GB1604631A

GB1604631A - X-ray microscopy

Info

Publication number: GB1604631A
Application number: GB1565777A
Authority: GB
Original assignee: NICOLET XRD CORP
Current assignee: NICOLET XRD CORP
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-12-09

Description

(54) IMPROVEMENTS IN OR RELATING TO X-RAY MICROSCOPY (71) We, NICOLET XRD COR PORATION, a company incorporated under the laws of the State of California, United States of America, of 10061 Bubb Road, Cupertino, California 95014, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to X-ray microscopy.

X-ray microscopy using a point focus Xray projection method provides a very valuable technique for the non-destructive examination in detail and depth of specimens which may be opaque to light or electrons. High magnification is obtainable. It is possible to have the specimen freely disposed in the atmosphere and this technique therefore lends itself to the observation of changes in specimens over a period of time.

In point focus X-ray projection apparatus, an electron beam from a gun is focussed onto a target of a suitable metal to emit X-rays; the electron beam must be sharply focussed to give a substantially point source of the X-rays. The specimen may be put closely adjacent a fluorescent viewing screen or other viewing device or a photographic emulsion but, with point focus projection techniques, by disposing the specimen between the point source and the screen or emulsion, magnification is obtained. Further magnification may be obtained by viewing the screen through a suitable optical or electronic viewing system or by photographic enlargement techniques of a photograph obtained by exposing photographic film to the X-rays which are passed through the specimen.

In all these arrangements, the sharpness of the image depends on the X-rays being a point source.

In X-ray microscopes, the source of the Xrays is a point source; in this context, a point source typically is a nearly circular source of X-rays of about 15 microns effective diameter.

The electron beam has to be sharply focussed on the target to produce this required point source. It is well-known that when a substantially circular and finely focussed beam electrons strikes a target, X-rays are produced in the form of a wide-angled cone. Point focus tubes designed to produce X-ray radiation at for example 10 to 90 kilovolts are fitted with a "window" of aluminium or similar foil which has to be sufficiently thin to be transparent to these X-rays. This window has to withstand the atmospheric pressure, as a vacuum has to be maintained within the envelope of the X-ray tube. For this reason the size of the window is limited and typically is 12 to 24 mm diameter. The limitation on the size of the window limits the size of the cone of X-rays from the target which can pass out of the tube envelope and be utilised for radiographic purposes.

According to the present invention, in apparatus for X-ray microscopy having an electron beam gun providing a beam of electrons focussed onto a target in an envelope to produce a point source of X-rays, there are provided two or more windows in the envelope arranged for the passage of X-rays from the target, auxiliary viewing means for monitoring the focus of the beam through one of said windows, means for viewing or photographing a specimen exposed to X-radiation from said source which has passed through another of said windows, and a mesh or other object forming a focussing device, a sharp image of which focussing device is to be observed by said auxiliary viewing means, said focussing device being in the X-ray path to said auxiliary means.Each window, in the known way, is preferably arranged in a plane normal to the line between the centre of the window and the target. The additional window or windows enable a further X-ray beam or beams from the target to pass out from the envelope and to be utilised for radiography. The windows have to be suitably positioned with respect to the direction of radiation of the X-rays from the target but, as previously explained, these X-rays are produced in the form of a wide angled cone and it is readily possible to provide two or more windows in the envelope to pass radiation from the focal spot on the target. Since this focal spot is nearly circular, the sharpness of focus of objects observed using one or more additional windows positioned near the main window will be substantially the same as that of the main beam.As previously explained, the sharpness of the image X-ray microscopy depends on the X-rays being from a point source and is primarily dependent on the electron beam focussing. For various reasons the focus may not remain constant during the making of a long exposure, as may occur, for example, in dynamic studies. The usual method of monitoring the focussing is by observation of the image of a focussing device, such as a fine metallic mesh, which can be moved into or out of the space between the target and the tube window using control means external of the envelope of the tube. Frequently the size and density of the specimen obscures the image of the focussing device and, for focussing purposes, it is necessary to move the specimen.

This may be undesirable for a number of reasons. In the X-ray apparatus of the present invention, a focussing device is placed in the X-ray beam path through a window separate from the window used for viewing the object under examination. The focussing device conveniently is arranged in the envelope between the target and the auxiliary window, means being provided for imaging the focussing device using said auxiliary viewing means. This auxiliary viewing means may comprise a suitable camera, e.g. a vidicon camera tube for providing an image on a cathode ray tube, or may comprise a fluorescent screen preferably with an optical magnifier.

It will be seen that, with the above-described construction, the focus can be monitored; if adjustment is necessary in the focussing of the electron beam on the target, this can be effected using electron beam focussing controls in the known way without interruption of observation of the specimen. This arrangement moreover permits of the possibility of using automatic focussing control; for this purpose the auxiliary viewing means may include optical or electronic means monitoring the image of the mesh or other focussing device on a screen and arranged to provide an output signal dependent on the sharpness of focus. Such a signal may be provided, for example, by scanning an image of the mesh. The signal can be used for controlling the electron beam focus in a feedback control system so as to maintain the required sharpness of focus on the auxiliary screen.

A further advantage of this arrangement is that it facilitates correct focussing if the kilo voltage used on the X-ray tube is changed; this may be desired if the penetration of the specimen is seen on the main beam viewing device to be either too little or too great. Rais ing or lowering of the kilovoltage alters the focus of the main beam. With the auxiliary viewing device, the necessary corrections to the focus can be effected readily without moving the specimen.

A further advantage of the arrangement of the present invention utilising a separate window in the envelope for focussing purposes is that the viewing device used for focussing, e.g. an X-ray sensitive television camera tube, may be positioned at the outer end of a tubular member extending through the envelope and vacuum sealed thereto, the tube being in vacuum connection with the envelope.

The auxiliary focussing window may then be at the outer end of this tubular member which, at its inner end, may be arranged to support the mesh or other object to be viewed for focussing. By this construction, it is possible to support a focussing mesh in a position as near the target as is electrically practical so that an image of the mesh is magnified as much as possible before reaching the camera. There are limitations to the possible distance between the mesh and the window because the intensity of the beam is reduced as the square of the distance; these limitations are particularly serious at comparatively weak long wavelength levels.

Air absorbs these longer wavelengths to much greater extent than the shorter wavelengths but, with the above construction, the tubular member provides an evacuated pathway for the beam from the envelope to the camera.

The viewing device for observing the specimen may be further away from the envelope than the focussing viewing camera; provision must be made for suitably holding the specimen in the X-ray beam between the viewing device and the envelope. A film for photographic recording will be sufficiently activated with a much weaker intensity than is required for viewing by an X-ray sensitive television camera. This construction thus enables the intensity of the X-rays generated at the secondary viewing device used for focussing to be much greater than that at the main viewing device. This is of particular importance when the X-ray tube is operated at a low voltage, e.g. 10 to 15 kv. Because of the greater intensity, it is much easier to focus accurately with the secondary camera.

By having a plurality of windows in addition to that used for focussing, independent radiography of a number of specimens is possible simultaneously. This is of particular importance in the industrial use of X-ray microscopy for inspection purposes. It enables a single X-ray tube to be used for the simultaneous inspection of a number of articles and thus considerably reduces the amount of equipment required when large numbers of articles have to be examined.

The following is a description of a number of embodiments of the invention, reference being made to the accompanying drawings in which: Figures 1 and 2 are diagrams showing in plan and elevation the conic form of X-rays produced in an X-ray tube; Figure 3 is a diagrammatic plan view of part of an X-ray microscope constituting one embodiment of the invention with the top cover of the target chamber removed; Figure 4 is a diagrammatic sectional elevation of part of the apparatus of Figure 3 along the lines A-A'; Figure 5 illustrates a modification of part of the construction of Figure 3; and Figure 6 shows diagrammatically a side view of part of another form of X-ray microscope constituting a further embodiment of the invention.

Referring to Figures 1 and 2 there is shown diagrammatically an electron beam gun 1 which produces electrons3 indicated by dashed lines which focus at a point 2 on a cylindrical target 3. From this point focus, X-rays are emitted in a wide angled cone as indicated by the full lines 4. A cylindrical target 3 is employed which is rotatable and slidable about its axis so that, if a point on the target becomes pitted by the electron beam, the target may be rotated and/or moved axially to bring a fresh portion of the target material into the focus position of the electron beam whilst still leaving a target surface at the same angle. The target can consist of several different elements, for example in the form of rings, so that the characteristic radiations of different material can be available if required.

The present invention makes use of two or more X-ray beams in different directions from the target. Referring to Figures 3 and 4, there is shown an X-ray microscope having an electron gun 10 with means 11 for focussing an electron beam onto a target 12 in an evacuated chamber 13. In this drawing, electromagnetic focussing means are shown. As explained above, the target 12 is conveniently cylindrical and rotatable and slidable about its axis. A main viewing device consists of either an X-ray sensitive vidicon camera 14 together with a television-type monitor (not shown) or a fluorescent screen with an optical magnifier. A specimen 15 is positioned between a window 16 and the viewing device 14, the position being in accordance with the X-ray enlargement required. An auxiliary window 17 provides an X-ray beam to an auxiliary viewing device 18.

As previously explained, this auxiliary window is used for monitoring the focussing.

For focussing, a fine metallic mesh is positioned either between the target 12 and the auxiliary window 17 or between the window 17 and the further viewing device 18. Preferably however the mesh is located inside the envelope so that the viewing device may be put close to the envelope when the X-ray intensity is greater, so facilitating focus adjustment, particularly when the tube is operated at low voltages.

A preferred construction for focussing is shown in Figure 5 which is a diagram showing a modified form of part of Figure 3. For focussing, a tubular member 20 extends through the envelope 13 and is sealed thereto.

At its inner end 21, it carries a mesh element which is to be viewed for focussing, this inner end being located as near the target 12 as is practical. The tubular member 20 is open to the evacuated envelope and is sealed at its outer end by a window 22 adjacent which is a focus viewing device including an X-ray sensitive television camera 23. As seen in Figure 5, the camera 14 for the main viewing beam may be put further away from the envelope 13 than is the camera 23; a photographic film will be sufficiently activated if placed just in front of the main camera whereas, as previously explained, a more intense beam is desirable for focussing purposes.

For the reasons previously discussed, the arrangement of Figure 5 is particularly suitable for longer wavelength work although it may be employed for shorter wavelength work. In the latter case, it is usual to draw the focussing camera back to give greater magnification of the mesh or sometimes to prevent burning of the vidicon faceplate of the focussing camera. Conviently a rotational disc is provided between the window and the camera, the disc having an aperture to permit of direct passage of the X-rays to the camera in one position of the disc but in other positions, e.g.

in two further positions, putting aluminium foil of different thicknesses between the window and the camera. By this arrangement, if the kilovolts on the tube is changed, then, by rotation of the disc, a suitable attenuation can be achieved without having to move the focussing camera.

The window 17 of Figure 3 permits of observation of the X-ray source from a direction at an angle to the axis of the main X-ray beam through the window 16. The windows must be positioned to lie in the path of the X-ray radiation3 which as explained with reference to Figures 1 and 2, is in the form of a wide angled cone. In the arrangement of Figures 3 and 4, the axis of the electron beam is substantially vertical and the axes of the X-ray beams defined by the windows 16, 17 are substantially horizontal. This is one convenient arrangement for use with a tube having a cylindrical envelope 13.

Another convenient arrangement however is illustrated in Figure 6 in which the electron beam and the utilised X-ray beam all lie in the same plane. Referring to Figure 6 there is shown an electron beam gun 30 for directing an electron beam onto a cylindrical target 12 in an envelope 13. In this particular embodiment, the gun 30 is shown as having a filament 24 in a Wehnelt cylinder 25 to give electrostatic focussing at a point 26 on the target. Two windows 27, 28 are illustrated in the housing through which pass electron beams from the source 26.

It will be readily apparent that more than two windows can readily be provided in all the above-described constructions. One window is used for focussing and the further windows are utilised for X-ray examination of separate specimens.

In the above-described arrangements, the electron beam produces a "point" source of X-rays which is a very small region on the target, substantially circular and typically of 15 microns diameter. It is readily possible to position the various windows so that this small region on the target gives substantially the same effective source size at all the various windows.

The various windows must be located within the region of X-ray radiation from the target.

As previously mentioned, this region is conical.

The X-rays are generated just below the surface of the target and not on the surface. Also a minute pit is formed in the target at an early stage. These two factors limit the cone angle and thus any auxiliary windows must not be too far from the plane defined by the target, gun and main window. The size of the envelope and of the windows will determine the maximum number of windows which can be provided but, typically, this maximum might be about 9 windows of 12 mm diameter.

The envelope in which the windows are located might be cylindrical but, if the windows are sealed with O-ring seals, fiats have to be formed on the envelope. To avoid having to have a large wall thickness in which a flat can be machined, it is often more convenient to use a cubic formation for the centre part of the X-ray tube, particularly for large tubes.

Cylindrical supports for the target and gun may be sealed to that centre portion.

WHAT WE CLAIM IS: 1. Apparatus for X-ray microscopy having an electron beam gun providing a beam of electrons focussed onto a target in an envelope to produce a point source of X-rays, wherein there are provided two or more windows in the envelope arranged for the passage of Xrays from the target, auxiliary viewing means for monitoring of the focus of the beam through one of said windows, means for viewing or photographing a specimen exposed to X-radiation from said source which has passed through another of said windows, and a mesh or other object forming a focussing device, a sharp image of which focussing device is to be observed by said auxiliary viewing means, said focussing device being in the X-ray path to said auxiliary viewing means.

2. Apparatus as claimed in claim 1 wherein each window is arranged in a plane normal to the line between the centre of the window and the target.

3. Apparatus as claimed in either of the preceding claims and having a plurality of further windows for the independent radiography of a number of specimens simultaneously.

4. Apparatus as claimed in any of the preceding claims wherein the focussing device is fixed in the envelope between the target and the window.

5. Apparatus as claimed in any of the preceding claims wherein the auxiliary viewing means comprises a camera tube for providing an image on a cathode ray tube.

6. Apparatus as claimed in any of claims 1 to 4 wherein the auxiliary viewing means comprises a fluorescent screen.

7. Apparatus as claimed in claim 6 wherein the auxiliary viewing means includes an optical magnifier.

8. Apparatus as claimed in any claims 1 to 4 wherein said auxiliary viewing means include optical or electronic means monitoring the image of the focussing device on a screen and arranged to provide an output signal dependent on the sharpness of focus.

9. Apparatus as claimed in claim 8 and having a feedback control system using said output signal for controlling the electron beam focus so as to maintain the required sharpness of focus at the auxiliary viewing means.

10. Apparatus as claimed in claim 1 wherein said auxiliary viewing means used for focussing is positioned at the outer end of a tubular member extending through the envelope and vacuum sealed thereto, the tube being in vacuum connection with the envelope and wherein said further window is at the outer end of said tubular member.

11. Apparatus as claimed in claim 10 wherein in said tubular member, at its inner end, is arranged to support the focussing device to be viewed for focussing.

12. Apparatus as claimed in either claim 10 or claim 11 wherein the auxiliary viewing means comprises an X-ray sensitive television camera tube.

13. Apparatus for X-ray microscopy substantially as hereinbefore described with reference to Figures 3 and 4 or Figure 5 or Figure 6 of the accompanying drawings.

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Claims

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in an envelope 13. In this particular embodiment, the gun 30 is shown as having a filament 24 in a Wehnelt cylinder 25 to give electrostatic focussing at a point 26 on the target. Two windows 27, 28 are illustrated in the housing through which pass electron beams from the source 26.

It will be readily apparent that more than two windows can readily be provided in all the above-described constructions. One window is used for focussing and the further windows are utilised for X-ray examination of separate specimens.

In the above-described arrangements, the electron beam produces a "point" source of X-rays which is a very small region on the target, substantially circular and typically of

15 microns diameter. It is readily possible to position the various windows so that this small region on the target gives substantially the same effective source size at all the various windows.

The various windows must be located within the region of X-ray radiation from the target.

As previously mentioned, this region is conical.

The X-rays are generated just below the surface of the target and not on the surface. Also a minute pit is formed in the target at an early stage. These two factors limit the cone angle and thus any auxiliary windows must not be too far from the plane defined by the target, gun and main window. The size of the envelope and of the windows will determine the maximum number of windows which can be provided but, typically, this maximum might be about 9 windows of 12 mm diameter.

The envelope in which the windows are located might be cylindrical but, if the windows are sealed with O-ring seals, fiats have to be formed on the envelope. To avoid having to have a large wall thickness in which a flat can be machined, it is often more convenient to use a cubic formation for the centre part of the X-ray tube, particularly for large tubes.

Cylindrical supports for the target and gun may be sealed to that centre portion.

WHAT WE CLAIM IS: 1. Apparatus for X-ray microscopy having an electron beam gun providing a beam of electrons focussed onto a target in an envelope to produce a point source of X-rays, wherein there are provided two or more windows in the envelope arranged for the passage of Xrays from the target, auxiliary viewing means for monitoring of the focus of the beam through one of said windows, means for viewing or photographing a specimen exposed to X-radiation from said source which has passed through another of said windows, and a mesh or other object forming a focussing device, a sharp image of which focussing device is to be observed by said auxiliary viewing means, said focussing device being in the X-ray path to said auxiliary viewing means.
2. Apparatus as claimed in claim 1 wherein each window is arranged in a plane normal to the line between the centre of the window and the target.
3. Apparatus as claimed in either of the preceding claims and having a plurality of further windows for the independent radiography of a number of specimens simultaneously.
4. Apparatus as claimed in any of the preceding claims wherein the focussing device is fixed in the envelope between the target and the window.
5. Apparatus as claimed in any of the preceding claims wherein the auxiliary viewing means comprises a camera tube for providing an image on a cathode ray tube.
6. Apparatus as claimed in any of claims

1 to 4 wherein the auxiliary viewing means comprises a fluorescent screen.
7. Apparatus as claimed in claim 6 wherein the auxiliary viewing means includes an optical magnifier.
8. Apparatus as claimed in any claims 1 to 4 wherein said auxiliary viewing means include optical or electronic means monitoring the image of the focussing device on a screen and arranged to provide an output signal dependent on the sharpness of focus.
9. Apparatus as claimed in claim 8 and having a feedback control system using said output signal for controlling the electron beam focus so as to maintain the required sharpness of focus at the auxiliary viewing means.
10. Apparatus as claimed in claim 1 wherein said auxiliary viewing means used for focussing is positioned at the outer end of a tubular member extending through the envelope and vacuum sealed thereto, the tube being in vacuum connection with the envelope and wherein said further window is at the outer end of said tubular member.
11. Apparatus as claimed in claim 10 wherein in said tubular member, at its inner end, is arranged to support the focussing device to be viewed for focussing.
12. Apparatus as claimed in either claim 10 or claim 11 wherein the auxiliary viewing means comprises an X-ray sensitive television camera tube.
13. Apparatus for X-ray microscopy substantially as hereinbefore described with reference to Figures 3 and 4 or Figure 5 or Figure 6 of the accompanying drawings.