1 ps, which is much faster than the average decay time in photoluminescence. Charge transport of excited carriers The second stage of scintillation is the charge transport of thermalized electrons and holes towards luminescence centers and the energy transfer to the atoms involved in the luminescence process. 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's 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. The charge transport can also become a bottleneck for the timing of the scintillation process. The charge transport phase is also one of the least understood parts of scintillation and depends strongly on the type material involved and its intrinsic charge conduction properties. Luminescence 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 recombine radiatively. The exact details of the luminescence phase also depend on the type of material used for scintillation. Inorganic Crystals 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. Organic scintillators In organic molecules scintillation is a product of π-orbitals. Organic materials form molecular crystals where the molecules are loosely bound by Van der Waals forces. The ground state of 12C is 1s2 2s2 2p2. 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. 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 p3 orbitals combine to produce four hybrid orbitals. In another configuration, known as trigonal configuration, one of the p-orbitals (say pz) remains unchanged and three hybrid orbitals are produced by mixing the s, px and py orbitals. The orbitals that are symmetrical about the