X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
1999-08-19
2001-03-06
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Target
C378S143000
Reexamination Certificate
active
06198805
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to X-ray tubes, and more particularly to a target assembly for an X-ray tube and to a method for making the target assembly.
X-ray equipment used in the medical field typically includes a rotating anode X-ray tube. Such X-ray tubes are vacuum tubes whose anodes each include a rotor having a rotatable rotor shaft and also include a stator which circumferentially surrounds, or is circumferentially surrounded by, the rotatable rotor shaft. A pair of bearings, such as rolling element bearings (e.g., ball bearings), is positioned radially between the rotor shaft and the stator.
The anode also has an X-ray target which includes a target cap attached to an annular target shaft. The target cap has a target track portion which produces X-rays and heat when struck by electrons emitted by the X-ray tube's cathode. The target cap may also have a central bore used to evacuate the anode when creating the vacuum during tube construction. The target typically also includes a graphite heat sink attached to the target cap. The target shaft is bolted to the rotor shaft either directly or through intermediate members. The bearings get heated to high temperatures since some of the heat produced by the target track region of the target cap flows by solid conduction from the target cap to the target shaft and then from the target shaft to the bearings. The bearings are poor thermal conductors which sets up a temperature differential between the rotating side and the stationary side of the bearings causing bearing misalignment and wear which shortens the operating life of the X-ray tube. Additional heat also comes from backscattered electrons entering the bore of the target cap and impinging on anode structure near the front bearing.
Known designs include X-ray-tube targets which have been constructed by bolting, or threading and brazing, together two target shaft members. Such bolted, or threaded and brazed, joints are under cyclic thermal and mechanical loads and have become loose over time causing tube vibrations requiring immediate system shutdown and tube replacement.
What is needed is an improved X-ray-tube target which reduces heat flow to the bearings without having a tendency to cause tube vibrations requiring unscheduled system shutdown for tube replacement.
BRIEF SUMMARY OF THE INVENTION
In one expression of the invention, an X-ray-tube target assembly includes an annular monolithic X-ray-tube target shaft and a monolithic X-ray-tube target cap. The target shaft has a generally longitudinal axis, has longitudinally-outermost first and second ends, and has longitudinally-extending first and second portions. The first portion extends longitudinally from proximate the first end toward the second portion, and the second portion extends longitudinally from proximate the second end toward the first portion. The first and second portions have inner and outer radii. The inner radius of the first portion is smaller than the inner radius of the second portion, and the outer radius of the first portion is smaller than the outer radius of the second portion. The target cap has a generally disk shape, is generally coaxially aligned with the longitudinal axis, and is inertially welded to the shaft proximate the first end.
In a second expression of the invention, a method for making an X-ray-tube target assembly includes steps a) through e). Step a) obtains a monolithic solid cylinder having a generally longitudinal axis and having a first radius, and step b) obtains a monolithic X-ray-tube target cap having a shape of a solid disk and having a second radius which is larger than the first radius. Step c) coaxially aligns the cylinder and the target cap, and step d), which is performed after step c), inertially welds together the cylinder and the target cap. Step e), which is performed after step d), machines the cylinder as required to define a hollow X-ray-tube target shaft having longitudinally-outermost first and second ends and having longitudinally-extending first and second portions, wherein the first portion extends longitudinally from proximate the first end toward the second portion, wherein the second portion extends longitudinally from proximate the second end toward the first portion, wherein the first and second portions have inner and outer radii, wherein the inner radius of the first portion is smaller than the inner radius of the second portion, and wherein the outer radius of the first portion is smaller than the outer radius of the second portion. In one variation of the method, another step is added. The added step, which is performed after step d), machines a through hole in the target cap such that the through hole is generally coaxially aligned with the longitudinal axis and such that the radius of the through hole is no larger than the inner radius of the first portion.
Several benefits and advantages are derived from the invention. The target shaft and cap assembly has no bolted, threaded and brazed, or other joints which eliminates the possibility of joint loosening causing tube vibrations requiring immediate system shutdown and tube replacement. Eliminating this cause of early tube failure will extend the life of the X-ray tube. The step target shaft design increases the length of the solid conduction thermal path reducing the heat flow to the bearings. When present, the through hole in the cap, having a small radius, allows anode evacuation when creating the vacuum during tube construction and, at the same time, reduces the number of backscattered electrons hence reducing the heat load on the bearings. Such heat flow and heat load reduction allows the X-ray tube to be run at higher energies while meeting temperature limits on the bearings.
REFERENCES:
patent: 5498187 (1996-03-01), Eggleston et al.
patent: 5592525 (1997-01-01), Reznikov et al.
patent: 5875227 (1999-02-01), Bhatt
patent: 4127414 A1 (1992-03-01), None
patent: 0055828 (1982-07-01), None
patent: 406076722 (1994-03-01), None
Bhatt Vivek
Dorri Bijan
Jambunathan Viswanathan
Vermilyea Mark Ernest
General Electric Company
Kiknadze Irakli
Porta David P.
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