X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2001-11-08
2004-01-06
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
Reexamination Certificate
active
06674838
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention generally relates to x-ray tube devices. More specifically, the present invention relates to an x-ray tube wherein the need for a fluid-filled outer housing is eliminated.
2. The Relevant Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. The entire outer housing is typically covered with a shielding layer composed of lead for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium, such as a dielectric oil, is typically disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. The oil may be cooled by circulating it to an external heat exchanger via a pump and fluid conduits disposed in the outer housing.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. In anode grounded x-ray tubes, a high negative electric potential is placed on the cathode while the anode is electrically grounded. This causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon approaching and striking the target surface, many of the electrons convert their kinetic energy and either emit, or cause the target surface material to emit, electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The x-rays are then collimated so that they exit the x-ray device through windows disposed in the vacuum enclosure and outer housing, and enter the x-ray subject, such as a medical patient.
A recurrent problem encountered with the operation of x-ray tubes deals with the removal of heat therefrom. In general, only a small percentage of the electrons that impact the anode target surface during x-ray production do, in fact, produce x-rays. The majority are instead absorbed into the anode target surface and surrounding areas, thereby creating large quantities of heat. This heat must be continuously and reliably removed from the anode and surrounding areas in order to prevent damage to critical tube components. To the extent that the heat is efficiently removed, less thermal and mechanical stress is imposed upon the x-ray tube, and its operation and performance will be enhanced. If the heat is allowed to buildup to detrimental levels, however, it can damage the anode and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Many approaches have been implemented to help alleviate the problems created by heating within the x-ray tube. For instance, in many x-ray tubes the anode, which typically comprises a substrate and a target surface disposed thereon, is formed in the shape of a disk. The rotary anode (also referred to as the rotary target or the anode disk) is then mounted on a supporting shaft and rotor assembly that can then be rotated by some type of motor, such as a stator. During operation of the x-ray tube, the rotary anode is rotated at high speeds, which causes successive portions of the target surface to continuously rotate into and out of the path of the electron beam produced by the cathode filament. In this way, the electron beam is in contact with any given point on the target surface for only short periods of time. This allows the remaining portion of the surface to cool during the time that it takes to rotate back into the path of the electron beam, thereby spreading the heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the target surface, a large amount of heat is still absorbed by the anode substrate, the rotor assembly, the cathode, and other components within the vacuum enclosure. This heat must be continuously and reliably removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube's efficiency and overall service life.
One approach has been to place the vacuum enclosure within an outer housing, as mentioned above. This outer housing must serve several functions. First, it must act as a radiation shield to prevent radiation leakage resulting from the production of x-rays within the vacuum enclosure. To do so, the can must include a radiation shield, which must be constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which surrounds and envelops the vacuum enclosure, and which may be continuously circulated by a pump about the outer surface thereof As heat is emitted from the x-ray tube components (anode, support shaft, etc.), it is radiated to the outer surface of the vacuum enclosure, and then at least partially absorbed by the dielectric oil. The heated oil is then passed to some form of heat exchange device, such as a radiative surface, and then cooled. The oil is then recirculated by the pump back through the outer housing and the process repeated.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquid coolants/dielectrics that surround and envelop the vacuum enclosure can be problematic in several respects. For example, use of large amounts of cooling fluid adds complexity to the construction and operation of the x-ray generating device. Use of fluid to envelop the vacuum enclosure requires that there be an outer housing as outlined above to retain the fluid. This outer housing must be constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing to retain the cooling fluid. This increases the cost and manufacturing complexity of the device. Also, the outer housing requires a large amount of physical space, resulting in the need for a larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner vacuum enclosure, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates.
In light of the above discussion, therefore, a need exists for an x-ray tube that eliminates the problems associated with fluid-filled outer housings. Further, a need exists to provide an x-ray tube whereby sufficient cooling of the vacuum enclosure is efficiently attained, thereby improving the performance and longevity of the x-ray tube. Moreover, an x-ray tube having a simple construction and flexible design would be an advancement in the
Church Craig E.
Varian Medical Systems Inc.
Workman Nydegger
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