RPC2010- Darmstadt- 9/12-Feb p.1 M. Abbrescia – University and INFN Bari New gas mixtures for Resistive Plate Chambers operated in avalanche mode New gas mixtures for Resistive Plate Chambers operated in avalanche mode M. Abbrescia, V. Cassano, S. Nuzzo, G. Piscitelli, D. Vadruccio, F. Zaza

RPC2010- Darmstadt- 9/12-Feb p.2 M. Abbrescia – University and INFN Bari The problem RPC are operated (e.g. at the large LHC experiments) with the so-called “standard” mixture C 2 H 2 F 4 /i-C 4 H 10 /SF /3.5/0.3 Quite successful mixture but:  operating voltage close to 10 kV;  expensive power suppliers, connectors, etc.;  production of HF during operation;  issues for aging;  isobutane content close to flammability limit;  need for sensors and external storage.

RPC2010- Darmstadt- 9/12-Feb p.3 M. Abbrescia – University and INFN Bari The “ideal” solution It is well known that avalanche processes in RPCs depend on environmental conditions:  i.e. temperature and pressure  because they change gas density Ideally, operating RPC a ½ Atm would reduce of a factor 2 the operating voltage and meet at least 2 of the wishlist  Not practically feasible in large systems M. Abbrescia et al., NIM A 359 (1995),

RPC2010- Darmstadt- 9/12-Feb p.4 M. Abbrescia – University and INFN Bari The “smart” solution Helium is a noble gas with no vibrational or rotational degrees of freedom, with an ionization potential greater than C 2 H 2 F 4  Undergoes mainly elastic scattering with electrons;  Takes part only partially in the avalanche processes;  In first approximation behaves only like a space holder; Essentially reduces the partial pressure of the active mixture Effect similar (in first approximation) to operate at a reduced pressure

RPC2010- Darmstadt- 9/12-Feb p.5 M. Abbrescia – University and INFN Bari New mixtures containing He Standard mixture used as a reference… then  “A” mixture  62.5/2.3/0.2/35 C 2 H 2 F 4 /C 4 H 10 /SF 6 /He  “B” mixture  48.1/1.75/0.15/50 C 2 H 2 F 4 /C 4 H 10 /SF 6 /He  “C” mixture  48/1.7/0.3/50 C 2 H 2 F 4 /C 4 H 10 /SF 6 /He “A” and “B” essentially standard mix. + Helium “C” increased SF 6 fraction

RPC2010- Darmstadt- 9/12-Feb p.6 M. Abbrescia – University and INFN Bari Experimental set-up 10 × 10 cm 2 × 2 mm bakelite RPC The trigger and the entire system HV and electronics

RPC2010- Darmstadt- 9/12-Feb p.7 M. Abbrescia – University and INFN Bari Data acquisition and analysis Induced signals readout by oscilloscope and analyzed with MatLab dedicated software Signal Noise Trigger

RPC2010- Darmstadt- 9/12-Feb p.8 M. Abbrescia – University and INFN Bari Noise analysis Substantially independent of mixture and operating voltage RMS ~ 0,6 mV Noise determined using the first 50 ns of each waveforms Gaussianly distributed Average noise

RPC2010- Darmstadt- 9/12-Feb p.9 M. Abbrescia – University and INFN Bari Induced charge (1/2) A mixture (35% He) HV = 7700 V A mixture (35% He) HV = 7700 V A mixture (35% He) HV = 8200V A mixture (35% He) HV = 8200V  A lot (~hundred) of charge distributions taken,  at different HV  with different gas mixtures  Interesting “by themselves”  whole information from RPC stored  Useful for any kind of analysis “Avalanche” events “Streamer” events Threshold chosen

RPC2010- Darmstadt- 9/12-Feb p.10 M. Abbrescia – University and INFN Bari Induced charge (2/2) Very similar trend (slope) of Q ind vs. HV for different mixtures  where equal Q ind scales down with He increase Induced charge reported here for avalanche events only Earlier streamer “onset” with B and C mixtures

RPC2010- Darmstadt- 9/12-Feb p.11 M. Abbrescia – University and INFN Bari Efficiency ~800 V ~600 V ~400 V Operating point and useful plateau width scale down with increased He percentage  Scaling only roughly proportional to He percentage for high He fraction;  Useful plateau wider increasing SF6 fraction

RPC2010- Darmstadt- 9/12-Feb p.12 M. Abbrescia – University and INFN Bari Time properties A mixture HV = 7.7 kV RMS = 1.8 ns  Gaussian distribution of time response  Time delays and resolution (roughly) scale with He percentage  Time resolution at plateau around 1.4 ns 1.4 ns 1 ns/100 V

RPC2010- Darmstadt- 9/12-Feb p.13 M. Abbrescia – University and INFN Bari Correlation studies (1/2) Induced charge directly related to efficiency (rather obvious…) and… To better compare performance with different mixture use efficiency as independent variable Identical behaviour = Curves superimposed (regardless the HV) Identical behaviour = Curves superimposed (regardless the HV) Q ind linearly increasing as efficiency increases Q ind vs. efficiency All events over thr., constant eff, Q ind increas

RPC2010- Darmstadt- 9/12-Feb p.14 M. Abbrescia – University and INFN Bari Correlation studies (2/2) Time properties substantially independent of gas mixtures once the operating point has been set Time resolution vs. efficiency Time delay vs. efficiency At full efficiency all mixtures with time resolution in the ns range

RPC2010- Darmstadt- 9/12-Feb p.15 M. Abbrescia – University and INFN Bari Conclusions It is a “smart” solution and works  general idea verified  corrections at high He content  inelastic scattering, Penning effect, etc. New gas mixtures allow:  reduction of the operating voltage (2-3 kV at least)  reduced flammability problems (increase i-C 4 H 10 ?)  good (> 95%) efficiency and time resolution (< 2ns) Good starting point for studies in this field definition for a new “standard” mixture