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  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

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: C 2 H 2 F 4 /Ar/i- C 4 H 10 =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/T 0 ) ×(P 0 /P) ; (T 0 =20 °C and P 0 =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  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

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