3 Scintillation Detectors for Charged Particles and Photons
55
Fig. 3.4 Schematics of
bunch crossing and TOF in
the forward and backward
directions of particles
generated by events created in
different positions of the
overlap region
Excellent timing resolution is therefore needed. It can be shown [10] that it is
related to the time density of the detected scintillation photons in the leading edge
of the scintillation pulse, which is given by the following formula:
σ t ≈
τ r τ d
N pe
where τ r and τ d are the scintillator rise time and decay time respectively and N pe is
the number of photoelectrons produced in the photodetector, which is proportional
to the light yield of the scintillator. A high light yield is therefore mandatory to
minimize the photo-statistic fluctuation influencing the time jitter of the detector.
An emission spectrum in the visible region is preferred as the quantum efficiency
of the majority of photodetectors is higher and the light is generally less attenuated
than in the UV region and hence more easily collected.
The energy resolution of the calorimeter is affected by all possible sources of
non-uniformity. The light collection in a pointing geometry of tapered crystals
introduces non-uniformity due to a focusing effect through the successive reflections
of the light on the lateral faces, which depends on the refractive index of the
crystal. Fluoride crystals and glasses, with low refractive index (around 1.5) have
smaller non-uniformities (and therefore are easier to correct) than BGO (index
2.15) or PWO (index 2.3). The material can be intrinsically luminescent if it holds
luminescent molecular complexes or ions, or is doped with a scintillating activator.
Intrinsic scintillators are generally preferred, as it is easier to control the light yield
uniformity in long crystals. On the other hand, a controlled distribution of the doping
could help correcting for the non-uniformity caused by the light collection in a
pointing geometry. Furthermore, the scintillation yield should be as independent as
possible from temperature. Large temperature coefficients increase the complexity
55
Fig. 3.4 Schematics of
bunch crossing and TOF in
the forward and backward
directions of particles
generated by events created in
different positions of the
overlap region
Excellent timing resolution is therefore needed. It can be shown [10] that it is
related to the time density of the detected scintillation photons in the leading edge
of the scintillation pulse, which is given by the following formula:
σ t ≈
τ r τ d
N pe
where τ r and τ d are the scintillator rise time and decay time respectively and N pe is
the number of photoelectrons produced in the photodetector, which is proportional
to the light yield of the scintillator. A high light yield is therefore mandatory to
minimize the photo-statistic fluctuation influencing the time jitter of the detector.
An emission spectrum in the visible region is preferred as the quantum efficiency
of the majority of photodetectors is higher and the light is generally less attenuated
than in the UV region and hence more easily collected.
The energy resolution of the calorimeter is affected by all possible sources of
non-uniformity. The light collection in a pointing geometry of tapered crystals
introduces non-uniformity due to a focusing effect through the successive reflections
of the light on the lateral faces, which depends on the refractive index of the
crystal. Fluoride crystals and glasses, with low refractive index (around 1.5) have
smaller non-uniformities (and therefore are easier to correct) than BGO (index
2.15) or PWO (index 2.3). The material can be intrinsically luminescent if it holds
luminescent molecular complexes or ions, or is doped with a scintillating activator.
Intrinsic scintillators are generally preferred, as it is easier to control the light yield
uniformity in long crystals. On the other hand, a controlled distribution of the doping
could help correcting for the non-uniformity caused by the light collection in a
pointing geometry. Furthermore, the scintillation yield should be as independent as
possible from temperature. Large temperature coefficients increase the complexity