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Study of gas mixture containing SF6 for the OPERA RPCs A. Paoloni, A. Mengucci (LNF) 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 • ZTFE = 50 : high primary ionisation better intrinsic efficiency and time resolution Unfortunately…. TFE + discharges in gas HF production, aggressive to electrodes surface Detector damaging 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 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; t=RC (R=50 W) • Charge-per-count=current/rate • Prompt charge measured integrating the average pad signal acquired with a scope Decreasing TFE without SF6 4% iso-butane fixed Lower operating voltage, but higher Currents !!! Very low ohmic currents current only due to discharges in the gas The effect of SF6 Baseline gas Ar/TFE/i-but=76/20/4 • The operating voltage is shifted depending on the SF6 quantity • The current is strongly reduced • The effect seems to be independent from the SF6 quantity • Also the prompt charge is reduced 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….. 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 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 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 W (to simulate OPERA read-out) • In order to measure efficiencies, the signals are taken from the 110 W side and sent to a discriminator with 50 W input impedance Threshold scan 200 V inside plateau Ar/TFE/I-but=48/48/4 150 V inside plateau Threshold=50 mV To be scaled by ~3 because of the discr. Rin (for OPERA read-out comparison) Ar/TFE/I-but=76/20/4 + 0.5% SF6 320 V inside plateau 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