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C. W. Fabjan and D. Fournier
6.3.4 Gas Detectors
Charge collection in gases, usually followed by some degree of internal amplification, forms the basis of another important category of ionization sampling
calorimetry. This method lends itself naturally to highly segmented construction
and has profited from the diversified developments of gaseous position detectors
(see Chap. 4). The relatively low costs of gaseous detectors favours their use in
large area applications such as calorimeters for neutrino physics.
While gaseous ionization calorimetry offers several of the advantages found in
ionization calorimetry with dense active materials, the low density of the gaseous
readout planes—even compensated by internal charge amplification—limits the
performance of such devices [29]. The low density has several disadvantages:
Landau fluctuations of the energy deposit in the active gaseous layers can be
comparable to the mean deposit and contribute to fluctuations at levels similar to
sampling fluctuations; low-energy shower-electrons may multiple-scatter into the
readout planes, where they may travel distances large compared to the gap thickness
of the active layer, resulting in path-length fluctuations. These effects are relatively
unimportant in dense materials, but may reach the level of Landau fluctuations
in gaseous readout. Soft particles in the shower will spiral in strong magnetic
fields, further increasing these path-length fluctuations. The absolute level of gas
amplification depends on external operating conditions (pressure, temperature,
gas composition) and is therefore difficult to control precisely. Variations of gas
amplification also contribute to worsening the resolution.
An illustration is the electromagnetic calorimeter of the Aleph experiment at
LEP [99]. The barrel part of the calorimeter consisted of 12 identical modules
surrounding the central tracking system (a Time Projection Chamber), immersed in
a solenoidal magnetic field of 1.5 T. The modules had 45 lead/wire-chamber layers
for a total of 22 X 0. The cathodes of the readout chambers were segmented into
pads of ~30 × 30 mm, providing energy and position information for each shower.
The calorimeter was operated with a xenon-CO 2 mixture to increase the density
of the active medium, thus reducing pathlength fluctuations. Wires connected to
the pads of each layer were brought to module edges, where they were grouped
into towers pointing to the vertex. The towers were segmented in three layers in
depth of 4, 9 and 9 X 0 , respectively. The connections of individual pads to the
module edges resulted in a large inductance and therefore limited the rise-time of the
readout signals (in the μs range). This was acceptable at LEP given the low event
rates. This calorimeter, segmented in 74,000 towers, had an energy resolution of
σ (E)/E = 0.18/
√
E ⊕ 1.9%, with E expressed in GeV (due to internal amplification,
the electronics noise term was negligible).
One of the weak points of this technique is the non-linearity of response. Test
beam studies showed that the energy E raw recorded for electromagnetic showers
needed to be corrected by:
E corr = E raw (1 + 0.00078 E raw (GeV)) ,
C. W. Fabjan and D. Fournier
6.3.4 Gas Detectors
Charge collection in gases, usually followed by some degree of internal amplification, forms the basis of another important category of ionization sampling
calorimetry. This method lends itself naturally to highly segmented construction
and has profited from the diversified developments of gaseous position detectors
(see Chap. 4). The relatively low costs of gaseous detectors favours their use in
large area applications such as calorimeters for neutrino physics.
While gaseous ionization calorimetry offers several of the advantages found in
ionization calorimetry with dense active materials, the low density of the gaseous
readout planes—even compensated by internal charge amplification—limits the
performance of such devices [29]. The low density has several disadvantages:
Landau fluctuations of the energy deposit in the active gaseous layers can be
comparable to the mean deposit and contribute to fluctuations at levels similar to
sampling fluctuations; low-energy shower-electrons may multiple-scatter into the
readout planes, where they may travel distances large compared to the gap thickness
of the active layer, resulting in path-length fluctuations. These effects are relatively
unimportant in dense materials, but may reach the level of Landau fluctuations
in gaseous readout. Soft particles in the shower will spiral in strong magnetic
fields, further increasing these path-length fluctuations. The absolute level of gas
amplification depends on external operating conditions (pressure, temperature,
gas composition) and is therefore difficult to control precisely. Variations of gas
amplification also contribute to worsening the resolution.
An illustration is the electromagnetic calorimeter of the Aleph experiment at
LEP [99]. The barrel part of the calorimeter consisted of 12 identical modules
surrounding the central tracking system (a Time Projection Chamber), immersed in
a solenoidal magnetic field of 1.5 T. The modules had 45 lead/wire-chamber layers
for a total of 22 X 0. The cathodes of the readout chambers were segmented into
pads of ~30 × 30 mm, providing energy and position information for each shower.
The calorimeter was operated with a xenon-CO 2 mixture to increase the density
of the active medium, thus reducing pathlength fluctuations. Wires connected to
the pads of each layer were brought to module edges, where they were grouped
into towers pointing to the vertex. The towers were segmented in three layers in
depth of 4, 9 and 9 X 0 , respectively. The connections of individual pads to the
module edges resulted in a large inductance and therefore limited the rise-time of the
readout signals (in the μs range). This was acceptable at LEP given the low event
rates. This calorimeter, segmented in 74,000 towers, had an energy resolution of
σ (E)/E = 0.18/
√
E ⊕ 1.9%, with E expressed in GeV (due to internal amplification,
the electronics noise term was negligible).
One of the weak points of this technique is the non-linearity of response. Test
beam studies showed that the energy E raw recorded for electromagnetic showers
needed to be corrected by:
E corr = E raw (1 + 0.00078 E raw (GeV)) ,