Patent Number: 047138335
Section: description

Referring to FIG. 1, an X-ray target 10 is illustrated located in a region of magnetic field H, produced by a super conducting solenoid 9, the direction of the field and of the lines of flux being indicated by an arrow 11. The target 10 comprises a block of metal, typically magnesium, having a face 12 exposed to be bombarded by energetic electrons. The target 10 is water cooled by means of pipes and conduits 13 and 14. In FIG. 1, the magnetic field H is illustrated as uniform and linear over an extended region. An electron source is shown generally at 15 also located in the region of magnetic field H and arranged to accelerate electrons towards the target in the direction parallel to the lines of flux indicated by the arrows 11. The magnetic field H and the positioning of the target 10 and electron source 15 is such that the source and the target are interlinked by lines of flux of the magnetic field H. The source 15 comprises a wire filament 16, typically of tungsten, supplied with DC current from a source illustrated by battery 17. The DC current heats the filament 16 to a temperature at which it emits thermionic electrons. A grid or iris 18 is located between the filament 16 and the X-ray target 10 across the lines of flux interlinking the target and filament. The grid or iris 18 is held at earth potential and the filament 16 is held at a relatively high negative potential, typically in excess of 15 kV, by means of a DC EHT supply indicated in FIG. 1 for convenience by the battery pile 19. Thus, an accelerating electric field is established between the grid or iris 18 and the filament 16 so that thermionic electrons from the filament are accelerated by the electric field towards the X-ray target 10. The operation of an electron gun of this general kind is well known and will not be described further herein. It is sufficient to note however that the electrons for bombarding the X-ray target 10 are accelerated by electric field between the filament 16 and the grid or iris 18. The target 10 itself is held at earth potential. The magnetic field H is arranged to be sufficiently strong to ensure that electrons accelerated from the filament 16 are constrained to spiral or execute helical paths about the flux lines towards the face 12 of the target 10. Since flux lines interlink the filament 16 and the target 10, the flux of electrons bombarding the target is maximised. The spacing between the target 10 and the source of electrons 15 is not critical and the two elements of the X-ray source may with advantage be at some distance, as compared with X-ray sources known hitherto. The proximity of the target 10 and electron source 15 as illustrated in FIG. 1 is exaggerated for simplicity and the flight path 20 of accelerated electrons towards the target 10 may be considerably longer. The source of electrons may thus be located in a region of lower magnetic field strength than the anode so that emission may take place over a relatively large area which is projected onto the anode at reduced size. In this way problems of space charge at the source of electrons can be minimised. In order to ensure that electrons accelerated to energies in excess of 15 kV and having components of these energies at angles to the lines of magnetic flux are fully constrained to spiral about the lines of flux, the magnetic field must be of sufficient strength over the entire flight path of the electrons. Magnetic fields of the order of 7 Tesla have been found satisfactory. It can be shown that the cyclotron orbit of an electron of an energy of 10 kV in a magnetic field of this magnitude has a diameter of only approximately 100 microns. Thus electrons travelling to the target at such energies in such a field are brought to the target with a spacial uncertainty of less than 100 microns. The magnetic field may be produced by superconducting solenoid magnets. Technology for this purpose is well established and no further details are given herein. Referring now to FIG. 2, a variation is illustrated of the arrangement shown in FIG. 1. The X-ray source of FIG. 2 may be used in a photo-electron spectroscope or photo-electron microscope as the electron source for irradiating specimens to emit photo-electrons for analysis purposes. Photo-electron spectroscopes are known and a particular form of photo-electron microscope is described in the specification of International patent application No. PCT/GB 82/00008. The X-ray source illustrated in FIG. 2 could be used in the photo-electron microscope described in the above-mentioned patent application. In that photo-electron microscope, the specimen is located in a region of high magnetic field which constrains photo-electrons emitted by the specimen to spiral around the flux lines of the field and thereby maximising the photo-electron flux for analysis purposes. Considering FIG. 2, a specimen 30, is located on the axis of an axially symmetrical magnetic field such as produced by a super-conducting solenoid 31. The specimen 30 is arranged to be irradiated with X-rays from an X-ray target 32 such as that illustrated in FIG. 1. The X-ray target 32 is located also in the region of high magnetic field close to the specimen 30 but slightly off the axis of the field. Energetic electrons from an electron gun illustrated generally at 33 are focused onto the target 32 by means of the magnetic field. The super-conducting solenoid 31 is arranged so that the field is weaker in the region of the electron gun 33 with the lines of magnetic flux diverging from the axis as illustrated in the drawing. Thus, the electron gun 33 is located rather further off the axis 34 than the target 32 such that the gun 33 and the target 32 are interlinked by the curved lines of flux of the magnetic field. In the same way as described above, electrons are accelerated by the gun 33 and constrained to travel along the curving lines of flux so as to bombard the target 32 to produce the desired X-rays which irradiate the specimen 30. The magnetic field strength is sufficient to constrain the electrons at the accelerated energy to follow the curved path 35 illustrated in FIG. 2. Again, the target 32 can be at earth potential because any elastically scattered electrons from the target are also constrained to spiral back along the lines of flux and therefor cannot contaminate the specimen 30 which is located off the flight path 35 of the electrons. An aperture 36 is provided along the flight path 35 to block the direct straight line of sight between the filament of the electron gun 33 and the target 32 and specimen 30. Thus, as a result of the curved path 35 of the electrons, neither the target 32 nor the specimen 30 can be contaminated by material evaporated off the filament. Because the target 32 is at earth potential, there is no need for the usual electrical screens necessary for X-ray sources having positive target anodes. As a result the target 32 can be positioned closer to the specimen 30 to maximise the X-ray flux onto the specimen. In the arrangement illustrated, the elements of the X-ray source and the specimen 30 of the photo-electron microscope or spectroscope share a common evacuated chamber. However, it may nevertheless be desirable to provide separate pumping for the X-ray source and for the spectroscope or microscope. It will be then necessary to provide a window between the X-ray source and the specimen 30 which is transparent to X-rays. An aluminum foil window may be used. The problem of bombardment of the aluminum window with scattered electrons is obviated so that the danger of excessive heating of the window or the generation of aluminum characteristic parasitic X-rays in the window is avoided. Referring now to FIGS. 3 and 4 two arrangements for the filament 16 of the electron gun or source 15 (FIG. 1) 33 (FIG. 2) are illustrated. Referring to FIG. 3, the filament 40 is arranged to extend in a straight line between support posts 41 and 42. The line of the filament 40 is arranged to be at an acute angle as illustrated to the direction of the magnetic field H. As a result the magnitude of Lorenz forces on the filament wire 40 caused by the DC current i flowing in the wire is reduced, thereby minimising the stress on the filament during operation and undesirable deviation of the filament. It will be understood that the smaller the angle between the line of the filament 40 and the field H the less is the Lorenz force on the wire. However, if the wire 40 is parallel to the field, then the field has the effect of preventing escape of thermionically emitted electrons from the wire. Thus, a compromise angle is employed at which the Lorenz force is satisfactorily reduced without excessive reduction in the electron flux from the filament. Angles between 5.degree. and 30.degree. to the field may be suitable. An alternative arrangement is illustrated in FIG. 4 in which the filament 50 extends in a circular path between the two supporting pillars 51 and 52 which are arranged side-by-side. The circular filament 50 is orientated in a plane at right angles to the direction of the field H. In operation, the DC voltage supply to heat the filament 50 is connected between the ends of the circular filament 50 so that the DC current flows about the filament in a direction relative to the direction of the field H which produces a force on the wire of the filament 50 directed radially outwards of the circular filament. In this way, the forces about the wire of the filament 50 do not cause the wire to deviate from the illustrated position, provided the wire of the filament has sufficient strength in tension when heated. Furthermore, forces applied by the ends of the filament 50 to the post 51, 52 are purely tension forces in the wire of the filament so that sheer forces between the ends of the wire and the connecting posts can be eliminated.