Cerron-Zeballos, E

Crotty, Ian

Hatzifotiadou, D

Lamas-Valverde, J

Neupane, S

Singh, S

Williams, M C S

Zichichi, Antonino

CERN Document Server

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHCERN-PPE/95-93CERN/LAA-MC 95-16February 6, 1995 HIGH RATE RESISTIVE PLATE CHAMBERSE. Cerron Zeballos1,2, I. Crotty1, D. Hatzifotiadou1,2, J. Lamas Valverde1,2, S. Neupane1,2, S. Singh1,2, M.C.S. Williams1 and A. Zichichi31) LAA project, CERN, Geneva, Switzerland2)World Laboratoty, Lausanne, Switzerland3)INFN and University of Bologna, ItalyAbstractIn this paper we consider some factors that could improve the high rate performance of theRPC; we consider the role of freon in the operation of RPCs and present results with anasymmetric RPC with one glass and one melamine resistive plate.Presented at the Vienna Wire Chamber Conference, February 13-17, 1995Proceedings to be published in  Nuclear Instruments and Methods in Physics Researchsection A21. INTRODUCTIONThe Resistive Plate Chamber is a simple gaseous detector; it has been the object ofextensive studies during the last years.  Essentially it is a gas volume enclosed by tworesistive plates.  High voltage is applied to electrodes on the outer surfaces of these resistiveplates, producing a high electric field across the gas gap.  Electrons liberated in the gas bythrough-going ionising particles avalanche in the high electric field, thus producing signalson the external electrodes.  If the electric field is even stronger, the avalanche can initiate aspark breakdown.  Due to its ease of construction and operation, the low cost and the goodtime resolution, it has been proposed as a suitable detector for the muon trigger for the LHCexperiments[1-2], where thousands of square metres will be needed.  The estimated flux dueto background radiation is up to thousands of Hz/cm2; therefore the device considered mustbe able to operate efficiently at such high rates.  In our effort to develop RPCs operatingwith maximum efficiency at high rates, we have undertaken systematic R & D in order tofind the most suitable plate material, gas mixture, width of the gas gap and mode ofoperation.  As described in our publications we have opted for the "avalanche" mode ofoperation (low gas gain); also we have chosen a wide gas gap, since this suppresses sparks[6].  The RPCs with which we performed the majority of the studies described here have an8 mm gas gap and resistive plates of melamine-phenolic laminate 0.8 mm thick.  Moredetails regarding the construction and operation of our RPCs can be found in our previouspublications [3-6].In this paper we discuss some factors that affect the high rate operation of the RPC.We first discuss, in section 2, the signal formation on pick up strips.  In section 3 we discussthe gas gain with freon as part of the gas mixture.  In section 4 we discuss the high rateoperation of the RPC and show that the flow of charge through a glass plate is differentfrom melamine laminate.2. SIGNAL FORMATION ON PICK UP STRIPSThe current in the pick up electrode is produced by the movement of charge.  Themagnitude and sign of this current can be calculated from the normalised ‘weighting field’,the concept of which has been described by Radeka [7].  The normalised weighting field iscalculated by applying 1 V to the electrode of interest and fixing all others at 0 V.  Its formmay have little relationship to the electric field needed to operate the device.  Current isproduced in the external electrodes by the movement of charge; the magnitude and sign canbe calculated from : i = q.EWv, where EW is the normalised weighting field and v is thevelocity of the charge q.  Thus the expected shape of the current pulse is shown in figure 1for an avalanche starting with one electron at the cathode, producing 4.4 107 electrons.  This3is the fast signal; by integration we find a charge of 460 fC on the pick up strip (1/15 of thetotal 7 pC charge of the electrons in the avalanche).  Our detection threshold of ~100 fC issimilar to the threshold mentioned by other researchers in the field of RPCs [8].  Theaverage charge on the strip is 1 pC, thus on average we have avalanches producing 15 pC ofelectron charge (108 electrons).3. GAS GAIN WITH FREON GAS MIXTURESIn the first studies of RPCs operated in spark mode, it was found necessary to addfreon to the gas mixture.  Otherwise the efficiency decreased with increasing voltage and itwas difficult to obtain high efficiencies, even at very low rates.During the avalanche process an electron can either multiply through an ionisingcollision or be captured by a freon molecule to form a negative ion.  The cross section forelectron capture by a freon molecule is strongly peaked towards low energies.  At lowelectric fields, where the kinetic energy gained by the electron between collisions is low, theattachment coefficient h  is high and the Townsend coefficient a  low [9].  The number ofelectrons produced in an avalanche is N = Noe a x, where No is the initial number ofelectrons, x is the distance and a  is the Townsend coefficient.  For freon we replace a in theabove with the effective a,  a-  = a-h , and this increases linearly with E [9] (rather than theexponential rise observed in gas mixtures not containing freon [10]).  So, for a gas mixturecontaining freon, we would expect a) faster drop in efficiency on reducing the voltage, b)longer efficiency plateau (since there is a slower rise in gas gain).  In figure 2 we show theefficiency curves for 10% isobutane with 90% Argon with and without the addition of 0.5%of C4F10.  The change is remarkable and as expected.Since wide gap RPCs have a smaller dynamic range in avalanche size [6], they areless likely to produce sparks; we have therefore chosen to work with wide gas gaps.However, it may be possible for chambers with narrow gaps (~2 mm) to be operated in a‘pure avalanche’ mode.  Duerdoth et al.[11] have measured the transition between‘proportional’ and ‘streamer’ mode (their nomenclature) with freon mixtures varyingbetween 4 and 40%.  They worked with a gas gap of 1.6 mm.  They find that there is athreshold of avalanche size above which a streamer is initiated.  This threshold slightlyincreases with higher freon concentrations.  With increasing freon concentration theymeasured a) a smaller rise in the magnitude of the streamer signal with increasing voltage,b) a smaller step between the proportional and the streamer signal.  In figure 3 we plot theratio of charge of the proportional / streamer signal as a function of the freon percentage, atthe knee of the plateau (lower curve) and at 1000 V above the knee of the plateau (uppercurve).  A linear extrapolation predicts that if one works with a very high fraction of freon(~80%), the streamer signal becomes equal in magnitude to the proportional signal.4However one should not overlook that the above measurement is of the ‘fast’ signal.  Thereis an increasing amount of charge carried by positive and negative ions; although this chargedoes not generate a fast signal, it does exist and has to be neutralised by current flowthrough the resistive plates.  Thus at high rates there will be a much larger voltage dropacross the resistive plate.  However this can be compensated by increasing the high voltageapplied to the chamber (this will increase the noise/dark current - at present we are aware ofno published results on such measurements).The avalanche process is terminated by the arrival of the electrons at the anode.  Thefast signal is generated by the movement of the electrons.  However with freon a largefraction of the electrons are captured and become immobile.  In figure 4 we show the chargeon the cathode observed from a 4 mm gap RPC using 3 different gas mixtures.  In the upperplot, the gas contained no freon; a clear step is observed due to the arrival of the electrons.The slow ramp is due to the drift of the positive ion cloud towards the cathode.  This rampflattens out when the positive ions arrive at the cathode.  The middle plot is with a gascontaining CF4, which is not so electronegative as other freons.  The fast step due to theelectrons is smaller in comparison with the ramp caused by the positive ions.  The lowerplot is for a gas containing 10% freon (CCl2F2).  The fast step can no longer be observed.Thus one sees that the ratio of ‘fast signal’ compared to slow deposition of charge is manyorders different if freon is part of the gas mixture.We have tested a chamber with freon C4F10.  It is a heavy gas and thus the numberof primary ionisation clusters/mm should be high (~15 clusters/mm).  This would haveallowed us to consider constructing RPCs with smaller gas gaps; however we found that ithad poor dielectric strength.  If we used a percentage higher than 3%, the chamber woulddraw continuous current at a voltage below normal avalanche operating mode.  However,like other freons, it is very electronegative.  We have discussed above that the total amountof generated charge in the avalanche process is much higher if freon is used.  At highparticle flux the efficiency is reduced by the voltage drop across the resistive plate.  Thus,we would expect with increasing rate, a faster fall in efficiency for gases containing freonthan those gases not containing freon.  We show in figure 5 the efficiency versus voltage forvarious rates.  The curves refer to 10% isobutane, 89.5% Argon and 0.5% C4F10.  We alsoshow the dark current drawn by the chamber.  In figure 6 we plot the efficiency versus rate.The lower curve is keeping the voltage constant (at 12.2 kV), while the upper curve is withan increasing voltage for the higher rate points.  We also plot the data for 40% DME60% Argon for a fixed high voltage.  This falls on the upper curve.  Thus the rate capabilityof a chamber operated with freon can be boosted by increasing the high voltage to mimic achamber operated without freon.  The price one has to pay is an increased dark current (thusincreased noise).54. HIGH RATE BEHAVIOUR AND THE ASYMMETRIC RPCAt the CERN PS the spill is 400 ms in length (however the majority of the particlesarrive during 250 ms).  Depending on the structure of the ‘super-cycle’ we can have eitherone or two spills every 15 to 20 s.  The results quoted from test beams are biased by thefinite length of the spill.  In our first studies of RPCs operated in spark mode we found alarge drop in efficiency during the spill [4].  In our later studies with avalanche modeoperation we found that the majority of the change occurred during the first 50 ms of thespill.  Recently we have made a measurement with an asymmetric RPC and studied thechange of efficiency during the spill.Our asymmetric RPC has the two resistive plates made from different materials: oneplate is Schott A4 glass*, 1.8 mm thick and the other plate is melamine-phenolic laminate,0.8 mm thick; the gas gap is 8 mm.  The cross section is shown in figure 7.  The resistivitiesare 1011 W  cm for the melamine laminate (with 1% water vapour) and 1012 W  cm for theglass (independent of water vapour).  Assuming a relative dielectric constant e =2 for themelamine laminate and e =4 for the glass, the capacitance per square centimetre is 2 pF/cm2.When high voltage is applied on the RPC, initially the resistive plates are in anelectric field.  The resistive plate is a dielectric, the electric field causes electric dipoleswithin the material to be aligned such that the voltage at the surface is reduced.  Since thereis an electric field inside the resistive sheet, any free charge carriers inside the plate willmigrate so that this field is neutralised.  Thus, eventually, the anode surface will have anexcess of holes, while the cathode surface an excess of electrons; the field inside the platewill be very weak, tending towards zero.In figure 8 we show the efficiency plateau for the asymmetric RPC; we see that adifferent voltage is required depending on whether the glass plate is the anode or the cathodeelectrode.  An extra 1000 V is needed if glass is used as the anode electrode.  A symmetricRPC constructed with both plates of melamine laminate operated at a similar voltage to thatof the asymmetric chamber with glass cathode electrode.Fixing the voltage and varying the rate gives similar results for both the glass anodeand cathode to previously measured chambers [6].  A notable exception is that it is easier toreach 100% efficiency at low rates when the glass is the cathode; also the plateau is longer.We have measured the variation of efficiency during the spill for the case of the glass* Schott produces dark glasses for various purposes, such as the fabrication of weldinggoggles.  The darkness is obtained by adding iron oxide to the glass, which also decreasesthe resistivity.  This glass is available with values of A1 through to A14; higher A valuescorrespond to darker, lower resistivity glass.6cathode (the case of the glass anode was not measured mainly for technical reasons relatedto the strip readout).  We show the results in figure 9a for three rates.  The rates we havequoted in previous papers are obtained by integrating the number of particles triggering thescintillators over a full spill and assuming a spill is 1/3 s.  The particle flux during the spill isshown in figure 9b.  It is clear that the spill is shorter than we previously estimated.For the 2 kHz/cm2 data there is a drop in efficiency to ~80% and then it stabilises.From figure 8 we see that the voltage has to be reduced by 600 V to give this efficiency.Since the resistance per cm2 is 20 times higher for the glass as compared to the melaminelaminate, we assume the cathode is responsible for this voltage drop; thus with a resistanceof 2 1011 W /cm2, 3 nA/cm2 produces the 600 V drop.  This is equivalent to an averagecharge of 1.5 pC/avalanche at a rate of 2 kHz/cm2.  We now consider an element of the plateas a capacitor (2 pF/cm2) and find that 1200 pC/cm2 gives a 600 V drop.  This happens in100 ms, i.e. 200 avalanches.  Thus one has on average 6 pC/avalanche.  Thus the averagecharge per avalanche drops from ~10 pC to ~2 pC when the voltage is reduced by 600 Vacross the gas gap.  This reduction is consistent with our expectations from the firstTownsend coefficient.5. SUMMARY AND CONCLUSIONSThe resistance per unit area (rather the resistivity) is important as this dictates thevoltage drop across the resistive plate for a given current flow.  Thus for a given platematerial, thinner sheets should give a higher rate cut-off.  The capacitance per unit area isalso important; a deposit of charge on the surface of the plate will cause a smallerdisturbance to the electric field if the capacitance is high; thus again thinnest possible platematerials should be employed.A wide gas gap allows operation of the RPC with low spark probability utilising gasmixtures with low (or zero) freon contents.  Narrow gap RPCs need a high fraction of freonto suppress sparks.  We have seen that there is an increase of ionic current when freon is partof the mixture; however this needs to be measured for gases with high freon content as theavalanche process could be different. The rate limit of the asymmetric chamber presented here is ~ 100 Hz/cm2(efficiency > 95%).  As discussed above, this is dominated by the resistance of the glassplate.  Glass with one order lower resistivity is available commercially in large sizes.  Wealso believe that we can increase the sensitivity of our preamplifiers by a factor 5.  Thisshould increase the rate limit to 5 kHz/cm2.  Higher rates can be obtained by using speciallyprepared materials.  It should be noted that the rate limit we quote is the most pessimistic(i.e. for flood illumination with the voltage set for low rate, low noise operation).7References1. ATLAS Technical Proposal, CERN/LHCC/94-43, LHCC/P2.2. CMS Technical Proposal, CERN/LHCC/94-38, LHCC/P1.3. I. Crotty et al., Nucl. Instr. and Meth. A 329 (1993) 133.4. I. Crotty et al., Nucl. Instr. and Meth. A 337(1994) 370.5. I. Crotty et al., Nucl. Instr. and Meth. A 346(1994) 107.6. I. Crotty, E. Cerron Zeballos, J. Lamas Valverde, D. Hatzifotiadou, M.C.S. Williamsand A. Zichichi, The Wide Gap Resistive Plate Chamber, CERN PPE/94-200,CERN/LAA-MC 94-33; submitted to Nucl. Instr. and Meth. A.7. V. Radeka, Ann. Rev. Nucl. Part. Sci. 38(1988) 217.8. C. Bacci et al., Nucl. Instr. and Meth. A 352 (1995) 552.9. Y. Qiu, X. Ren, Z. Y. Liu and M. C. Zhang, J. Phys. D: Appl. Phys. 22 (1989) 1553.10. A. Sharma and F. Sauli, Nucl. Instr. and Meth. A 334 (1993) 420.11. I. Duerdoth et al., Nucl. Instr. and Meth. A 348 (1994) 303.8FIGURE CAPTIONS1. Expected shape of current pulse on pick up strip for a gas avalanche producing4.4 107 electrons.2. Efficiency versus voltage for Argon-isobutane, with and without the addition of0.5% C4F10.3. Ratio of charge of proportional /streamer signals versus freon percentage in gasmixture (data from ref. [11]).4. The signal from an RPC (4 mm gas gap) observed with a charge amplifier attachedto the cathode.  The upper plot is for a gas not containing freon, the middle plot is fora gas containing CF4; the lower plot is for a gas containing freon (CCl2F2).5. Efficiency versus voltage for various particle fluxes.  The gas mixture is 89.5%Argon, 10% isobutane and 0.5% C4F10.6. Efficiency versus particle flux.  The upper curve is for 40% DME 60% Argon andkeeping the voltage fixed and also for 89.5% Argon 10% isobutane and 0.5% C4F10but with an increased voltage for higher rates.  The lower curve corresponds to89.5% Argon 10% isobutane and 0.5% C4F10 with a fixed voltage.7. Cross section of the asymmetric RPC.8. Efficiency versus voltage for asymmetric glass/melamine RPC at (upper) 1 Hz/cm2,and (lower) 1 kHz/cm2.  The gas mixture was 42% Argon, 19% DME, 39% CO2 .9. (a) Efficiency during the spill for 3 particle fluxes.  The gas mixture was 42% Argon,19% DME, 39% CO2.  (b) Measurement of incident particle flux during spill in 4 x 4cm2 scintillator.9-50-40-30-20-10070 90 110 130 150 170current [mA]Time [ns]Figure 1Avalanche producing 4.4 107 electronsVelocity of electrons in avalanche  5 106 cm/s8 mm gap RPC CURRENT PULSE ON PICKUP ELECTRODE1002040608010011 11.5 12 12.5 13 13.5Voltage [kV]89.5% Argon 10% iso C4H10  0.5% C4F1090.0% Argon 10% iso C4H10  Efficiency [%]Figure 211Percent of CF3Br [%]RATIO11010 210 30 20 40 60 80 100At knee of plateau1000 V above kneeCHARGE OF STREAMER / CHARGE OF PROPORTIONAL PULSEFigure 3122 m s / div500  mV / divArgon  =  87%Ethane =  13%6.5 kV / 4 mmCF4  =  14%Ethane =  11%Argon = 75%8.8 kV / 4 mmFreon-12 = 10%Isobutane = 13%Argon  = 77%12 kV / 4 mmFigure 413Figure 511.5 12 12.5 13 13.5Voltage [kV]0510152025303540Dark current [mA]10 Hz/cm2 900 Hz/cm 21800 Hz/cm2020406080100Efficiency [%]Dark current147075808590951000 500 1000 1500 2000Rate [Hz/cm2]Voltage  increased for higher ratesVoltage  fixed - no freon in gas mixtureVoltage fixed at 12.2 kVEfficiency [%]Figure 615Pick-up strips (0v)Melamine-Phenoliclaminate (0.8 mm  thick)A4 Schott Glass (1.8 mm thick)High Voltage Electrode(+VE or -VE)8 mmFigure 716Figure 8020406080100Efficiency [%]Glass AnodeGlass Cathode02040608010015 15.5 16 16.5 17 17.5 18Voltage [kV]Efficiency [%]RATE  ~1 kHz/cm2RATE  ~1 Hz/cm2Glass AnodeGlass Cathode176065707580859095100100 Hz/cm 2400 Hz/cm 22 kHz/cm 2Efficiency [%]0 100 200 300 400 5003020100Rate in 4*4 cm2  [kHz]Time [ms]Figure 9

High rate resistive plate chambers

E. Cerron Zeballos

I. Crotty

D. Hatzifotiadou

J. Lamas Valverde

S. Neupane

S. Singh

M.C.S. Williams

A. Zichichi

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High rate resistive plate chambers

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