3 Scintillation Detectors for Charged Particles and Photons
85
continuously and the growing crystal starts substituting for the melt. This method
is relatively inexpensive and multiple crystal pulling is possible by moving
several crucibles together through the temperature gradient zone of a single
oven [31]. If the simplicity and reliability of the Bridgeman and Stockbarger
methods make them particularly attractive for many applications, these methods
suffer from several drawbacks, such as large variations of the temperature
field parameters during the crystal growth and strong non-uniformities in the
distribution of doping ions, impurities and defects in the crystal.
• Establishing a temperature gradient between the crystal and the melt in an
open crucible by progressive cooling of the melt after seeding or extracting the
growing crystal from melt (Kyropoulos and Czochralski methods, respectively).
In the classical Kyropolos method [32] the entire crystallization process starts
with the seeding and propagates through the melt as a result of a continuous
temperature decrease applied during the process. There is no relative movement
of the seed and the crucible. In the Czochralski method [33] the crystal is pulled
from the melt. The seed is attached to a Platinum rod and put in contact with
the melt in the crucible. The rod or the crucible (sometimes both) are rotating
at a few rpm to maintain a good homogeneity of the melt in contact with the
crystallized phase. The rod is simultaneously pulled up at a speed of typically
1–10 mm/h depending on the crystal. This method is the most widely used for
growing oxide scintillators and several other types of scintillators because of its
potential to grow high quality crystals by concentrating impurities and defects in
the bottom part of the crucible.
More details about crystal engineering techniques are given in ref. [11].
Technologies for the production of crystals are rapidly evolving. The impressive
progress in nanotechnologies in particular open new perspectives for the production
of pre-reacted raw materials of excellent quality with a high uniformity of the grain
sizes. With these new materials, transparent ceramics of heavy scintillators can be
produced (Fig. 3.20), with the advantage over standard crystal growth techniques to
be much more cost effective: not only the scintillator can be produced to its final
shape, saving on the cost of mechanical processing, but also the temperature for
sintering is usually much lower than for standard crystal growth.
The recently developed pulling-down technology from a shape-controlled capillary die gives the possibility to produce elongated crystals with dimensions that
are not accessible using traditional cutting and polishing of bulk crystals grown by
the more standard Czochralski or Bridgeman methods (Fig. 3.21). This approach
has important advantages, such as growing the crystal in the final shape (round,
oval, square, rectangular, hexagonal), very rapidly (several millimeters per minute
instead of millimeters per hour), simultaneous multifibre pulling, increased activator
doping concentration, etc. Excellent quality BGO, YAG and LSO fibers have been
grown with a length of up to 2 m and a diameter between 0.3 and 3 mm. Some
other materials are being studied, in particular from the very interesting perovskite
family: YAP and LuAP [34].
85
continuously and the growing crystal starts substituting for the melt. This method
is relatively inexpensive and multiple crystal pulling is possible by moving
several crucibles together through the temperature gradient zone of a single
oven [31]. If the simplicity and reliability of the Bridgeman and Stockbarger
methods make them particularly attractive for many applications, these methods
suffer from several drawbacks, such as large variations of the temperature
field parameters during the crystal growth and strong non-uniformities in the
distribution of doping ions, impurities and defects in the crystal.
• Establishing a temperature gradient between the crystal and the melt in an
open crucible by progressive cooling of the melt after seeding or extracting the
growing crystal from melt (Kyropoulos and Czochralski methods, respectively).
In the classical Kyropolos method [32] the entire crystallization process starts
with the seeding and propagates through the melt as a result of a continuous
temperature decrease applied during the process. There is no relative movement
of the seed and the crucible. In the Czochralski method [33] the crystal is pulled
from the melt. The seed is attached to a Platinum rod and put in contact with
the melt in the crucible. The rod or the crucible (sometimes both) are rotating
at a few rpm to maintain a good homogeneity of the melt in contact with the
crystallized phase. The rod is simultaneously pulled up at a speed of typically
1–10 mm/h depending on the crystal. This method is the most widely used for
growing oxide scintillators and several other types of scintillators because of its
potential to grow high quality crystals by concentrating impurities and defects in
the bottom part of the crucible.
More details about crystal engineering techniques are given in ref. [11].
Technologies for the production of crystals are rapidly evolving. The impressive
progress in nanotechnologies in particular open new perspectives for the production
of pre-reacted raw materials of excellent quality with a high uniformity of the grain
sizes. With these new materials, transparent ceramics of heavy scintillators can be
produced (Fig. 3.20), with the advantage over standard crystal growth techniques to
be much more cost effective: not only the scintillator can be produced to its final
shape, saving on the cost of mechanical processing, but also the temperature for
sintering is usually much lower than for standard crystal growth.
The recently developed pulling-down technology from a shape-controlled capillary die gives the possibility to produce elongated crystals with dimensions that
are not accessible using traditional cutting and polishing of bulk crystals grown by
the more standard Czochralski or Bridgeman methods (Fig. 3.21). This approach
has important advantages, such as growing the crystal in the final shape (round,
oval, square, rectangular, hexagonal), very rapidly (several millimeters per minute
instead of millimeters per hour), simultaneous multifibre pulling, increased activator
doping concentration, etc. Excellent quality BGO, YAG and LSO fibers have been
grown with a length of up to 2 m and a diameter between 0.3 and 3 mm. Some
other materials are being studied, in particular from the very interesting perovskite
family: YAP and LuAP [34].