3 Scintillation Detectors for Charged Particles and Photons
75
interesting applications in ultrafast X-Ray imaging as well as providing a fast time
tag in γ imaging if used in hetero-structures in combination with dense scintillators
like LSO with a structuration dimension of the order of the recoil electron range, as
suggested in Ref [25].
3.3 Role of Defects on Scintillation Properties
and on Radiation Damage in Inorganic Scintillators
3.3.1 Structural Defects in a Crystal
The properties of a scintillator strongly depend on the structural quality of the
crystal lattice. The presence of defects influences all stages of the scintillation
process. They play also an important role in the light transport to the photodetector,
as well as in the generation of optically active defects under radiation exposure.
They continuously exchange charge carriers and phonons with the crystal lattice
and are therefore in thermodynamic equilibrium with the medium. This can have
a number of consequences such as reduced or enhanced scintillation efficiency if
the charge carriers are channelled through these defects to non-radiative or radiative
traps respectively, modification of the scintillation kinetics, afterglow, creation of
perturbed emission centres, self-absorption, emission wavelength shift, radiation
damage, radiation damage recovery. Depending on their size and physical nature,
one can distinguish two main classes of structural defects, namely point size defects
and impurities. Larger scale defects such as dislocations, twins, voids and other
macroscopic defects also exist. They will not be described here, as their influence
on the crystal properties is usually limited to the mechanical ruggedness and to a
small extent to the optical homogeneity.
3.3.1.1 Point Size Defects
A perfect crystal is a virtual object that can only exist at absolute zero temperature.
At higher temperature, a thermodynamic equilibrium is obtained by exchange of
energy quanta (in the form of phonons) between the environment and the crystal
lattice. Moreover, the finite dimensions of the crystal imposes conditions on the
surface to compensate the electrostatic field unbalance for the atoms at the interface.
This requires some level of plasticity of the lattice, which is generally achieved by
a certain concentration of cation and anion vacancies. Thermodynamics imposes
a relatively low concentration of such defects at room temperature, typically of
the order of 10 12 cm −3 . For comparison, the atomic density of the majority of
known heavy scintillators is about 10 23 cm −3 . In practise, the concentration of
vacancies is determined by the crystal growth technology. The melt is a mixture
of several chemical components, each of them with a different melting temperature
75
interesting applications in ultrafast X-Ray imaging as well as providing a fast time
tag in γ imaging if used in hetero-structures in combination with dense scintillators
like LSO with a structuration dimension of the order of the recoil electron range, as
suggested in Ref [25].
3.3 Role of Defects on Scintillation Properties
and on Radiation Damage in Inorganic Scintillators
3.3.1 Structural Defects in a Crystal
The properties of a scintillator strongly depend on the structural quality of the
crystal lattice. The presence of defects influences all stages of the scintillation
process. They play also an important role in the light transport to the photodetector,
as well as in the generation of optically active defects under radiation exposure.
They continuously exchange charge carriers and phonons with the crystal lattice
and are therefore in thermodynamic equilibrium with the medium. This can have
a number of consequences such as reduced or enhanced scintillation efficiency if
the charge carriers are channelled through these defects to non-radiative or radiative
traps respectively, modification of the scintillation kinetics, afterglow, creation of
perturbed emission centres, self-absorption, emission wavelength shift, radiation
damage, radiation damage recovery. Depending on their size and physical nature,
one can distinguish two main classes of structural defects, namely point size defects
and impurities. Larger scale defects such as dislocations, twins, voids and other
macroscopic defects also exist. They will not be described here, as their influence
on the crystal properties is usually limited to the mechanical ruggedness and to a
small extent to the optical homogeneity.
3.3.1.1 Point Size Defects
A perfect crystal is a virtual object that can only exist at absolute zero temperature.
At higher temperature, a thermodynamic equilibrium is obtained by exchange of
energy quanta (in the form of phonons) between the environment and the crystal
lattice. Moreover, the finite dimensions of the crystal imposes conditions on the
surface to compensate the electrostatic field unbalance for the atoms at the interface.
This requires some level of plasticity of the lattice, which is generally achieved by
a certain concentration of cation and anion vacancies. Thermodynamics imposes
a relatively low concentration of such defects at room temperature, typically of
the order of 10 12 cm −3 . For comparison, the atomic density of the majority of
known heavy scintillators is about 10 23 cm −3 . In practise, the concentration of
vacancies is determined by the crystal growth technology. The melt is a mixture
of several chemical components, each of them with a different melting temperature