76
P. Lecoq
and vapour pressure, which leads to segregational evaporation of some components.
Furthermore, close-to-surface vacancies can be partially compensated by absorption
of ions or radicals from the surrounding atmosphere. Typically the concentration
of such defects is at the level of 10 18 cm −3 (10 ppm atomic) or even more.
At such concentration, some collective effects can take place, leading to more
complex molecular or cluster defects. Another typical point defect results from
the displacement of an ion of the lattice to an interstitial position. The electrically
neutral system behaves as a dipole and is called a Frenkel defect. In the case of Lead
Tungstate an oxygen-based Frenkel defect is responsible for an absorption band at
360 nm and for an increased susceptibility to radiation damage.
3.3.1.2 Impurities
Impurities are ions of different nature than the constituents of the crystal lattice.
They are generally introduced from imperfectly purified raw materials or by
contamination, for instance from the crucible material, during the crystal growth
process. Doping ions acting as luminescence activators, such as Ce 3+ in LSO,
LuAP and many other fast scintillators, can be considered as impurities with a
positive role. Ions from the lattice, but in a different valence state than required
by the electric charge balance, are another type of impurity. As an example, Ce 4+
has been considered by some authors as a possible scintillation quencher in CeF 3
crystals. Two important parameters influence the way impurities can be introduced
in a crystal: their electric charge and their ionic radius. If the ionic radius is close to
the one of ions from the lattice, impurities can easily replace these ions, producing
only a small distortion of the lattice. Isovalent ions will then easily produce a solid
solution as is the case for LYSO or LuYAP when Y 3+ ions substitute Lu 3+ in
LSO and LuAP crystals, producing locally a mixed compound of LSO-YSO and
LuAP-YAP, respectively. If heterovalent impurities are introduced in the crystal their
charge excess or deficit must be compensated by other impurities or by lattice ion
vacancies. This mechanism can be used to suppress the detrimental role of some
defects, which cannot be eliminated. A good example are the lead vacancies in
PWO, which are efficient hole traps responsible for radiation damage and which can
be compensated by substituting trivalent ions such as Y 3+ or La 3+ to neighbouring
Pb 2+ ions in the lattice.
Impurities with too large an ionic radius have generally little chance to be
introduced in the lattice, whereas small ions can find interstitial positions and create
strong local distortion of the crystal electronic configuration.
In practice it is difficult, or at least very expensive, to purify raw materials
to the sub-ppm level. Most of the scintillators grown in good conditions have
therefore an impurity concentration of about 10 −17 –10 −19 cm −3 , comparable to
the concentration of point defects.
P. Lecoq
and vapour pressure, which leads to segregational evaporation of some components.
Furthermore, close-to-surface vacancies can be partially compensated by absorption
of ions or radicals from the surrounding atmosphere. Typically the concentration
of such defects is at the level of 10 18 cm −3 (10 ppm atomic) or even more.
At such concentration, some collective effects can take place, leading to more
complex molecular or cluster defects. Another typical point defect results from
the displacement of an ion of the lattice to an interstitial position. The electrically
neutral system behaves as a dipole and is called a Frenkel defect. In the case of Lead
Tungstate an oxygen-based Frenkel defect is responsible for an absorption band at
360 nm and for an increased susceptibility to radiation damage.
3.3.1.2 Impurities
Impurities are ions of different nature than the constituents of the crystal lattice.
They are generally introduced from imperfectly purified raw materials or by
contamination, for instance from the crucible material, during the crystal growth
process. Doping ions acting as luminescence activators, such as Ce 3+ in LSO,
LuAP and many other fast scintillators, can be considered as impurities with a
positive role. Ions from the lattice, but in a different valence state than required
by the electric charge balance, are another type of impurity. As an example, Ce 4+
has been considered by some authors as a possible scintillation quencher in CeF 3
crystals. Two important parameters influence the way impurities can be introduced
in a crystal: their electric charge and their ionic radius. If the ionic radius is close to
the one of ions from the lattice, impurities can easily replace these ions, producing
only a small distortion of the lattice. Isovalent ions will then easily produce a solid
solution as is the case for LYSO or LuYAP when Y 3+ ions substitute Lu 3+ in
LSO and LuAP crystals, producing locally a mixed compound of LSO-YSO and
LuAP-YAP, respectively. If heterovalent impurities are introduced in the crystal their
charge excess or deficit must be compensated by other impurities or by lattice ion
vacancies. This mechanism can be used to suppress the detrimental role of some
defects, which cannot be eliminated. A good example are the lead vacancies in
PWO, which are efficient hole traps responsible for radiation damage and which can
be compensated by substituting trivalent ions such as Y 3+ or La 3+ to neighbouring
Pb 2+ ions in the lattice.
Impurities with too large an ionic radius have generally little chance to be
introduced in the lattice, whereas small ions can find interstitial positions and create
strong local distortion of the crystal electronic configuration.
In practice it is difficult, or at least very expensive, to purify raw materials
to the sub-ppm level. Most of the scintillators grown in good conditions have
therefore an impurity concentration of about 10 −17 –10 −19 cm −3 , comparable to
the concentration of point defects.