Editing Compton scattering (section)

== Introduction ==
[[File:Compton-en.svg|thumb|right|Fig. 1: Schematic diagram of Compton's experiment. Compton scattering occurs in the [[graphite]] target on the left. The slit passes X-ray photons scattered at the selected angle and their average energy rate is measured using [[Bragg scattering]] from the crystal on the right in conjunction with an ionization chamber.]]

[[File:Dominant Photon-Matter Interaction.svg|thumb|Plot of photon energies calculated for a given element (atomic number ''Z'') at which the [[Cross section (physics)|cross section]] value for the process on the right becomes larger than the cross section for the process on the left. For calcium ({{nowrap|1=''Z'' = 20}}), Compton scattering starts to dominate at {{nowrap|1=''hυ'' = 0.08&nbsp;MeV}} and ceases at 12&nbsp;MeV.<ref>{{Cite journal |date=2009-09-17 |title=XCOM: Photon Cross Sections Database |url=https://dx.doi.org/10.18434/T48G6X |journal=NIST |doi=10.18434/T48G6X |language=en |last1=Seltzer |first1=Stephen }}</ref>]]
In Compton's original experiment (see Fig.&nbsp;1), the energy of the X ray photon (≈&nbsp;17&nbsp;keV) was significantly larger than the binding energy of the atomic electron, so the electrons could be treated as being free after scattering. The amount by which the light's wavelength changes is called the '''Compton shift'''. Although nucleus Compton scattering exists,<ref>{{cite journal|title=Nuclear Compton scattering|author=P. Christillin|year=1986|journal= J. Phys. G: Nucl. Phys.|volume=12|pages=837–851|url=https://iopscience.iop.org/article/10.1088/0305-4616/12/9/008/meta|doi=10.1088/0305-4616/12/9/008|bibcode = 1986JPhG...12..837C|issue=9 |s2cid=250783416 |url-access=subscription}}</ref> Compton scattering usually refers to the interaction involving only the electrons of an atom. The Compton effect was observed by [[Arthur Holly Compton]] in 1923 at [[Washington University in St. Louis]] and further verified by his graduate student [[Wu Youxun|Y. H. Woo]] in the years following. Compton was awarded the 1927 [[Nobel Prize in Physics]] for the discovery.

The effect is significant because it demonstrates that light cannot be explained purely as a [[wave]] phenomenon.<ref>
{{cite book
 | last = Griffiths | first = David
 | title = Introduction to Elementary Particles
 | publisher = Wiley
 | date = 1987
 | pages = 15, 91
 | isbn = 0-471-60386-4
}}</ref> [[Thomson scattering]], the classical theory of an [[electromagnetic wave]] scattered by charged particles, cannot explain shifts in wavelength at low intensity: classically, light of sufficient intensity for the electric field to accelerate a charged particle to a relativistic speed will cause radiation-pressure recoil and an associated Doppler shift of the scattered light,<ref>{{cite web|title=Observation of the Transition from Thomson to Compton Scattering in Optical Multiphoton Interactions with Electrons|author=C. Moore|year=1995|url=http://www.lle.rochester.edu/media/publications/documents/theses/Moore.pdf}}</ref> but the effect would become arbitrarily small at sufficiently low light intensities ''regardless of wavelength''. Thus, if we are to explain low-intensity Compton scattering, light must behave as if it consists of particles. Or the assumption that the electron can be treated as free is invalid resulting in the effectively infinite electron mass equal to the nuclear mass (see e.g. the comment below on elastic scattering of X-rays being from that effect). Compton's experiment convinced physicists that light can be treated as a stream of particle-like objects (quanta called photons), whose energy is proportional to the light wave's frequency.

As shown in Fig. 2, the interaction between an electron and a photon results in the electron being given part of the energy (making it recoil), and a photon of the remaining energy being emitted in a different direction from the original, so that the overall [[momentum]] of the system is also conserved. If the scattered photon still has enough energy, the process may be repeated. In this scenario, the electron is treated as free or loosely bound. Experimental verification of momentum conservation in individual Compton scattering processes by [[Walther Bothe|Bothe]] and [[Hans Geiger|Geiger]] as well as by Compton and Simon has been important in disproving the [[BKS theory]].

Compton scattering is commonly described as [[inelastic scattering#Photons|inelastic scattering]]. This is because, unlike the more common Thomson scattering that happens at the low-energy limit, the energy in the scattered photon in Compton scattering is less than the energy of the incident photon.<ref>{{cite book |last1=Carron |first1=NJ |title=An Introduction to the Passage of Energetic Particles through Matter |date=2007 |publisher=CRC Press |location=Boca Raton, FL |isbn=9781420012378 |page=61}}</ref><ref>{{cite book |last1=Chen |first1=Sow-Hsin |last2=Kotlarchyk |first2=Michael |title=Interactions of Photons and Neutrons with Matter |date=2007 |publisher=World Scientific |location=Singapore |isbn=9789810242145 |page=271 |edition=2nd}}</ref> As the electron is typically weakly bound to the atom, the scattering can be viewed from either the perspective of an electron in a potential well, or as an atom with a small ionization energy. In the former perspective, energy of the incident photon is transferred to the recoil particle, but only as kinetic energy. The electron gains no internal energy, respective masses remain the same, the mark of an [[elastic collision]]. From this perspective, Compton scattering could be considered elastic because the internal state of the electron does not change during the scattering process. In the latter perspective, the atom's state is changed, constituting an [[inelastic collision]]. Whether Compton scattering is considered elastic or inelastic depends on which perspective is being used, as well as the context.

Compton scattering is one of four competing processes when photons interact with matter. At energies of a few eV to a few keV, corresponding to [[visible light]] through soft X-rays, a photon can be completely absorbed and its energy can eject an electron from its host atom, a process known as the photoelectric effect. High-energy photons of {{val|1.022|u=MeV}} and above may bombard the nucleus and cause an electron and a positron to be formed, a process called [[pair production]]; even-higher-energy photons (beyond a threshold energy of at least {{val|1.670|u=MeV}}, depending on the nuclei involved), can eject a nucleon or [[alpha particle]] from the nucleus in a process called [[photodisintegration]]. Compton scattering is the most important interaction in the intervening energy region, at photon energies greater than those typical of the photoelectric effect but less than the pair-production threshold.