4 Gaseous Detectors
129
4.3.4 Parallel Plate Geometries, Resistive Plate Chambers
(RPCs)
Parallel plate devices offer fast response, as there is no drift delay and the avalanche
amplification starts immediately.
Keuffel’s spark counter [6] featured two metal electrodes at millimeter distance
in a gaseous atmosphere, where the primary electrons deposited in the gap provoke
a fast discharge and therefore a detectable signal. By the end of the 1960s the spark
counters had arrived at time resolutions around 100 ps, the rates however were
limited to 1 kHz and small areas of about 30 cm 2 , since after each discharge the
entire counter was insensitive during the recharge time of typically a few hundred
microseconds. Parallel Plate Chambers (PPCs) use the same geometry but operate
below the discharge voltage. The avalanche therefore induces a signal but does not
create a discharge, allowing a rate capability of 100 kHz for a 80 cm 2 detector
[6]. Still, the fact that the detector mechanics and especially the detector boundaries
have very carefully controlled to ensure stability, limits this detector to a rather small
surface.
The Pestov spark counter [66] uses the same parallel plate geometry, with one
electrode made from resistive material having a volume resistivity of ρ = 10 9 − 10 10
cm. The charge deposited locally on this resistive layer takes a time of τ ≈ ρε to
be removed, where ε is the permittivity of the resistive plate. This time is very long
compared to the timescale of the avalanche process, the electric field is therefore
reduced at the location of the avalanche, avoiding a discharge of the entire counter.
This allows stable operation of the detector at very high fields and particle rates.
A counter with a size of 600 cm 2 and a gas gap of 1 mm, operated at atmospheric
pressure achieved a time resolution of <0.5 ns and efficiency of 98%. By decreasing
the size of the gas gap to 0.1 mm and operating the detector at 12 bar pressure, a
time resolution of 27 ps was achieved with this detector [67].
Resistive Plate Chambers (RPCs) [68] are building on this same principle and
they are widely used as trigger detectors and for time-of-flight measurements, as
they allow relatively cheap large area construction. Large detector systems of several
hundred m 2 surface have been built with Bakelite plates (ρ = 10 10 − 10 12 cm) or
window glass (ρ = 10 12 − 10 13 cm). Tetrafluorethane (C 2 F 4 H 2 ) is nowadays
widely used as the main component of the RPC gas mixture due to the large number
of primary ionization clusters (8–10/mm) leading to large detection efficiency
and due to it’s electronegativity that reduces the probability for the formation of
streamers. Small additions of SF 6 are further reducing this streamer probability.
The time resolution for RPCs is given by σ t ≈ 1.28/αv, where α is the effective
Townsend coefficient of the gas mixture and v is the drift-velocity of the electrons.
RPCs with a single gas gap of 2 mm at a field of 50 kV/cm (α ≈ 10/mm,
v ≈ 130 μm/ns) provide a time resolution of ≈1 ns and efficiency close to 100%.
Figure 4.28a shows the geometry as used for the muon system of the ATLAS
experiment. In addition to collider experiments, similar geometries are used as
129
4.3.4 Parallel Plate Geometries, Resistive Plate Chambers
(RPCs)
Parallel plate devices offer fast response, as there is no drift delay and the avalanche
amplification starts immediately.
Keuffel’s spark counter [6] featured two metal electrodes at millimeter distance
in a gaseous atmosphere, where the primary electrons deposited in the gap provoke
a fast discharge and therefore a detectable signal. By the end of the 1960s the spark
counters had arrived at time resolutions around 100 ps, the rates however were
limited to 1 kHz and small areas of about 30 cm 2 , since after each discharge the
entire counter was insensitive during the recharge time of typically a few hundred
microseconds. Parallel Plate Chambers (PPCs) use the same geometry but operate
below the discharge voltage. The avalanche therefore induces a signal but does not
create a discharge, allowing a rate capability of 100 kHz for a 80 cm 2 detector
[6]. Still, the fact that the detector mechanics and especially the detector boundaries
have very carefully controlled to ensure stability, limits this detector to a rather small
surface.
The Pestov spark counter [66] uses the same parallel plate geometry, with one
electrode made from resistive material having a volume resistivity of ρ = 10 9 − 10 10
cm. The charge deposited locally on this resistive layer takes a time of τ ≈ ρε to
be removed, where ε is the permittivity of the resistive plate. This time is very long
compared to the timescale of the avalanche process, the electric field is therefore
reduced at the location of the avalanche, avoiding a discharge of the entire counter.
This allows stable operation of the detector at very high fields and particle rates.
A counter with a size of 600 cm 2 and a gas gap of 1 mm, operated at atmospheric
pressure achieved a time resolution of <0.5 ns and efficiency of 98%. By decreasing
the size of the gas gap to 0.1 mm and operating the detector at 12 bar pressure, a
time resolution of 27 ps was achieved with this detector [67].
Resistive Plate Chambers (RPCs) [68] are building on this same principle and
they are widely used as trigger detectors and for time-of-flight measurements, as
they allow relatively cheap large area construction. Large detector systems of several
hundred m 2 surface have been built with Bakelite plates (ρ = 10 10 − 10 12 cm) or
window glass (ρ = 10 12 − 10 13 cm). Tetrafluorethane (C 2 F 4 H 2 ) is nowadays
widely used as the main component of the RPC gas mixture due to the large number
of primary ionization clusters (8–10/mm) leading to large detection efficiency
and due to it’s electronegativity that reduces the probability for the formation of
streamers. Small additions of SF 6 are further reducing this streamer probability.
The time resolution for RPCs is given by σ t ≈ 1.28/αv, where α is the effective
Townsend coefficient of the gas mixture and v is the drift-velocity of the electrons.
RPCs with a single gas gap of 2 mm at a field of 50 kV/cm (α ≈ 10/mm,
v ≈ 130 μm/ns) provide a time resolution of ≈1 ns and efficiency close to 100%.
Figure 4.28a shows the geometry as used for the muon system of the ATLAS
experiment. In addition to collider experiments, similar geometries are used as