tests of rpcs (resistive plate chambers) for the argo experiment at ybj g. aielli¹, p.camarri¹, r....
DESCRIPTION
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)TRANSCRIPT
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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
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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
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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)
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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
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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
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Cosmic ray tests at sea level
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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
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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)
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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
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Time resolution (I)
• Strip to strip time of flight distribution between 2 RPCs operated at 9.7 kV
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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
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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
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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
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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)
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Intrinsic noise studies
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Read out (I)
• Performed with small pads• thickness = few mm• area ~ 10 cm²
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Read out (II)
• In the previous schematisation, C ~ pF and R = 50 ohm
• RC << streamer duration
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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
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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
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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