3 Scintillation Detectors for Charged Particles and Photons
77
3.3.2 Impact of Defects on Optical Properties
Defects in a crystal influence its optical properties in a number of ways, affecting
the charge carriers or the photon transport.
3.3.2.1 Charge Carrier Traps
Most point defects or impurities are electron or hole traps. They reduce therefore
the transfer efficiency of charge carriers to the luminescent centres and therefore
also the scintillation efficiency. For good quality crystals the density of defects
(at 1–100 ppm level) is several orders of magnitude smaller than the density of
luminescent centres, which is very high for intrinsic scintillators (about 10 22 cm −3 )
but also quite high for extrinsic scintillators, for which the activator concentration is
typically at the atomic percent level. Under normal excitation conditions, it would
look therefore rather unlikely that charge carriers are trapped by defects before they
convert on luminescent centres. This does not take into account the charge carrier
capture cross-section, which can vary by large factors for different kinds of traps. A
typical example is given by the molybdenum molecular complex MoO 4
2− , which
is a very efficient and stable electron trap with a radiative decay at 508 nm in PWO.
At the level of only a few ppm it gives rise to a slow (500 ns) additional green
component to the regular fast PWO emission band at 420 nm. As molybdenum is
isomorphic to tungsten it can easily enter into the PWO lattice and locally produce
a solid solution (PbWO 4 -PbMoO 4 ). This slow green component is negligible if the
molybdenum contamination of the tungsten oxide raw material is less than 1 ppm
[26].
In some cases, the traps are non-radiative but have energy levels close enough
to the valence or conduction bands so that the carriers can be released by thermal
activation, eventually converting on the luminescent centres. If the trap is close to
the radiative centre this thermally assisted transfer can take place directly between
them without involving the valence or conduction bands. As a result, the regular
emission will take place but with some delay associated with the transit of the carrier
via the trap. This is the origin of the well-known afterglow or phosphorescence.
When afterglow effects are undesirable, for instance for high X-ray counting rates in
CT scanners, additional impurities can help opening some non-radiative relaxation
channels for these traps. As an example, afterglow in (Y,Gb) 2 O 3 :Eu scintillators
can be significantly reduced by the addition of heterovalent Pr 3+ or Tb 3+ ions to
the lattice [27]. The Pr 3+ and Tb 3+ additives readily trap holes to form Pr 4+ and
Tb 4+ , which compete with the intrinsic traps responsible for afterglow. This energy
trapped in the Pr or Tb sites decays non-radiatively in the presence of the Eu 3+ ion.
As a consequence, afterglow emission is suppressed by one order of magnitude or
more.
77
3.3.2 Impact of Defects on Optical Properties
Defects in a crystal influence its optical properties in a number of ways, affecting
the charge carriers or the photon transport.
3.3.2.1 Charge Carrier Traps
Most point defects or impurities are electron or hole traps. They reduce therefore
the transfer efficiency of charge carriers to the luminescent centres and therefore
also the scintillation efficiency. For good quality crystals the density of defects
(at 1–100 ppm level) is several orders of magnitude smaller than the density of
luminescent centres, which is very high for intrinsic scintillators (about 10 22 cm −3 )
but also quite high for extrinsic scintillators, for which the activator concentration is
typically at the atomic percent level. Under normal excitation conditions, it would
look therefore rather unlikely that charge carriers are trapped by defects before they
convert on luminescent centres. This does not take into account the charge carrier
capture cross-section, which can vary by large factors for different kinds of traps. A
typical example is given by the molybdenum molecular complex MoO 4
2− , which
is a very efficient and stable electron trap with a radiative decay at 508 nm in PWO.
At the level of only a few ppm it gives rise to a slow (500 ns) additional green
component to the regular fast PWO emission band at 420 nm. As molybdenum is
isomorphic to tungsten it can easily enter into the PWO lattice and locally produce
a solid solution (PbWO 4 -PbMoO 4 ). This slow green component is negligible if the
molybdenum contamination of the tungsten oxide raw material is less than 1 ppm
[26].
In some cases, the traps are non-radiative but have energy levels close enough
to the valence or conduction bands so that the carriers can be released by thermal
activation, eventually converting on the luminescent centres. If the trap is close to
the radiative centre this thermally assisted transfer can take place directly between
them without involving the valence or conduction bands. As a result, the regular
emission will take place but with some delay associated with the transit of the carrier
via the trap. This is the origin of the well-known afterglow or phosphorescence.
When afterglow effects are undesirable, for instance for high X-ray counting rates in
CT scanners, additional impurities can help opening some non-radiative relaxation
channels for these traps. As an example, afterglow in (Y,Gb) 2 O 3 :Eu scintillators
can be significantly reduced by the addition of heterovalent Pr 3+ or Tb 3+ ions to
the lattice [27]. The Pr 3+ and Tb 3+ additives readily trap holes to form Pr 4+ and
Tb 4+ , which compete with the intrinsic traps responsible for afterglow. This energy
trapped in the Pr or Tb sites decays non-radiatively in the presence of the Eu 3+ ion.
As a consequence, afterglow emission is suppressed by one order of magnitude or
more.