RPC HV efficiency scan
2011 HV scan
SF6 flow-meter accident
2011 Results
2010-2011 comparison
Pigi Paolucci on behalf of RPC collaboration
RPC – Why an HV scan ?

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
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We decided to do an HV scan per year:
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ΔP = 20 mbar
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
2010 efficiency
Δε=10%
RPC monitor stream is vital.

2
2010 pressure
Very high statistic in 1 hour at present L
Pigi - RPC project - XEB 12 April
11/04/11
HV scan – first 8 points
18-19 March: HV scan (8 points – HV corrected with pressure)
plateau curves repository
27 March: full analysis completed
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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 !!!
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barrel
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RPC project - CMS GM 4 May 2011
endcap
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Pigi on behalf of RPC group
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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%))
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Pigi - RPC project - XEB 12 April
11/04/11
HV scan – 11 points
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15 April: 3 more points taken – scan completed (3.3 pb-1 BAD)
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95% of the rolls have now a very good efficiency sigmoid fit
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RPC project - CMS GM 4 May 2011
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Pigi on behalf of RPC group
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barrel
HV scan – Chi2 distribution
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – all curves shifted
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it is now clear that was not an overall efficiency decrease but that
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all curves were shifted forward about 120 ± 10 V (respect 2010) ??
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We double checked HV, pressure, temperature, timing, Trigger, m selection checked
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Everything seems to be ok – what about the GAS ?
2010
Artificial shift of 110 Volts
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – curve shifted
HV scan 2010
Fit HV Scan
2011
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – SF6 shift measurement
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16 April: Gas expert called  SF6 mass flow-meter replaced after few hours
18 April: System came back at nominal gas mixture
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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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – working points
30 April:
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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
Nominal
voltage
2010
2011
2011
2010 DP
2010 DP
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – final results I

2172 (99%) fits are good (51 recovered deleting 1-2 bad points)
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Only 23 OFF rolls are without plateau
All Barrel HV scan 2011
HV value at 50% eff
Barrel
RMS 64 Volts
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
Endcap
RMS 84 Volts
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
Public results on WEB page
Cluster size
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RPC project - CMS GM 4 May 2011
noise
Pigi on behalf of RPC group
HV scan – final results I

Also low efficiency rolls have a very nice plateau
1 Front End board broken
barrel
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
HV scan – gas monitoring improvements
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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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group
Conclusions
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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
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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
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RPC project - CMS GM 4 May 2011
Pigi on behalf of RPC group