1-9, arXiv:physics/0701014. Publisher: Los AlamosNational Laboratoty, CODEN: LNPHF9
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A. Colaleo

A. Grelli

A. Ranieri

B. Marangelli

B. Ortenzi

B. Ponzio

C. Pucci

C. Riccardi

C. Sciacca

D. Colonna

D. Donisi

D. Piccolo

F. Fabozzi

F. L. Fabbri

F. Loddo

F. Romano

FELLI, Ferdinando

G. Belli

G. Iaselli

G. Polese

G. Pugliese

I. Segoni

L. Benussi

L. Passamonti

M. A. Caponero

M. Abbrescia

M. Bertani

M. Giardoni

M. Maggi

M. Necchi

M. Pallotta

N. Cavallo

P. Paolucci

P. Torre

P. Vitulo

PAOLOZZI, Antonio

R. Guida

R. Liuzzi

R. Trentadue

S. Bianco

S. Natali

S. Nuzzo

S. P. Ratti

SAVIANO, Giovanna

M. ABBRESCIA

A. COLALEO

R. GUIDA

G. IASELLI

R. LIUZZIA

F. LODDO

M. MAGGI

B. MARANGELLI

S. NATALI

S. NUZZO

G. PUGLIESE

A. RANIERI

F. ROMANO

R. TRENTADUE

L. BENUSSI

M. BERTANI

M. A. CAPONERO

D. COLONNA

D. DONISI

F. L. FABBRI

M. GIARDONI

B. ORTENZI

M. PALLOTTA

L. PASSAMONTI

B. PONZIO

C. PUCCI

G. POLESE

I. SEGONI

N. CAVALLO

F. FABOZZI

P. PAOLUCCI

D. PICCOLO

C. SCIACCA

G. BELLI

A. GRELLI

M. NECCHI

S. P. RATTI

C. RICCARDI

P. TORRE

P. VITULO

Archivio della ricerca- Università di Roma La Sapienza

arXiv:physics/0701014v1  [physics.ins-det]  30 Dec 2006 LNF-04/25 (P)27 dicembre 2006GAS ANALYSIS AND MONITORING SYSTEMSFOR THE RPC DETECTOR OF CMS AT LHCM.Abbresciaa, A.Colaleoa , R.Guidaa, G.Iasellia, R.Liuzzia, F.Loddoa, M.Maggia,B.Marangellia, S.Natalia, S.Nuzzoa, G.Pugliesea, A.Ranieria, F.Romanoa, R.Trentaduea,L.Benussib, M.Bertanib, M.A.Caponeroc, D.Colonnad, D.Donisic, F.L.Fabbrib, F.Fellid, M.Giardonib,B.Ortenzib, M.Pallottab, A.Paolozzie, L.Passamontib, B.Ponziob, C.Puccie, G.Savianod,G.Polesef,g, I.Segonig, N.Cavalloh, F.Fabozzih, P.Paoluccii, D.Piccoloi, C.Sciaccai,G.Bellil, A.Grellil, M.Necchil, S.P.Rattil, C.Riccardil, P.Torrel, P.VitulolAbstractThe Resistive Plate Chambers (RPC) detector of the CMS experiment at the LHC proton collider (CERN,Switzerland) will employ an online gas analysis and monitoring system of the freon-based gas mixtureused. We give an overview of the CMS RPC gas system, describe the project parameters and first results ongas-chromatograph analysis. Finally, we report on preliminary results for a set of monitor RPC.Manuscript received on November 17, 2006.Stefano Bianco is with the Laboratori Nazionali di Frascati dell’INFN, v.E.Fermi 40, 00044 Frascati(Rome) Italy (telephone: +39-06-94032793, e-mail: stefano.bianco@lnf.infn.it).a Dipartimento Interateneo di Fisica and Sezione INFN, Bari, Italy;b Laboratori Nazionali di Frascati dell’INFN, Italy;c Laboratori Nazionali di Frascati dell’INFN and ENEA Frascati, Italy;d Laboratori Nazionali di Frascati dell’INFN and Facolta’ di Ingegneria Roma1, Italye Laboratori Nazionali di Frascati dell’INFN and Scuola di Ingegneria aerospaziale Roma 1, Italyf CERN, Geneva, Switzerland;g Lappeenranta University of Technology, Finland;h Universita’ degli Studi della Basilicata and Sezione INFN, Napoli, Italy;i Dipartimento di Fisica and Sezione INFN, Napoli, Italy;l Dipartimento di Fisica Nucleare e Teorica and Sezione INFN, Pavia, Italy1 INTRODUCTIONResistive Plate Chambers (RPC) detectors are widely used in HEP experiments for muon detection andtriggering at high-energy, high-luminosity hadron colliders, in astroparticle physics experiments for thedetection of extended air showers, as well as in medical and imaging applications. While gain and efficiencystability are always a must, in the case of RPC detectors in high-rate experiments which use freon-basedgas mixtures, utmost care has to be paid also for the possible presence of gas contaminants. The RPCdetector of experiment CMS at the LHC proton collider (CERN, Switzerland) will employ a gas analysis andmonitoring system for the online monitor of the freon-based gas mixture used. The gas monitoring system isbased on small RPC detectors whose working point (gain and efficiency) is continuously monitored online.The gas monitoring system is designed to provide fast and accurate determination of any shift in workingpoint conditions. Quantitative gas chemical analysis is then performed online by a complete system whichincludes gas-chromatography, pH sensors and contaminants (notably HF) detectors.2 THE CMS EXPERIMENT AT LHCThe Compact Muon Solenoid (CMS) experiment [1] will search for the missing block of Nature - theHiggs boson - and for new exotic elementary particles that are predicted by theory and by cosmologicalobservations. The CMS detector uses Resistive Plate Chambers (RPC) as muon detectors, coupled to DriftTubes in the barrel region, and to Cathode Strip Chambers in the endcaps. The RPC detector in CMS wasdescribed elsewhere at this conference [2].RPC counters [3] are fast, efficient and economical charged particles detectors, well-suited for op-eration in high magnetic field. The elementary component is a gap, a gas volume enclosed between tworesistive plates. Resistive plates are made of bakelite, coated with lineseed oil for surface uniformity. Gasmix used is 96.2% C2H2F4 / 3.5% Iso-C4H10 / 0.3% SF6, with a 45% relative humidity.Signal pulses are picked up by readout strips. In CMS, RPC counters are operated in avalanchemode to sustain high-rate operation, with the streamer suppressed by the addition of SF6 gas in the mixture.3 THE CMS CLOSED LOOP GAS SYSTEMBecause of high costs and huge volumes of the freon-based gas mix used, CMS will use a recirculation(Closed Loop) gas system developed by the CERN gas group [4]. The Closed Loop is a critical componentof RPC. CMS has accumulated experience on its use and performances during the test at the GammaIrradiation Facility at CERN in 2001 [5], and currently at the ISR where chambers are tested in CL priorto installation. At the GIF facility we observed substancial production of HF, linearly correlated with thesignal current.In the Closed Loop (CL) system, purifiers are the crucial componente. Purifiers were determined af-ter tests at the GIF in order to minimize the unknown contaminants which showed as spurious peaks besidesthe known gas mix components. Three filters were selected: 5A molecular sieve, Cu/Cu-Zn, Ni/Al2O3. A2small scale CL system is currently in use at the ISR test station, where RPC chambers are tested at CERNprior to installation in the CMS detector. The system total flux is 110 l/h, with a 10% fraction of fresh mix.Our operational experience showed how the CL system works well as long as the purifiers are not saturated(about 20 days). When purifiers are saturated, contaminants are not filtered and currents in chambers startincreasing (Fig.1). Currents return to standard after purifiers regeneration. We do not observe any trace inGC analysis of either impurities or pollutants.A measurement campaign[6] on purifiers is in progress, using chemical, SEM/EDS (Scanning Elec-tron Microscopy/Energy Dispersive Spectroscopy), XRD (x-ray Diffrattometry) analyses. We plan to char-acterize the CL gas system in three phases: at ISR during chamber testing, at ISR at testing finished withdedicated gaps and dedicated gas system, at the GIF in high-radiation environment. Many open questionsdo exist: why only some 30-40% of chambers show current increase with saturated purifiers? Why oftenonly one gap out of two is affected? It is clear how full understanding of the CL system is crucial for areliable operation of CMS RPCs.4 GAS QUALITY MONITORING SYSTEMGas quality in CMS RPC will be monitored by a dedicated system able to accomplish a full analysis of thegas quality. The gas quality monitoring system will use specific electrodes for hydro-fluoridric acid (HF)detection, as well as SEM/EDS, XRD analyses for purifiers. The system will be flexible and open, so as tointegrate other analysis devices such as GC (Gas Chromatography) and MS (Mass Spectrometry). In caseof anomalies detected, we shall have the possibilities of accessing the experimental cavern where manualpickup points (one per half wheel) will be setup. Several results have been produced over the last few yearswith subsystems planned to be used in the gas quality monitoring system. Production of HF was found inchambers irradiated at the GIF[5], with the concentration of HF produced in the gap found proportional tothe signal charge. We recently performed new studies on irradiated RPCs. We opened small (50cm x 50cm)chambers irradiated at the GIF and observed defects on the inner surfaces (Fig.2). We performed SEM-EDSanalyses on- and off-defect. The presence of Na in defects is confirmed. The origin of Na is supposedlythe bakelite bulk, where NaOH is used as a catalyst in production. We performed XRD analysis of defects,preliminary results [7] show a good match of diffrattogram with the lines characteristic of NaF (Fig.3).5 GAS GAIN MONITORING SYSTEMThe gas gain monitoring system will monitor the RPC working point faster and more precisely than whatone could get by using the CMS RPC system, and provide a warning in case of shifts caused by the gasmixture changes. The system is designed to monitor efficiency and charge continuously in one-hour cycleswith a 1The system (Fig.4) is composed of three subsystem of RPC single gaps, readout by 45cm x 45cm3pads in a cosmic ray telescope located in the SGX5 gas building. Each subsystem is flushed with a differentgas. The Reference subsystem is flushed with fresh open loop gas mixture. The MonitorOut subsystem isflushed with CL gas downstream of CMS RPCs. The MonitorIn subsystem is flushed with CL gas upstreamof CMS RPCs. Each subsystem is composed of three gaps, whose high voltage is set to the standard workingpoint voltage at the efficiency knee, and to 200V above and below the knee respectively. Each cosmic raytrack therefore provides completely correlated pulses in the three subsystems, allowing one to study thedifferential response of gaps and by disentangling any effect due to changes in the gas mixture. In casea working point change is detected, an alarm condition is released and the gas quality monitoring systemdescribed in Sec.IV will verify what the change of work point is due to.5.1 PrototypesSeveral readout options have been considered for the RPC single gaps composing each subsystem. Adouble-pad readout was investigated, with each single pad read by both sides by 45cm x 45cm pads. Anexploded view is shown in Fig.5.5.2 Preliminary ResultsSingle gap prototypes were exposed to cosmic ray tracks triggered by a scintillator counter hodoscope.The negative and positive pads are sent to two input channels of a Tektronix TDS5600 digital samplingoscilloscope. Event-by-event pulse charges are measured and stored via LabView custom applications. Thestandard CMS RPC gas mixture is used, with 45% relative humidity to keep the bakelite resistivity.Fig.6 shows the charge distributions of avalanches from cosmic rays for positive and negative pads,at high voltages from 9.5kV to 10kV. Fitting the charge distributions with truncated Gaussians shows theexpected linear dependance of charge at the peak of distribution on the high voltage applied in case ofsaturated avalanche (Fig.7).Several amplification schemes are being explored. Even a simple passive sum of positive and nega-tive pads, and feeding to NIM LeCroy LRS612AM amplifier improves the S/N ratio (Fig.8).6 CONCLUSIONSThe CMS RPC group is performing a detailed and complete analysis campaign since early tests at the GIFin 2001, to guarantee high-purity gas mixture for a reliable operation of the detector. A lot of work is beingspent into the full understanding of the chemistry of purifiers in CL gas system. A gas analysis systemhas been designed, SEM-EDS analysis observes presence of Na in RPC gaps irradiated at GIF confirmingprevious results, while XRD analysis shows the presence of NaF. A gas gain monitoring system utilizingsmall RPC gaps has been designed, prototypes have been tested and preliminary results show the expectedresponse to cosmic rays.4References[1] MS Collaboration home page is /cms.cern.ch/.[2] . Pugliese for the CMS RPC Collaboration, N24-3, this Conference.[3] . Abbrescia et al., Nucl. Instr. And Meth. A533 (2994) 102-106; M.Abbrescia et al., Nucl. Instr. AndMeth. A515 (2993) 342-347.[4] .Hahn, CERN Closed Loop Gas System: Operating Instructions (unpublished).[5] .Abbrescia et al., HF production in CMS Resistive Plate Chambers, submitted to NIM, presented byR.Guida at the RPC Conference, Seoul (Korea) 2005.[6] .Abbrescia et al., Proposal for a systematic study of the CERN closed loop gas system used by the RPCmuon detectors in CMS, Frascati preprint LNF-06/26(IR).[7] .Pucci, Master’s Degree Thesis, University of Rome La Sapienza 2006 (in Italian). Frascati preprintLNF-06/31(Thesis).5Figure 1: Increase of currents in RPC chambers under test at the ISR in CL gas system when purifiers aresaturated. Currents start decreasing when chamber is set in Open Loop, and decrease further when chamberis returned to CL with regenerated purifiers. Top inset show results of a best fit of an exponenential curveto data.Figure 2: SEM image of a defect in a RPC chamber irradiated at the GIF under two magnifications. Thedefect is identified as NaF.6Figure 3: XRD spectrum of defects in a chamber irradiated at GIF. A very good match is observed for twocharacteristic peaks of NaF.Figure 4: Gas gain monitoring system conceptual layout.7Figure 5: Exploded view of a 50cm x 50cm single gap which composes a subsystem of the gas gainmonitoring system. Each single gap is read by two 45cm x 45cm pads on both sides, each one picking upnegative and positive pulses from the avalanche developing in the gas gap upon the crossing of a cosmic raytrack.Figure 6: Charge distribution of single gap operated in avalanche regime and read by two 45cm x 45cmpads. The high voltage is increased from 9.5kV to 10kV. Pedestal is subtracted.8Figure 7: Dependence of peak values of charge distributions from high voltageFigure 8: Charge distribution from the double pad readout scheme. A transformer (inset) sums positive andnegative pads outputs. Transformer output is filtered and sent to NIM LeCroy amplifier. Pedestal is notsubtracted and shown in arbitrary vertical scale at about -4pC.9

Gas analysis and monitoring systems for the RPC detector of CMS at LHC

The Resistive Plate Chambers (RPC) detector of the CMS experiment at the LHC proton collider (CERN, Switzerland) will employ an online gas analysis and monitoring system of the freon-based gas mixture used. We give an overview of the CMS RPC gas system, describe the project parameters and first results on gas-chromatograph analysis. Finally, we report on preliminary results for a set of monitor RPC. ©2006 IEEE

http://arxiv.org/PS_cache/physics/pdf/0701/0701014v1.pdf

Gas analysis and monitoring systems for the RPC detector of CMS at LHC

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Archivio della ricerca- Università di Roma La Sapienza

Archivio della ricerca- Università di Roma La Sapienza