Scintillator detector materials...

An ideal scintillator detector material will:

• provide as many electron-hole pairs as possible per unit of gamma-ray energy (low Î, where Î is the average energy needed to create an electron-hole pair);
• have high stopping power for gamma radiation (high density and atomic number);
• have a response proportional to energy;
• be transparent to light;
• have a short decay time for excited states to allow high count rates;
• be available in optical quality, in reasonable amounts at reasonable cost; and
• have a refractive index close to that of glass to allow efficient coupling to photomultipliers.

Materials which are used for gamma-ray purposes include inorganic crystals for example: NaI, CsI, CaF₂ and BGO. 

For some materials the band gap is large and photons emitted by the de-excitation of electrons from the conduction band would be outside the visible range of light. Also the bulk of the material would absorb the emitted photons before they reach the photomultiplier. Both problems are solved using an activator. For NaI this is Tl and for CsI it is Tl or Na.

The introduction of small amounts of the activator material as an impurity produces defect lattice sites which give rise to extra energy levels within the forbidden band. The ground state of these activator sites lies just above the valence band and the excited states lie just below the conduction band. Electrons within the conduction and exciton band will tend to be captured by the excited activator states. When the electron de-excites the photon energy released is lower and now in the visible range.

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