HIGH ENERGY X-RAY TUBE

Abstract: A high energy X-ray tube has a cold cathode and anode in opposed relationship for the electron emission X-rays formed by the electron beam to travel directly to the cathode to produce further electron emission. The electron beam is collimated to be concentrated in a spot size on the anode with the result that the resulting X-rays are emitted at 180 DEG to the direction in which the electrons are emitted. A Pierce-type electron gun design is used for the collimation.

Description: HIGH ENERGY X-RAY TUBE TECHNICAL FIELD OF THE INVENTION This invention relates to a high energy x-ray tube. BACKGROUND OF THE INVENTION The conventional x-ray tube includes a target which is at an angle to the electrons emitted from the cathode and the characteristic spectrum of a sample is detected by a detector located in the path of the reflected energy. The Applicant believes that the efficiency of this could be greatly improved as about 99% of the energy is lost as waste heat. If it is desired to use high energy it is necessary to provide an ultra-high vacuum in the tube. It is an object of the present invention to provide a high energy x-ray tube which does not require such high vacuum and which is far more efficient than conventional tubes in that ray scattering from the target is greatly reduced. DISCLOSURE OF THE INVENTION According to the invention a high energy x-ray tube includes a cold cathode and an anode located in opposed relationship thereto so that electron emission X-rays formed by the electron beam travel directly to the cathode to produce further electron emission. The electron beam is preferably collimated so that it is concentrated in a predetermined pattern, for example in a particular spotsize of the produced X-ray beam on the anode; the resulting x-rays being thereby emitted at 180° to the direction in wliich the electrons are emitted. In a preferred form of the invention the collimation is achieved by means of a Pierce-type electron gun design. When dealing with high electron densities a space charge effect is established which will detract from the efficacy of the X-ray tube of the present invention which relies on the production of an electron flow effect such as the one found in an idealised diode. Pierce's theory of electron beam design comprises the insertion of a unipotential electrode, called the beam forming or focusing electrode which causes fields outside the electron beam to be formed to satisfy the proper boundary conditions in other words the electrons still behave as though they were flowing in a complete diode - that is, they pursue rectilinear trajectories. As far as the Applicant is aware, such as electron gun arrangement has not been used in X-ray tubes, nor in conjunction with a cold cathode. A further discussion of this aspect of the invention will be found in the description of the embodiment of the invention below. In a preferred form of the invention the cathode includes a window for transmission of the x-rays produced. The geometry of the tube is arranged to prevent arcing across the vacuum or across the material of the tube and the electrodes, and at the same time to achieve optimum focusing of the emitted electrons so that they can strike the anode surface at a right angle and hence produce x-rays whose maximum intensity is orientated at 180° to the electron beam. The cathode may be a cold photocathode made from earth alkali oxides on a metallic base, for example Ni- based MgO due to the fact that a self-sustained electron emission from cold magnesium oxide films applied to a metal sleeve or mesh can be achieved by applying a minimum of 300V to the anode. A positive charge is developed on the magnesium oxide layer producing a high electrical field of the order of 54 kV/cm inside the layer. Electrons which are liberated in the magnesium oxide coating are multiplied by an avalanche effect to gain sufficient energy to leave the surface. This emission current can be controlled by varying the voltage of the anode and/or of the ring electrode so that currents of several tens of milliamperes can be achieved over a period of several thousand hours without any decrease in emission. The cathode may comprise a thin and porous magnesium oxide layer which is applied to the nickel mesh. 5 The tube may be of joined ceramic and metal or glass and metal. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is described below with reference to the accompanying drawings, in which Figure 1 is a sectional side view of an X-ray tube according to the invention; i Q Figure 2 is a diagrammatic representation of a Pierce-type planar electron gun adapted for use for the X-ray tube of the invention; And Figure 3 is a graph of the equipotential lines external to the planar space charge limited electron beam at differing distances from the edge of the beam and from the 15 cathode. DESCRIPTION OF MODES OF THE INVENTION In Figure 1 a ceramic sleeve 10 encloses an anode 12 and a cathode indicated by reference 14. A focusing lens 16 for the X-rays emitted by the anode is located above the cathode. An X-ray window 20 and a cathode and X-ray window structure 22 are also 20 provided at the base of the tube. The cathode is made from an alkaline earth oxide on a metallic base, such as magnesium oxide on a nickel base. The anode may be W-Cu and is parallel with the cathode. In general, the operation of the tube relies on the establishment of an electron path which strike the anode at right angles. Some of the X-rays pass through the window 20 for 2 analyses while those that do not pass through the window strike the cathode and cause further electrons to be emitted therefrom. As indicated above a Pierce-type electron gun arrangement is provided and this is illustrated in Figure 2 where the cathode 30 is surrounded by a focusing electrode 32 which diverts random electrons back to the desired electron path between cathode and anode 34.

Claims: CLAIMS: 1. A high energy x-ray tube characterised in that it comprises a cold cathode and an anode located in opposed relationship so that electron emission X-rays formed by the electron beam travel directly to the cathode to produce further electron emission. 2. The x-ray tube according to claim 1 characterised in that the electron beam is collimated so that it is concentrated in a predetermined pattern. 3. The x-ray tube according to claim 2 characterised in that that collimations are assisted by a Pierce-type planar electron gun. 4. The x-ray tube according to any of the above claims characterised in that the beam is concentrated in a particular spotsize of the produced X-ray beam on the anode; the resulting x-rays being thereby emitted at 180° to the direction in which the elect ons are emitted. 5. The x-ray tube according to any of the above claims characterised in that the cathode includes a window for transmission of the x- ays produced. 6. The x-ray tube according to any of the above claims characterised in that the geometry of the tube is arranged to prevent arcing across the vacuum or across the material of the tube and the electrodes, and at the same time to achieve optimum focusing of the emitted electrons so that they can strike the anode surface at a right angle and hence produce x-rays orientated at 180° to the electron beam 7. The x-ray tube according to any of the above claims characterised in that the cathode is a cold photo cathode made from a metal-based alkaline earth oxide. 8. The x-ray tube according to claim 7 characterised in that the cathode comprises a thin and porous magnesium oxide layer applied to a nickel mesh. 9. The x-ray tube according to any of the above claims characterised in that the tube is of joined ceramic and metal or glass and metal.

Inventors: DAMJANOVIC DANIEL, HUNT JOHN WILLIAM, THEBOCK PETER, DAMJANOVIC, DANIEL, HUNT, JOHN, WILLIAM, THEBOCK, PETER
Applicants: HUNT JOHN WILLIAM, HUNT, JOHN, WILLIAM
Application year: 2000
Main classification: H01J35
IPC codes: H01J35