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32 4 Scintillation detectors
activator should lie energetically above the valence band, but should
lie low enough to maintain a high probability for hole capture. Vice
versa, this applies for electrons and the energy gap between the bottom
of the conduction band and the radiation level of the doping ion. In
general, the energy of the excited activator state should be smaller than
the bandgap energy Eg to prevent self-absorption of the scintillation
light by the host lattice.
Instead of exciting luminescence centres, the electrons, holes and excitons
can be bound by traps. These are metastable levels from which the
charge carriers may subsequently return to the conduction band by acquiring
thermal energy from the lattice vibrations or fall to the valence
band by a radiationless transition.
Q is the luminescence quantum yield, which is the efficiency for the luminescence
emission of an activated luminescence centre and depends
on the kind of excited state of a luminescence centre.
Scintillation yields can be a few thousand to much more than 60.000
photons per deposited MeV for efficient scintillators [26, 27].
The amount of produced photons varies and leads to an intrinsic scintillator
resolution. The variation results from three factors. There is a
nonproportionality of the number of emitted photons to the incident
energy. The importance of this factor depends on the scintillator material
and the considered energy region. Also, the electronic excitations
of various energies lead to a distribution of light yields, which increase
the energy resolution. Additionally, local variations of the light yield
are caused by inhomogeneity of the crystal.
DECAY TIME
The time characteristic of the scintillator is determined by the half-life
characteristic of the excited states. Mostly, the migration time does
not influence the time characteristic, because it is much shorter than
the half-life. The impurity configurations are formed essentially after
10 −12 -10 −8 sec and then de-excite with the corresponding half-life of
typically 50-500 ns. Inorganic scintillators can have more than one decay
time, originating from different excitations of luminescence centres.
In general, the time evolution of the luminescence emission can be described
by one or two exponential decay forms [26, 27, 28, 29].
TEMPERATURE DEPENDENCE
The light output of scintillators also depends on temperature. While
organic scintillators have a temperature independent light output, the
light output of inorganic scintillators can vary strongly with temperature.
As shown in figure 4.5 NaI:Tl has a stable light output between 20
and 40 ◦ C, increasing with higher and lower temperature. Bi4Ge3O12