50
P. Lecoq
However, contrary to a common assumption, the optimum conditions are not
necessarily achieved with the highest Z ions, because in addition to a small X 0 , the
density ρ should be high. This reduces the lateral shower size given by the Moliere
radius:
R M ≈ X 0 · (Z + 1.2) /37.74 ∼ 1/ρ
(3.6)
The stability of the physico-chemical parameters is also important for the
detector design. Scintillation crystals are very stable materials, at least in the bulk,
if grown under conditions allowing a good structural quality. This provides a high
degree of internal symmetry in the material together with high energetic stability.
However, the charge unbalance on the surface can be at the origin of different
problems, such as a concentration of impurities or crystallographic defects. As
a result, the material can interact with its environment and locally change its
properties. The majority of halide crystals have the anions weakly bound to the
cations at the surface. They are therefore easily replaced by OH − radicals from the
atmosphere, which have strong optical absorption bands in the visible spectrum.
This causes a progressive brownish discoloration of the crystal surface, a well know
feature of hygroscopic materials. Encapsulating the crystal in an inert atmosphere
avoids this effect.
3.1.2.2 Optical Properties
Inorganic scintillators usually show wide emission bands because of multi-site
emission centres differently distorted by the crystal field, as well as by temperature
broadening of the optical transitions through vibronic coupling of the emission
centres with the crystal lattice. These emission bands are situated in the optical
window of the scintillator and produce light in the visible, near infrared or near
ultraviolet part of the spectrum. One of the objectives of scintillator development
is to design scintillators with emissions peaks matching the maximum quantum
efficiency of photodetectors, typically 250–500 nm for photomultipliers and 450–
900 nm for solid state photodetectors (pin diodes and avalanche photodiodes).
Light yield (LY) is an essential parameter for a scintillator as it directly influences
the energy resolution at low or medium energy through the photostatistic term
proportional to (LY) −1/2 and the timing resolution proportional to (τ sc /LY) −1/2 , with
τ sc being the scintillation decay time. The scintillation mechanism is a multi-step
process, which will be described in detail in Sect. 3.2. The overall scintillation yield
is determined by the product of efficiencies for all these steps. The dominant factor,
which sets the fundamental limit on the light output of a given scintillator, is the
number n eh of thermalized electron-hole pairs (active for scintillation) produced in
the ionization track of the incoming particle:
n eh =
E α
β · E g
(3.7)
P. Lecoq
However, contrary to a common assumption, the optimum conditions are not
necessarily achieved with the highest Z ions, because in addition to a small X 0 , the
density ρ should be high. This reduces the lateral shower size given by the Moliere
radius:
R M ≈ X 0 · (Z + 1.2) /37.74 ∼ 1/ρ
(3.6)
The stability of the physico-chemical parameters is also important for the
detector design. Scintillation crystals are very stable materials, at least in the bulk,
if grown under conditions allowing a good structural quality. This provides a high
degree of internal symmetry in the material together with high energetic stability.
However, the charge unbalance on the surface can be at the origin of different
problems, such as a concentration of impurities or crystallographic defects. As
a result, the material can interact with its environment and locally change its
properties. The majority of halide crystals have the anions weakly bound to the
cations at the surface. They are therefore easily replaced by OH − radicals from the
atmosphere, which have strong optical absorption bands in the visible spectrum.
This causes a progressive brownish discoloration of the crystal surface, a well know
feature of hygroscopic materials. Encapsulating the crystal in an inert atmosphere
avoids this effect.
3.1.2.2 Optical Properties
Inorganic scintillators usually show wide emission bands because of multi-site
emission centres differently distorted by the crystal field, as well as by temperature
broadening of the optical transitions through vibronic coupling of the emission
centres with the crystal lattice. These emission bands are situated in the optical
window of the scintillator and produce light in the visible, near infrared or near
ultraviolet part of the spectrum. One of the objectives of scintillator development
is to design scintillators with emissions peaks matching the maximum quantum
efficiency of photodetectors, typically 250–500 nm for photomultipliers and 450–
900 nm for solid state photodetectors (pin diodes and avalanche photodiodes).
Light yield (LY) is an essential parameter for a scintillator as it directly influences
the energy resolution at low or medium energy through the photostatistic term
proportional to (LY) −1/2 and the timing resolution proportional to (τ sc /LY) −1/2 , with
τ sc being the scintillation decay time. The scintillation mechanism is a multi-step
process, which will be described in detail in Sect. 3.2. The overall scintillation yield
is determined by the product of efficiencies for all these steps. The dominant factor,
which sets the fundamental limit on the light output of a given scintillator, is the
number n eh of thermalized electron-hole pairs (active for scintillation) produced in
the ionization track of the incoming particle:
n eh =
E α
β · E g
(3.7)