Tests of RPCs (Resistive Plate Chambers) for the ARGO experiment at YBJ

1. 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

2. 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

3. 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)

4. 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

5. 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

6. Cosmic ray tests at sea level

7. 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

8. 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)

9. 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

10. Time resolution (I) • Strip to strip time of flight distribution between 2 RPCs operated at 9.7 kV

11. 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

12. 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

13. 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

14. 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)

15. Intrinsic noise studies

16. Read out (I) • Performed with small pads • thickness = few mm • area ~ 10 cm²

17. Read out (II) • In the previous schematisation, C ~ pF and R = 50 ohm • RC << streamer duration

18. 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

19. 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

20. 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