X-ray or gamma ray systems or devices – Source
Patent
1985-11-14
1987-12-15
Fields, Carolyn E.
X-ray or gamma ray systems or devices
Source
378137, 378138, G21G 400, H01J 3500, H01J 3530
Patent
active
047138335
DESCRIPTION:
BRIEF 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 hav
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Beamson et al, "The Collimating and Magnifying Properties of a Superconducting Field Photoelectron Spectrometer", J. Phys. E: Sci. Instrum., vol. 13, 1980, pp. 64-66.
Dixon Andrew J.
Gehring Karl A.
Keenlyside Michael
Turner David W.
Fields Carolyn E.
Hynds Joseph A.
Kevex Corporation
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