1. Study of gas mixture containing SF6 for the OPERA RPCs A.Paoloni, A. Mengucci (LNF)

2. The present gas mixture for streamer mode operation Ar/TFE/i-but = 48/48/4 Argon : high charge multiplication at low voltages Iso-butane : quenching, but limited by flammability requirements Tetrafluoroethane : electronegative gas, good quenching power Z TFE = 50 : high primary ionisation  better intrinsic efficiency and time resolution

3. Unfortunately…. TFE + discharges in gas HF production, aggressive to electrodes surface Detector damaging

4. A possible solution Decrease TFE and increase Ar, with fixed i-butane quantity: 1.The operating voltage decreases, and so does the streamer charge 2.The loss of quenching power can be compensated with the addition of little quantities ( <1% ) of SF6, a strongly electronegative gas already used for RPCs working in avalanche mode 3.Lower charge released in the gas + less TFE  much less HF  less chamber damaging 4.Moreover the gas mixture is less expensive

5. Experimental Set-up M1, M2, M3 trigger RPCs (50*50) cm², flushed with the standard gas mixture RPC1, (60*70) cm², under test with different gas mixtures RPC1 current read out with the scope on a resistor placed in the HV circuit RPC signals are read out with a single pad covering the entire surface (Threshold=45 mV) Signal shape: RC > streamer duration  peak=Q/C;  RC (R=50  Charge-per-count=current/rate Prompt charge measured integrating the average pad signal acquired with a scope

6. Decreasing TFE without SF6 4% iso-butane fixed Lower operating voltage, but higher Currents !!!

7. Very low ohmic currents  current only due to discharges in the gas

8. The effect of SF6 The operating voltage is shifted depending on the SF6 quantity The current is strongly reduced Baseline gas Ar/TFE/i-but=76/20/4

9. The effect seems to be independent from the SF6 quantity Also the prompt charge is reduced

10. Decreasing TFE with SF6 4% iso-butane and 1% SF6 fixed From the point of view of the current, the chamber behaviour seems to be independent from the TFE concentration in a wide range…..

11. Final comparison (I) Ar/TFE/I-but=48/48/4 Ar/TFE/I-but=76/20/4 + 0.5% SF6 The operating point is about 2 kV lower

12. Final comparison (II) Ar/TFE/I-but=48/48/4 Ar/TFE/I-but=76/20/4 + 0.5% SF6 No loss of efficiency with half charge-per-count and prompt charge

13. Strip signals study Cosmic rays are triggered with two scintillators RPC strips are terminated on their characteristic impedance only on one side; on the other side they are terminated on 110  (to simulate OPERA read-out) In order to measure efficiencies, the signals are taken from the 110  side and sent to a discriminator with 50  input impedance

14. Threshold scan Ar/TFE/I-but=48/48/4 Ar/TFE/I-but=76/20/4 + 0.5% SF6 Threshold=50 mV To be scaled by ~3 because of the discr. R in (for OPERA read-out comparison) 200 V inside plateau 150 V inside plateau 320 V inside plateau

15. Conclusions HF produced by discharges in gases containing TFE can damage electrodes surface TFE concentration can be reduced compensating the loss of quenching power by adding little quantities (<1%) of SF6, a strongly electronegative gas With a gas mixture Ar/TFE/i-but=76/20/4 + 0.5% SF6 the operating voltage is ~2 kV lower than with the standard mixture (Ar/TFE/i-but=48/48/4) No loss of efficiency is observed and both the current and the prompt charge are about one half with respect to the standard mixture More to come: time resolution and afterpulses studies