3 Scintillation Detectors for Charged Particles and Photons
83
related to structural defects, impurities and anion or cation vacancies induced by
differential evaporation of the chemical components during the crystal growth. This
can be achieved for the majority of crystals, through different cycles of purification
of the raw materials, multiple crystal growth and annealing of the crystals in
specific atmosphere and temperature conditions. This approach is however costly
and limited to defect concentration levels in the ppm range. For some applications,
such as in high luminosity collider experiments, this is sometimes not enough to
guarantee the optical stability of the crystals over long periods.
In another approach additional well selected defects are produced in the crystal,
which compete with the uncontrollable defects and reduce their influence. This
so-called co-doping strategy has been the result of improved understanding of the
mechanisms of light production and charge carrier transport and trapping, opening
the way to a defect engineering of the crystals. It has been shown for instance that
divalent doping with Ca 2+ or Mg 2+ in some Ce 3+ activated crystals (in particular
in ortho-silicates and aluminium garnets), not only increases the light yield, but also
suppresses slow scintillation components and improves the radiation hardness [30].
This is the result of easier charge carrier transport to the luminescent centres through
the energy levels of these impurities and easier delocalization of trapped carriers due
to the smaller energy gap between these traps and the conduction band, which may
even absorbed in the conduction band.
3.4 Crystal Engineering. Impact of New Technologies
The conditions of synthesis of the chemical components of a crystal are governed
by thermodynamic relations between composition, temperature and pressure of the
mixture. At a given pressure, the composition-temperature equilibrium for both
the liquid and solid phases is represented by a phase diagram. The phase diagram
shows the domains of stability of a given chemical composition and the influence
of deviations from stoichiometry (composition of the mixture), unwanted impurities
or specific doping. An example of such a phase diagram is shown in Fig. 3.19 for
PWO crystals.
Two stable compositions can be grown from a PbO-WO 3 mixture, namely
PbWO 4 (PWO) and Pb 2 WO 5 . The PbWO 4 melts congruently, i.e. without decomposition of the compound, at 1123 ◦ C. The analysis of this phase diagram helps to
define some practical parameters for the PbWO 4 crystals. First of all the melting
temperature restricts the choice of the crucibles to metals with melting points
much higher than 1123 ◦ C, such as platinum, iridium and their various alloys.
Moreover, such crucibles must be chemically inert with melts of similar oxides
like PbMoO 4 , CaMoO 4 , ZnWO 4 , as Mo, Ca and Zn are impurities likely to be
present in the raw materials. Secondly, the possibility to deviate from the perfect
stoichiometric composition of the raw material with some excess of either WO 3 , or
PbO is of great importance to compensate for a strong differential evaporation of
the different components of the melt during the growth process. An initial deviation
83
related to structural defects, impurities and anion or cation vacancies induced by
differential evaporation of the chemical components during the crystal growth. This
can be achieved for the majority of crystals, through different cycles of purification
of the raw materials, multiple crystal growth and annealing of the crystals in
specific atmosphere and temperature conditions. This approach is however costly
and limited to defect concentration levels in the ppm range. For some applications,
such as in high luminosity collider experiments, this is sometimes not enough to
guarantee the optical stability of the crystals over long periods.
In another approach additional well selected defects are produced in the crystal,
which compete with the uncontrollable defects and reduce their influence. This
so-called co-doping strategy has been the result of improved understanding of the
mechanisms of light production and charge carrier transport and trapping, opening
the way to a defect engineering of the crystals. It has been shown for instance that
divalent doping with Ca 2+ or Mg 2+ in some Ce 3+ activated crystals (in particular
in ortho-silicates and aluminium garnets), not only increases the light yield, but also
suppresses slow scintillation components and improves the radiation hardness [30].
This is the result of easier charge carrier transport to the luminescent centres through
the energy levels of these impurities and easier delocalization of trapped carriers due
to the smaller energy gap between these traps and the conduction band, which may
even absorbed in the conduction band.
3.4 Crystal Engineering. Impact of New Technologies
The conditions of synthesis of the chemical components of a crystal are governed
by thermodynamic relations between composition, temperature and pressure of the
mixture. At a given pressure, the composition-temperature equilibrium for both
the liquid and solid phases is represented by a phase diagram. The phase diagram
shows the domains of stability of a given chemical composition and the influence
of deviations from stoichiometry (composition of the mixture), unwanted impurities
or specific doping. An example of such a phase diagram is shown in Fig. 3.19 for
PWO crystals.
Two stable compositions can be grown from a PbO-WO 3 mixture, namely
PbWO 4 (PWO) and Pb 2 WO 5 . The PbWO 4 melts congruently, i.e. without decomposition of the compound, at 1123 ◦ C. The analysis of this phase diagram helps to
define some practical parameters for the PbWO 4 crystals. First of all the melting
temperature restricts the choice of the crucibles to metals with melting points
much higher than 1123 ◦ C, such as platinum, iridium and their various alloys.
Moreover, such crucibles must be chemically inert with melts of similar oxides
like PbMoO 4 , CaMoO 4 , ZnWO 4 , as Mo, Ca and Zn are impurities likely to be
present in the raw materials. Secondly, the possibility to deviate from the perfect
stoichiometric composition of the raw material with some excess of either WO 3 , or
PbO is of great importance to compensate for a strong differential evaporation of
the different components of the melt during the growth process. An initial deviation