Patent Number: 047138335
Section: summary

The present invention is concerned with X-ray source apparatus. A typical form of X-ray source available hitherto has an anode or anodes which are normally water cooled and at ground potential and which are bombarded with electrons from an electron gun having a filament biased at a high negative potential with respect to the anode. Typically the electrons travel in straight lines from the electron gun filament to the anode or anodes. Commonly, X-rays generated by the electron bombardment of the target are emitted from the source through a thin metal window (typically 0.004" thick aluminum). The target and electron source are, of course, in an evacuated chamber. This kind of X-ray source has disadvantages in certain applications. Firstly, because of the straight line (line of sight) arrangement of the electron gun and target, material evaporated from the filament can contaminate the anode which attenuates the flux of X-rays at the characteristic wavelength of the target and introduces impurity lines into the X-ray spectrum. Secondly, high energy elastically scattered electrons may be emitted from the surface of the target anode and strike the aluminum window. Such elastically scattered electrons may have energies of the order of 15 keV. These can result in melting of the window during high power operations and also the production of X-rays at wavelengths characteristic of aluminum. Furthermore, secondary electrons may be ejected from the aluminum of the window into the region to be irradiated by the X-rays. The above disadvantages are particularly important where the X-ray source is used to irradiate a sample for analytical purposes, particularly in photo-electron spectrometry. In such instruments, a specimen to be analysed is irradiated with characteristic X-rays from the X-ray source and any irradiation with stray electrons such as emitted from the aluminum window can degrade the sample. An existing form of X-ray source which avoids a number of the above disadvantages uses a target anode held at a positive potential with the electron source filament maintained at or close to ground potential. The filament is also located out of the line of sight to the target anode and focusing shields are provided to produce an electric field which focuses electrons emitted by the filament onto the target anode as desired. With this arrangement material evaporated from the filament does not contaminate the target anode and the high positive voltage of the target anode draws back elastically scattered electrons and prevents them from striking the aluminum window. With this positive anode X-ray source, however, it is essential to ensure good electrical screening of the anode when the source is being used to irradiate a specimen for example in an electron spectrometer. It is then important to ensure that the specimen is isolated from the electric field of the source so that electrons emitted by the specimen are not deviated. Because of the need for electrical shields, there is a limit to how close the target anode can be placed to a specimen to be irradiated. Also, in a practical source, a defined area of the anode produces X-rays able to illuminate the specimen. The useful X-ray intensity therefore depends on the electron current density at the anode. In a conventional source using electric field focusing, the current density is limited amongst other things by space charge spreading of the electron beam. An example of positive anode X-ray source is described in Handbook of X-ray and Ultra-Violet Photo-Electron Spectroscopy, edited by D. B. Briggs Heyden, published 1978 (pages 81-84). According to the present invention, X-ray source apparatus comprises, in an evacuated chamber, an X-ray target of a selected material which emits X-rays when bombarded with electrons of at least a predetermined energy, a source of electrons and means for accelerating electrons from the source to at least said predetermined energy, means for generating a magnetic field with lines of flux interlinking said target and said electron source and having sufficient strength that electrons of the energies of those accelerated from the source with components at angles to the magnetic field are constrained by the field to execute a helical motion along the direction of the magnetic field, with the radius of the helix being small compared to the dimensions of the apparatus. By employing a strong magnetic field in this way to "focus" or constrain electrons emitted by the source and accelerated towards the target to spiral along the lines of flux to the target, the spacing between the target and the source may be considerably increased without loss of electron flux onto the target. Very importantly, the fact that the target is in the strong magnetic field ensures also that any elastically scattered electrons from the target are similarly constrained to move back along the flux lines. Thus by suitably orientating the target relative to the flux lines (and the general direction of bombarding electron flux) X-rays can be emitted from the target to irradiate a nearby sample whilst the sample is positioned clear of the path of electrons bombarding the target and of any scattered electrons leaving the target. Thus, in the absence of any window separating the X-ray target and the specimen to be irradiated, irradiation of the specimen with elastically scattered electrons from the target is avoided. If a metal window is used between specimen and target, then the window can be positioned also so as not to be bombarded by scattered electrons. The magnetic field also limits expansion of the electron beam by space charge spreading and allows a higher current density at the X-ray anode. Conveniently, said means for generating a magnetic field is arranged such that the lines of flux interlinking said target and said electron source are curved and the apparatus includes aperture means blocking straight line paths between the source and target but permitting passage of electrons from the source along the flux lines to the target. It is relatively straightforward to arrange for the lines of flux interlinking target and source to be curved as envisaged in the above. This can be done by employing an axially symmetric magnetic field and locating the target slightly off axis in a region of strong field and locating the electron source in a region of relatively weaker field and appropriately further off axis such that the flux lines interlink target and source. By then employing the aperture means to restrict line of sight between target and source and permit only passage of electrons travelling along the flux lines, contamination of the X-ray target with material evaporated from the filament is avoided. The target may be at earth potential and the means for accelerating may then comprise an earthed grid or iris along the lines of flux interlinking said source and said target and means for producing an electron accelerating electric potential gradient between the source and the grid or iris. It will be appreciated that with the arrangement of the present invention, contamination of the specimen with elastically scattered electrons is avoided even when using an X-ray target at earth potential. There is thus no need for the positive target anode arrangement employed hitherto. Thus, the usual electrical shielding for such positive anode arrangements can be dispensed with thereby permitting the X-ray target to be positioned much closer to the specimen with attendent increases in X-ray flux onto the specimen. In one arrangement the electron source is a wire filament arranged to extend in a line at an acute angle to the lines of magnetic flux at the source and a DC voltage source to heat the filament. It will be appreciated that the filament is located in a region of relatively high mangetic field (though possibly weaker than the field of the target). Thus the DC current flowing in the filament will cause Lorenz forces to be exerted on the filament wire. By arranging th filament at an acute angle to the lines of flux the magnitude of Lorenz forces on the wire filament can be reduced. However, if the filament is too close to being parallel to the lines of flux, then thermal electrons are emitted from the filament with negligible velocity along the lines of flux and are prevented by the magnetic field from escaping the region of the filament. A compromise between these conflicting requirements is reached with typical filament angles of between 5.degree. and 30.degree. to the magnetic field. In an alternative arrangement, the electron source is a wire filament arranged to extend in a circle in a plane perpendicular to the lines of flux at the source and a DC voltage source connected to heat the filament with a DC current directed about the filament such that Lorenz forces on the filament are directed radially outwards. With this arrangement, the Lorenz forces should not produce undesirable deviation of the wire filament provided the wire has sufficient strength in tension to withstand the forces when heated. The present invention further envisages a photoelectron spectroscope or microscope having means for generating a magnetic field in the region of the specimen and X-ray source apparatus as claimed in any preceding claim having said target located adjacent the specimen in the magnetic field to irradiate the specimen.