X-ray Techniques - Overview
- Author: Ron Jenkins
- Date: 15/09/2006
- Link: analyticalscience.wiley.com - PDF
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Abstract
The use of X-ray methods in the field of materials analysis is now entering its seventh decade. While the broad definition of X-ray methods covers many techniques based on the scatter, emission and absorption properties of X-radiation, the two most common are X-ray fluorescence (XRF) spectrometry and X-ray powder diffractometry (XRD). When a sample of material is bombarded with energetic radiation (X-rays, γ-rays, electrons, protons, etc.) vacancies may arise from the removal of inner orbital electrons. One of the processes by which the atom regains stability is by transference of electrons from outer to inner electron shells. Each of these transitions is accompanied by the emission of an X-ray photon having an energy equal to the energy difference between the two states. The X-ray emission wavelengths are characteristic of the atom in question and there is a simple relationship (Moseley’s law) between the wavelength of the emission line and the atomic number of the atom. Thus when a sample is made up of many different types of atoms, each atom will produce a series of wavelengths, and all of the contributions add up to become the total X-ray emission from the sample. XRF is a technique which utilizes the diffracting power of a single crystal, or the proportional characteristics of a photon detector, to separate the polychromatic beam of radiation from the sample into separate wavelengths, thus allowing qualitative and quantitative elemental measurements to be made.
A beam of monochromatic radiation may also be scattered when X-ray photons collide with atomic electrons. Where the scattered wavelengths interfere with one another diffraction of X-rays occurs. All substances are built up of individual atoms and nearly all substances have some degree of order of periodicity in the arrangement of these atoms. It is the scattering from these periodic arrays that leads to the diffraction effect, and there is a simple relationship (Bragg’s law) between the scattering angle, the wavelength of the radiation and the spacings between the planes of atoms. Since the distances between the atomic planes are dependent on the size and distribution of atoms – i.e. the structure of the material, XRD can be used for qualitative and quantitative phase identification.
Ron Jenkins
This paper provides a concise but comprehensive introduction to the diverse family of X‑ray analytical methods, spanning techniques based on scattering, emission, and absorption of X‑radiation.
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