The Process of Scintillation, Types of Scintillators

The gamma photons arriving at the detector generate high-energy free electrons by photoelectric effect or Compton scattering, which excite electrons until they give off their energy. The relaxation of the excited electrons is the process that produces visible light signals. Parallel to this, several other processes occur with a lower probability, which appear as noise in the signal. Examples include Bremsstrahlung, the emission of Auger electrons, electron-electron collisions or the relaxation of energy in nonradiative transitions. The electrons excited in the basic process get into so-called luminescence centres, from where they relax and emit light, hence the name.

The process of scintillation

 
Types of scintillators:

 
Fundamentally there are two types of scintillators based on whether the existence of luminescence centres is the property of a certain material (intrinsic scintillators), or if the luminescent centres appear due to the addition of some sort of contamination (extrinsic scintillators).
An example of the inorganic intrinsic scintillators is the so-called BGO: Bi4Ge3O12 – Bi3+.
Extrinsic (or contaminated) scintillators include:

• NaI:Tl-Tl+: very widespread due to the fact that they can be produced easily and at a low price
• CsI:Na-Na+
• Ce3+: its advantage is the fact that transition (relaxation) happens very fast, many photons are emitted, so its energy resolution is fairly good

 
The shape of a light pulse in the scintillator depends mainly on the mobility of charge carriers within the particular material.