X-ray Crystal Spectrometers and Monochromators in Microanalysis

Abstract

Castaing’s successful implementation and application of the electron probe microanalyzer in 1950 stimulated a flurry of development activity around the world. The later versions of this instrument represented a truly international effort, with significant contributions by scientists from Europe, Asia, and North America. If the probe-forming system of the instrument was its heart, the X-ray wavelength spectrometer was its soul. This article reviews some of the history of spectrometer developments—lthrough the “golden years” of microprobe development, namely the dozen or so years following the publication of Castaing’s thesis, to the present. The basic physics of spectrometer and crystal design is reviewed. Early experimental devices, such as those developed by Castaing, Borovskii, Wittry, Duncumb, and Ogilvie are reported. Examples of commercial spectrometers such as those by ARL, MAC, Microspec, and Peak are described. Recent developments such as the combination of grazing-incidence optics with flat crystal spectrometers are noted, and the properties and uses of doubly curved crystals are discussed. Finally, the continued development of doubly curved crystal configurations, such as the “Wittry geometry” for scanning monochromators, and point-to-point focusing diffractors for producing small monochromatic X-ray probes to provide improved detection limits for microanalysis are considered.

A LiF(200) double‐curved crystal for performance improvement of a monochromatic X‐ray fluorescence source

“…Hence, an attenuator was placed at the second slit to reduce the direct count rate; it was made of a 0.5‐mm‐thick copper sheet with a hole that was 0.71% of the slit area. The maximum diffraction intensity from the DCC approached 5.3 × 10 8 cps, which was close to the XOS Cr source with Ge(220), [ 11 ] as shown in Table 2.…”

“…When the X‐ray source target is very small, a curved crystal can focus incident rays in a manner similar to that of a curved grating for visible light. [ 11 ] Chen designed a compact focused‐beam, total‐reflection X‐ray instrument with DCC optics, with three different X‐ray energies, which exhibited high‐intensity and excellent sensitivity. [ 12 ] The DCC was the key component for diffracting the monochromatic X‐rays.…”

Source misalignment effects on crystal collection solid angle and intensity profile

“…The ability of a crystal to collect X-rays from a point source is limited by the solid angle subtended at the source by the effective area of the crystal and by the peak reflectivity in the crystal's rocking curve (Wittry & Barbi, 2001). For X-rays divergent from a point source, the diffracting region on a curved crystal surface subtends a solid angle ðx P ; z P Þ at the source.…”

Atomic spectrometry update. X-ray fluorescence spectrometry

“…The history of spectrometer development through what the author calls the 'golden age' was reviewed by Wittry and Barbi. 6 Although devoted to the development of the microprobe spectrometer, emphasising advances in the dozen or so years following publication of Castaing's thesis, this review covers more modern developments that will be of interest to X-ray spectroscopists.…”