6 Calorimetry
239
6.3 Readout Methods in Calorimeters
6.3.1 Scintillation Light Collection and Conversion
Scintillator materials used in calorimetry are inorganic crystals, organic compounds
and noble liquids. Dense inorganic crystals represent one of the best techniques
for homogeneous electromagnetic calorimetry. These crystals are insulators with
a normally empty conduction band. When energy is deposited in the crystal, an
electron can jump into the conduction band and cascade to the valence band by
intermediate acceptor levels, part of the energy being emitted as light. The emitted
light needs to be in the wavelength range where good photodetectors are available,
and the crystal must be transparent to this wavelength range. The lifetime of the
light emission depends on the concentration of acceptor levels, and temperature. In
general, different decay times are present in the light luminescence spectrum of a
given crystal (see also Chap. 3).
A list of commonly used scintillators, with some of their characteristic properties
is given in Table 6.2. Crystals for homogeneous calorimetry are usually shaped
as bars, typically of ~25 X 0 length and ~ 1 × 1 ρ M transverse size. In colliding
beam detectors, the cylindrical geometry leads in general to the use of tapered
bars, with the incident radiation impinging on the smaller face. The growth of good
quality ingots, followed by sawing and polishing to the needed size and surface
quality requires specialized tooling available in industry. Careful packaging of the
crystal in appropriate material (Tywek or equivalent) and sometimes lateral masking
are needed to minimize the response dependence on position, transversally and
longitudinally. The light detector (photomultiplier, photodiode, . . . ) is optically
coupled to the back face of the crystal. The overall light yield, including the area and
quantum efficiency of the transducer, influences the achievable energy resolution. A
light yield of 1 photoelectron per MeV implies that the energy resolution cannot be
better than σ (E)/E = 3%/
√
E (GeV). The number of emitted photons per MeV is
in general much larger, being for example 4·10 4 in NaI doped with Thallium, one
of the best scintillating crystal in terms of light yield. PbWO 4 produces ~150 times
less light than NaI, but is far superior in other aspects (density, radiation resistance).
New (and expensive) materials, like LYSO (a compound of Lutetium) are being
developed for applications requiring fast response and high light yield.
A photomultiplier is schematically sketched in Fig. 6.30. All elements are located
in an evacuated glass envelope. At the photocathode an electron is extracted by
the photo-electric effect. A voltage difference accelerates the electron towards the
first dynode out of which several electrons are extracted by secondary emission.
This process is repeated over ~10 dynodes up to the anode at the highest (~1000
to 2000 volts) positive potential. With a sufficiently large gain at the first dynode
the fluctuation of the number of electrons in the final charge pulse is dominated by
the Poisson fluctuation of the number of photo-electrons. Amplification factors of
several thousands are typical. A careful design of the High Voltage divider chain
is mandatory to avoid non-linear effects. With recently developed “super bi-alkali”
239
6.3 Readout Methods in Calorimeters
6.3.1 Scintillation Light Collection and Conversion
Scintillator materials used in calorimetry are inorganic crystals, organic compounds
and noble liquids. Dense inorganic crystals represent one of the best techniques
for homogeneous electromagnetic calorimetry. These crystals are insulators with
a normally empty conduction band. When energy is deposited in the crystal, an
electron can jump into the conduction band and cascade to the valence band by
intermediate acceptor levels, part of the energy being emitted as light. The emitted
light needs to be in the wavelength range where good photodetectors are available,
and the crystal must be transparent to this wavelength range. The lifetime of the
light emission depends on the concentration of acceptor levels, and temperature. In
general, different decay times are present in the light luminescence spectrum of a
given crystal (see also Chap. 3).
A list of commonly used scintillators, with some of their characteristic properties
is given in Table 6.2. Crystals for homogeneous calorimetry are usually shaped
as bars, typically of ~25 X 0 length and ~ 1 × 1 ρ M transverse size. In colliding
beam detectors, the cylindrical geometry leads in general to the use of tapered
bars, with the incident radiation impinging on the smaller face. The growth of good
quality ingots, followed by sawing and polishing to the needed size and surface
quality requires specialized tooling available in industry. Careful packaging of the
crystal in appropriate material (Tywek or equivalent) and sometimes lateral masking
are needed to minimize the response dependence on position, transversally and
longitudinally. The light detector (photomultiplier, photodiode, . . . ) is optically
coupled to the back face of the crystal. The overall light yield, including the area and
quantum efficiency of the transducer, influences the achievable energy resolution. A
light yield of 1 photoelectron per MeV implies that the energy resolution cannot be
better than σ (E)/E = 3%/
√
E (GeV). The number of emitted photons per MeV is
in general much larger, being for example 4·10 4 in NaI doped with Thallium, one
of the best scintillating crystal in terms of light yield. PbWO 4 produces ~150 times
less light than NaI, but is far superior in other aspects (density, radiation resistance).
New (and expensive) materials, like LYSO (a compound of Lutetium) are being
developed for applications requiring fast response and high light yield.
A photomultiplier is schematically sketched in Fig. 6.30. All elements are located
in an evacuated glass envelope. At the photocathode an electron is extracted by
the photo-electric effect. A voltage difference accelerates the electron towards the
first dynode out of which several electrons are extracted by secondary emission.
This process is repeated over ~10 dynodes up to the anode at the highest (~1000
to 2000 volts) positive potential. With a sufficiently large gain at the first dynode
the fluctuation of the number of electrons in the final charge pulse is dominated by
the Poisson fluctuation of the number of photo-electrons. Amplification factors of
several thousands are typical. A careful design of the High Voltage divider chain
is mandatory to avoid non-linear effects. With recently developed “super bi-alkali”