3 Scintillation Detectors for Charged Particles and Photons
65
very high light yield because the Ce 3+ 4f is lying around 3–4 eV above the valence
band, which strongly reduces the hole capture probability.
It is also important to avoid the delocalization of electrons from the activator
excited state to the conduction band. This is achieved if the energy gap E between
the radiating level of the doping ion and the bottom of the conduction band is large
enough. If E >> kT, or the radiative decay τ γ << τ d , where the delocalization time
τ d ≈ (1/S)exp(–E /kT), with S—the frequency factor, k—the Boltzman constant,
and T—the temperature, the scintillation yield is not strongly dependent on the
temperature. In the reverse case one can expect a reduction of the scintillation
yield when the temperature increases (temperature quenching). Similarly, when the
ground state is located in or very close to the valence band, the hole is weakly
trapped and can be easily delocalized to the valence band.
Besides these different processes, a number of competing channels can limit the
probability of charge carrier capture by the luminescent centres. Impurities or ions
in the lattice can act as specific killer ions and compete with active ions for the
capture of charge carriers and/or interact with them, inducing severe limitations
in scintillation efficiency. For example, in cerium-doped crystals the presence of
ions or molecular groups with two or more stable valence states is generally to be
avoided. This is due to the fact that cerium has two stable valence states, Ce 3+
and Ce 4+ , but Ce 3+ only gives rise to luminescence. If a possibility exists for
Ce 3+ to transfer one electron to these killers it will transform into Ce 4+ and no
longer scintillate. This is the case for Ce-doped tungstates and vanadates, which
do not exhibit cerium scintillation because of such Ce-W and Ce-V interactions.
For the same reason the good electron acceptor Yb 3+ severely quenches the Ce 3+
scintillation.
Self-trapping is also a very frequent source of efficiency loss in insulating
materials. Indeed, some of the electrons and holes can be trapped by impurity
or crystal defect related acceptors and cannot excite directly luminescent centres
through sequential capture. If the trap is very shallow it will quickly release the
charge carriers and will slightly delay scintillation. However, in deep traps strong
quenching of the fast luminescence components is observed. Very long components
in the fluorescence decay appear when the temperature is raised to the point, where
trapped electrons can be released by thermal energy (glow peaks).
The interaction between closely spaced electronic excitations (in a few nanometre range) may lead to luminescence quenching, also-called local density-induced
quenching. For electronic excitations created through the different mechanisms of
photon absorption, the probability to produce excitations at such short distances is
very low if the excitation source has a limited intensity. On the contrary, secondary
electronic excitations created by inelastic scattering of photoelectrons or Auger
decay of core holes can be quite closely spaced. In these clusters of high local e and
h density, the interaction between excitations can modify their localization and can
even create defects in crystals. In addition, these clusters can excite closely spaced
luminescent centres, which can interact with each-others, giving rise to faster and
65
very high light yield because the Ce 3+ 4f is lying around 3–4 eV above the valence
band, which strongly reduces the hole capture probability.
It is also important to avoid the delocalization of electrons from the activator
excited state to the conduction band. This is achieved if the energy gap E between
the radiating level of the doping ion and the bottom of the conduction band is large
enough. If E >> kT, or the radiative decay τ γ << τ d , where the delocalization time
τ d ≈ (1/S)exp(–E /kT), with S—the frequency factor, k—the Boltzman constant,
and T—the temperature, the scintillation yield is not strongly dependent on the
temperature. In the reverse case one can expect a reduction of the scintillation
yield when the temperature increases (temperature quenching). Similarly, when the
ground state is located in or very close to the valence band, the hole is weakly
trapped and can be easily delocalized to the valence band.
Besides these different processes, a number of competing channels can limit the
probability of charge carrier capture by the luminescent centres. Impurities or ions
in the lattice can act as specific killer ions and compete with active ions for the
capture of charge carriers and/or interact with them, inducing severe limitations
in scintillation efficiency. For example, in cerium-doped crystals the presence of
ions or molecular groups with two or more stable valence states is generally to be
avoided. This is due to the fact that cerium has two stable valence states, Ce 3+
and Ce 4+ , but Ce 3+ only gives rise to luminescence. If a possibility exists for
Ce 3+ to transfer one electron to these killers it will transform into Ce 4+ and no
longer scintillate. This is the case for Ce-doped tungstates and vanadates, which
do not exhibit cerium scintillation because of such Ce-W and Ce-V interactions.
For the same reason the good electron acceptor Yb 3+ severely quenches the Ce 3+
scintillation.
Self-trapping is also a very frequent source of efficiency loss in insulating
materials. Indeed, some of the electrons and holes can be trapped by impurity
or crystal defect related acceptors and cannot excite directly luminescent centres
through sequential capture. If the trap is very shallow it will quickly release the
charge carriers and will slightly delay scintillation. However, in deep traps strong
quenching of the fast luminescence components is observed. Very long components
in the fluorescence decay appear when the temperature is raised to the point, where
trapped electrons can be released by thermal energy (glow peaks).
The interaction between closely spaced electronic excitations (in a few nanometre range) may lead to luminescence quenching, also-called local density-induced
quenching. For electronic excitations created through the different mechanisms of
photon absorption, the probability to produce excitations at such short distances is
very low if the excitation source has a limited intensity. On the contrary, secondary
electronic excitations created by inelastic scattering of photoelectrons or Auger
decay of core holes can be quite closely spaced. In these clusters of high local e and
h density, the interaction between excitations can modify their localization and can
even create defects in crystals. In addition, these clusters can excite closely spaced
luminescent centres, which can interact with each-others, giving rise to faster and