GB2034105A

GB2034105A - Improvements in or relating to radiography

Info

Publication number: GB2034105A
Application number: GB7843565A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1978-11-07
Filing date: 1978-11-07
Publication date: 1980-05-29
Also published as: GB2034105B

Abstract

In scanning x-ray tubes distortion by eddy currents of the scanning fields which intrude into anode shroud 9 region is countered by lining the shroud 9 with a magnetic shield 10 to reduce the field component in that region. The shield may be formed integrally of the shroud during casting. Shield materials are instanced and anode 5 may be oil cooled. <IMAGE>

Description

SPECIFICATION Improvements in or relating to radiography The present invention relates to X-ray tubes and, in particular, those X-ray tubes in which an electron beam is scanned relative to an X-ray target.

In X-ray tubes, a conventional electron gun provides a beam of electrons which is incident on a target to generate X-ray therefrom. It has been proposed to provide an X-ray tube in which the electron beam can be scanned, by suitable scanning coils, along an extended X-ray target so that the X-ray origin on the target, and hence the X-rays, can be scanned relative to the tube position. A typical scan may be for a distance of 40mm along the target, and suitable scanning coils are provided to sufficiently deflect the electron beam.

In applications for which such X-ray tubes are intended, it is desirable to achieve the scan with as much precision as possible. Various factors can cause scan distortion, and it is an object of this invention to provide an improved X-ray tube for which the scan can be more precise.

According to the invention, there is provided an X-ray tube anode assembly including an anode shroud integral with the anode to enclose and withdraw heat from a region through which an electron beam passes to be incident on an X-ray target, and a magnetic shield also surrounding said region and in heat conducting contact with said shroud.

According to another apsect of the invention, there is provided an X-ray tube including: an electron gun arranged to provide an electron beam; an anode assembly including an X-ray target on which said beam is incident and an anode shroud, surrounding a region through which the beam passes immediately prior to incidence on said target, to withdraw heat from said target; means for deflecting said electron beam to scan the area of incidence thereof over a said target; and a magnetic shield also surrounding said region to reduce field therein due to said scanning means, the shield being in intimate contact with said shroud to transfer heat thereto.

In order that the invention may be clearly understood and readily carried into effect, an example thereof will now be described with reference to the accompanying drawings of which, Figure 1 shows in section an X-ray tube with which the invention may be used, Figure 2 shows an anode assembly incorporating the invention in section on AA of Figure 3, and Figure 3 shows the same anode assembly in section on BB of Figure 2.

The tube shown in simplified sectional form in Figure 1 is typical of scanning X-ray tubes. The gun part includes an electron emitter (fiiament) 1 which provides electrons focussed in part by a cathode cup 2 to form a beam 3. The electrons are incident on a target region 4 of a copper anode 5 to emit X rays. In this exampie, the X-rays are selected so that those emitted substantially perpendicular to the plane of the drawing are allowed to leave the tube by a window, not shown. The anode 5 includes cooling arrangements, such as oil cooling, also not shown.

The electron beam is to be scanned over the target as indicated at 6, thus moving the X-ray origin, and for this purpose scanning coils 7 are provided to deflect beam 3.

The tube also includes an anode cone 8, and the anode cone and copper anode are generally designed so that they do not affect the field from coils 7. It is nevertheless found that distortion of the scanning can occur.

It has now been realised that a proportion of the scanning field spreads into the area occupied by the copper shroud part 9, of anode 5, which is in front of the target region. This part of the field can contribute typically 5% to the beam deflection. When the scanning waveform is composed of a mixture of frequencies, as is often the case, it is found that high frequency components are attenuated in the enclosed space as a result of eddy currents in the thick copper shroud. This causes distortion of the waveform and consequent distortion of the scan.

In a typical tube employing a linear sawtooth scan waveform of about 1 00Hz frequency, the motion of the spot at which the beam strikes the target may depart from linearity by at least 1 q/o.

It is conventional to remove eddy currents by slotting copper, but in this application it is not practicable as the consequent mechanical instabilities would also introduce distortion. It is, fulthermore, not possible to remove the copper shroud 9 as this thick section is required to collect reflection energy from the target and to return it to the anode cooling system which is situated behind the target.

To reduce the problem, this invention places a magnetic shield inside the aperture of the shroud.

The arrangement is shown in a side elevational section in Figure 2 and an end elevational section in Figure 3, both of which show the anode part, including shroud 9, only. The shield shown in these Figures at 10 is a frusto-conical mumetal shield of 1/32 inch thickness formed integrally with the shroud during the casting of the copper.

Such a shield can allow non linearity errors to be reduced by a factor often due to a reduction of the amplitude of the deflecting field within the shield and a reduction in harmonic distortion.

It should be noted that the shield should be in intimate contact with the anode shroud, perhaps by being cast within the shroud as shown, to allow proper transfer of heat to the anode for cooling. If in particular examples the temperature would exceed the Curie temperature of mu-metal or other alloys, pure iron can be used for the shield.

Other embodiments of the invention will be apparent to those skilled in the art, to suit various particular designs of X-ray tubes.

1. An X-ray tube including: an electron gun

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to radiography The present invention relates to X-ray tubes and, in particular, those X-ray tubes in which an electron beam is scanned relative to an X-ray target. In X-ray tubes, a conventional electron gun provides a beam of electrons which is incident on a target to generate X-ray therefrom. It has been proposed to provide an X-ray tube in which the electron beam can be scanned, by suitable scanning coils, along an extended X-ray target so that the X-ray origin on the target, and hence the X-rays, can be scanned relative to the tube position. A typical scan may be for a distance of 40mm along the target, and suitable scanning coils are provided to sufficiently deflect the electron beam. In applications for which such X-ray tubes are intended, it is desirable to achieve the scan with as much precision as possible. Various factors can cause scan distortion, and it is an object of this invention to provide an improved X-ray tube for which the scan can be more precise. According to the invention, there is provided an X-ray tube anode assembly including an anode shroud integral with the anode to enclose and withdraw heat from a region through which an electron beam passes to be incident on an X-ray target, and a magnetic shield also surrounding said region and in heat conducting contact with said shroud. According to another apsect of the invention, there is provided an X-ray tube including: an electron gun arranged to provide an electron beam; an anode assembly including an X-ray target on which said beam is incident and an anode shroud, surrounding a region through which the beam passes immediately prior to incidence on said target, to withdraw heat from said target; means for deflecting said electron beam to scan the area of incidence thereof over a said target; and a magnetic shield also surrounding said region to reduce field therein due to said scanning means, the shield being in intimate contact with said shroud to transfer heat thereto. In order that the invention may be clearly understood and readily carried into effect, an example thereof will now be described with reference to the accompanying drawings of which, Figure 1 shows in section an X-ray tube with which the invention may be used, Figure 2 shows an anode assembly incorporating the invention in section on AA of Figure 3, and Figure 3 shows the same anode assembly in section on BB of Figure 2. The tube shown in simplified sectional form in Figure 1 is typical of scanning X-ray tubes. The gun part includes an electron emitter (fiiament) 1 which provides electrons focussed in part by a cathode cup 2 to form a beam 3. The electrons are incident on a target region 4 of a copper anode 5 to emit X rays. In this exampie, the X-rays are selected so that those emitted substantially perpendicular to the plane of the drawing are allowed to leave the tube by a window, not shown. The anode 5 includes cooling arrangements, such as oil cooling, also not shown. The electron beam is to be scanned over the target as indicated at 6, thus moving the X-ray origin, and for this purpose scanning coils 7 are provided to deflect beam 3. The tube also includes an anode cone 8, and the anode cone and copper anode are generally designed so that they do not affect the field from coils 7. It is nevertheless found that distortion of the scanning can occur. It has now been realised that a proportion of the scanning field spreads into the area occupied by the copper shroud part 9, of anode 5, which is in front of the target region. This part of the field can contribute typically 5% to the beam deflection. When the scanning waveform is composed of a mixture of frequencies, as is often the case, it is found that high frequency components are attenuated in the enclosed space as a result of eddy currents in the thick copper shroud. This causes distortion of the waveform and consequent distortion of the scan. In a typical tube employing a linear sawtooth scan waveform of about 1 00Hz frequency, the motion of the spot at which the beam strikes the target may depart from linearity by at least 1 q/o. It is conventional to remove eddy currents by slotting copper, but in this application it is not practicable as the consequent mechanical instabilities would also introduce distortion. It is, fulthermore, not possible to remove the copper shroud 9 as this thick section is required to collect reflection energy from the target and to return it to the anode cooling system which is situated behind the target. To reduce the problem, this invention places a magnetic shield inside the aperture of the shroud. The arrangement is shown in a side elevational section in Figure 2 and an end elevational section in Figure 3, both of which show the anode part, including shroud 9, only. The shield shown in these Figures at 10 is a frusto-conical mumetal shield of 1/32 inch thickness formed integrally with the shroud during the casting of the copper. Such a shield can allow non linearity errors to be reduced by a factor often due to a reduction of the amplitude of the deflecting field within the shield and a reduction in harmonic distortion. It should be noted that the shield should be in intimate contact with the anode shroud, perhaps by being cast within the shroud as shown, to allow proper transfer of heat to the anode for cooling. If in particular examples the temperature would exceed the Curie temperature of mu-metal or other alloys, pure iron can be used for the shield. Other embodiments of the invention will be apparent to those skilled in the art, to suit various particular designs of X-ray tubes. CLAIMS

1. An X-ray tube including: an electron gun arranged to provide an electron beam: an anode assembly including an X-ray target on which said beam is incident and an anode shroud, surrounding a region through which the beam passes immediately prior to incidence on said target, means for deflecting said electron beam to scan the area of incidence thereof over said target; and a magnetic shield also surrounding said region to reduce magnetic field therein due to said scanning means, the shield being in intimate contact with said shroud to transfer heat thereto.

2. An X-ray tube according to claim 1 in which the shield is formed integrally with the shroud.

3. An X-ray tube according to either of the preceding claims in which the shield is made of a magnetic alloy.

4. An X-ray tube according to claim 3 in which the magnetic alloy is a Nickel-lron alloy.

5. An X-ray tube according to claim 4 in which the alloy is Mumetal.

6. An X-ray tube according to either of claims 1 or 2 in which the shield is made of iron.

7. An X-ray tube substantially as herein described with reference to the drawings accompanying the specification.

8. A method of manufacturing an anode shroud member for an X-ray tube incorporating means for scanning an incident electron beam over an extended target, the method including the step of casting the shroud with a magnetic shield formed integrally therewith.

9. A method according to claim in which the shield is made of a magnetic alloy.

10. A method of manufacturing an anode shroud member for an X-ray tube, the method being substantially as herein described with reference to the drawings accompanying the specification.