Editing Photoelectric effect (section)

==Competing processes and photoemission cross section==
[[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 (Z=20), Compton scattering starts to dominate at ''hυ''=0.08 MeV and ceases at 12 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>]]

When photon energies are as high as the electron rest energy of {{val|511|u=keV}}, yet another process, [[Compton scattering]], may occur. Above twice this energy, at {{val|1.022|u=MeV}}, [[pair production]] is also more likely.<ref name="Evans1955">{{cite book|title = The Atomic Nucleus|author = Evans, R. D.|year = 1955|page= [https://archive.org/details/atomicnucleus032805mbp/page/n735 712]|url = https://archive.org/details/atomicnucleus032805mbp|isbn = 0-89874-414-8|publisher = Krieger|location = Malabar, Fla.}}</ref> Compton scattering and pair production are examples of two other competing mechanisms.{{citation needed|date=November 2023}}
Even if the photoelectric effect is the favoured reaction for a particular interaction of a single photon with a bound electron, the result is also subject to quantum statistics and is not guaranteed. The probability of the photoelectric effect occurring is measured by the [[Cross section (physics)|cross section]] of the interaction, σ. This has been found to be a function of the atomic number of the target atom and photon energy. In a crude approximation, for photon energies above the highest atomic binding energy, the cross section is given by:<ref name="Davisson1965">{{cite book |chapter= Interaction of gamma-radiation with matter|title= Alpha-, Beta- and Gamma-ray Spectroscopy: Volume 1  |editor=Kai Siegbahn |publisher=North-Holland Publishing Company |location=Amsterdam |last= Davisson |first=C. M. |journal=<!-- -->|year = 1965 |volume= 1 |pages = 37–78|bibcode = 1965abgs.conf...37D}}</ref>

:<math> \sigma = \mathrm{constant} \cdot \frac{Z^n}{E^3} </math>

Here ''Z'' is the [[atomic number]] and ''n'' is a number which varies between 4 and 5. The photoelectric effect rapidly decreases in significance in the gamma-ray region of the spectrum, with increasing photon energy. It is also more likely from elements with high atomic number. Consequently, high-''Z'' materials make good [[Gamma ray|gamma-ray]] shields, which is the principal reason why lead (''Z''&nbsp;=&nbsp;82) is preferred and most widely used.<ref name="Knoll1999">{{cite book|title = Radiation Detection and Measurement|author = Knoll, Glenn F.|year = 1999|page = [https://archive.org/details/radiationdetecti00knol_0/page/49 49]|url = https://archive.org/details/radiationdetecti00knol_0/page/49|isbn = 0-471-49545-X|publisher = Wiley|location = New York}}</ref>