GB2428866A

GB2428866A - Improvements in x-ray tubes

Info

Publication number: GB2428866A
Application number: GB0515534A
Authority: GB
Inventors: William Thomas Luke Walker
Original assignee: Dage Precision Industries Ltd
Current assignee: Dage Precision Industries Ltd
Priority date: 2005-07-28
Filing date: 2005-07-28
Publication date: 2007-02-07
Anticipated expiration: 2025-07-28
Also published as: GB0515534D0; WO2007012817A1; GB2428866B

Abstract

An x-ray tube assembly 10 has a focussing magnet for the electron beam 12. The magnet is an electromagnet whose winding 16 projects axially of an air gap 21 between the inner pole 14 and outer pole 15. The outer pole is inturned at the radially inner edge. The arrangement provides a substantial increase in focussing power for a given diameter of electromagnet.

Description

Improvements in X-Ray Tubes This invention relates to x-ray tubes, and

particularly to reducing the diameter thereof whilst maintaining performance.

A typical x-ray tube comprises a means of producing an electron beam, which impinges on a target to produce x-rays. As is well-known x-rays are used to image objects placed between the x-ray source and an image intensifier; the x-rays are absorbed to a greater or less extent so as to cast a shadow from which internal detail of the imaged object can be determined. A typical use of x-ray imaging is to determine hidden structure, for example the bones of a human.

It will be appreciated that the quality of an image is to some extent dependant on the size of the electron beam spot from which the x-rays emanate. The smaller the spot, the more distinct will be the edge definition of the object being imaged. Accordingly an x-ray tube will typically include electromagnetic coils to focus the electron beam onto the target so as to give a small spot size. The ability to focus the electron beam is dependant on the magnetic field developed by the coils, which is in turn influenced by both the number of windings (and hence size) of the coil, and the electrical power applied to the windings. In inspection apparatus, the image may be highly magnified, in which case the smallest possible spot size is desirable in order to give sharp edge definition in the magnified image.

In an x-ray imaging device, it is also necessary to provide a very rigid tube mounting, and in some cases it may be necessary to move the tube in order to image the object from different directions. Accordingly it would be desirable to reduce the tube mass, so as generally to minimize the problems of rigidity, inertia and vibration.

According to the invention there is provided an x-ray tube assembly including a tubular focus magnet and a target at one end thereof, the focus magnet acting in use to focus an electron beam passing axially therethrough onto the target, so that x-rays are emitted, wherein the focus magnet is an electromagnet comprising an annular inner pole, an annular outer pole, and an electrical winding therebetween, the outer pole extending radially inwardly at said one end and projecting axially inwardly adjacent the target to define a circular air gap with said inner pole, and wherein said winding projects axially of said air gap.

The x-ray tube assembly of the invention permits greatly increased magnetic flux in the air gap between the inner and outer poles thereby substantially increasing the focussing power of the electromagnet as compared with prior art designs in which the winding is axially inwardly of the air gap. The inward return of the outer pole permits a longer winding whilst still allowing effective focussing of the electron beam onto the target.

Preferably the winding projects axially across the air gap, and most preferably the winding extends axially of the inward projection of the outer pole, so that the outer pole wraps around the end of the winding.

In the preferred embodiment, the axially inward projection of the outer pole projects inwardly of the target, and the air gap is also preferably inwardly of the target.

Other features of the invention will be apparent from the following description of a preferred embodiment of the invention, which is illustrated in the accompanying schematic cross-section.

The accompanying drawing illustrates in schematic form the target end of an x-ray tube assembly 10, and comprising the usual jet tube 11 through which an electron beam 12 is projected onto a target 13. Around the jet tube is the electromagnet comprising an annular inner pole 14, an annular outer pole 15, and an electrical coil 16 which for assembly purposes may be mounted on an annular carrier of e.g. aluminium. The tube assembly 10 is generally rotationally symmetrical about the axis of the electron beam.

The inner and outer poles are of soft iron.

As is well understood the electromagnet (14,15,16) can be energized to strongly focus the electron beam 12 onto the target 1 3 so as to stimulate the emission of x-rays 20 from the smallest possible area.

The focussing power of the electromagnet is dependent on the strength of the field which can be generated, and that in turn is dependant on the physical size of the coil and the electrical power supplied to the coil. A large coil is heavy, and high levels of power lead to considerable generation of heat.

In the preferred embodiment illustrated, the outer pole 15 is thickened at its radially inner edge 18, adjacent the target 13. Thickening of the inner edge 18 also permits the coil 16 to be axially longer than with prior art arrangements, so that it bridges the air gap 21 between the inner and outer poles 14,15, and overlaps to some extent the target 13.

This configuration considerably enhances field strength in the region of the target so that for a given size of coil, or for a given electrical power, the focussing power of the electromagnet is significantly increased.

For example, if the inner edge 18 is thickened to about double the thickness of the radial end wall 19, and the coil is extended, it has been found that the strength of the

magnetic field can be substantially doubled.

Field enhancement is valuable because it means that for a given focussing power, the overall diameter of the x-ray tube assembly can be significantly reduced. In the preferred embodiment the invention permits a doubling enhancement of magnetic field strength, which in turn allows a reduction of overall diameter from 150 mm to 100 mm for an assembly of substantially the same length and performance.

Alternatively, the doubling of magnetic field strength allows the current to be halved whilst maintaining tube performance at substantially the original level. A halving of current has the effect of reducing the consequent heat input by about 75%, which itself is beneficial because excessive heating is deleterious to permanent magnets. A reduction in heating also has benefits to the necessary control electronics, and avoids the need for special cooling arrangements therefor.

In the embodiment illustrated in the accompanying drawing, the diameter of the assembly 10 is 95 mm, and the axial length 150 mm. The thickness of the end wall 19 is about 3.5 mm, and the thickened inner edge 18 has a width of about 3.5 mm, and an axial length of about 7 mm. The air gap 21 is about 5 mm. A suitable ratio of diameters of inner pole, outer pole and coil is as illustrated, and the skilled man will incorporate the necessary clearances to facilitate assembly.

The dimensions quoted may of course be varied empirically in order to suit particular performance requirements, keeping in mind that the air gap 21 should be significantly axially inwardly of the end wall 19 so as to permit the coil to be substantially radially co-extensive therewith. The benefits of reduced size andlor reduced power consumption can be adapted to suit the particular installation requirements.

Claims

Claims 1. An x-ray tube assembly including a tubular focus magnet and a

target at one end thereof, the focus magnet acting in use to focus an electron beam passing axially therethrough onto the target, so that x-rays are emitted, wherein the focus magnet is an electromagnet comprising an annular inner pole, an annular outer pole, and an electrical winding therebetween, the outer pole extending radially inwardly at said one end and projecting axially inwardly adjacent the target to define a circular air gap with said inner pole, and wherein said winding projects axially of said air gap.
2. An assembly according to claim 1 wherein said winding projects axially across said air gap.
3. An assembly according to claim 2 wherein said winding extends axially of the axial projection of the outer pole.
4. An assembly according to any preceding claim wherein the target comprises an inturned cup, and said air gap is axially inwardly thereof.
5. An assembly according to any of claims 1-3 wherein the target comprises an inturned cup, and the axial projection of the outer pole extends inwardly thereof.
6. An assembly according to any preceding claim wherein the axially inward projection of the outer pole is in the range 5-10 mm, and the outer diameter of the tubular magnet is 90-110 mm.
7. An assembly according to any preceding claim wherein the axial projection of the winding is in the range 3-10 mm.
8. An assembly according to any preceding claim wherein said air gap is 5 mm or less.
9. An assembly according to any preceding claim wherein said target is circular and the outer radial edge thereof is immediately adjacent the inner radial edge of said outer pole.
10. An x-ray tube assembly substantially as described and illustrated herein with reference to the accompanying drawings. * S. * S S es..

SSCS * S * *e * S S S...

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S S * S S 5.5 *

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10. An x-ray tube assembly substantially as described and illustrated herein with reference to the accompanying drawings.

Amendments to the claims Claims 1. An x-ray tube assembly including a tubular focus magnet and a target at one end thereof, the focus magnet acting in use to focus an electron beam passing axially therethrough onto the target, and said target being perpendicular to said electron beam, so that x-rays are emitted in the direction of said beam, wherein the focus magnet is an electromagnet comprising an annular inner pole, an annular outer pole, and an electrical winding therebetween, the outer pole extending radially inwardly at said one end and projecting axially inwardly adjacent the target to define a circular air gap with said inner pole, and wherein said winding projects axially of said air gap, and said winding axially overlaps said target to some extent. * S.

2. An assembly according to claim 1 wherein said winding projects axially across said air gap. * *S * . S S...

*:. 15 3. An assembly according to claim 2 wherein said winding extends axially of the .: axial projection of the outer pole.

S.....

* 4. An assembly according to any preceding claim wherein the target comprises an inturned cup, and said air gap is axially inwardly thereof.

5. An assembly according to any of claims 1-3 wherein the target comprises an inturned cup, and the axial projection of the outer pole extends inwardly thereof 6. An assembly according to any preceding claim wherein the axially inward projection of the outer pole is in the range 5-10 mm, and the outer diameter of the tubular magnet is 90-110 mm.

7. An assembly according to any preceding claim wherein the axial projection of the winding is in the range 3-10 mm.

8. An assembly according to any preceding claim wherein said air gap is 5 mm or less.

9. An assembly according to any preceding claim wherein said target is circular and the outer radial edge thereof is immediately adjacent the inner radial edge of said outer pole.