====== 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 [[phylabs:lab_courses:supplemental-material:physics-and-mathematics-references:interactions_of_photons_with_matter:start|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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| {{ phylabs:lab_courses:phys-211-wiki-home:x-ray_studies:copper_electron_levels_v2.png |}} |
| **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.) |
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For an idea of where absorption edges occur, see the [[https://physics.nist.gov/cgi-bin/XrayTrans/search.pl?element=All&trans=Kedge&lower=&upper=&units=eV|NIST list of K-edge energies]].  You can also look at the energy levels by element by use of the more general [[https://physics.nist.gov/PhysRefData/XrayTrans/Html/search.html| search form]].