Editing Scintillation (physics) (section)

==Luminescence==
[[File:Pistates.svg|thumb|350px|right|π-electronic energy levels of an organic molecule. S<sub>0</sub> is the ground state. S<sub>1</sub>, S<sub>2</sub>, S<sub>3</sub> are excited singlet states. T<sub>1</sub>, T<sub>2</sub>, T<sub>3</sub> are excited triplet states. S<sub>00</sub>, S<sub>01</sub>, S<sub>10</sub>, S<sub>11</sub> etc. are vibrational sublevels.]]Once the electrons and holes reach the luminescence centers, the third and final stage of scintillation occurs: luminescence. In this stage the electrons and holes are captured potential paths by the luminescent center, and then the electrons and hole [[Radiative recombination|recombine radiatively]].<ref>{{Cite journal |last=Blasse |first=G. |date=1989-05-01 |title=New luminescent materials |url=https://pubs.acs.org/doi/abs/10.1021/cm00003a005 |journal=Chemistry of Materials |language=en |volume=1 |issue=3 |pages=294–301 |doi=10.1021/cm00003a005 |issn=0897-4756|url-access=subscription }}</ref> The exact details of the luminescence phase also depend on the type of material used for scintillation.

=== Inorganic crystals ===
{{further|Scintillation counter#Detection materials}}For photons such as gamma rays, thallium activated [[NaI]] crystals (NaI(Tl)) are often used. For a faster response (but only 5% of the output) [[CsF]] crystals can be used.<ref>Introductory Nuclear Physics. Krane. 1987.</ref>{{rp|211}}

=== Organic scintillators ===
In organic molecules scintillation is a product of [[pi bond|π-orbitals]]. Organic materials form molecular crystals where the molecules are loosely bound by [[Van der Waals force]]s. The ground state of <sup>12</sup>C is 1s<sup>2</sup> 2s<sup>2</sup> 2p<sup>2</sup>. In valence bond theory, when carbon forms compounds, one of the 2s electrons is excited into the 2p state resulting in a configuration of 1s<sup>2</sup> 2s<sup>1</sup> 2p<sup>3</sup>. To describe the different valencies of carbon, the four valence electron orbitals, one 2s and three 2p, are considered to be mixed or hybridized in several alternative configurations. For example, in a tetrahedral configuration the s and p<sup>3</sup> orbitals combine to produce four hybrid orbitals. In another configuration, known as trigonal configuration, one of the p-orbitals (say p<sub>z</sub>) remains unchanged and three hybrid orbitals are produced by mixing the s, p<sub>x</sub> and p<sub>y</sub> orbitals. The orbitals that are symmetrical about the bonding axes and plane of the molecule (sp<sup>2</sup>) are known as σ-electrons and the bonds are called σ-bonds. The p<sub>z</sub> orbital is called a π-orbital. A π-bond occurs when two π-orbitals interact. This occurs when their nodal planes are coplanar.

In certain organic molecules π-orbitals interact to produce a common nodal plane. These form delocalized π-electrons that can be excited by radiation. The de-excitation of the delocalized π-electrons results in luminescence.

The excited states of π-electron systems can be explained by the perimeter free-electron model (Platt 1949). This model is used for describing polycyclic hydrocarbons consisting of condensed systems of benzenoid rings in which no C atom belongs to more than two rings and every C atom is on the periphery.

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:

:<math>\psi(x)=\psi(x+l) \,</math>

The corresponding solutions to the [[Schrödinger equation|Schrödinger wave equation]] are:

:<math>\begin{align}
 \psi_0    &= \left( \frac{1}{l} \right)^{\frac{1}{2}} \\
 \psi_{q1} &= \left( \frac{2}{l} \right)^{\frac{1}{2}} \cos{\left( \frac{2\pi\ qx}{l} \right)} \\
 \psi_{q2} &= \left( \frac{2}{l} \right)^{\frac{1}{2}} \sin{\left( \frac{2\pi\ qx}{l} \right)} \\
 E_q       &= \frac{q^2\hbar^2}{2m_0l^2}
\end{align}</math>

where q is the orbital ring quantum number; the number of nodes of the wave-function. Since the electron can have spin up and spin down and can rotate about the circle in both directions all of the energy levels except the lowest are doubly degenerate.

The above shows the π-electronic energy levels of an organic molecule. Absorption of radiation is followed by molecular vibration to the S<sub>1</sub> state. This is followed by a de-excitation to the S<sub>0</sub> state called fluorescence. The population of triplet states is also possible by other means. The triplet states decay with a much longer decay time than singlet states, which results in what is called the slow component of the decay process (the fluorescence process is called the fast component). Depending on the particular energy loss of a certain particle (dE/dx), the "fast" and "slow" states are occupied in different proportions. The relative intensities in the light output of these states thus differs for different dE/dx. This property of scintillators allows for pulse shape discrimination: it is possible to identify which particle was detected by looking at the pulse shape. 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.