposition, or a technique used in micro-electro-mechanical systems (MEMS) fabrication.
Once the disk 205 is fabricated with the passivated area 210, the emission portion 215 of the cathode 110 may be activated using any appropriate technique, such as depositing carbon on the emission portion 215 of the cathode 110, yielding an activation layer 410. Most activation techniques cause carbon 415 to also be deposited on top of the passivation layer 210. However, platinum and tantalum are not activated by carbon. Thus, the platinum or tantalum passivation layer 210 serves as a passivator and prevents a halo, even if the platinum or tantalum is coated with carbon 415.
Temperature-related factors may be considered when choosing a passivation material. For example, while platinum has a higher work function than tantalum (and, therefore, is a more effective passivator), platinum has a lower melting temperature (about 1,770° C.) than tantalum. Furthermore, tantalum forms a carbide at temperatures normally used to activate thoriated tungsten. The tantalum carbide offers protection for the tantalum and has a melting temperature above about 3,800° C.
A cathode with a passivated area has at least two desirable features. First, such a cathode has a well-defined emission area. The remainder of the cathode area is passivated; thus, for all intents and purposes, no, or significantly less, thermionic emissions take place from the passivated area. Second, a surface that is covered with platinum or tantalum is more resistant to damage from ion bombardment.
A miniature x-ray tube typically requires only about 10-100 microamperes of current. The small emission portion of the cathode, i.e., the activated tungsten portion, is large enough to provide the required current. For example, the graph in
While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example, although a tubular x-ray source is described, a spherical or other shaped housing may be used, depending on an intended use of the x-ray source. Moreover, disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the exemplary embodiments.
This application claims the benefit of U.S. Provisional Patent Application No. 60/969,926, filed Sep. 4, 2007, titled “X-Ray Tube with Enhanced Small Spot Cathode and Methods for Manufacture Thereof,” the entire contents of which are hereby incorporated by reference herein, for all purposes.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6320932 | Dinsmore | Nov 2001 | B2 |
| 7236568 | Dinsmore et al. | Jun 2007 | B2 |
| 20050136185 | Ramanathan et al. | Jun 2005 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20090060142 A1 | Mar 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60969926 | Sep 2007 | US |
