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Abstract-- Resistive Plate Chambers are rugged and
affordable gas detectors that have found extensive use in High
Energy Physics and Astroparticle experiments. The main
features of these counters are the very large pulse height,
reduced cost per unit area and good (about 1 ns) time
resolution.
The field has enjoyed very lively progress in recent years,
including the introduction of a new (avalanche) mode of
operation, extension of the counting rate capabilities to levels
around 10 MHz/cm
2
, improvement of the time resolution for
MIPs to 50 ps
s
and the achievement of position resolutions of
a few tens of µm.
These new developments have extended the range of HEP
applications and promise new applications in medical imaging.
I.
I
NTRODUCTION
Resistive Plate Counters (RPCs) were introduced in 1981 [1]
as a practical alternative to the remarkable "Localized
discharge spark counters" [2], which ultimately achieved a
time resolution of 25 ps
s
[3]. The resulting detector, being
by construction free from damaging discharges and enjoying
a time resolution of the order of 1 ns, has found very good
acceptance in High Energy and Astroparticle Physics.
In modern language the original RPCs were single-gap
counters operated in streamer mode. Soon the double-gap
structure was introduced [4] to improve the detection
efficiency along with the avalanche mode of operation [5],
which extends its counting rate capabilities.
An imaginative construction method, denominated
"multigap RPC" was introduced in 1996 [6], being specially
suited for the construction of counters with more than a
single gas gap.
Recent innovations in detector construction and readout
electronics have extended the timing resolution of RPCs for
minimum ionizing particles (MIPs) to 50 ps
s
[7], the rate
capability to
5
2
10 Hz/mm
[8] and the position resolution for
X-rays to 30 µm FWHM in digital readout mode [9].
Single and double-gap streamer-mode RPCs have so far
found application in cosmic ray experiments, like
COVER_PLASTEX [10] and EAS-TOP [11] being also used
in the High Energy Physics experiments L3 at CERN,

Manuscript received November 25, 2001. This work was supported by
Fundação para a Ciência e Tecnologia in the framework of the project
CERN/P/FIS/40111/2000.
P. Fonte is with ISEC and LIP, Coimbra University, Coimbra P-3000,
Portugal (e-mail: .fonte@lipc.fis.uc.pt).
BABAR at SLAC, USA and BELLE at KEK, Japan. Future
applications will include the ARGO experiment at the
"YangBaJing High-altitude Cosmic Ray Laboratory" [12]
and the OPERA [13] and MONOLITH [14] cosmic ray
experiments in LNGS, Italy. The Muon Arm of the ALICE
experiment at LHC [15] will also be equipped with
streamer-mode RPCs.
Avalanche-mode RPCs will be used for the muon trigger
systems of the ATLAS [16], CMS [17] and LHCb [18]
experiments at LHC.
Timing RPCs, a recent development [19], are already in use
by the HARP experiment at the CERN PS accelerator [20]
and will equip the 160
2
m
TOF barrel of ALICE's Particle
Identification Detector [21].
II. RPC
DESIGNS

The combination of resistive and metallic electrodes with
signal-transparent semi-conductive layers, highly isolating
layers and different kinds of pickup electrodes endows the
RPCs with a rich variety of configurations, tunable to a
variety of requirements.
A. Single gap
The original RPC design [1], included a single gas gap
delimited by bakelite resistive electrodes. Naturally the
counter design has evolved since then and a modern example
is shown in Fig. 1.
The application of the polarizing potential to the resistive
electrode via an electrode with a lower resistivity, but still
transparent to the induced signals (see for instance [22],
[23]) allows to operate both signal pickup electrodes at
ground potential, saving the utilization of high-voltage
capacitors and avoiding the need for high voltage insulation
of the strips.
Glass electrodes, enjoying a mechanical stiffness and surface
quality much superior to bakelite, have also been considered
in the past and remain in use today (eg. [2], [24], [25]).
B. Double gap
Double-gap designs [4], having a larger number of elements
(gas gaps, pickup electrodes), allow for more varied
structures than the single gap ones and two such designs are
presented as examples in Fig. 2.
C. "Multigap"
A construction method denominated "multigap RPC" was
introduced in 1996 [6], being especially well suited for the
Applications and new developments in Resistive
Plate Chambers

P. Fonte P.Fonte, presented at the 2001 IEEE Nuclear Science Symposium, submitted to IEEE Trans. Nucl. Sci.
2
construction of counters with more than a single gas gap. A
schematic drawing is shown in Fig. 3.
The most preeminent feature of this design is the inclusion
of resistive, electrically floating, electrodes that divide the
gas volume into a number of individual gas gaps, without the
need of any conductive electrodes. According to its inventors
the steady-state requirement for a null total current on each
of the dividing electrodes stabilizes their potential at a value
that equalizes the currents flowing in and out by adjusting
the gas gain in the neighboring gaps.
Possible drawbacks of this design are the large voltages
required and the fact that at low ionizing particle fluxes the
stabilizing mechanism may be dominated by the dark
counting rate.
D. Hybrid
designs
Metallic and resistive electrodes may be combined and still
retain the main property of the RPC: the total absence of
violent discharges. The only requirement is that no gas gap
will be delimited by two metallic electrodes.
Actually the gas counter formed by two parallel metallic
electrodes defining a gas gap is denominated Parallel Plate
Chamber (PPC), which has found wide application in the
detection of heavy ions and, with wire-mesh electrodes,
proposed long ago for high-rate applications [26]. However,
possibly due to the violent nature of the discharges, this type
of counter never found wide acceptance in HEP.
A few schematic examples of hybrid RPCs can be seen in
Fig. 4.
III. M
ODES OF
O
PERATION

A. Nature of the operation modes
RPCs may be operated in avalanche mode or discharge
mode.
The avalanche mode corresponds to the generation in the gas
gap of a Townsend avalanche, following the release of
primary charge by the incoming ionizing radiation.
In discharge mode the avalanche is followed by a "streamer"
discharge [27]. In a metallic counter the discharge will
evolve via a sequence stages comprising avalanche,
streamer, glow discharge, filamentary discharge and spark
[28]. However the later discharge stages require a
considerable current, up to few Amperes, to flow in the gap,
which is forbidden by the high resistivity of the RPC
electrodes.
Optical observations suggest that in glass RPCs the
discharge is quenched at the filamentary discharge stage
([29], [30]). However, for electrode resistivities of the order
of
7
4 10
cm
× , a permanent glow discharge could
sometimes be observed [8].
B. Space charge effect
The space charge present in a Townsend avalanche creates
its own electric field that is summed to the applied electric
field. Three regions can be identified: the total field is larger
than the applied field immediately upstream and downstream
from the avalanche and lower than the applied field over the
main avalanche body [31]. For avalanches approaching
8
10
electrons (the Raether limit [27]), the high-field regions
generate the conditions for the development of the cathode
and anode (backward and forward) streamers, while the
lower field region causes the reduction of gas gain seen by
the avalanche (avalanche saturation effect) [27]. Both
phenomena, avalanche saturation and streamers, share a
common physical origin and are normally simultaneously
present.
The existence of a strong avalanche saturation effect in RPCs
has been experimentally verified and has been shown to play
a central role in the interpretation of the charge spectra and
efficiency characteristics, both for millimetric ([32], [33])
and sub-millimetric ([34], [35]) gas gaps.
C. Available signal
The charge signal ranges from a few pC for the fast
(electron) component of the signal in avalanche mode to a
range between 50 pC [36] and a few nC ([37] for instance) in
streamer mode.
Naturally, due to the high resistivity of the electrodes, there
is a tradeoff between the available signal charge and the
counting rate capability (see section V).
D. Gas mixtures
Modern standard RPCs working in avalanche mode use
mostly mixtures of tetrafluoroethane (
2
2 4
C H F
) with 2 to 5%
of isobutane (iso-
4
10
C H
) and 0.4% to 10% of sulphur
hexafluoride (
6
SF
). The addition of
6
SF
has been shown to
extend the streamer-free operation region and to reduce the
amount of charge in the streamer ([32], [38]).
In streamer mode mixtures of argon with isobutane and
tetrafluoroethane in widely varying proportions tend to be
used.
It was recently shown the addition of
6
SF
(4%) to the
remaining constituents allowed to reduce the streamer
charge to 50 pC and extend the counting rate capability to
300 Hz/cm
2
, while keeping efficient streamer-mode
operation [36].
IV. E
FFICIENCY FOR
MIP
S

In Fig. 5 a survey of recently published results concerning
the effici