Several technologies are under consideration for the muon system of the LHCb experiment. Resistive Plate Chambers (RPCs) are one of the favourite candidates for the outer areas where the particle fluxes are expected to be at most some kHz/cm/sup 2/. This work describes the results obtained with a multigap RPC prototype under various beam conditions at the CERN facilities. (9 refs)

Colrain, P

Corti, G

De Paula, L S

Gandelman, M

Lamas-Valverde, J

Maréchal, B

Moraes, D

Polycarpo, E

Schmidt, B

Schneider, T

Wright, A

P Colrain

G Corti

L de Paula

M Gandelman

J Lamas-Valverde

B Marechal

D Moraes

E Polycarpo

B Schmidt

T Schneider

A Wright

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Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Performance of a Multigap RPC prototype for the LHCb Muon system

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LHCb 99-050MuonDecember 2, 1999.Performance of a Multigap RPC prototype forthe LHCb Muon SystemP. Colrain a, G. Corti a, L. de Paula b,1,2, M. Gandelman b,1,2,J. Lamas-Valverde a, B. Marechal b,1,2, D. Moraes b,1,2,E. Polycarpo b,1,2,3,4, B. Schmidt a, T. Schneider a, A. Wright aaCERN, CH - 1211 Geneve 23, SwitzerlandbLAPE/IF-UFRJ, CP 68528 Rio de Janeiro, BrazilAbstractSeveral technologies are under consideration for the muon system of the LHCbexperiment. RPCs are one of the favourite candidates for the outer areas where theparticle fluxes are expected to be at most some kHz/cm2. This work describes theresults obtained with a Multigap Resistive Plate Chamber prototype under variousbeam conditions at the CERN facilities.1 IntroductionThe LHCb experiment [1] is a single-arm spectrometer with forward angularcoverage from 10 mrad to 300 mrad. The LHCb Muon System is composedof ve muon tracking stations interleaved in a longitudinally-segmented shield.It must provide muon identication and L0 muon trigger formation. For thelatter, good time resolution for bunch-crossing identication and reasonablemomentum resolution are required. Due to their excellent timing properties,RPCs are candidates for the outer part of the muon system (starting from apolar angle of 150 mrad). In this region, the rate capability required is about1 Partially supported by European Community (contract CI1*-CT94-0118)2 Partially supported by CNPq, Brazil3 Partially supported by CAPES, Brazil4 Corresponding author. Fax +41 22 7679425; e-mail Erica.Polycarpo@cern.chPreprint submitted to Elsevier Preprint 2 December 19991.6-4.0 kHz/cm2, depending on the LHCb operating luminosity. This numberhas been conservatively estimated, in order to guarantee safe operation.It has been shown by many authors that the rate capability of RPCs canbe improved by operating them in avalanche or low gas amplication mode(e.g. [2]), using gas mixtures with a high content of freon gases, in particulartetrafluoroethane [3]. The addition of small amounts of SF6 has increased therange of voltages in which the chambers can operate without streamer forma-tion [4]. Further improvements have been achieved by using lower resistivitybakelite plates (  1010Ωcm) [5]. It has also been shown that dividing the gasvolume into several gaps [6] results in higher rate capability. The advantagesof using several gas gaps are that the time walk and resolution are limited bythe small size of the subgaps and that the tolerance to irregularities is larger.With this motivation, two multigap RPC prototypes have been built, withactive areas 2424 cm2 and 1.22.2 m2.This paper describes the tests of the small prototype. In the CERN PS (3GeV/c ) and SPS (120 Gev/c ) beam lines, special emphasis has been givento the rate capability studies. The performance of the chamber has also beentested under continuous and uniform irradiation over the whole surface inthe Gamma Irradiation Facility (GIF), where timing properties and cross-talkeects were also investigated.2 Characteristics of the MRPC prototypeThe chamber, whose cross section is shown in gure 1, is composed of fourgas gaps, each of 0.66 mm, preserved with nylon button spacers. The platesare 0.7 mm thick melamine-phenolic laminates, with bulk resistivity 5 − 6 1011Ωcm. The HV plane consists of a graphite layer sprayed on the outersurface of the external resistive plate, while the readout plane is made of kap-ton foil with copper gold coated strips (1.3 cm wide and 24 cm long), eachconnected to an amplier shaper discriminator chip ASD8-B [7]. The thresh-olds used were always 120 mV. The gas mixture was 96% C2H2F4/3% iso-Butane/1% SF6. In addition, 1% of water vapour was added to this gas mix-ture, in order to ensure stability of the electrical and mechanical properties ofthe plates.23 Tests carried out in the PS and SPS beam lines3.1 Experimental setupThe trigger system was composed of two scintillators (1515 cm2 and 2020cm2) centered on the beam line. A hodoscope composed of 8 horizontal (Hch1−8)and 8 vertical (Vch1−8) scintillator strips (0.8 cm wide and 8.0 cm long) with 1cm pitch was placed between the large scintillators and used in the oine anal-ysis. The MRPC prototype was placed immediately behind the hodoscope. Aducial region of three chamber strips was used to measure the eciency forminimum ionizing particles which crossed the intersection (Hch5,Vch5) of thehodoscope, corresponding to the peak of the two-dimensional beam prole ingure 2. The signal from the strips was discriminated before feeding it to a16-hit TDC (LeCroy 1176), operating in common-stop mode. A 15 ns timewindow was used to calculate the eciency.3.2 Rate capability studies at the PS and SPSFigure 3 shows the eciency measured at the CERN PS as a function of theapplied high voltage for dierent rates measured in the (Hch5,Vch5) cell of thehodoscope. The eciency is integrated over the spill duration of 400 ms. Theknee of the plateau is shifted towards higher voltages when the rate increases,due to the increasing voltage drop across the resistive plates, which reducesthe eective eld in the gas volume.In order to determine the eect of the irradiation time on the performanceof the chamber, a further test was done at the CERN SPS, which has a spillduration of 2500 ms. The eciency was measured for several time intervals.The result is shown in gure 4, together with the rate as measured in thehodoscope peak. The drop in eciency is on one hand due to the variationof the rate within the burst; on the other hand it is possible to see that forsimilar rates a signicant eciency drop with time occurs (e.g., for a rate ofapproximately 7.5 kHz/cm2, the eciency drops from 90% to 70%). Thereforewe conclude that the rate capability as measured at the PS is not valid for highrates. A precise quantitative measurement of the eciency drop would requiremore careful investigation. Since in GIF the chamber can be continuouslyirradiated at the same rate, it is the best available facility at CERN to test theperformance of the present MRPC prototype with the given bulk resistivity.34 Tests in the Gamma Irradiation Facility4.1 Experimental SetupThe experimental setup in GIF was similar to the one used in the PS andSPS. The chamber was placed at 1.08 m from the 137Cs source, between thetwo large scintillators. In such a position, the whole surface of the chambercould be uniformly irradiated. A set of lead lters with dierent attenuationfactors (AF) allowed the reduction of the gamma flux through the chamber.The muon beam coming from the SPS beam dump was used to monitor theperformance of the prototype.4.2 Experimental Results4.2.1 Measurement of rate and efficiencyFigure 5 shows the eciency for various voltages and several attenuation fac-tors. The eciency is corrected for the probability of nding a spurious hitin the same time window (typical values  0.1%). The eective rate in thechamber was measured using the method described in [8] and a γ sensitivityof approximately #gaps/800 = 1/200 has been observed, which is in goodagreement with other measurements [9]. The chamber reaches 97% eciencyat 18 kV for a rate of approximately 1.9 kHz/cm2.4.2.2 Timing PropertiesFigure 6 shows the time walk in the chamber as a function of the measuredeective rate. The mean time measured without source (lowest rate) was takenas reference. The time resolution, shown in the same plot, lies in the range1.5-1.9 ns. For the two highest rates, the voltages are 18 kV, otherwise it is17 kV, which is already well within the eciency plateau. It is clear that thetiming requirements of LHCb are fullled by this prototype.4.2.3 Cross Talk and NoiseThe cluster size is taken as a measurement of the cross talk in the chamber. Thesame time window used to measure the eciency was used to build the clusters(15 ns). The average cluster size has a very stable behaviour as function of thevoltage and rate. For voltages in the range within which the chamber usuallyoperates, the maximum cluster size is 1.2 strips, which indicates very small4cross-talk.The noise rate was estimated by subtracting the muon flux coming from theSPS beam dump ( 30 Hz/cm2) from the rate measured with no source. Thisgives a noise rate of about 60 Hz/cm2, which is rather high. The cosmic rayflux is negligible.5 ConclusionsWe have tested a four-gap RPC prototype in three facilities at CERN. It hasbeen shown that the rate capability measurements at the PS are not reliablefor high rates. From the measurements at the SPS, it was possible to seethat an irradiation time longer than 1 s is needed for a stable operationof the chamber and that the variation of the rate within the burst must betaken into account when describing the eciency drop. In GIF, the continuousirradiation allowed correct measurements of the rate capability to be made. Inspite of the high resistivity of the melamine-phenolic plates of the prototype,the chamber reached 97% eciency at 1.9 kHz/cm2. The time properties andcross-talk match the requirements for the LHCb muon system well.References[1] LHCb Technical Proposal, CERN/LHCC 98-4.[2] R. Cardarelli et al., Nucl. Instr. and Meth. A 333 (1993) 39.[3] M. Abbrescia et al., Nucl. Instr. and Meth. A 392 (1997)155-160[4] P. Camarri et al., ATLAS Internal Note MUON-NO-226 (1998).[5] F. Ceradini et al., Nucl. Instr. and Meth. A 340 (1994) 466-473[6] E.Cerron-Zeballos et al., IV International Workshop on Resistive PlateChambers and related Detectors, Naples, Italy, 15-16 October 1997.[7] F. M. Newcomer et al., IEEE Trans. Nucl. Sci. 40(4) (1993) 630.[8] M. Maggi et al., IV International Workshop on Resistive Plate Chambers andrelated Detectors, Naples, Italy, 15-16 October 1997.[9] G. Carboni, talk at this workshop.5Fig. 1. Schematic of the resistive plate chamber. The HV plane is at the externalface of the fth melamine plate, from the top to the bottom part of the chamber.The honeycomb ensures the mechanical rigidity of the system.x channely channel1234567812345678100200300400500600Fig. 2. Two-dimensional prole of the PS beam, determined by the hodoscope.60.40.50.60.70.80.911.19 10 11 12 13 14 15 16 17 181.5 kHz/cm23.0 kHz/cm25.5 kHz/cm28.0 kHz/cm211.5 kHz/cm2MRPC HV (kV)EfficiencyFig. 3. Eciency as a function of the high voltage, for several rates measured bythe hodoscope in the PS.00.20.40.60.810 250 500 750 1000 1250 1500 1750 2000 2250 2500Time (ms)Efficiency024681012141618200 250 500 750 1000 1250 1500 1750 2000 2250 2500Rate (kHz/cm2 )Fig. 4. Eciency as a function of the time within the burst in the SPS hall (stars).The triangles are the rates for the same time intervals, measured at the hodoscopecell.70.20.30.40.50.60.70.80.911.110 12 14 16 18 20HV (kV)Efficiency1000   110∞         90100     22050       36010     12005       1900Fig. 5. Eciency as a function of the eective rate of the MRPC, in the GammaIrradiation Facility at CERN. The attenuation labelled \1" refers to the data takenwithout source.00.250.50.7511.251.51.7522.252.5102 10 3Rate (Hz/cm2)Time walk (ns)Time resolution (ns)-1-0.500.511.522.533.5410 2 10 3Fig. 6. Time walk (stars) and time resolution (triangles) as a function of the rate.8

Performance of a multigap RPC prototype for the LHCb muon system

Performance of a multigap RPC prototype for the LHCb muon system

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