Editing X-ray (section)

==Interaction with matter==
[[File:Attenuation.svg|thumb|Attenuation length of X-rays in water showing the oxygen [[absorption edge]] at 540&nbsp;eV, the energy<sup>−3</sup> dependence of [[Photoelectric effect|photoabsorption]], as well as a leveling off at higher photon energies due to [[Compton scattering]]. The attenuation length is about four orders of magnitude longer for hard X-rays (right half) compared to soft X-rays (left half).]]

X-rays interact with matter in three main ways, through [[Photoelectric effect|photoabsorption]], [[Compton scattering]], and [[Rayleigh scattering]]. The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies. Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. At higher energies, Compton scattering dominates.{{cn|date=December 2024}}

===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}}

===Compton scattering===
Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.<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= 38}}</ref> Compton scattering is an [[inelastic scattering]] of the X-ray photon by an outer shell electron. Part of the energy of the photon is transferred to the scattering electron, thereby ionizing the atom and increasing the wavelength of the X-ray. The scattered photon can go in any direction, but a direction similar to the original direction is more likely, especially for [[high-energy X-rays]]. The probability for different scattering angles is described by the [[Klein–Nishina formula]]. The transferred energy can be directly obtained from the scattering angle from the [[conservation of energy]] and [[conservation of momentum|momentum]].{{cn|date=December 2024}}

===Rayleigh scattering===
Rayleigh scattering is the dominant [[elastic scattering]] mechanism in the X-ray regime.<ref>{{cite journal |last1=Kissel |first1=Lynn |title=RTAB: the Rayleigh scattering database |journal=Radiation Physics and Chemistry |date=August 2000 |volume=59 |issue=2 |pages=185–200 |doi=10.1016/S0969-806X(00)00290-5 |bibcode=2000RaPC...59..185K }}</ref> Inelastic forward scattering gives rise to the refractive index, which for X-rays is only slightly below 1.<ref>{{Cite book |author= Attwood, David |title= Soft X-rays and extreme ultraviolet radiation |publisher= Cambridge University Press |date= 1999 |isbn= 978-0-521-65214-8 |chapter-url= http://ast.coe.berkeley.edu/sxreuv/ |chapter= 3 |access-date= 4 November 2012 |archive-url= https://web.archive.org/web/20121111141255/http://ast.coe.berkeley.edu/sxreuv/ |archive-date= 2012-11-11 }}</ref>