X-ray spectroscopy

X-ray spectroscopy is a gathering name for several spectroscopic techniques for characterization of materials by using x-ray excitation.

When an electron from the inner shell of an atom is excited by the energy of a photon, it moves to a higher energy level, when it returns to the low energy level the energy which it previously gained by the excitation is emitted as a photon which has a wavelength that is characteristic for the element (there could be several characteristic wavelengths per element). Analysis of the X-ray emission spectrum produces qualitative results about the elemental composition of the specimen. Comparison of the specimen's spectrum with the spectra of samples of known composition produces quantitative results (after some mathematical corrections for absorption, fluorescence and atomic number). Atoms can be excited by a high-energy beam of charged particles such as electrons (in an electron microscope for example), protons (see PIXE) or a beam of X-rays (see X-ray fluorescence, or XRF). These methods enable elements from the entire periodic table to be analysed, with the exception of H, He and Li. In electron microscopy an electron beam excites X-rays; there are two main techniques for analysis of spectra of characteristic X-ray radiation: energy-dispersive X-ray spectroscopy (EDS) and wavelength dispersive X-ray spectroscopy (WDS).

In an energy-dispersive X-ray spectrometer, a semiconductor detector measures energy of incoming photons. To maintain detector integrity and resolution it should be cooled with liquid nitrogen or by Peltier cooling. EDS is widely employed in electron microscopes (where not spectroscopy but imaging is a main task) and in cheaper and/or portable XRF units.

In a wavelength dispersive X-ray spectrometer the single crystal diffracts the photons (Bragg's law) which are collected by a detector. Without any motion there will be just one wavelength detected. By moving crystal and detector, a wide region of spectrum is observed (to collect all parts of spectrum three of four different single crystals may be needed). In contrast to EDS, WDS method is a method of sequential spectrum acquisition. While WDS is slower than EDS and more sensitive to positioning specimen in the spectrometer, it has superior spectral resolution and sensitivity. WDS is widely used in microprobes (where X-ray microanalysis is the main task) and in XRF; it is widely used in the field of x ray diffraction to calculate various data such as interplanar spacing and wavelength of the incident x ray using Bragg's law.

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