Wavelength-dispersive X-ray spectroscopy

Wavelength-dispersive X-ray spectroscopy (WDXS or WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging X-ray and the crystal's lattice spacings are related by Bragg's law and produce constructive interference if they fit the criteria of Bragg's law. Unlike the related technique of energy-dispersive X-ray spectroscopy (EDS), WDS reads or counts only the X-rays of a single wavelength at a time, not producing a broad spectrum of wavelengths or energies simultaneously. WDS is mainly used in chemical analysis, in an X-ray fluorescence spectrometer, in an electron microprobe, and may be used in a scanning electron microscope.

The X-rays emitted by the sample being analyzed are collimated by parallel copper blades (called collimator or Soller slits), and irradiate a known single crystal at a precise angle. The single crystal diffracts the photons (Bragg's law) which are collected by a detector, usually a scintillation counter or a proportional counter.

The single crystal, the specimen, and the detector are mounted precisely on a goniometer with the distance from the source of X-rays (the specimen) and the crystal equal to the distance from the crystal to the detector. It is usually operated under vacuum to reduce the absorption of soft radiation (low-energy photons) by the air and thus increase the sensitivity for the detection and quantification of light elements (between boron and oxygen).

Modern systems contain a small number of crystals of known but differing properties, with automated changing of the crystal depending on the energy being analysed, enabling elements from the entire periodic table to be analyzed, with the exception of hydrogen, helium, and lithium, as their atomic number, and by extension x-ray cross section, is too low to analyze via x-ray methods.

...
Wikipedia