64
P. Lecoq
One such case are so-called cross-luminescent materials, of which one wellknown example is Barium Fluoride (BaF 2 ). Such systems are characterized by a
specific configuration of the energy bands, such that the width of the forbidden gap
(between the valence and conduction bands) is larger than the energy gap between
the uppermost core band (5pBa in the case of BaF 2 ) and the bottom of the valence
band. When a hole produced in this core band recombines with an electron of the
valence band there is not enough energy available to eject an Auger electron from
the valence to the conduction band. The core-valence transition can therefore only
be radiative giving rise to a scintillation in the UV, which is usually very fast (subnanosecond).
3.2.2 Scintillation Efficiency
The overall scintillation efficiency η is generally given by the product of three terms:
η = β· S· Q
(3.10)
where β represents the conversion efficiency for the production of electron-hole
pairs, S the excitation transport efficiency, including thermalization of electric
carriers, localization and transfer to the luminescent centre, and Q is is the quantum
efficiency of the radiative transition of the luminescent centre. If we consider, as
discussed in Sect. 3.1.2.2, the number of 140,000 ph/MeV as an upper limit for the
scintillation yield of an ideal scintillator with an emission peak around 600 nm the
maximum scintillation efficiency is less than 30%. In reality, for the majority of
existing scintillators it is less than 5%, mostly because of important losses during
the thermalization and transport process.
At the end of the first phase of inelastic scattering the holes and electrons have
reached an energy below the Auger and ionization thresholds respectively. Their
thermalization to the top of the valence band for holes and to the bottom of the
conduction band for electrons can only take place by heat dissipation through
coupling to the phonon modes of the lattice. This is an unavoidable part of energy
loss for the scintillation process. The energy gap between these two thresholds being
of the order of 2.3E g for ionic crystals one concludes that an ideal scintillator cannot
convert more than 43% of the absorbed energy into light.
Another important loss is related to the transfer of the excitations to the
luminescent centres. A frequent channel of excitation for acceptors is a charge
transfer process with a sequential capture of charge carriers. In Ce 3+ -doped crystals,
the hole is first captured with its capture probability strongly depending on the
position of the Ce 3+ ground level (4f) in the forbidden band gap. In cerium-doped
oxides and halides, this level is usually lying very low in the gap close to the top
of the valence band, and these systems can lead to very efficient scintillation (LSO,
LuAP, LaCl 3 , etc.). On the other hand, Ce 3+ -doped fluoride crystals cannot exhibit
P. Lecoq
One such case are so-called cross-luminescent materials, of which one wellknown example is Barium Fluoride (BaF 2 ). Such systems are characterized by a
specific configuration of the energy bands, such that the width of the forbidden gap
(between the valence and conduction bands) is larger than the energy gap between
the uppermost core band (5pBa in the case of BaF 2 ) and the bottom of the valence
band. When a hole produced in this core band recombines with an electron of the
valence band there is not enough energy available to eject an Auger electron from
the valence to the conduction band. The core-valence transition can therefore only
be radiative giving rise to a scintillation in the UV, which is usually very fast (subnanosecond).
3.2.2 Scintillation Efficiency
The overall scintillation efficiency η is generally given by the product of three terms:
η = β· S· Q
(3.10)
where β represents the conversion efficiency for the production of electron-hole
pairs, S the excitation transport efficiency, including thermalization of electric
carriers, localization and transfer to the luminescent centre, and Q is is the quantum
efficiency of the radiative transition of the luminescent centre. If we consider, as
discussed in Sect. 3.1.2.2, the number of 140,000 ph/MeV as an upper limit for the
scintillation yield of an ideal scintillator with an emission peak around 600 nm the
maximum scintillation efficiency is less than 30%. In reality, for the majority of
existing scintillators it is less than 5%, mostly because of important losses during
the thermalization and transport process.
At the end of the first phase of inelastic scattering the holes and electrons have
reached an energy below the Auger and ionization thresholds respectively. Their
thermalization to the top of the valence band for holes and to the bottom of the
conduction band for electrons can only take place by heat dissipation through
coupling to the phonon modes of the lattice. This is an unavoidable part of energy
loss for the scintillation process. The energy gap between these two thresholds being
of the order of 2.3E g for ionic crystals one concludes that an ideal scintillator cannot
convert more than 43% of the absorbed energy into light.
Another important loss is related to the transfer of the excitations to the
luminescent centres. A frequent channel of excitation for acceptors is a charge
transfer process with a sequential capture of charge carriers. In Ce 3+ -doped crystals,
the hole is first captured with its capture probability strongly depending on the
position of the Ce 3+ ground level (4f) in the forbidden band gap. In cerium-doped
oxides and halides, this level is usually lying very low in the gap close to the top
of the valence band, and these systems can lead to very efficient scintillation (LSO,
LuAP, LaCl 3 , etc.). On the other hand, Ce 3+ -doped fluoride crystals cannot exhibit