tests of rpcs (resistive plate chambers) for the argo experiment at ybj g. aielli¹, p.camarri¹, r....
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Tests of RPCs (Resistive Plate Chambers) for the Tests of RPCs (Resistive Plate Chambers) for the ARGO experiment at YBJARGO 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 2²Università 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 astrophysics)
• Direction of primaries reconstructed from the time profile of the shower front (time resolution ~ ns required)
• Discrimination /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/i-C4H10=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
• 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 • RC » streamer duration (20 ns),
so big pads integrate the signal: amplitude proportional to Q/C and exponential tail with =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
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