X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-07-29
2001-05-08
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S119000, C378S121000
Reexamination Certificate
active
06229876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an x-ray tube. More specifically, the present invention relates to an x-ray tube configuration capable of generating high intensity x-rays that emanate from a small focal spot without a loss of reliability.
2. Description of the Related Art
X-ray tubes have a number of applications which involve the treatment or analysis of a sample, for example, industrial imaging, analytical instruments and medical imaging. For such applications, it is often desirable to have an x-ray tube which has a long service life, which is capable of forming a small focal spot and which is also capable of generating a high intensity of x-radiation at the sample.
X-ray tubes generally include an electron gun and an anode. A beam of electrons generated by the electron gun is focused to a focal spot on the anode, and x-rays are generated by the interaction of the beam of electrons with the atoms of the anode. These x-rays are generated in all directions emitting from the anode in the region surrounding the focal spot. Typically, the anode is substantially surrounded by an evacuated housing in which a window is formed to permit some of the x-rays to pass out of the housing, the window typically comprising a thin foil of a low atomic number metal, such as beryllium or aluminum, having a high transmission coefficient for x-radiation. Illustrative prior art x-ray tube structures in this regard are described, for example, in U.S. Pat. Nos. 5,751,784 and 5,563,923.
A commercially available x-ray tube
1
is schematically illustrated in
FIG. 1
which employs a gun container
2
having a flat surfaced discharge end
3
, i.e., gun snout, relative to the electron beam direction x, and x-rays emitted from anode
4
pass through an x-ray transmissive window
5
formed in a side of the hermetically sealed and evacuated envelope
6
. The gun container
2
houses a cathode and filamentary heater used to generate an electron beam in a generally known manner. The gun snout
3
is flat in the sense it is a planar surface oriented perpendiculary to the beam direction x with a central aperture (not shown in the view) for transmission of the electron beam out of the gun container
2
. Using the flat surfaced discharge end
3
, focal spot sizes are yielded on the anode
4
(i.e., target) which tend to be relatively large, e.g., exceeding 20 microns in largest diameter, and which tend to be heavily limited by chromatic aberration. One approach to reducing the spot size is to reduce the working distance between the gun snout and anode. Smaller working distances produce smaller magnification for the beam; hence, a smaller spot at the anode. However, investigations conducted by the present inventors have shown that a flat gun snout tends to produce high field emission and instability when positioned nearer a target due to high electric fields. Consequently, such x-ray tubes with flat gun container snouts are focus limited due to chromatic aberration and instability issues, especially if attempts are made to position the gun assembly in closer proximity to the target.
It would be desirable to be able to position a gun snout in very close proximity to the target in order to reduce beam size and provide small spot focusing yet without unduly increasing the structural or operational complexity of the x-ray tube. An object of this invention is to provide an x-ray tube endowed with enhanced electron beam focusing performance and capabilities, and, more particularly, provide an x-ray tube with an electron gun positionable in close proximity to the target at reduced surface fields to achieve smaller focal spots without incurring instability.
SUMMARY OF THE INVENTION
According to the present invention, an x-ray tube is provided having an electron gun container having a spherical shaped snout for discharging electrons from the gun container towards an anode (target) where x-rays are generated.
More particularly, in one embodiment of this invention, there is an x-ray tube including:
an electron gun assembly comprising an electron gun container housing an electron generator for generating electrons in a first direction along a first axis, where the gun container has a gun snout with an egress aperture through which the electrons are discharged from the gun container; and
a target for generating x-rays upon being impinged by the electrons that are discharged from the gun snout;
and in which the gun snout of the x-ray tube is characterized by having a solid spherical shape.
The spherical surface of the gun snout has its concave (inner) side facing the electron generating means within the gun container and its convex (outer) side facing the anode (target) located outside the gun container. For purposes of this application, the term “spherical” refers to a gun snout having a surface profile all points of which are substantial equidistant from a common imaginary center of radius located inside the gun container, i.e. comprises a substantially constant radius of curvature. Thus, the edge profile of the spherical snout of the invention is curvilinear, and not a straight line such as would be the case with a conical shape. As a matter of course, the gun snout will be semi-spherical, as it does not form a complete sphere. Preferably, the spherical surface is arranged such the electron beam passes through the geometric center of the space defined within the solid spherical surface before exiting from the snout aperture. The spherical surface preferably has a decreasing cross-sectional diameter in the direction of the electron beam towards the exit aperture of the snout. In this way, the spherical gun snout presents an overall “lens” profile. The gun snout preferably is a substantially hemispherical dome-shape with the gun container having a separate cylindrical portion merged with the dome to form an overall tubular-shaped gun container with a rounded, aperture tip for discharging the electron beam towards the anode (target).
Advantageously, the x-ray tube also includes at least one accelerating electrode to provide a focus grid through which the electrons pass before reaching the snout situated within the gun container at a location between the electron generating means (e.g. a heated cathode), and the egress aperture of the gun snout of the gun container. The focus grid includes a central opening through which the electrons pass and accelerates the electrons emitted by the cathode and converges the electrons into an electron beam along the first axis. A separate electrode situated between the cathode and the focus electrode functions to control the beam current.
The inventive x-ray tube, as equipped with such a spherical discharge snout on the gun container, enhances the electron beam focusing capability of the x-ray tube. The inventive x-ray tube with the spherical gun can be operated in a highly stable manner with beam diameters generated at the x-ray target of relatively small dimension, e.g., less than approximately 10 micron focal spot sizes in largest dimension. The spherical shape provides the lowest possible surface field on the snout, thereby reducing field emission, even at very close distances and high field conditions. The inventive spherical gun snout configuration enables the closest possible spacing and approach of the electron gun to the high electric fields of the target (anode) without suffering focus aberration.
REFERENCES:
patent: 5751784 (1998-05-01), Enck
Enck Richard S.
Johnson Richard G.
Benman William J.
Ho Allen C.
Kevex X-Ray, Inc.
Kim Robert H.
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