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 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 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