Scintillation (physics)
In condensed matter physics, scintillation (/ˈsɪntɪleɪʃən/ SIN-til-ay-shun) is the physical process where a material, called a scintillator, emits ultraviolet or visible light under excitation from high energy photons (X-rays or gamma rays) or energetic particles (such as electrons, alpha particles, neutrons, or ions).
[3][4] Scintillation is an example of luminescence, whereby light of a characteristic spectrum is emitted following the absorption of radiation.
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 holes are created in the material.
> 100 keV), three types of interactions are responsible for the energy conversion process in scintillation: photoelectric absorption,[6] Compton scattering,[7] and pair production,[8] which only occurs when
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 K- or L-shell of the atom, and then ejected, leading to the ionization of the host atom.
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.
The linear attenuation coefficient contribution for Compton scattering is given by:[7][9] 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.
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
term includes other (minor) contributions, such as Rayleigh (coherent) scattering at low energies and 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, plasmons, phonons), 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 thermalization, i.e. dissipation of part of their energy through interaction with phonons in the material The resulting large number of energetic charge carriers will then undergo further energy dissipation called thermalization.
This occurs via interaction with phonons for electrons and Auger processes for holes.
In this stage, the large number of electrons and holes that have been generated during the conversion process, migrate inside the material.
This is probably one of the most critical phases of scintillation, since it is generally in this stage where most loss of efficiency occur due to effects such as trapping or non-radiative recombination.
These are mainly caused by the presence of defects in the scintillator crystal, such as impurities, ionic vacancies, and grain boundaries.
[11] The exact details of the luminescence phase also depend on the type of material used for scintillation.
Organic materials form molecular crystals where the molecules are loosely bound by Van der Waals forces.
In valence bond theory, when carbon forms compounds, one of the 2s electrons is excited into the 2p state resulting in a configuration of 1s2 2s1 2p3.
In certain organic molecules π-orbitals interact to produce a common nodal plane.
The excited states of π-electron systems can be explained by the perimeter free-electron model (Platt 1949).
The ring can be approximated as a circle with circumference l. The wave-function of the electron orbital must satisfy the condition of a plane rotator: The corresponding solutions to the Schrödinger wave equation are: where q is the orbital ring quantum number; the number of nodes of the wave-function.
Depending on the particular energy loss of a certain particle (dE/dx), the "fast" and "slow" states are occupied in different proportions.
Of course, the difference in shape is visible in the trailing side of the pulse, since it is due to the decay of the excited states.
