6.3. Inorganic Scintillators 351
Figure 6.2.12: Light yield of an-
thracene and stilbene to differ-
ent charged particles (40). The
absicssas have been reduced by
MZ^2 ,withMandZbeing the
mass and charge of the incident
particles.radiation is desired. Another advantage is their higher light output than organic
scintillators.
6.3.A Scintillation Mechanism
A.1 ExcitonLuminescence
When ionizing radiation passes through an inorganic scintillator crystal it produces
electron hole pairs. Depending on the energy departed to the molecule the pair can
become free to move around or remainpartial lybound by the Coulomb attraction.
If the energy is high enough for the electron to jump to the conduction band then
it becomes essentially free. In such a case the hole, having an effective positive
charge, becomes free to move around as well. However if the energy departed is
not that high then the electron becomes partially bound to the hole. This can be
viewed as jumping of the electron to a band just below the conduction band (see
Fig:6.3.1). The bondage between the electron and hole is not very strong, though,
since a small amount of additional energy transferred to the electron can elevate it
to the conduction band. In this bound state, the electron and hole are said to form a
system calledexciton. Exciton has an interesting property that it can move around
in the material as an entity and can get trapped by an impurity or a defect site.
If this site constitutes a luminous center, it can lead to the emission of scintillation
photons when the electron falls into the lower level. This process, generally known
as self trapping of excitons, is graphically depicted in Fig.6.3.1. The trapping of
excitons can also occur through another process calledcharge transfer,inwhich
case the luminescence is called charge transfer luminescence.