Editing Scintillation (physics) (section)

== Conversion processes ==
The first stage of scintillation, conversion, is the process where the energy from the incident radiation is absorbed by the scintillator and highly energetic electrons and [[Electron hole|holes]] are created in the material. The energy absorption mechanism by the scintillator depends on the type and energy of radiation involved. For highly energetic photons such as X-rays (0.1 keV < <math>E_{\gamma}</math> < 100 keV) and γ-rays (<math>E_{\gamma}</math> > 100 keV), three types of interactions are responsible for the energy conversion process in scintillation: [[Photoelectric effect|photoelectric absorption]],<ref name=":3">{{Cite journal |last=Hall |first=Harvey |date=1936-10-01 |title=The Theory of Photoelectric Absorption for X-Rays and $\ensuremath{\gamma}$-Rays |url=https://link.aps.org/doi/10.1103/RevModPhys.8.358 |journal=Reviews of Modern Physics |volume=8 |issue=4 |pages=358–397 |doi=10.1103/RevModPhys.8.358|url-access=subscription }}</ref> [[Compton scattering]],<ref name=":4">{{Cite journal |last1=Eisenberger |first1=P. |last2=Platzman |first2=P. M. |date=1970-08-01 |title=Compton Scattering of X Rays from Bound Electrons |url=https://link.aps.org/doi/10.1103/PhysRevA.2.415 |journal=Physical Review A |volume=2 |issue=2 |pages=415–423 |doi=10.1103/PhysRevA.2.415|bibcode=1970PhRvA...2..415E |url-access=subscription }}</ref> and [[pair production]],<ref name=":5">{{Cite journal |last=Hubbell |first=J. H. |date=2006-06-01 |title=Electron–positron pair production by photons: A historical overview |url=https://www.sciencedirect.com/science/article/pii/S0969806X0500263X |journal=Radiation Physics and Chemistry |language=en |volume=75 |issue=6 |pages=614–623 |doi=10.1016/j.radphyschem.2005.10.008 |bibcode=2006RaPC...75..614H |issn=0969-806X|url-access=subscription }}</ref> which only occurs when <math>E_{\gamma}</math> > 1022 keV, i.e. the photon has enough energy to create an electron-positron pair.

These processes have different [[attenuation coefficient]]s, which depend mainly on the energy of the incident radiation, the average [[atomic number]] of the material and the [[density]] of the material. Generally the absorption of high energy radiation is described by:

:<math>I= I_0\cdot e^{-\mu d}</math>

where <math>I_0</math> is the intensity of the incident radiation, <math>d</math> is the thickness of the material, and <math>\mu</math> is the linear attenuation coefficient, which is the sum of the attenuation coefficients of the various contributions:

:<math>\mu = \mu_{pe} + \mu_{cs} + \mu_{pp} + \mu_{oc}</math>

At lower X-ray energies (<math>E_{\gamma} \lesssim</math> 60 keV), the most dominant process is the photoelectric effect, where the photons are fully absorbed by bound electrons in the material, usually core electrons in the [[Electron shell|K- or L-shell]] of the atom, and then ejected, leading to the ionization of the host atom. The linear attenuation coefficient contribution for the photoelectric effect is given by:<ref name=":3" /><ref name=":6">{{Cite book |last=Rodnyi |first=Piotr A. |url=https://www.worldcat.org/oclc/36240945 |title=Physical processes in inorganic scintillators |date=1997 |publisher=CRC Press |isbn=0-8493-3788-7 |location=Boca Raton |oclc=36240945}}</ref>

:<math>\mu_{pe} \propto {\rho Z^n \over E_{\gamma}^{3.5}}</math>

where <math>\rho</math> is the density of the scintillator, <math>Z</math> is the average atomic number, <math>n</math> is a constant that varies between 3 and 4, and <math>E_{\gamma}</math> is the energy of the photon. At low X-ray energies, scintillator materials with atoms with high atomic numbers and densities are favored for more efficient absorption of the incident radiation.

At higher energies (<math>E_{\gamma}</math> <math>\gtrsim</math> 60 keV) Compton scattering, the inelastic scattering of photons by bound electrons, often also leading to ionization of the host atom, becomes the more dominant conversion process. The linear attenuation coefficient contribution for Compton scattering is given by:<ref name=":4" /><ref name=":6" />

:<math>\mu_{cs} \propto {\rho \over \sqrt{E_{\gamma}}}</math>

Unlike the photoelectric effect, the absorption resulting from Compton scattering is independent of the atomic number of the atoms present in the crystal, but linearly on their density.

At γ-ray energies higher than <math>E_{\gamma}</math> > 1022 keV, i.e. energies higher than twice the rest-mass energy of the electron, pair production starts to occur. Pair production is the relativistic phenomenon where the energy of a photon is converted into an electron-positron pair. The created electron and positron will then further interact with the scintillating material to generate energetic electron and holes. The attenuation coefficient contribution for pair production is given by:<ref name=":5" /><ref name=":6" />

:<math>\mu_{pp} \propto \rho Z \ln \Bigl( {2 E_{\gamma} \over m_e c^2}\Bigr)</math>

where <math>m_e</math> is the [[Rest Mass|rest mass]] of the electron and <math>c </math> is the [[speed of light]]. Hence, at high γ-ray energies, the energy absorption depends both on the density and average atomic number of the scintillator. In addition, unlike for the photoelectric effect and Compton scattering, pair production becomes more probable as the energy of the incident photons increases, and pair production becomes the most dominant conversion process above <math>E_{\gamma}</math>~ 8 MeV.

The <math>\mu_{oc}</math> term includes other (minor) contributions, such as [[Rayleigh scattering|Rayleigh (coherent) scattering]] at low energies and [[Photodisintegration|photonuclear reactions]] at very high energies, which also contribute to the conversion, however the contribution from Rayleigh scattering is almost negligible and photonuclear reactions become relevant only at very high energies.

After the energy of the incident radiation is absorbed and converted into so-called hot electrons and holes in the material, these energetic charge carriers will interact with other particles and quasi-particles in the scintillator (electrons, [[plasmon]]s, [[phonon]]s), leading to an "avalanche event", where a great number of secondary electron–hole pairs are produced until the hot electrons and holes have lost sufficient energy. The large number of electrons and holes that result from this process will then undergo [[Thermalisation|thermalization]], i.e. dissipation of part of their energy through interaction with phonons in the material

The resulting large number of energetic [[charge carrier]]s will then undergo further energy dissipation called thermalization. This occurs via interaction with phonons for electrons and [[Auger effect|Auger processes]] for holes.

The average timescale for conversion, including energy absorption and thermalization has been estimated to be in the order of 1 ps,<ref name=":2" /><ref>{{Cite journal |last1=Lempicki |first1=A. |last2=Wojtowicz |first2=A. J. |last3=Berman |first3=E. |date=1993-09-01 |title=Fundamental limits of scintillator performance |url=https://dx.doi.org/10.1016/0168-9002%2893%2991170-R |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |language=en |volume=333 |issue=2 |pages=304–311 |doi=10.1016/0168-9002(93)91170-R |bibcode=1993NIMPA.333..304L |issn=0168-9002|url-access=subscription }}</ref> which is much faster than the average [[decay time]] in [[photoluminescence]].