X-ray Absorption
| Absorption of radiation may be considered as any mechanism which removes some radiation from a directed beam. For x-rays – those photons with energies from about 100 eV to 100 keV (higher energy than ultraviolet light, but lower than what are typically termed gamma rays), – the two most common interaction modes in the absorber for removing x-rays from a beam are the photoelectric effect and Rayleigh scattering, of which the photoelectric dominates. Since these mechanisms are energy-dependent, the effect on an absorber is also energy-dependent. (See Interactions of Photons with Matter.) |
The photoelectric cross-section decreases with increasing photon energy. However, as the photon energy approaches the binding energy of an atomic electron, a new mechanism for photon absorption becomes possible. Above this energy there is an abrupt increase in absorption, called an absorption edge. There is a distinct absorption edge for each distinct atomic electron binding energy. (See, e.g. Fig. 3).
The energy at which the absorption edges occur increases with atomic number of the absorber.
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| Figure 3: For copper, the electron energy levels are displayed with gamma emissions highlighted on the left and gamma absorptions on the right. (Note that the vertical energy axis is plotted on a logarithmic scale.) Importantly, gamma emissions occur when an electron falls from a higher energy level into a lower energy level (releasing a photon equal to the difference in energy levels), while a gamma absorption knocks a bound electron completely out of the atom (with the electron carrying away any excess energy from the absorbed photon in the form of kinetic energy.) |
For an idea of where absorption edges occur, see the NIST list of K-edge energies. You can also look at the energy levels by element by use of the more general search form.