Editing X-ray (section)

===Photoelectric absorption===
The probability of a photoelectric absorption per unit mass is approximately proportional to <math display="inline">Z^3/E^3</math>, where <math display="inline">Z</math> is the [[atomic number]] and <math display="inline">E</math> is the energy of the incident photon.<ref>{{Cite book |author1=Bushberg, Jerrold T. |author2=Seibert, J. Anthony |author3=Leidholdt, Edwin M. |author4=Boone, John M. |title= The essential physics of medical imaging |publisher= Lippincott Williams & Wilkins |date= 2002 |isbn= 978-0-683-30118-2 |page= 42}}</ref> This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called [[absorption edges]]. However, the general trend of high [[absorption coefficient]]s and thus short [[penetration depth]]s for low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26&nbsp;keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount of [[calcium]] (<math display="inline">Z=20</math>) in bones, together with their high density, is what makes them show up so clearly on medical radiographs.{{cn|date=December 2024}}

A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path. An outer electron will fill the vacant electron position and produce either a characteristic X-ray or an [[Auger electron]]. These effects can be used for elemental detection through [[X-ray spectroscopy]] or [[Auger electron spectroscopy]].{{cn|date=December 2024}}