X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
2000-04-29
2002-08-06
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Target
C378S143000
Reexamination Certificate
active
06430264
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to x-ray producing equipment. More particularly, the invention relates to an improved anode target structure present on an x-ray tube of the sort that is commonly used in such x-ray producing equipment. In addition, the present invention relates to a method of manufacturing an improved anode target structure for use in an x-ray tube.
BACKGROUND OF THE INVENTION
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic operation for producing x-rays in the equipment used in these different industries and applications is very similar. X-rays, or x-radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. Typically, this entire process takes place in a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode, which is the source of electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode.
The three elements are usually positioned within an evacuated glass tube, and connected within an electrical circuit. The electrical circuit is connected so that the voltage generation element can apply a very high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The high voltage differential causes a thin stream, or beam, of electrons to be emitted at a very high velocity from the cathode towards an x-ray “target” positioned on the anode. The x-ray target has a target surface (sometimes referred to as the focal track) that is comprised of a refractory metal. When the electrons strike the target, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target, and are then collimated for penetration into an object, such as an area of a patient's body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
In general, a very small part of the electrical energy used for accelerating the electrons is converted into x-rays. The remainder of the energy is dissipated as a large amount of heat in the target region and the rest of the anode. This heat can damage the anode structure over time, and can negatively affect the operating life of the x-ray tube and/or the performance and operating efficiency of the tube. To alleviate this problem the x-ray target, or focal track, is typically positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is mounted on a supporting shaft that is rotated by a motor. The motor is used to rotate the disk at high speeds (often in the range of 10,000 RPM), thereby causing the focal track to rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam.
While the rotation of the track helps reduce the amount and duration of heat dissipated in the anode target, the focal track is still exposed to very high temperatures—often temperatures of 2500° C. or higher are encountered at the focal spot of the electron beam. Thus the rotary anode must still be constructed of a material that is both resistant to heat, and that can effectively block an impinging high velocity electron beam. Moreover, since the disk is rotated at high rotational speeds, it must be capable of withstanding high mechanical stresses. One commonly used material for an anode disk is a refractory metal, such as a molybdenum alloy, an example of which is known as TZM (titanium-zirconium-molybdenum). Refractory metals are, however, expensive, and require complex manufacturing and processing procedures to be used for fabrication of an anode disk. Also, such metal alloys are quite dense and thus can be very heavy, which can be especially problematic when a larger anode disk is used. For instance, the higher weight requires a larger motor and stronger rotor assembly to rotate the anode disk, resulting in higher costs, and greater wear and tear on the components. Moreover, the increased weight of a metal anode disk makes it more difficult to rotate at high speeds, especially in x-ray devices that require the anode disk to be accelerated quickly to high operational speeds in short periods of time.
One approach to address the problems encountered when a refractory metal is used, has been to use a graphite material. Graphite offers several advantages over metal. It has a significantly higher heat storage capacity than metal, and thus can operate at higher temperatures for longer periods of time. Graphite also has a much lower density (lighter weight) than metal, so it can be more easily rotated at higher speeds, allows for the use of bigger targets, and puts less mechanical stress on the anode assembly (such as the rotor, bearings and motor).
Graphite, however, has a low mechanical strength and can be brittle, especially pressed and sintered graphite. As such, mechanical loading—for example, tangential loading during starting and stopping of rotation—can cause fracturing of the graphite disk, especially with the high rotational speeds encountered by the rotating anode. Also, a focal track constructed of a material that is capable of blocking an impinging high velocity electron beam must be applied directly to the graphite substrate. Typically, this results in an anode where the rate of heat transfer from the focal track to the substrate is slower than when a focal track is attached to a metal substrate, such as TZM. Under certain operating conditions, this can cause an overheating of the focal track and resultant damage to the graphite target disk, such as bonded layer failure.
It has also been proposed that a carbon-carbon composite material be used in place of graphite. Such a material exhibits the same heat storage capacity and low weight characteristics of graphite, but is much stronger than graphite, and is better able to withstand the mechanical stresses imposed. As with graphite, a suitable metal material must be bonded to the carbon-carbon disk to function as the anode focal track. The material must be of sufficient thickness so as to effectively block an impinging high velocity electron beam and generate usable x-ray output, and must also be capable of withstanding the high temperatures that are dissipated on the track during operation. At the same time, the focal track material must remain bonded to the underlying carbon-carbon composite disk. This gives rise to the primary problem with the carbon-carbon material, in that its thermal expansion rate differs significantly from the metal materials that are commonly used for the focal track on the disk. Maintaining a bond is thus difficult to achieve. When exposed to high temperatures, the different thermal expansion rates result in a macroscopic buildup of stresses across the bonding surface between the focal track target material and the carbon-carbon composite material. These stresses often result in a debonding, peel-off, or cracking of the target layer, which can render the x-ray tube inoperable, shorten its operating life, or reduce its operating efficiency.
As such, there is a need in the art to provide a rotating anode disk that is constructed of a material that has a low de
Kim Robert H.
Song Hoon K.
Varian Medical Systems Inc.
Workman & Nydegger & Seeley
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