6 Calorimetry
245
(1/βn) with respect to the incident particle direction, and with a number N of emitted
photons in the visible range (400 to 700 nm) per unit length:
dN/dx = 490 sin
2 θ c
cm
−1
.
(6.29)
Lead glass, a dense material with a high index of refraction, has been used in
several experiments (in particular OPAL [83] at LEP) with very similar geometries
(tapered bars) as described above for scintillating crystals. The energy resolution is
limited by the number of electrons and positrons in the shower above the Cherenkov
threshold, resulting in a stochastic term σ (E)/E of > ~ 5–6%/
√
E, comparable
to very good sampling calorimeters. Given the small number of photons, readout
with photomultipliers is mandatory. As for crystals, longitudinal segmentation is in
general not feasible. In several cases, “preshowers” of a few X 0 depth, instrumented
with another higher granularity readout technique, have been used in front of lead
glass arrays, in order to improve particle identification (see Sect. 6.4.3). Another
limitation for large collider systems is the reduced response of lead glass to
hadronic showers (a large fraction of the hadronic cascade is made of non-relativistic
particles), inducing a performance limitation for hadronic calorimetry. However,
the preponderance of Cherenkov-light production from electrons and positrons, i.e.
the electromagnetic part of the hadronic shower, offers an interesting possibility.
A hadronic sampling calorimeter instrumented with two sets of fibres—one set
sensitive to Cherenkov-light only, the other set consisting of scintillating fibres,
sensitive to all charged particles—can measure separately the electromagnetic
component of the hadronic shower. This possibility is being studied in the dualreadout “DREAM” project. Test beam results are reported in Ref. [84].
Exploiting only the Cherenkov component, an hadronic calorimeter made of
quartz fibers (parallel to the beam axis) embedded in an iron matrix has been chosen
for the very forward calorimeter of the CMS experiment (for the pseudorapidity
region up to 5). This choice was motivated by the high radiation resistance of quartz
fibers, well adapted to this harsh environment [85].
Energy measurement with Cherenkov light produced in water was used with
great success in very large detectors for nucleon decay and solar neutrino experiments, like Superkamiokande [86]. For the required detector volume of 50,000
tons water, the Cherenkov light was read out using large photomultipliers. In
Superkamiokande, 50% of the outer surface of the detection volume is covered by
50 cm diameter phototubes. Electrons of 10 MeV are reconstructed with an energy
resolution of about 15%. Their position in the detector volume is reconstructed
with an accuracy of 70 cm and their direction with an accuracy of ~25 degrees.
The detector also provides some discrimination between electrons (showering) and
muons (single Cherenkov cone).
245
(1/βn) with respect to the incident particle direction, and with a number N of emitted
photons in the visible range (400 to 700 nm) per unit length:
dN/dx = 490 sin
2 θ c
cm
−1
.
(6.29)
Lead glass, a dense material with a high index of refraction, has been used in
several experiments (in particular OPAL [83] at LEP) with very similar geometries
(tapered bars) as described above for scintillating crystals. The energy resolution is
limited by the number of electrons and positrons in the shower above the Cherenkov
threshold, resulting in a stochastic term σ (E)/E of > ~ 5–6%/
√
E, comparable
to very good sampling calorimeters. Given the small number of photons, readout
with photomultipliers is mandatory. As for crystals, longitudinal segmentation is in
general not feasible. In several cases, “preshowers” of a few X 0 depth, instrumented
with another higher granularity readout technique, have been used in front of lead
glass arrays, in order to improve particle identification (see Sect. 6.4.3). Another
limitation for large collider systems is the reduced response of lead glass to
hadronic showers (a large fraction of the hadronic cascade is made of non-relativistic
particles), inducing a performance limitation for hadronic calorimetry. However,
the preponderance of Cherenkov-light production from electrons and positrons, i.e.
the electromagnetic part of the hadronic shower, offers an interesting possibility.
A hadronic sampling calorimeter instrumented with two sets of fibres—one set
sensitive to Cherenkov-light only, the other set consisting of scintillating fibres,
sensitive to all charged particles—can measure separately the electromagnetic
component of the hadronic shower. This possibility is being studied in the dualreadout “DREAM” project. Test beam results are reported in Ref. [84].
Exploiting only the Cherenkov component, an hadronic calorimeter made of
quartz fibers (parallel to the beam axis) embedded in an iron matrix has been chosen
for the very forward calorimeter of the CMS experiment (for the pseudorapidity
region up to 5). This choice was motivated by the high radiation resistance of quartz
fibers, well adapted to this harsh environment [85].
Energy measurement with Cherenkov light produced in water was used with
great success in very large detectors for nucleon decay and solar neutrino experiments, like Superkamiokande [86]. For the required detector volume of 50,000
tons water, the Cherenkov light was read out using large photomultipliers. In
Superkamiokande, 50% of the outer surface of the detection volume is covered by
50 cm diameter phototubes. Electrons of 10 MeV are reconstructed with an energy
resolution of about 15%. Their position in the detector volume is reconstructed
with an accuracy of 70 cm and their direction with an accuracy of ~25 degrees.
The detector also provides some discrimination between electrons (showering) and
muons (single Cherenkov cone).