Links in this section:

Introduction
Evolution of WDS technique
Basics of WDS
Diffraction
Crystals
Detectors & Geometry
Comparison of EDS and WDS
Qualitative Analysis
Quantitative Analysis
Mapping
Summary

Crystals

There are two types of crystal geometry in use today. In the first, called Johann geometry, the diffracting crystal is bent to a radius of 2R, where R is the radius of the focusing circle, called the Rowland circle (Fig. 2a). The second, called Johansson geometry, is more precise. It has the crystal bent to radius 2R and then ground to radius R, so that all of the points of reflection lie on the Rowland circle (Fig. 2b). The result of this geometry is that all of the X-rays originating from the point source on the sample are diffracted over a greater percentage of the crystal surface and are brought to focus at the same point on the detector, thus maximizing the collection efficiency of the spectrometer.

Several different diffracting crystals with different crystal lattice spacings are normally used for WDS, in order to cover all of the wavelengths (energies) of interest, as well as to optimize performance in the different wavelength ranges. Some of the crystals commonly in use are listed in Table 1. Of these crystals, only the LIF is naturally occurring. Low energy (long wavelength) X-rays require larger d-spacing for diffraction and LSM (layered synthetic microstructure) crystals are often used for this purpose. These pseudo-crystals are built up by physical vapor deposition of alternating layers of heavy and light elements. The elements are chosen to maximize scattering efficiency, and the effective d-spacing is dictated by the thickness of the alternating layers.

Table 1: Common diffracting crystals used for WDS

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