X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-04-21
2001-08-28
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S113000
Reexamination Certificate
active
06282263
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an X-ray generator and in particular to an X-ray generator suitable to be closely coupled to a focusing X-ray device.
X-ray generators comprise an electron gun, an X-ray target and an X-ray exit window, generally in a sealed evacuated X-ray tube. Prior art generators produce X-ray beams having a relatively large focal spot or line. Many applications require a precisely collimated X-ray beam. To achieve this relatively small apertures are coupled with the generator to restrict beam diameter and divergence, but this results in a large loss of X-ray intensity.
For many applications the most effective way of using the X-rays emitted from the target of an X-ray generator is to form an image of the source, i.e. of the electron focus on the target, on the specimen. For crystallographic applications, it is normally essential that the convergence or divergence of the rays incident on the sample be very small. To maximise the X-ray intensity at the sample the angle of collection at the source should be as large as possible. The combination of these two requirements implies that the imaging optics should magnify. The sample size determines the maximum useful image size (see FIG.
3
).
FIG. 3
shows that the ratio of the collecting angle a at the source S to the beam convergence angle &bgr; at the image I is equal to the magnification of the focusing collimator or focusing mirror F. In single-crystal diffractometry, for example, the specimen crystal is frequently about 300 &mgr;m in diameter. The X-ray source should, therefore, be much smaller than 300 &mgr;m.
Maximum power loading of the target, without damage to its surface is greatest when the source is a line focus at a small take-off angle to give a foreshortening of about 10 times.
It is an object of the present invention to provide an X-ray generator which produces an X-ray source having a focal spot or line of very small dimensions. It is a further object of the present invention to provide an X-ray generator capable of producing a high intensity X-ray beam at a relatively small point of application using a low operating power.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided an X-ray tube, X-ray generator comprising an electron gun, an X-ray tube, electron focusing means and a target, the electron focusing means being arranged such that the X-ray source on said target may be varied in size and/or shape and/or position.
Preferably the X-ray source on said target may be varied from a small diameter spot to a line of small width.
Preferably the generator further comprises an X-ray exit window comprising a tube of material with low X-ray absorption and of a small diameter to allow close coupling of X-ray focusing devices.
Preferably the electron focusing means comprises an electron beam focusing means mounted around the X-ray tube. The electron beam focusing means may comprise an x-y deflection system for centring the electron beam in the X-ray tube. The electron beam focusing means may further comprise at least one electron lens, preferably an axially symmetric or round lens, and at least one quadrupole or multipole lens for focusing the electron beam to a line focus. The line focus preferably has an aspect ratio in the range 1:1 to 1:20.
The electron beam lenses may be magnetic or electrostatic and are preferably electronically controlled.
Preferably the material of the exit window has a high mechanical strength and is preferably beryllium. The exit window may form part of the mechanical structure of the X-ray tube and preferably connects the X-ray tube and the target.
Preferably the target is metal, most preferably a metal selected from the group Cu. Ag, Mo, Rh, Al, Ti, Cr, Co, Fe, W, Au. In a preferred embodiment the target is copper. The target surface may be orientated such that the plane of the target surface is perpendicular or at an angle to the axis of the X-ray tube.
The target may comprise a thin metal layer deposited on a thicker substrate of a material with high thermal conductivity. Preferably the substrate material is diamond.
Preferably the generator further comprises a target cooling means. According to a first embodiment the cooling means may comprise means for directing a jet of fluid onto the target, on the opposite side of the target to the side on which the electron beam impinges. The fluid is preferably air or water. According to a second embodiment the cooling means may comprise means for effecting heat transfer by conduction or convection from the target.
Preferably the generator further comprises a deflection means which spatially scans the position of the electron beam over the face of the target.
Preferably the generator further comprises an electron mask having an aperture adapted to align the focal spot of the electron beam.
According to a second aspect of the invention there is provided an X-ray generator comprising an electron gun, an X-ray tube, a target and an X-ray exit window comprising a tube of material with low X-ray absorption and of small diameter to allow close coupling of X-ray focusing devices.
According to a third aspect of the invention the generator according to the first or second aspects is coupled with an X-ray focusing means. The X-ray focusing means preferably comprises a mirror.
The X-ray source according to the invention is designed specifically to be closely coupled to focusing X-ray devices. It is able to produce a focal spot or line of very small dimensions, and thus maximise the benefit of the focusing methods.
The distance from the electron focus to the exit window exterior is very small, and can be as low as 7 mm or less for a reflection target, or less than 1 mm for a foil transmission target.
The X-ray generator according to the invention is compact and provides a sealed tube.
The X-ray generator according to the invention needs only low power because of the efficiency of the collection and subsequent delivery of X-rays to the sample.
The generator achieves a high brilliance, defined as X-ray power per unit area per steradian.
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Arndt Ulrich Wolfgang
Duncumb Peter
Long James Victor Percival
Bede Scientific Instruments Limited
Kiknadze Irakli
Kim Robert H.
Ratner & Prestia
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