Performance and Aging Studies of BaBar RPC’s Henry R. Band University of Wisconsin for the BaBar IFR-RPC group RPC 2005 VIII Workshop On Resistive Plate Chambers and Related Detectors Seoul, Korea

Oct. 11, 2005H. R. Band 2 BaBar Resistive Plate Chambers I nstrumented F lux R eturn BaBar has > 650 bakelite RPCs –Streamer mode –60% Ar, 35.5% C 2 H 2 F 4,4.5% C 4 H 10 Original BaBar RPCs –Since 1998 –Multiple problems Growing currents & declining efficiencies 1/3 of barrel replaced in 2004 with LSTs Forward Endcap RPC’s –Replaced in 2002 as part of upgrade Improved QA, graphite –Operating Conditions –Backgrounds & rates –Current, efficiency, and rate trends –Humidity study –Small radii inefficiency –Fluorine measurements –Avalanche mode tests

Oct. 11, 2005H. R. Band 3 RPC Efficiency from  pairs Barrel and Bkwd RPC efficiency continue slow decline Most forward RPCs have stable/high efficiency Higher rate Forward Endcap RPCs show decline –Mix of known and unknown causes –Concern about long term performance All Forward Endcap FEC layers 1-12 Barrel RPCs Backward Endcap Nov.1, 1999 Run 1 Run 2 Run3 Run 4 Run

Oct. 11, 2005H. R. Band 4 Original Production RPCs Original RPCs –Constructed at General Tecnica Factory mm gas gap with polycarbonate spacers Bakelite electrode –  V =  cm Graphite paint –  S = 100 K  /  Exhibit many failure modes –Nearly all RPCs show a slow decline in efficiency (linseed oil, debrii) –Many also suffer a complete efficiency loss in 1-2 months (graphite) –Correlated with position in gas chain (barrel 3 RPCs in series) First = 52% Middle = 39% last = 26%

Oct. 11, 2005H. R. Band 5 Layer 13,14 Layer 15,16 Upgrade –Added 2 of brass/steel absorber –Double gaps in outer layers –New RPCs Molded gas inlets Thinner linseed oil Improved graphite coating Stringent QC/QA –Increased gas flows 4-6 vol/day –Software improvements – Kalman track fitter & NeuralNet muon selection algorithm Forward Endcap Upgrade 2002

Oct. 11, 2005H. R. Band 6 IFR Backgrounds & rates Signal and background rates vary widely with position Shield wall + local lead added for Run 5 Shielding Wall High rate around beamline in inner layers L ~11-20 Hz cm 2 Backgrounds more uniform in outer layers, much reduced in recent run by trickle injection and shield wall ~ 1-4 Hz cm 2

Oct. 11, 2005H. R. Band 7 Forward RPCs Previous RPC problems were signaled by increased i & rates and then loss of efficiency. Many new chambers also show a systematic increase in current & rates Correlated with position and noise rate. Rate dependent inefficiency seen –Some due to lack of humidity –Others – integrated rate ? Other RPCs are stable Small number of RPCs have gas flow or HV problems (6%) Rates, currents, efficiency, temperature, and gas flow monitored and recorded 12 RPC HV modules per layer, grouped into 6 gas volumes WestEast Gas flow B A C D

Oct. 11, 2005H. R. Band 8 Trends Top-6 & bottom-1 RPCs of inner layers have very low rates Currents, rates, & efficiency stable WestEast West Current East Rate Red – beam Blue - cosmic Efficiency Nov, 02 days  days  Layer 2 Rate (Hz) I (  A) Efficiency

Oct. 11, 2005H. R. Band 9 Trends(2) RPCs in positions 3&4 show clear aging Module 4 has large increases in I and rate Sensitive to gas flow changes Module 3 very stable Both are now ~80% efficient WestEast Current Rate Red – beam Blue - cosmic Efficiency Nov, 02 days  days  Layer 1 west Rate (Hz) I (  A) Efficiency Layer 3 west Gas flow reversed

Oct. 11, 2005H. R. Band 10 Trends(3) Outer Layers Backgrounds much reduced from peak rates of >200 kHz, 200  A Rate (>10 Hz/cm 2 ) induced inefficiencies No significant increase in cosmic rates and current WestEast Current Rate Red – beam Blue - cosmic Efficiency Nov, 02 days  days  Layer 14 west Rate (Hz) I (  A) Efficiency Layer 14 east Trickle injection

Oct. 11, 2005H. R. Band 11 FW EC efficiency – RUN 5 HV sparks FWM-15. leaky chamber FET-16. FWM-4 inner ring FWM-5. leaky chamber FWM-3. geometry +inner ring FEM-15. inner ring Most top or bottom Efficiency distribution for all the modules

Oct. 11, 2005H. R. Band 12 Gas Humidity tests During Runs 3-4 –Found inefficient regions near gas inlets Input IFR gas ~0% RH RPC exhaust ~30% RH Started – humid gas on Feb 18 –At first to odd layers FET & FEM –On May 5 added FEB-15, FEB,M,T-16 –On June 8 added all Forward East and Layers Forward West Except FWM-15, FWT14 Monitor input and output RH Look for changes Days  Mixer Output IR2 Hall Temperature Run 5  Feb RH % oCoC Mixer IR2 Hall

Oct. 11, 2005H. R. Band 13 Gas RH extrapolated to RPC exhaust RH reading depends on temperature Temperature at mixer, RPCs, and output bubblers are all different IR Hall and steel temps increase with time General trend to larger RH not due to wet gas Input Top out Middle out Bottom out Belt out East West Feb Days  RH%

Oct. 11, 2005H. R. Band 14 2Deff. Vs. time Clear Effect of humidified gas Dry gas Layer 14Layer 15 Layer 16 Run April, 05 Run Aug. 23, 05

Oct. 11, 2005H. R. Band 15 Effect on overall efficiency DRY gas Improved efficiency well correlated with the start of the humidified gas

Oct. 11, 2005H. R. Band 16 Small Radii Many of the middle RPCs have lost efficiency near the beamline Eff. OK with cosmics only Not improved by humid gas occupancy efficiency

Oct. 11, 2005H. R. Band 17 Efficiency loss at small radii Efficiency loss in highest rate regions growing worse with time Not solely a rate effect Layer 1Layer 4Layer 10 Feb Oct Dec Jan Jun Jun Geometry Measured rate vs. luminosity Black - Oct. 03 Blue - July 05

Oct. 11, 2005H. R. Band 18 Summary Humidified gas –Clearly necessary for long term operation –No negative effects (so far) Still studying efficiency at small radii –Bakelite resistance? –Graphite damage? Good gas flow crucial At low rate - bakelite RPCs in streamer mode have proven to be reliable and efficient Future plans BaBar plans to run until fall 2008 with >*2 luminosity Rates rise proportionately Provide humid gas to all forward RPCs (no improvement seen in barrel or backward endcap RPCs) Peak expected integrated charges to exceed 0.5 C/cm 2 Explore converting highest rate RPCs to avalanche mode

Oct. 11, 2005H. R. Band 19 RPCs in Avalanche Mode Test results with a BaBar endcap spare chamber give good efficiency and very small streamer contamination. 0%, 0.3%, 0.6% and 1% of SF6 in R134A/Isobutane/Ar (~75.6/4.5/18.9) gas mixture has been tested. w/o SF6 the streamer contamination is too large to be useful. The benefit on the rate capability and on the ageing will be in a range of 3 ~ 10. C. Lu, Princeton