Patent Number: 058928090
Section: summary

This application is based on provisional application Ser. No. 60/058,409 which was filed Sep. 10, 1997 and bears the same title. BACKGROUND--FIELD OF THE INVENTION This invention relates to instruments for microanalysis in which a small region at the surface of a specimen is bombarded with monochromatic x-rays. Measurement of the scattered or emitted x-rays or charged particles is used to characterize the material making up the specimen. BACKGROUND--PRIOR ART It is well known that microanalysis can be performed by bombarding a small region with a focused beam of charged particles or electromagnetic radiation. At the present time, most commercially available instruments for accomplishing this in the laboratory use electron or ion beams for excitation because of the ease with which charged particles can be focused. The use of X-rays has been limited up to now because of the difficulties of focusing X-rays. However, some success has been achieved in localizing the analysis in instruments for X-ray fluorescence analysis by the use of apertures for the X-ray beam or by using total reflection inside a capillary or a monolithic polycapillary optic. Unfortunately these approaches do not provide monochromatic radiation. This results in the detection limits in microanalysis by X-ray fluorescence being degraded by the presence of background due to scattering by the specimen of the X-ray continuum from the source. The use of doubly curved X-ray diffractors has been investigated for a long time as a way to obtain focussing of a monochromatic X-ray beam for microanalysis. A suitable geometry for point-to-point focussing ideally involves rotating either the Johann or the Johansson geometries about a line joining the source and image. In the Johann geometry, a curved crystal has lattice planes curved to a radius 2R and the source and its image lie on a focal circle of radius R that is tangent to the crystal's surface. In the Johansson geometry, the crystal planes are similarly shaped but the crystal surface is curved to a radius of R. Some of the other approaches that have been proposed have been referenced in Wittry's U.S. Pat. No. 4,599,741 issued in 1986, for example patents by Berreman (1958), Hammond (1973), Furnas (1975), and Carrol (1980). A paper describing the focusing of monochromatic radiation was presented by P. S. Ong at the 29th Annual conference of the Microbeam Analysis Society in 1974. This paper described an experimental set-up for X-ray fluorescence analysis that had been constructed using a singly curved diffractor and another one under construction that would use a doubly curved diffractor. Both diffractors employed the Johansson geometry in the plane of the focal circle but satisfactory operation of the doubly curved diffractor was never reported subsequently. Practical development of point-focusing diffractors for obtaining a small spot of monochromatic X-radiation was accomplished by Larson and Palmberg as described in U. S. Pat. Nos. 5,315,113 and 5,444,242. Their diffractor was used for aluminum K.sub..alpha. radiation in a commercial instrument for ESCA. Because of the relatively long wavelength of radiation needed for ESCA, it was possible to use a Bragg angle close to 90 degrees. The large Bragg angle facilitates implementation of the point focussing diffractor which was made of quartz. For X-ray fluorescence analysis, the use of Bragg angles close to 90 degrees is not usually possible. In this application, the desired radiation has sufficiently small wavelengths that no commonly available crystal materials yield high diffraction efficiency at large Bragg angles. Also, for this application, it appeared that the Johansson geometry would be needed in the plane of the focal circle in order to obtain a sufficiently high collection solid angle. As a result, much of the work on point-focusing diffractors during the past 10 years was concentrated on trying to construct diffractors that had this geometry and also subtended a large solid angle at the source. The difficulties of achieving this configuration greatly inhibited the development of practical diffractors for X-ray microprobe fluorescence analysis. A different approach which was used recently by Chen and Wittry led to a successful demonstration of microprobe X-ray fluorescence analysis as described in the Journal of Applied Physics vol. 84, pp 1064-1073 (1998). In this approach, the emphasis was on utilizing a diffractor that was more accurately made and aligned, rather than one that had the largest possible collection solid angle. It was found that a small toroidally curved diffractor based on the Johann geometry when used with a 3 watt microfocus X-ray source provided enough intensity in a focused beam of Cu K.sub..alpha. radiation for X-ray fluorescence analysis. The successful results were due to a number of factors. First, work by Wittry and his coworkers provided a theoretical basis for understanding the precision required in the fabrication and alignment of curved diffractors (Journal of Applied Physics: vol. 67, pp 1633-38, 1990; vol. 71, pp 564-8, 1992; vol. 73, pp 601-07, 1993; vol. 74, pp 2999-3008, 1993). Second, detection of the fluorescence-excited radiation by an energy dispersive spectrometer with its high collection solid angle reduced the X-ray microprobe intensity required compared with the intensity required if a wavelength dispersive spectrometer were used. Finally, comparison data for X-ray fluorescence analysis obtained with an X-ray microprobe based on the use of single and polycapillary optics were available to indicate that the diffractor was capable of providing superior performance than these other methods. The present invention is an improvement and simplification over the Wittry U.S. Pat. No. 4,599,741. As in U.S. Pat. No. 4,599,741, several diffractors are provided. But selection of one of the multiple diffractors can be accomplished without the need for the diffractors to move. This results in simpler and more reliable operation. OBJECTIVES AND ADVANTAGES OF THE PRESENT INVENTION The objectives of the present invention are to provide a simplified system whereby microanalysis with high sensitivity and low detection limits for impurities can be performed in the laboratory. These characteristics are obtained by the used of monochromatic x-rays from characteristic x-ray lines which can be selected to optimize the photon energy for excitation of particular ranges of energy levels of elements in the specimen. In contrast to similar systems that have previously been described, the present system is more compact, less expensive to manufacture, and easier to operate.