4 Gaseous Detectors
131
trigger or veto detectors in neutrino experiments like OPERA [69] and Daya Bay
[70] and as large area cosmic ray detectors like ARGO [71].
Using a small gas gap of 0.25–0.3 mm with a field around 100 kV/cm
(α ≈ 113/mm, v ≈ 210 μm/ns) results in a time resolution of ≈50 ps, making the
detector well suited for time-of-flight measurements. The reduced efficiency due to
the narrow gas gap is overcome by using a multi-gap structure [83]. Figure 4.28b
shows the geometry as used for the time of flight system of the ALICE experiment.
The avalanche process in RPCs is significantly affected by spacecharge effects.
After the initial exponential increase of the electron number, the ions produced in
the avalanche are significantly reducing the electric field and therefore resulting in
strong slow down of the avalanche growth. This results in moderate signal charges
in the pC range even for very large Townsend coefficients [72].
The rate capability of RPCs is defined by the thickness d of the resistive plates
and their volume resistivity ρ. The current I produced per unit area inside the
gas gap is flowing through these plates, which results in an effective voltage drop
of V = Iρd across a single plate. The rate limit of the RPC is reached at the
point where the effective voltage across the gas gap moves outside the efficiency
plateau. For the values and geometries quoted above, this limit is in the range of
10–1000 Hz/cm 2 .
4.3.5 Micropattern Devices
The constantly increasing particle rates and track densities in modern day experiments exceed the capabilities of standard gaseous detectors. Semiconductor technology dominates this regime. On the other hand, numerous novel designs of gaseous
detectors have been studied. Two have emerged and attract much attention, the socalled GEM and Micromegas devices. Offering small ExB track distortions and low
ion feedback, they are also being used for the readout of TPCs.
4.3.6 Gas Electron Multiplier (GEM)
In a thin metal-coated polymer foil, holes are chemically etched at high special
density [73], see Fig. 4.29. A voltage applied to the metal layers produces gas
amplification in the holes. Typical parameters are: Foil thickness = 50 μm, inner
hole diameter = 70 μm, hole pitch = 140 μm, voltage = 400 V. To achieve a
practical gas gain of the order of 10 4 − 10 5 with an acceptable low discharge
probability, usually three GEMs are put in series. In COMPASS [74], a system of 20
triple-GEMs with an active area of 31 × 31 cm 2 each was operated in a very high
intensity muon beam. With 2-D readout via superposed orthogonal strips, a space
resolution of 70 μm was achieved at rates up to 2.5 MHz/cm 2 . The efficiency was
99% with 50 ns pulse shaping at an effective gain of 8000.
131
trigger or veto detectors in neutrino experiments like OPERA [69] and Daya Bay
[70] and as large area cosmic ray detectors like ARGO [71].
Using a small gas gap of 0.25–0.3 mm with a field around 100 kV/cm
(α ≈ 113/mm, v ≈ 210 μm/ns) results in a time resolution of ≈50 ps, making the
detector well suited for time-of-flight measurements. The reduced efficiency due to
the narrow gas gap is overcome by using a multi-gap structure [83]. Figure 4.28b
shows the geometry as used for the time of flight system of the ALICE experiment.
The avalanche process in RPCs is significantly affected by spacecharge effects.
After the initial exponential increase of the electron number, the ions produced in
the avalanche are significantly reducing the electric field and therefore resulting in
strong slow down of the avalanche growth. This results in moderate signal charges
in the pC range even for very large Townsend coefficients [72].
The rate capability of RPCs is defined by the thickness d of the resistive plates
and their volume resistivity ρ. The current I produced per unit area inside the
gas gap is flowing through these plates, which results in an effective voltage drop
of V = Iρd across a single plate. The rate limit of the RPC is reached at the
point where the effective voltage across the gas gap moves outside the efficiency
plateau. For the values and geometries quoted above, this limit is in the range of
10–1000 Hz/cm 2 .
4.3.5 Micropattern Devices
The constantly increasing particle rates and track densities in modern day experiments exceed the capabilities of standard gaseous detectors. Semiconductor technology dominates this regime. On the other hand, numerous novel designs of gaseous
detectors have been studied. Two have emerged and attract much attention, the socalled GEM and Micromegas devices. Offering small ExB track distortions and low
ion feedback, they are also being used for the readout of TPCs.
4.3.6 Gas Electron Multiplier (GEM)
In a thin metal-coated polymer foil, holes are chemically etched at high special
density [73], see Fig. 4.29. A voltage applied to the metal layers produces gas
amplification in the holes. Typical parameters are: Foil thickness = 50 μm, inner
hole diameter = 70 μm, hole pitch = 140 μm, voltage = 400 V. To achieve a
practical gas gain of the order of 10 4 − 10 5 with an acceptable low discharge
probability, usually three GEMs are put in series. In COMPASS [74], a system of 20
triple-GEMs with an active area of 31 × 31 cm 2 each was operated in a very high
intensity muon beam. With 2-D readout via superposed orthogonal strips, a space
resolution of 70 μm was achieved at rates up to 2.5 MHz/cm 2 . The efficiency was
99% with 50 ns pulse shaping at an effective gain of 8000.