Tests of RPCs (Resistive Plate Chambers) for the
ARGO experiment at YBJ
G. Aielli¹, P.Camarri¹, R. Cardarelli¹, M. Civardi²,
L. Di Stante¹, B. Liberti¹, A. Paoloni¹, E. Pastori¹,
R. Santonico¹
¹Università di Roma “Tor Vergata” and I.N.F.N. sez. Roma 2
²Università statale di Milano
On behalf of ARGO Collaboration
7th International Conference on Advanced Technology and
Particle Physics
Villa Olmo, October 15-19, 2001
ARGO Experiment (I)
• ARGO is an Extensive Air Showers detector being
installed in YBJ laboratories (4300 m a.s.l., Tibet, P.R.
China)
• Energy reconstructed from hit multiplicity (in the range
100 GeV to several TeV for g astrophysics)
• Direction of primaries reconstructed from the time profile
of the shower front (time resolution ~ ns required)
• Discrimination g/p from particle density
Such a low energy thresold for an EAS detector is obtained
with:
 High altitude
 Full coverage over 74×78 m²
ARGO Experiment (II)
Hit multiplicity measurements:
• # pads (56×64 cm², the basic readout cell)
• # strips (8 for each pad with pitch=7 cm)
• Analog signal on 1.4×1.28 m² ‘‘big pad’’ electrodes (energies > 10 TeV)
RPC description
• RPC are gaseous ionisation
detectors with parallel resistive
electrodes
• ARGO gas mixture: C2H2F4 /Ar/iC4H10=75/15/10 (to be operated at
600 mbar)
• Gas gap thickness: 2 mm
• Electrodes are 2 mm thick and made
of phenolic/melaminic polymers
• High Voltage (about 7 kV) applied
on a graphite layer (E=3.5 kV/mm)
• Gap uniformity due to a 10 cm pace
polycarbonate spacer lattice
• Signal picked up with strips or pads
Operating principles
• In absence of ionisation in the gas, the voltage
is applied entirely on the gas gap
• In presence of a discharge in the gas, the
voltage is transferred to the resistive plates
• Because of the high resistivity of the electrodes,
the RPC is divided into a large number of small
discharge cells of area ~ 1 cm²
• Time resolution ~ 1 ns due to the uniformity of
the electric field
Cosmic ray tests at sea level
Experimental lay-out
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Four RPCs with strip pitch 3 cm are used for tracking cosmic rays
Trigger area = (50×50) cm²
Tracking resolution = 1 cm
Operating voltages rescaled: Vr=Vop ×(T/T0) ×(P0/P) ; (T0=20 °C and
P0=1010 mbar)
• Expected conditions at YBJ : T=8÷25 °C and P=600 mbar
Efficiency
• Measured efficiencies for 10
different pads with 500 mV
threshold on amplified signals
• 60% efficiency voltage
dispersion: ±75 V on 8.8 kV
(spacers thickness tolerance:
±15 micron on 2 mm)
Cluster size
• Cluster size distributions for different operating voltages: a) 8.8 kV,
b) 9.0 kV, c) 9.4 kV, d) 9.6 kV
• Events with cluster size > 2 in d) are less than 3% of the total
• Cluster sizes = 2 events are due to particles crossing the detector in the
interstices between adiacent strips
Time resolution (I)
• Strip to strip time of flight
distribution between 2 RPCs
operated at 9.7 kV
Time resolution (II)
• The time resolution improves with increasing voltages
• Inside the efficiency plateau it reaches values ~ 1 ns
• The dispersion on arrival time among the 8 strips considered is ~1ns
Pads counting rate
• Almost threshold independent
because of saturated signals in
streamer operation
• Low counting rate => lower
energy threshold
• Charged part rate=130 Hz/m²
(coincidence between 2 RPCs
overlapped)
• Counting rate measured at YBJ
~1.3 kHz/m²
Big Pad Readout
• Read out on HV side performed
with 200 µm thick “big pads”,
covering half detector (1.4×1.28
m²)
• C ~100 nF and R=50 W
• RC » streamer duration (20 ns),
so big pads integrate the signal:
amplitude proportional to Q/C
and exponential tail with t=RC
• Big Pad is a powerful tool for
measuring particle densities of
very high energy showers
Operating current
• The current exhibits a linear
behaviour at low voltages, with
a slope increasing with the
temperature
• dI/dV < 35 nA/kV (T=32°C)
• At higher voltages, in presence
of charge multiplication in the
gas, the current increases
exponentially
• At operating voltages I < 4 µA
• Charge-per-count ~ 600 pC
(obtained from the ratio
between the current and the
counting rate)
Intrinsic noise studies
Read out (I)
• Performed with small pads
• thickness = few mm
• area ~ 10 cm²
Read out (II)
• In the previous schematisation,
C ~ pF and R = 50 ohm
• RC << streamer duration
Spacers (I)
• Policarbonate spacers ensure gas gap uniformity
• Spacers have a cilindric body with 4 mm radius, surrounded by a 12
mm diameter guard ring
• Distance between contiguous spacers = 10 cm
Spacers (II)
• Pad diameter = 4 cm; V=9.6 kV; threshold=200 mV
• By comparing the distributions it is evident that spacers are a potential
source of noise
Conclusions
• ARGO is an EAS detector which, due to high altitude and
full coverage, is characterized by an energy threshold of
100 GeV, accessible to satellite experiments
• Resistive Plate Chambers are well suited for
implementation because of their time resolution ( ~ 1 ns),
robustness and low cost
• Tests on prototypes, performed at sea level with cosmic
rays and reported on this presentation, confirm the above
statement
• Small pads pick-up is an original and powerful tool for
noise investigations on the cm² scale
• ARGO time schedule: full installation within 3 years