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30 4 Scintillation detectors
emission and leads to a slow component of light, which can be as
after-glow an significant source of background.
Figure 4.4: Scheme of light emission in an scintillation crystal, with
traps, luminescence and quenching mode [27]
.
5. De-excitation of luminescence centres: The emission of light by the
luminescence centres is the last step of the scintillation process.
The deexcitation mechanism depends on the electronic structure
of the luminescence centre and the surrounding host lattice.
Thereby, radiative and nonradiative transitions can compete.
Radiative transitions between excited luminescence centre states
and their ground states lead to luminescence emission. Nonradiative
transitions are called quenching processes and are
possible with intermediate excitation levels between the ground
state and the emission level and over multi-phonon emission.
Ions like Tl + , Bi 3+ and Ce 3+ with a large free gap between their
6p or 5p emitting level and their ground state are therefore more
efficient luminescence centres than ions like Pr 3+ with many 4f
levels lying between the 5d emission level and the ground state.