Title:
Absorption layer for use during x-ray fluorescence analysis which prevents hard x-rays
United States Patent 3925678


Abstract:
An absorption layer for use during x-ray fluorescence analyses which prevents hard x-ray radiation comprising coating a specimen holder with a first layer of high-purity aluminum and a second layer of high-purity anodic oxidized aluminum.



Inventors:
Eberspaecher, Otto (Munich, DT)
Pfisterer, Herman (Munich, DT)
Application Number:
05/488076
Publication Date:
12/09/1975
Filing Date:
07/12/1974
Assignee:
SIEMENS AKTIENGESELLSCHAFT
Primary Class:
Other Classes:
378/44, 378/45, 976/DIG.328
International Classes:
G01N23/223; G21F1/08; (IPC1-7): H01J35/16
Field of Search:
250/508,515
View Patent Images:
US Patent References:



Primary Examiner:
Willis, Davis L.
Attorney, Agent or Firm:
Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson
Claims:
We claim as our invention

1. A layered structure for x-ray absorption formed on and covering a base member comprising a first layer of high-purity aluminum formed on said base member, and a second layer of high-purity anodic oxidized aluminum formed on said first layer.

2. A layered structure according to claim 1, wherein an additional layer consisting of iron, nickel, or copper is formed between said base member and said first layer of high-purity aluminum.

3. A layered structure according to claim 1, wherein said first layer of aluminum is produced by electro-deposition from an aprotic, oxygen-free media.

4. A layered structure according to claim 1, wherein said first layer of aluminum has a purity of better than 99.999%.

5. A layered structure according to claim 1, wherein said first layer has a thickness of about 200 μm.

6. A layered structure according to claim 1, wherein said second layer has a thickness of about 13 μm.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to layers of material for preventing undesirable effects in x-ray applications.

2. Description of the Prior Art

In x-ray fluorescence analysis, interfering radiation often is encountered due to the radiation from the specimen holder and the results obtained are ambiguous since it is impossible to determine which radiation originated from the specimen and which radiation originated from the specimen holder.

SUMMARY OF THE INVENTION

The present invention provides coding specimen holders with layers so as to prevent undesired radiation during x-ray fluorescence analysis. X-ray fluorescence analysis is very versatile and can be used in the analysis of substances which consist of chemical elements with order numbers of z>8. Because of the high sensitivity of this method, in addition, this method is used in quantitative determinations of low concentrations, small specimen quantities, and for determination of mass populations in thin layers and in many other applications. Thus, highly accurate measurements must be made of the low intensities of the characteristic line radiation in these applications.

In such measurements, the results can be inaccurate due to parasitic signals in that it is not possible to distinguish whether the measured intensity of a spectral line originates from the specimen under examination or from other sources. With regard to parasitic signals, these may originate from inherent, scatter, or fluorescent radiation from components which are either located inside the x-ray tube or fall within the path of the primary beam. The specimen holder, which holds the specimen being examined, is particularly critical. In order to limit the contour of the primary beam between the x-ray tube and the specimen under examination, a gold-plated diaphragm is inserted and is designed to operate with the specimen holder. However, it is inevitable that the specimen holder will be struck by at least a part of the primary beam. This excites fluorescent radiation from the elements in the specimen holder. If the specimen holder is by way of example made of AlMgSi 1 or of gilded or silvered brass, the fluorescent radiation produced by the alloying constituents of the AlMgSi 1 can be absorbed by a galvanically deposited layer of gold or silver. However, the layer material must be of extremely high purity.

In some usages, the material of the specimen holder is subjected to a very heavy mechanical load on its surface due to sliding friction during rotation of the specimen, for example. The surface of the specimen holder may also be roughened by repeated cleaning operations and can become damaged and contaminated. For this reason, the relatively soft, very high-purity gold and silver layers, which have Vickers hardnesses of VH<1000 N/mm2 are not optimum materials for the absorption of the parasitic radiation emanating from the specimen holder. Also, radiation of wide line spectrum excited from the gold and silver in this layer material will contribute to the parasitic radiation problem.

It is an object of the present invention to provide an absorption layer for x-rays in which the surface layer has a Vickers hardness of VH>4000 N/mm2. This is achieved by utilizing a layer of high-purity aluminum over a holder or other device and coating the first layer with a second layer of high-purity Eloxal which is anodic oxidized aluminum.

The invention is based upon the realization that high-purity layers of aluminum will absorb high-energy x-rays and electrons and the aluminum simply produces low-energy Al x-ray fluorescent radiation which is not a source of serious interference and is not dangerous.

In a preferred embodiment of the invention, the aluminum will be applied galvanically from oxygen-free aprotic media over the specimen holder or other device and the aluminum layer will then be anodically oxidized in a GX bath and for reasons of purity, an oxalic bath can be used. This method of anodizing is described relative to semiconductor components in the publication entitled "Chemic-Ingenieur-Technik 36," (1964), pages 616-637. The thickness of the aluminum layer should be around 200 μm and that of the Eloxal or anodically oxidized layer should be around 13 μm.

It is an advantage that the thickness of the layer is sufficient to fully absorb the spectrum lines of the disturbing fluorescent radiation from all elements having an order number less than 31. Thus, in the K spectrum, the radiation of all elements up to gallium will be abosrbed. Also, the spectrum lines of the L spectra of all elements up to iridium will be absorbed. The very long wave AlK radiation usually does not interfere with most kinds of investigations and is not a problem. It is only where investigations involving the element aluminum are being conducted that the specimen holder must be made of an aluminum-free material as for example from sintered carbon.

It has been discovered that using specimen holders coded according to the invention results in substantially improved results over analysis not using the invention in that the intensity of the background radiation is reduced and the parasitic signals disappear. It has also been found advantageous to use cleaning agents for cleaning the specimen holder which are free of elements in order numbers of z>8. For example, distilled water or hydro-carbons in association with ultrasonic cleaning devices have been used.

The surface of the layers of the present invention has very low adhesive power and attracts virtually no impurities.

Also, in the present invention, the thickness of the layers can be very accurately controlled; and thus precision components can be constructed.

Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawing, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view illustrating a first embodiment of the invention; and

FIG. 2 is a sectional view illustrating a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view according to the invention. The material 1 which might, for example, be AlMgSi 1 is utilized as a specimen holder and the x-ray fluorescence radiation from this material is to be absorbed so as to prevent contamination and spurious radiation. A layer 2 of high-purity aluminum which, for example, might have a purity of better than 99.999 percent, may be applied galvanically from an oxygen-free aprotic media to a thickness of around 200 μm. Then the outer surface of the aluminum layer 2 is subjected to a high-purity anodic oxidation referred to as the Eloxal process so as to produce an Eloxal layer 21. The Eloxal or anodically oxidized layer may have a thickness of around 13 μm. The arrows 30 represent the primary x-ray beam and the arrows 31 represent a reflected x-ray beam.

FIG. 2 illustrates a modification of the invention wherein before depositing the aluminum layer 2 on the material 1 there is deposited a layer 4 of nickel or iron or copper.

In order to measure the effectiveness of the layer system of the invention, comparative measurements were carried out utilizing the following test arrangement.

X-ray fluorescence spectrograms were recorded, first using a specimen holder of uncoated material, and second, with a specimen holder having the layer system according to the invention. In both specimen holders, high-purity aluminum was used as a dummy specimen. The net intensity was measured, in other words, the gross intensity minus the background intensity for Kα lines of Mg, Si, Cr, Mn, Fe, and Ga. In this manner the intensity Is of the parasitic signals emanating from the specimen holder were measured. Thereafter, the intensity Im of these lines in solid specimens of pure samples of the above elements were measured. Then the quotient Is /Im was calculated and this ratio indicates the ratio between the intensity of the parasitic radiation and the intensity of the radiation to be measured. This quotient is defined in this application as A in the case of the uncoated specimen holder and is designated as B in the case of the specimen holder coated with the layer system of the invention. Then the quotient (A - B) /A was calculated which is a measure of the proportion of the parasitic radiation intesity absorbed by the layer system of the invention. The following table lists these quotients for the different kinds of radiation.

______________________________________ Radiation A B (A - B) /A ______________________________________ Mg K 1.00 . 10-1 7.03 . 10-4 99.3 % Si K 2.16 . 10-2 1.24 . 10-2 42.6 % * Cr K 4.04 . 10-3 1.23 . 10-4 97.0 % * Mn K 2.41 . 10-2 1.99 . 10-4 99.2 % Fe K 3.72 . 10-2 7.28 . 10-4 98.0 % * Ga K 1.38 . 10-3 1.45 . 10-5 98.9 % ______________________________________

The figures followed by an asterisk, in particular the figure for SiKα radiation, were too low because the specimen of the pure elements cannot be manufactured free of impurities. For example, the dummy specimen of pure aluminum is not free of silicon.

Insofar as the material whose x-ray fluorescence radiation is to be absorbed consists of elements having order numbers of z>31, the additional layer 4 consisting of iron, nickel, or copper are provided. These layers can be electro-deposited on the material 1 before the layer 2.

It is to be noted that the layer systems of the present invention are advantageous not only for the specimen holder used for x-ray fluorescence analysis, but they can also be used for diaphragms and other components subject to the effect of a primary beam as well as in x-ray apparatus of all kinds and for electron beam equipment.

Although the invention has been described with respect to preferred embodiments, it is not to be so limited as changes and modifications may be made which are within the full intended scope as defined by the appended claims.