1. The gas system for the CMS-RPC detector Roberto Guida Collaborazione CMS-RPC Riunione CSN1 Roma, 2 Aprile 2007

2. Introduction to the CMS-RPC gas system R&D studies on the closed loop gas system: Development of the purifier module Gas monitoring systems: Gas quality monitoring system Gas gain monitoring system Conclusions Outlook

3. Characteristics and requirements Total volume of the CMS-RPC detector: 18 m 3 Gas mixture: 96.2 % C 2 H 2 F 4, 3.5 % iC 4 H 10, 0.3% SF 6 Mixture relative humidity: 40% Number of channels: 360 Nominal gas flow: 8 m 3 /h (0.5 volume change/h) Gas recirculation factor (CLOSED LOOP MODE): ~95% Fresh gas replenishing rate: 1 m 3 /h (~5%)

4. Humidity Suppliers Mixer Humidifier Purifier module Pre-distribution and pump Final distribution Layout of the system

5. Schedule Apr-May 07 (installation) May 07 (CL commissioning) almost ready All gas racks are installed except purifiers and analysis All racks are leak tested and functionally tested in the lab Control software 95% ready PVSS user interface ready

6. Gas racks layout RPC Modules: Pre-distribution; Pump; Analysis; Control rack RPC Modules: Mixer; Exhaust; Humidifier; Purifiers; Analysis; Control rack SG5 (surface) USC55 (service cavern) …at the end the RPC system will be the biggest

7. CMS UX cavern The last step is the final distribution of the gas mixture on the experiment 11 gas distribution racks: 50 channels each Gas flow read-out by means of flowcells Manual adjustment between channels

8. A careful equalization of the gas flow is needed in order to exploit the full capacity of the system -Between stations (i.e. wheel level): Results independent on the number of channels connected Optimized at the minimum pressure needed The key parameter is the number of volume change/h -Station level: When needed a needle valve has been inserted in order to equalize the flow between the two chambers Final gas distribution

9. Present specifications The system described is the result of an intense R&D program. In fact, the results from the GIF ageing test carried out by the CMS-RPC and ATLAS-RPC collaborations (1999-2004) as well as the test of many RPC with cosmic have implied an important evolution in the requirement for the RPC gas system and mixture composition in CMS: TodayCMSin 1999/018 Gas volume18 m 3 Gas mixtureR134a/iC 4 H 10 /SF 6 96.2/3.5/0.3 R134a/iC 4 H 10 96.5/3.5 Tollerable contamination: H 2 O Mixture RH 40% (7,000-10,000 ppm) <1000 ppm Chamber pressure<3 mbar Maximum flow rate18 m 3 /h2.2 m 3 /h Flow rate at operating conditions 8 m 3 /h1.2 m 3 /h Fresh gas replenishing≤1 m 3 /h<0.12 m 3 /h

10. The CMS-RPC collaboration started an intense R&D program since 2003 using a closed loop gas system prototype, provided by the CERN gas group, connected with two final RB1 detectors under irradiation at the CERN-GIF. After, in a second phase (started in 2005) the system was moved to the ISR area and used for the mass production chamber test. The aim of this phase was a further optimization of the purifier effectiveness (Preprint LNF 06/27). Phase 1: test at the GIF facility Phase 2: test at ISR Further studies of the materials properties: Bakelite, purifiers, gas mixture R&D on the closed loop system

11. The purifier module is a fundamental part of the closed loop gas system. 1999: Standard purifier using a twin column (6 l) filled with e.g. Molecular sieve and activated copper can be used to remove water and oxygen. Since the expected contaminant were H 2 O and O 2 2003: first hints from the GIF first results. However the idea of an upgrade was not considered in detail. 2005: The final results from the GIF ageing test have confirmed that: –RPCs need a humidified gas mixture (RH about 40%) –The gas flow should be 0.5 – 1 volume change/hour –New component in the gas mixture is needed (0.3% SF 6 ) –The outlet gas from a reference chamber (not irradiated) has a similar gas chromatogram as the inlet gas (except air and H 2 O) –In the outlet gas from a chamber inside GIF 5-7 extra components are visible in the gas chromatogram –The best configuration found for the purifier is: 1.Molecular Sieve 5A 2.Cu-Zn / Cu 3.Ni – Al 2 O 3 Purifier R&D on the closed loop system

12. Gas chromatogram: GIF results Fresh mixture purifiers Closed loop recirculation (95%) after about one month: many small extra signals are visible especially at low retention time Results obtained with a MicroGC Varian CP2002P (PoraPLOT Q and TCD detector). Fresh mixture Mixture in the closed loop circuit

13. Final RPCs in closed loop mode with cosmic ray System stable from November 2005, but just before the purifier regeneration … slight increase in the currents. Reversible after regeneration. Log scale Gas chromatogram: ISR results Usually no trace of impurities and pollutants in GC analysis. Seldom something is visible only in log scale.

14. Upgrade of the Purifier Module First possibility: 2 purifiers, the second one including the two metallic filters In case of necessity a further upgrade including a third module will be considered for the LHC high luminosity phase An upgrade of the purifier module is mandatory for both capacity and purifiers material reasons Purifier #1: Molecular Sieve 5, 24 l cartridge. Expected lifetime at operating conditions 1.5 day of running time Purifier #2: Cu-Zn/Cu and Ni-Al 2 O 3 Adsorption capacity x 50 with respect to the GIF setup. Expected lifetime at operating conditions 15 day P #1P #2 P #3 ?

15. 8 double gaps are now under test in ISR (they are equivalent to 4 RB1 detectors) We started with a open mode gas system and we will move soon in closed loop mode Results expected in 1-2 months New more gaps in 2 months (?) ISR test: second phase, a dedicated test

16. Development of the gas system Extra signals in the closed loop gas Increase of the gas flow New component (SF 6 ) Humidified mixture Humidity

17. Gas monitoring for CMS-RPC We foreseen two systems able to monitor the gas mixture for the RPC detector: Gas quality and composition monitoring system based on some chemical analyses of the gas: F- electrode pH electrode Pick-up point for a possible use of a Gas Chromatograph (GC) Gas gain monitoring system based on three sets of small RPCs supplied with fresh mixture, input mixture to CMS, return mixture from CMS respectively. It will be also possible to scan all the gas lines. Results from both the monitoring systems need be recorded in the DCS system.

18. Gas Monitoring Systems USC55UXC55 SG5 RPCs #1 Fresh gas Purifiers Mixer Humidifier GC F-F- pH sample RPCs #2 Before purifiers RPCs #3 After purifiers Gas quality monitoring Gas gain monitoring GC+others (half wheel pick- up points) CMS

19. Specific for fluoride/HF concentration monitoring. Analysis station with two independent input channels (possibility to monitor simultaneously different sampling points). F - electrode GIF results

20. Low-cost and very simple system, with PC readout, already used during tests at GIF, in our labs and in ISR … nevertheless quite sensitive pH meter

21. Gas Chromatograph purifiers GC Results: Signals from impurities produced by RPCs working in Closed loop mode (95%) after about one month. GC is the most flexible system  it has already demonstrated at GIF its ability to detect produced pollutants in the gas mixture

22. A monitoring system of the RPC working point able to provide much faster and sensitive response than the CMS RPC system Monitor of efficiency and charge continuously Monitoring of charge and efficiencies with cosmic rays in SG5 (surface gas building) Three sub-sets of 45x45 cm 2 Single Gaps 1 Reference RPCs, i.e. fresh mixture 2Monitor “IN” RPCs, i.e. mixture for CMS-RPC 3Monitor “OUT” RPCs, i.e. mixture downstream of CMS-RPC In case work point changes –A warning goes on  action needed –The Gas quality monitoring system (i.e. GC,probes, etc) will understand the reason Gas gain monitoring system

23. The R&D program carried out by the RPCs community since 1999 has allowed to found the proper solution in order to guarantee the detector functionality for a long term period (increase of the gas flow, humidified mixture, a more complex purifier module, ….) However the results of this R&D program have produced many important changes on the CMS-RPC gas system Now the system is much more complex, but the changes are effective Further studies are needed especially for the purifier module (most of this work is already going on or well defined) Conclusions

24. Recovery of RPC9 with the moist mixture: maximum efficiency vs time at Source Off and ABS1 Necessity of a humid gas mixture dry moist mixture

25. Detail of extra signals before and after purifiers (best configuration found) 1 2 3 4 5 11 12 10 9 8 7 6 13

27. RPCs integrated charge at GIF Closed loop status: recirculation factor 50 % 90 % 95 % Actual production RB1: CH 26 and CH 45 In gas recirculation mode: 30 mC/cm2 ~ 6 CMS eq. years 6 CMS equivalent years

28. Peak #Retention time (s) OriginMoleculeAbsorbed with filter Approx. concentration 118.9LeaksAir 322.2Air-leaksCO 2 30-40 ppm 1238.8H2OH2OMol 5A, Ni- Al2O3, Ni- SiO2, R12, R3-11G 1000-3000 ppm 525.3MixtureSF 6 No0.3 % 1137.4MixtureC2H2F4C2H2F4 No300-600 ppm 13MixtureC2H2F4C2H2F4 No96.2 % 14MixtureC2H2F4C2H2F4 No100-300 ppm 18178.2MixtureiC 4 H 10 Ni-SiO23.5 % 1572.5MixtureC2H2F4C2H2F4 R12, R311G (?) 10 ppm GC signals from mixture + leak (detail)

29. Peak #Retention time (s) OriginMoleculeAbsorbed with filter Approx. concentration 219.9RPC or SF 6 or filter No10-30 ppm 424.3RPCNo10-40 ppm 627.6RPCMol 5A20-30 ppm 728.5RPC Ni-Al2O3, Ni- SiO2, R311G 20-50 ppm 933.0RPCNi-Al2O3, Ni- SiO2 30-70 ppm 1036.0RPCNi-Al2O3, Ni- SiO2, R12, R311G 10 ppm 829.3Ni filtersNo20-80 ppm 1677.5RPCR12, R311G (?)10 ppm 17103.6RPCR12, R311G (?)10 ppm GC signals from RPC and other (detail)

30. –Reading RPCs with two pads placed on both sides of detectors for timing, efficiency and charge knee +200 -200 Three monitoring RPC with independent HV supply, on and off knee Clean cosmic rays peak provide accurate monitor of working point Ref.: S.Nuzzo 2006 10x10cm 2 pad Gas gain monitoring system

31. Pads 45x45cm 2 positive and negative Sum pulse DEVELOPING DIFFERENTIAL AMPLIFICATION SCHEME TO REDUCE COHERENT NOISE Cosmic ray event triggered by scintillation counters hodoscope Gas gain monitoring system Double pad read-out

32. Charge distribution of avalanche from cosmic rays at voltages from 9.5kV to 10kV. Charge readout from 45x45cm 2 pads fed to TEKTRONIX TDS5000 scope. Gas mix used is 96.2% C 2 H 2 F 4 / 3.5% Iso-C 4 H 10 / 0.3% SF 6 Positive pad Negative pad Gas gain monitoring system CMS RPC preliminary

33. Avalanche to streamer transition Charge distribution on 45x45cm 2 pad showing transition from avalanche to streamer. At t=0 standard gas mix contains 0.3% SF 6, which is progressively removed. As SF 6 is decreased, the streamer peak appears. 0.3% SF 6 0.0% SF 6

34. References 1) “HF production in CMS-Resistive Plate Chambers” Pubblicato su Nucl.Phys.B (Proc.Suppl.) 158:30-34,2006 2) “Cosmic ray test of Double gap Resistive Plate Chambers for the CMS experiment.” Pubblicato su Nucl.Instrum.Meth.A550:116-126,2005 3) VIII International Workshop on Resistite Plate Chambers and related detectors application in particle physics and astrophysics. Seoul (Corea Sud), Ottobre 2005. “Results from the CMS-RPC ageing test at the CERN Gamma Irradiation Facility” 4) VIII International Workshop on Resistite Plate Chambers and related detectors application in particle physics and astrophysics. Seoul (Corea Sud), Ottobre 2005. “New results about HF production, chemical analysis of gas and bakelite samples from CMS RPC detectors.” 5) C.Pucci “Gas monitoring system” CMS week March 2006 http://indico.cern.ch/materialDisplay.py?contribId=5&materialId=1&confId=3754;C.Pucci “CMS RPC gas gain system”, Annual Congress of Italian Physical Society, Torino 2006; S.Bianco on behalf of the CMS RPC Collaboration, “The gas gain monitoring system for the CMS RPC detector”, IEEE06, San Diego (USA) http://arxiv.org/pdf/physics/0701014. 6) F.Hahn, Closed Loop Users’ Guide 7) M. Abbrescia et al., “Study of long-term performance of CMS RPC under irradiation at the CERN GIF, Nucl. Instrum. And Meth. A533 (2004) 102-106. 8) http://indico.cern.ch/conferenceDisplay.py?confId=a045466http://indico.cern.ch/conferenceDisplay.py?confId=a045466 9)M.Abbrescia et al., Proposal for a detailed study of Closed Loop… Frascati Preprint LNF 06/27 (IR) available at http://www.lnf.infn.it/sis/preprint/http://www.lnf.infn.it/sis/preprint/ 10) G.Saviano CMS week Dec 06 11) S.Bianco CMS week Dec 06 12) R. Guida CMS week Dec 06