X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2002-07-17
2003-11-18
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S084000
Reexamination Certificate
active
06650728
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for the analysis of atomic and/or molecular elements by wavelength dispersive X-ray spectrometric devices comprising at least one reflection or focussing device including a multi-layer structure, particularly an apparatus wherein fluorescence rays generated by a sample to be analyzed when subjected to incident primary X-ray or electron radiation are directed onto a mirror or focussing device before reaching a measuring or analysis detector. The mirror or focussing device is formed by a multi-layer structure comprising layer pairs each including a first layer element formed by lanthanum. The invention also resides in an analysis method employing such apparatus.
Apparatus and methods of this type are known for example from DE OS 199 26 036. They are used in scientific analyses but also in industrial applications for the detection of atomic and/or molecular elements in various areas for example when impurities or disturbances present in examples in only small amounts are to be detected or analyzed.
In that case, X-ray or electron beams from any type of X-ray or electron source are directed onto a sample whereby, among others, fluorescence rays are returned from the sample which are induced by the incident X-rays by a well-known physical processes. These fluorescence rays are directed onto a suitable crystal where they are reflected and then directed onto a measuring and analysis arrangement for example in the form of a fluorescence radiation-selective detector. The crystals act as analyzers. These crystals which can be artificial crystals. may consist of thin multiple alternate layers of two or more materials with different X-ray optical properties. In connection with the above example, the incident fluorescent rays are reflected from these layers but only that part of the fluorescent rays for which the Bragg equation.
n&lgr;=2d sin &thgr;
is fulfilled,
Herein: &lgr;(nm)=1.24/E(keV)
wherein n is a natural number (n=
1, 2, 3, 4 . . . )
&lgr;=the wavelength of the x rays, that is,
d=the periodicity (lattice parameter) of the analyzer crystal,
2&thgr;=diffraction angle, and
E=energy of the X-rays.
Taking into consideration the effect of the refraction, which is very small for X-rays, results in an equation which is modified from the first equation whereby from the set angles &thgr; and the lattice parameter d of the analyzer the wavelength of the reflected X-rays can be determined from the first equation or the modification thereof. By varying the angle therefore the wavelength of the reflected rays, that is in the above example the fluorescence rays, can be selected in a controlled manner.
The big advantage of the artificial crystals which consist of many regularly changing layers—called in this connection also multi-layer arrangement—is that the materials, of which the multi-layer arrangement consists, can be selected and optimized for best results. This is an essential advantage of the manufactured multi-layer arrangement as compared to natural crystals.
The intensity of the reflected radiation depends greatly on the materials used for the multi-layer. In addition, the lattice parameters of the multi-layer can be modified within a larger range than it is possible with natural crystals.
It is a particular advantage of the multi-layer analyzer that it facilitates the analysis of light elements with uniform intensity and without health-endangering side effects.
In many cases so far the multi-layer structure or, respectively, the individual layers of the multi-layer structure, have been adapted specifically to the element expected to be analyzed. For the special case of an energy range of 100 to 180 eV, that is, particularly for the detection of beryllium and boron, in the past, tungsten-carbon multi-layers have been used. Newer developments of lanthanum boron carbide multi-layer arrangements further improve the detection limit of boron, see DE OS 199 26056 referred to earlier. For the detection of carbon at energies of about 277 eV, multi-layer arrangements of vanadium-, or respectively, nickel-carbon have been used, see also U.S. Pat. No. 4,785,470.
Multi-layer arrangements which are utilized mainly in x-ray spectrometers for the detection of boron and beryllium (Mo—B
4
C) or which provide the best detection limits (La—B
4
C) have only a small reflectivity of 3 to 0.9% for carbon. The reason herefor is that, in the light layer material B
4
C, boron is contained which is an element whose absorption edge is at an energy below the carbon emission line of 277 eV. Therefore the carbon radiation of multi-layer structures including B
4
C is highly absorbed. In contrast, the multi-layers V—C and Cr—C, which are optimized for the carbon detection are not well suited for the detection of the light elements beryllium and boron, because their reflectivity in comparison with lanthanum containing multi-layer arrangements is lower by more than the factor 2. This low reflectivity is insufficient for the detection of the light element B and Be, mainly because the light elements have a substantially smaller fluorescence yield than the heavier elements so that the expected count rates are comparatively low anyhow.
For the detection of the light elements beryllium to carbon, which have their highest reflectivity at energies of 108 183 and 277 eV, so far at least two different optimized multi-layer arrangements are necessary.
It is therefore the object of the present invention to provide an apparatus and a method with which a very much improved x-ray analysis for the detection of beryllium, boron and carbon is possible, and wherein the apparatus as well as the method are to be set up and operated in a simple manner utilizing means known from the state of the art, so that present analysis apparatus and methods can be utilized without major changes whereby the apparatus and the method can be established relatively inexpensively and operated in a simple manner and at low costs by research institutions and industrial installations.
SUMMARY OF THE INVENTION
In an apparatus and a method for the analysis of atomic or molecular elements contained in a sample by wavelength dispersive X-ray spectrometry, wherein primary x ray or electron radiation is directed onto the sample whereby fluorescence radiation is emitted from the sample, the fluorescence radiation is directed onto a mirror or focussing device consisting of a multi-layer structure including pairs of layers of which one layer of a pair consists of lanthanum and the other consists of carbon and the fluorescence radiation is reflected from the mirror or focussing device onto an analysis detector for the analysis of the atomic or molecular elements contained in the sample.
The advantage of the apparatus and method according to the invention resides essentially in the fact that the combination of lanthanum and carbon according to the invention for the layer elements forming the layer pair in multi-layer arrangements for wavelength dispersive analyzers of x-rays in the energy range of about 108 eV for the detection of boron and in the range of about 277 eV are used for the detection of carbon. The particularly advantageous x-ray optical properties of the layer elements of lanthanum and carbon result, in comparison with the best conventional analyzers referred to above with optimized multi-layer arrangements, in reflectivities which are comparable for the detection of beryllium, slightly lower than La—B
4
C for boron and also slightly lower for carbon. La—C is the only material combination which has high reflectivity for all three elements Be, B and C at the same time. Furthermore, LaC multi-layer arrangements, for example as La-B
4
C—multi-layer structures, provide during boron detection for a substantially improved suppression of the oxygen K— as well as the silicon-L lines because of the use of lanthanum as heavy material (reflector). This suppression is also improved for the C-radiation in comparison with the special mirrors of Ni—C, or re
Bormann Rudiger
Michaelsen Carsten
Wiesmann Jörg
Bach Klaus J.
Church Craig E.
Forschungszentrum Geesthacht
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