23.2 Scintillators
645
Table 23.4 Emission peak wavelength and decay time of various scintillator materials
Material
λ p (nm)
Decay time
Zn 2 SiO 4 :Mn
525
24 ms
ZnS:Cu
543
35–100 µs
CdWO 4
475
5 µs
CsI:Tl
540
1 µs
CsI:Na
425
630 ns
Y 3 Al 5 O 12 :Ce
550
65 ns
Lu 2 SiO 5 :Ce
400
40 ns
YAlO 3 :Ce
365
30 ns
ZnO:Ga
385
2 ns
Large-volume plastic scintillators have been used in gamma detection, particularly for dose-related
measurements.
Other inorganic crystalline scintillators, especially sodium iodide activated with thallium, NaI:Tl,
have been used for gamma-ray energy measurements. Such detectors can be grown as large single
crystals that have a reasonably high efficiency for absorbing all of the energy from incident gamma
rays. There exists a rather large number of inorganic scintillators; some examples of these include
cesium iodide activated with thallium, CsI:Tl, bismuth germanate, Bi 4 Ge 3 O 12 , and barium fluoride,
BaF 2 . These are mostly used for gamma measurements but can also be prepared with thin windows and
have been used for charged particle (e.g. alpha and beta) counting. A number of scintillator materials
including tungstates like CdWO 4 has been reviewed in [1932].
In Table 23.4, the peak emission wavelength and the characteristic decay time are listed for a
variety of scintillator materials. Direct semiconductors, although not offering the highest efficiency,
are particularly useful for high time resolution in, e.g., time-of-flight measurements or fast scanning
electron microscopy.
23.2.4 Luminescence Mechanisms
Self-Trapped Excitons
In a strongly ionic crystal, such as NaI, a hole becomes localized to an atomic site via the polaron
effect. A spatially diffuse electron is attracted, and a self-trapped exciton is formed that can recombine
radiatively.
Self-Activated Scintillator
In such material, the luminescent species is a constituent of the crystal. The emission involves an
intraionic transition, e.g. 6p→6s in Bi
3+ of Bi 4 Ge 3 O 12 , or a charge-transfer transition in the case of
(WO 4 )
2− in CaWO 4 . At room temperature, nonradiative competing processes limit the efficiency.
Activator Ions
For dopant ions such as Eu
2+ in YO 2 S:Eu, Ce
3+ in YAlO 3 :Ce or Tl
+ in NaI:Tl, the hole and electron
excited by the radiation are sequentially trapped by the same ion that then undergoes a radiative
transition, in the case of Eu and Ce
10 5d→4f, for Tl
3 P 0,1 →S 0 . CsI:Tl has one of the highest efficiencies
of 64.8 photons/keV [1933].
10 This transition is dipole allowed for Ce and partially forbidden for Eu.
645
Table 23.4 Emission peak wavelength and decay time of various scintillator materials
Material
λ p (nm)
Decay time
Zn 2 SiO 4 :Mn
525
24 ms
ZnS:Cu
543
35–100 µs
CdWO 4
475
5 µs
CsI:Tl
540
1 µs
CsI:Na
425
630 ns
Y 3 Al 5 O 12 :Ce
550
65 ns
Lu 2 SiO 5 :Ce
400
40 ns
YAlO 3 :Ce
365
30 ns
ZnO:Ga
385
2 ns
Large-volume plastic scintillators have been used in gamma detection, particularly for dose-related
measurements.
Other inorganic crystalline scintillators, especially sodium iodide activated with thallium, NaI:Tl,
have been used for gamma-ray energy measurements. Such detectors can be grown as large single
crystals that have a reasonably high efficiency for absorbing all of the energy from incident gamma
rays. There exists a rather large number of inorganic scintillators; some examples of these include
cesium iodide activated with thallium, CsI:Tl, bismuth germanate, Bi 4 Ge 3 O 12 , and barium fluoride,
BaF 2 . These are mostly used for gamma measurements but can also be prepared with thin windows and
have been used for charged particle (e.g. alpha and beta) counting. A number of scintillator materials
including tungstates like CdWO 4 has been reviewed in [1932].
In Table 23.4, the peak emission wavelength and the characteristic decay time are listed for a
variety of scintillator materials. Direct semiconductors, although not offering the highest efficiency,
are particularly useful for high time resolution in, e.g., time-of-flight measurements or fast scanning
electron microscopy.
23.2.4 Luminescence Mechanisms
Self-Trapped Excitons
In a strongly ionic crystal, such as NaI, a hole becomes localized to an atomic site via the polaron
effect. A spatially diffuse electron is attracted, and a self-trapped exciton is formed that can recombine
radiatively.
Self-Activated Scintillator
In such material, the luminescent species is a constituent of the crystal. The emission involves an
intraionic transition, e.g. 6p→6s in Bi
3+ of Bi 4 Ge 3 O 12 , or a charge-transfer transition in the case of
(WO 4 )
2− in CaWO 4 . At room temperature, nonradiative competing processes limit the efficiency.
Activator Ions
For dopant ions such as Eu
2+ in YO 2 S:Eu, Ce
3+ in YAlO 3 :Ce or Tl
+ in NaI:Tl, the hole and electron
excited by the radiation are sequentially trapped by the same ion that then undergoes a radiative
transition, in the case of Eu and Ce
10 5d→4f, for Tl
3 P 0,1 →S 0 . CsI:Tl has one of the highest efficiencies
of 64.8 photons/keV [1933].
10 This transition is dipole allowed for Ce and partially forbidden for Eu.