RPC HV efficiency scan

1. RPC HV efficiency scan 2011 HV scan SF6 flow-meter accident 2011 Results 2010-2011 comparison Pigi Paolucci on behalf of RPC collaboration

2. RPC – Why an HV scan ? 2010 pressure ΔP = 20 mbar • In 2010 we had not enough statistic to make a full efficiency scan • RPC system worked with 2 common HV working point 9.35 & 9.55 kV • RPC performance depends on P and T • We decided to do an HV scan per year: • To define a WP per chamber in order to improve the performance and avoid to stress the detector. • Choose a WP in the plateau region in order to avoid instability with pressure • RPC monitor stream is vital. • Very high statistic in 1 hour at present L 2010 efficiency Δε=10% Pigi - RPC project - XEB 12 April

3. HV scan – first 8 points plateau curves repository • 18-19 March: HV scan (8 points – HV corrected with pressure) • 27 March: full analysis completed • About 60% of the rolls have a very good efficiency sigmoid fit – more points needed ! • An overall efficiency decrease of 3% seen in the only run comparable with 2011 WP !!! endcap endcap endcap barrel barrel barrel RPC project - CMS GM 4 May 2011

4. HV scan – preliminary results Working Point defined as Knee + 150 V taking into account the DP of 2010 We can reduce to 100 in case of rate problem and even less when we will correct the Effective HV with the pressure variations. Defined plateau region Knee 95% Working = knee + 150 V HV scan fit ε(HV) = εmax/(1 + eSlope (HV-HV50%)) Pigi - RPC project - XEB 12 April

5. HV scan – 11 points • 15 April: 3 more points taken – scan completed (3.3 pb-1 BAD) • 95% of the rolls have now a very good efficiency sigmoid fit endcap endcap endcap barrel barrel barrel RPC project - CMS GM 4 May 2011

6. HV scan – Chi2 distribution Chi2 sigmoid fit distribution RPC project - CMS GM 4 May 2011

7. HV scan – all curves shifted • it is now clear that was not an overall efficiency decrease but that • all curves were shifted forward about 120 ± 10 V (respect 2010) ?? • We double checked HV, pressure, temperature, timing, Trigger, mselection checked • Everything seems to be ok – what about the GAS ? 2010 Artificial shift of 110 Volts RPC project - CMS GM 4 May 2011

8. HV scan – curve shifted HV scan 2010 Fit HV Scan 2011 RPC project - CMS GM 4 May 2011

9. HV scan – SF6 shift measurement • 16 April: Gas expert called  SF6 mass flow-meter replaced after few hours • 18 April: System came back at nominal gas mixture • SF6 mass flow-control analyzed by Gas Group: not working properly - 30% more !!! 0.39% not 0.30% (flow-meter monitoring was stable and at 0.3%) • Only way to detect the failure was an external measurement with chromatograph • 24 April: mini-scan in 2 sectors (39 rolls) to calibrate the shift due to the SF6 DHV(0.39% SF6 – 0.30% SF6) at 50% eff HV50% shifted of 100 Volts RMS is 19 Volts 100 Volts renormalization will be applied to all the curves RPC project - CMS GM 4 May 2011

10. HV scan – working points • 30 April: • Rolls WP merged in chamber and then in HV channel (1 HV ch.  4 bi-gap rolls) • 2011 final HV working point table is ready (average of 4 rolls WP) • System should be now more stable .vs. pressure • About 65% were working closer to the knee in the 2010 Nominal voltage 2010 2011 Nominal voltage 2010 2011 2010 DP 2010 DP RPC project - CMS GM 4 May 2011

11. HV scan – final results I • 2172 (99%) fits are good (51 recovered deleting 1-2 bad points) • Only 23 OFF rolls are without plateau All Barrel HV scan 2011 HV value at 50% eff Barrel RMS 64 Volts Endcap RMS 84 Volts RPC project - CMS GM 4 May 2011

12. RPC project - CMS GM 4 May 2011

13. Public results on WEB page noise Cluster size RPC project - CMS GM 4 May 2011

14. HV scan – final results I • Also low efficiency rolls have a very nice plateau 1 Front End board broken barrel RPC project - CMS GM 4 May 2011

15. HV scan – final results II 2010 Average Effective voltage over 1 month <9.35 & 9.55> kV PTcut > 0 GeV Systematic error of about 1% 2011 Effective voltage in 1 hour 9.35 & 9.55 kV PTcut > 7 GeV RPC project - CMS GM 4 May 2011

16. 2010 Average Effective voltage over 1 month <9.35 & 9.55> kV PTcut > 0 GeV Cluster Size Barrel 2011 Effective voltage in 1 hour 9.35 & 9.55 kV PTcut > 7 GeV RPC project - CMS GM 4 May 2011

17. Cluster size distribution at WP • Cluster size distribution at the 2011 HV channel WP • Measurement will be re-done in 1 day after the TS <CS> = 1.6 RMS 0.23 RPC project - CMS GM 4 May 2011

18. HV scan – gas monitoring improvements • Central gas group is helping a lot in understanding the SF6 flow-meter failure and in improving the monitoring of this crucial component – Thanks • SF6 flow-meter is today monitored by the gas group with a gas chromatograph until the automatic procedure will be ready (June)” • RPC Gas system review 3 May (today): • Gas Gain Monitor could be used to monitor the gas mixture (under study) • Gas monitor parameters alarm/warning have been reviewed • Instructions for the RPC shifter will be updated RPC project - CMS GM 4 May 2011

19. Conclusions • Barrel Gaps have been produced from 2003 and after 8 years • Stability and uniformity of 2172 bi-gap rolls is now evident: • All the efficiency curve fits are good – 23 (1%) rolls are OFF • Average efficiency ≈ 95% - stable in time (2010-2011) • Efficiency RMS = 5% • HV50% distribution has RMS = 64 Volts and 84 Volts (very uniform set) • HV working point table is now ready. It will be uploaded after TS • Expected <efficiency> at WP are 95-96 % and more stable .vs. P & T • SF6 is a critical component for RPC detector 30% more corresponds to 110 Volts • Gas chromatograph automatic measurement needed to control the gas mixture • Online HV correction with pressure is needed to reduce the operating voltage and keep safe the detector (under-study) Final results WEB page RPC project - CMS GM 4 May 2011