6.1. Scintillation Mechanism and Scintillator Properties 333
Figure 6.1.10: Temperature
dependence of two decay con-
stants of pureCsI(2).6.1.K Scintillation Efficiency
Scintillation efficiency is a term that is used to characterize the efficiency by which
the energy lost by the incident particle is converted into scintillation photons. Gen-
erally it is described as a ratio of the total energy of scintillation photons and the
total energy deposited by the incident radiation, that is
η =Total Energy of Scintillation Photons
Energy Deposited by Incident Radiation=Es
Ei, (6.1.6)
where the subscriptssandistand for scintillation and incident respectively.
In order to determine the energy of the scintillation photons in terms of the energy
deposited by the incident radiation we first realize that scintillation is a complex and
multi-step process. The reason is that the energy transfer to the luminous centers
(that is the energy traps that are responsible for emission of scintillation light)
follows the charge pair production by the incident radiation. The complexity of the
scintillation process can be appreciated by looking at the following simplified version
of it.
γi+X → X++e
X++X → X 2 +∗+heat
X 2 +∗+e → 2 X+γs (6.1.7)HereXandX∗symbolically represent the scintillator in ground and excited states
respectively.γiandγsare the incident and scintillation photons respectively, though
the incident particle does not necessarily have to be a photon. The first step of the
scintillation process depicted here involves production of charge pairs. We already