X-ray or gamma ray systems or devices – Specific application – Fluorescence
Patent
1989-03-10
1991-05-28
Fields, Carolyn E.
X-ray or gamma ray systems or devices
Specific application
Fluorescence
378 45, 378156, G21K 300
Patent
active
050200844
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and apparatus for the analysis of samples of materials in which a finely divided heavy metal is dispersed in a non-metallic matrix, especially mineral ores containing gold, uranium, lead or platinum, using the technique of X-ray fluorescence (XRF).
The expression "heavy metal" used herein means tungsten and metals of higher atomic number.
A "non-metallic matrix" means a matrix consisting predominantly of non-metallic elements of low atomic number and/or their compounds, such as silica, but may also contain metals which are not "heavy metals" as defined above, either in elemental or in combined form, such as iron or barium.
The phenomenon of energy-dispersive X-ray fluorescence (XRF) is well-known. A sample, for example, of a mineral ore, is bombarded by high-energy X-rays and the fluorescence spectrum is analysed by counting the rate of emission of photons over a range of photon energies.
Assay laboratories of gold mines have a daily chore of analysing large numbers of samples of gold ore; these include both exploration samples taken to locate the gold-bearing ore, and random samples of ore actually mined, for accounting purposes. The total number of samples to be analysed per day typically ranges from a few hundred to several thousand depending on the size of the mine. A high throughput of samples is therefore highly desirable, in the order of say one sample per hundred seconds, and the only practicable assay method which can achieve this is the traditional fire assay method. This method, however, is expensive in time, labour, capital and running costs, and less accurate than desirable. It is essentially a manual process requiring large quantities of electricity and other consumables and in a typical laboratory dealing with 1,000-2,000 samples per day it is difficult to keep track of individual samples.
The use of X-ray fluorescence techniques to analyse ores for heavy metals is known, and is described for example in U.S. Pat. No. 3,404,275 (Martinelli) UK Patent Patent Specification 1080346 (VEB Vacutronic), German Offenlegungschriften 2046606 and 2140794 (Siemens AG), U.S. Pat. No. 4,224,517 (Lubecki) and European Patent Application 0166914 (Kernforschungzentrum Karlsruhe GmbH). Most of these disclosures contemplate the use of the characteristic K-emission peaks of the elements of interest. Such fluorescent emission may be stimulated by various sources, principally X-ray tubes (as in UK Patent Specifications 1070337 (Laurila) and 1017595 (UKAEA), and German Offenlegungsschriften 2054464) and radionucleides (as in U.S. Pat. No. 3,404,275). None of these disclosures however uses high energy X-radiation within the meaning of the present disclosure.
None of the above prior art discloses the particular technique of counting the emitted fluorescence photons in energy bands selected in relation to the K-emission bands of the elements under analysis according to the present invention.
The use of metallic filters to shape the radiation incident upon the sample under analysis is known, for example, from U.S. Pat. No. 3,404,275 where a cadmium filter is used, but not for the particular purpose relevant to the present invention; this U.S. Patent, for example, uses for exciting the sample not high-energy X-rays, but .delta.-rays. Again, UK Patent 1070337 discloses the use of nickel or cobalt filters but in the context of analysis for lighter metals than those contemplated by the present invention.
It is especially significant that none of the prior art discloses the preferred method according to the invention of producing the stimulating radiation--that is the exploitation of the broad bremsstrahlung peaks, filtered through a suitable metallic filter to excite the characteristic K-emission peaks of the metals under analysis. The use of germanium detectors, both of high purity (European Patent Application 0166 914 and UK Patent 4224571) and silicon or lithium-drifted (Offenlegungsschriften 2046606 and 2054464) is known but not in the context of the particular energy profile of the f
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Translation of the Baxmann et al. article titled "Basic Principles of X-ray Fluoresence Analysis".
Translation of W. German OffenLegungsschrift 2,649,512 by Denbsky.
Rolf Woldseth, Ph.D., "All You Ever Wanted to Know About X-Ray Energy Spectrometry", Jun., 1973, pp. 2.52-2.55.
Fields Carolyn E.
National Research Development Corporation
Porta David P.
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