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 Z TFE = 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;  RC (R=50  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 The operating voltage is shifted depending on the SF6 quantity The current is strongly reduced Baseline gas Ar/TFE/i-but=76/20/4

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/ % 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/ % 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  (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

Threshold scan Ar/TFE/I-but=48/48/4 Ar/TFE/I-but=76/20/ % 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

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/ % 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