Optics: measuring and testing – By dispersed light spectroscopy – With sample excitation
Patent
1995-06-14
1997-05-06
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
With sample excitation
G01N 2163
Patent
active
056276419
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
TECHNICAL FIELD
The present invention relates to an isotopic analysis process by optical emission spectrometry on a laser-produced plasma.
It is used in the measurement of the isotopic concentrations of elements. One of the preferred fields of the invention is the nuclear industry.
PRIOR ART
Measurements of isotopic concentrations of elements are generally performed by mass spectrometry. The sample to be analyzed is dissolved, then atomized and then thermally ionized. The ions are then separated in accordance with their mass before being counted.
Mass spectrometry is characterized by a very good sensitivity and an excellent precision of the measurements. However, apart from the fact that it requires preparative chemistry for dissolving the samples, it involves the use of sophisticated and expensive equipment, which can only be used in the laboratory.
Another procedure for measuring isotopic concentrations consists of the spectral analysis of the emission radiation of a source of excited atoms. In this case, separation takes place with the aid of a spectrometer, of the emission wavelengths, which can differ very significantly for several isotopes of the same element (isotopic shift effect). The measurement of the isotopic concentrations is then obtained by measuring the emission signal corresponding to each isotope.
Optical emission spectrometry on high frequency-induced plasma (or OES/HFIP for short) uses this principle and has e.g. been used for the isotopic analysis of uranium. This procedure is fast and relatively economic. However, it is not very precise and, like mass spectrometry, it cannot be envisaged for use outside the laboratory.
Alpha counting spectrometry can also be considered for radio-active elements, because it is not very onerous and is relatively simple to perform. It can also be used for the direct analysis of solid samples. However, its field of use is very restricted. The alpha particles detected are those which emit the surface atomic layers, the others being absorbed in the sample. This means that the measurement is only representative of the sample surface. Mass analysis consequently requires a preparative chemistry (dissolving the sample and preparing deposits of very small thickness).
Another limiting factor is linked with the radioactive material concentration. For example, in the case of uranium, if the concentration is too high (a few mg/cm.sup.2), interferences occur (overlap of the corresponding peaks of U.sub.234, U.sub.235 and U.sub.238). For low concentrations, the counting times are very long (several minutes and even several dozen minutes). In the best of cases, the precision level of alpha spectrometry for isotopic analysis is a few per cent.
Gamma counting spectrometry has promising performance characteristics for the isotopic analysis of uranium. It is a non-destructive analysis process, which also offers the possibility of a direct determination of the uranium mass. However, the counting and therefore analysis times are very long. It only relates to radioactive elements and does not permit a precise cartography of the isotopic composition of the samples. Finally, it cannot be envisaged as an in situ analysis method.
A spectral analysis process is also known, which consists of irradiating a sample by a pulse laser beam in order to produce a plasma, analyzing the spectrum of the light emitted by said plasma and deducing from said spectrum the elementary composition of the sample.
By analogy with the usual terminology in the field, this process can be called optical emission spectrometry on laser-produced plasma or OES/LPP for short. Such a process is e.g. described in GB-A-1,024,687.
This process is e.g. used for copper, stainless steel, molybdenum, tungsten and graphite. It is suitable for determining the elementary composition of the sample, but a priori does not make it possible to carry out an isotopic analysis which requires a much greater analysis precision. Thus, the isotopic shifts are generally smaller than the spectral shifts corresponding
REFERENCES:
patent: 4794230 (1988-12-01), Seliskar et al.
patent: 5133901 (1992-07-01), Peterson et al.
Watcher et al, Applied Spectroscopy, vol. 41, No. 6, 1987, pp. 1042-1048.
Patent Abstracts of Japan -Janvier 1988 -vol. 12, No. 224 (P-721 (3071) 25 Jun. 1988 & JP-A-63 018 249 (Kawasaki Steel Corp) 26.
Determination of Uranium Isotopes in a Complex Matrix by Optical Spectroscopy -G.M. Murray et al., -Journal of Alloys and Compounds -vol. 181, 1992 -pp. 57-62.
Briand Alain
Mauchien Patrick
Petit Alain
Pietsch Walter
Commissariat a l''Energie Atomique
Evans F. L.
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