Abstract   In this paper a model simulating the main aspects of avalanche growth and signal development in Resistive Plate Chambers (RPCs) is presented. The model has been used to compute the performances, in particular, charge distribution and efficiency of single- double- and multi-gap RPCs, and to compare them with the available experimental results.  This model could be used to optimize the characteristics of this type of detector with a view to its use in the future large experiments at LHC: ATLAS and CMS

A. Ranieri

Anna Colaleo

B. Marangelli

F. Loddo

F. Romano

Gabriele Gianini

Gabriella Pugliese

Giuseppe Iaselli

Marcello Abbrescia

Mario Maggi

Paolo Vitulo

S. Natali

S. P. Ratti

Salvatore Nuzzo

English

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*Corresponding author. Tel.: #39 80 5443140; fax: #39 805442470; e-mail: marcello.abbrescia@ba.infn.it.1Talk presented by M. Abbrescia at the 7th Meeting onadvanced detectors, La Biodola, Isola d’Elba, Italy, 25—31 May1997.Nuclear Instruments and Methods in Physics Research A 409 (1998) 1—5Resistive Plate Chambers in avalanche mode: a comparisonbetween model predictions and experimental results1M. Abbrescia!,*, A. Colaleo!, G. Iaselli!, F. Loddo!, M. Maggi!, B. Marangelli!,S. Natali!, S. Nuzzo!, G. Pugliese!, A. Ranieri!, F. Romano!, G. Gianini",S.P. Ratti", P. Vitulo"! Dipartimento Interateneo di Fisica, Sezione INFN, Via G. Amendola 173, 70126 Bari, Italy" Dipartimento di Fisica Nucleare e Teorica, Sezione INFN, Pavia, ItalyAbstractIn this paper a model simulating the main aspects of avalanche growth and signal development in Resistive PlateChambers (RPCs) is presented. The model has been used to compute the performances, in particular, charge distributionand efficiency of single- double- and multi-gap RPCs, and to compare them with the available experimental results.This model could be used to optimize the characteristics of this type of detector with a view to its use in the future largeexperiments at LHC: ATLAS and CMS. ( 1998 Published by Elsevier Science B.V. All rights reserved.1. IntroductionResistive Plate Chambers (RPCs) were de-veloped at the beginning of 1980s [1], and wereoriginally thought to be operated in streamermode, meaning that the electric field inside the gasgap is so intense as to generate limited dischargeslocalized near the track of an ionizing crossingparticle. This operation mode allows us to obtaingood detection efficiency ('95%) and time resolu-tion (&1 ns), in particular, when a low flux ofincident particles is involved. At higher fluxes(*200Hz/cm2) RPCs begin to lose their efficiency,due to the fact that, after the streamer development,the area of the electrode plates involved in thedischarge needs a certain time (&10ms) to getcharged again.A way to overcome this problem is to operateRPCs in avalanche mode; the idea consists in usinglower electric fields, to reduce, as a consequence,the size and charge of the avalanches taking placein the gas gap and the corresponding electrodedischarged area. In this way a consistent improve-ment in the rate capability can be achieved. Satis-factory results up to a rate of about 1 kHz/cm2 havebeen reported [2]; for this reason, RPCs operatedin avalanche mode have been proposed as the dedi-cated detector for the generation of the first-levelmuon trigger, both in CMS and ATLAS at LHC.0168-9002/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reservedPII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 2 5 - 4 I. TRACKING (GAS)2. The modelA detailed description of the model used in thesimulation and its results will be reported ina forthcoming paper; here just the main items willbe repeated. The program starts from consideringan ionizing particle which crosses the gas gap andgenerates a certain number of clusters of ion—elec-tron pairs. The electrons contained in the clustersdrift towards the anode and, if the electric field issufficiently high, give rise to the avalanche pro-cesses.The primary cluster positions and the avalanchegrowth are assumed to follow, respectively, simplePoisson statistics and the usual exponential law.Avalanche gain fluctuations have been taken intoaccount using a Polya distribution [3]. After thesimulation of the drifting avalanches, the programcomputes, by means of the Ramo theorem, thecharge q*/$induced on the external pick-up elec-trodes (strips or pads) by the avalanche motion.Under certain approximations, this is given by theformula [4]q*/$"kgg+#-645%34q%-n0Mk[eg(g~x0)!1],where q%-is the electron charge, g the 1st effectiveTownsend coefficient, g the gap width, x0the clus-ter initial distance from the anode, n0the number ofinitial electrons in the considered cluster, M theavalanche gain fluctuation factor. k takes into ac-count the fact that not the whole charge can actual-ly be read out on the pick-up electrodes. In additionto q*/$, the current i*/$(t) induced on the same elec-trodes (as a function of time) by the drifting chargeq$(t)"q$(x/v$) may be computed asi*/$(t)"!kMq$(t)$)E8,where $is the electron drift velocity and E8is theweighting field inside the gap. The computation ofi*/$(t) allows to reproduce the whole informationcoming out from RPCs; it is possible to input thesimulated signals in simulated amplifiers, discrimi-nators, etc., reproducing, with accuracy, the datataking conditions of a real experiment.3. ResultsThe comparison between the simulated chargespectra for single narrow (g"2mm) and wide(g"9mm) gap RPCs is reported in Fig. 1 (the gasmixture is 90% C2H2F4and 10% i-C4H10, and willremain the same in all the following plots); thevalue of the 1st effective Townsend coefficient g hasbeen chosen in such a way that the averages of thetwo distributions are roughly the same: 0.6 pC theformer, 0.75 pC the latter. A change in the gas gapwidth has the effect to change the shape of thecharge distributions; more precisely, as it can beseen from the figure, in the case of a narrow gapthe charge distribution tends to diverge near theorigin, while the opposite is true for a wide gap.This has deep implications for the efficiency and therate capability that can be obtained with the twodifferent kinds of chamber.It is worth noting that, under certain approxima-tions, the shape of these distributions can be pre-dicted by means of simple analytical calculations;for instance, if only the contribution of the firstcluster (i.e. the closest to the cathode) is considered,the probability distribution function f1(q*/$) of theinduced charge q*/$can be expressed asf1(q*/$)"Aq((j@g)~1),where A includes the normalization constant andj is the primary cluster density. This function isreported in Fig. 1, and superimposes well to thesimulated spectra, in particular, in the region nearthe origin, which is the region of main interest; fora better agreement in the other regions the contri-butions coming from the other clusters should beconsidered. A comparison between the predictedand the experimental charge distribution for singlegap 2 mm RPCs is reported in Ref. [5].The events characterized by an induced chargeq*/$lower than the discrimination threshold ac-count for the chamber inefficiency; such events, infact, are not experimentally detected. This meansthat, starting from charge distributions, it is pos-sible to compute chamber efficiency; this is shownin Fig. 2 for single 2 and 3 mm gap 10]10 cm2RPCs. The chamber efficiency has been measuredwith cosmic k, by counting the coincidences be-tween signals coming from the chamber (with2 M. Abbrescia et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 1—5Fig. 1. Comparison between the simulated charge spectra for single (a) narrow (g"2mm) and (b) wide (g"9mm) gap RPCs.Fig. 2. Simulated and experimental efficiency and streamer probability for single 2 and 3mm RPCs.a 1mV discrimination threshold, i.e. 3 times morethan the noise level) and 4 trigger scintillators. Theexperimental 1mV amplitude threshold has beensimulated with a 100 fC charge threshold, for bothchambers. The predicted and the experimentalcurve superimpose very well, in particular the onesconcerning the 2 mm chamber. Also the predictedstreamer probability is reported in Fig. 2; it hasM. Abbrescia et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 1—5 3I. TRACKING (GAS)Fig. 3. Monte Carlo charge spectra for single- double- and multi-gap RPCs.been computed by counting the number of eventswith at least one avalanche characterized by a gaingreater than e20K4.9]108, which are comparedwith events experimentally characterized by an am-plitude greater than 30mV.The leap from the case of single gap RPCs todouble or multi-gap RPCs is, in principle, simple. Itis sufficient to simulate two or more identical gapsand sum the signals (instant by instant) comingfrom the gaps on the common read out electrode.However, the ratio k/gg between the chargeq*/$which is actually read out on the strips and theelectron charge which reaches the anode dependson the type of RPC considered. In particular, asshown in Ref. [4], in a single gap 2 mm RPCkK0.71, in a double gap kK1.42 and in a multi-gap kK0.78; this has the effect that, given a certainoperating condition for a single-gap RPC, a doublegap or a multi-gap made out of two or three gapswill have a different average value of the inducedcharge q*/$. This is shown in Fig. 3, where, in fact,the reported simulated charge spectra for single-double- and multi-gap 2mm RPCs are character-ized, respectively, by an average value of 0.34, 0.68and 0.37pC. In addition, from the figure it can benoted that the shape of the charge spectra changes,passing from the usual inverse power law of thesingle gap to a “Landau” shape for the double andthe multi-gap.The comparison between Monte-Carlo and ex-perimental charge spectra for a single and a double2mm gap 50]50 cm2 RPC is shown in Fig. 4; datawere taken with the CMS-H2 beam line at CERN-SPS, using 300 GeV/c muons: experimental datataking conditions are reported in detail in Ref. [2].The single-gap distribution was obtained using the4 M. Abbrescia et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 1—5Fig. 4. Simulated and experimental charge distributions for single and double 2mm RPCs.double-gap chamber with a gap turned off; theagreement between the predictions and the experi-mental data is very good and, in particular, it hasverified the change in the spectra shapes.4. ConclusionsA model describing the basic processes takingplace in RPCs in avalanche mode has been de-veloped. This model has been used to reproduce theavailable experimental data, and explains most re-sults, taken in many different conditions of opera-tion and with different kinds of chambers (single-double- and multi-gap). The agreement between thesimulation and the real data is very good where itconcerns charge distribution and efficiency. Thesimulation will be a very useful tool to exercisea choice among different RPC types, depending onthe performances required.References[1] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. 187(1981) 377—380.[2] M. Abbrescia et al., CMS Note/1997-062-Muons, Nucl.Instr. and Meth., submitted.[3] H. Genz, Nucl. Instr. and Meth. 112 (1973) 83—90.[4] M. Abbrescia et al., CMS Note/1997-004-Muons, Nucl.Instr. and Meth., acc. for publ. in press.[5] M. Abbrescia et al., Nucl. Instr. and Meth. A 392 (1997)155—160.M. Abbrescia et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 1—5 5I. TRACKING (GAS)

Resistive Plate Chambers in avalanche mode: a comparison between model predictions and experimental results

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Resistive Plate Chambers in avalanche mode: a comparison between model predictions and experimental results

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