This paper describes the multigap resistive plate chamber (RPC). The goal is to obtain a much improved time resolution, keeping the advantages of the wide gap RPC in comparison with the conventional narrow gap RPC (smaller dynamic range and thus lower charge per avalanche which gives higher rate capability and lower power dissipation in the gas gap)

Cerron-Zeballos, E

Crotty, Ian

Hatzifotiadou, D

Lamas-Valverde, J

Neupane, S

Williams, M C S

Zichichi, Antonino

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

CERN Document Server

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHCERN PPE/95-166 CERN/LAA-MC 95-23November 21, 1995A NEW TYPE OF RESISTIVE PLATE CHAMBER:THE MULTIGAP RPCE. Cerron Zeballos1,2, I. Crotty1, D. Hatzifotiadou1,2, J. Lamas Valverde1,2,3, S. Neupane1,2, M.C.S. Williams1and A. Zichichi41) LAA project, CERN, Geneva, Switzerland2) World Laboratory, Lausanne, Switzerland3) University Louis Pasteur, Strasbourg, France4) University of Bologna and INFN, Bologna, ItalyAbstractThis paper describes the multigap resistive plate chamber (RPC).  The goal is toobtain a much improved time resolution, keeping the advantages of the wide gap RPC incomparison with the conventional narrow gap RPC (smaller dynamic range and thus lowercharge per avalanche which gives higher rate capability and lower power dissipation in thegas gap).2To be submitted to Nuclear Instruments and Methods in Physics Research section A1. INTRODUCTIONResistive plate chambers have been actively developed during the last few years inorder to meet the challenge of the future hadron collider (LHC).  The goal is to produce alow cost detector that has good timing, space resolution sufficient for trigger purposes(readout strips of several cm width) and can withstand a flux of several kHz/cm2.  Twotypes of RPCs can be considered as candidates.  The more conventional RPC with a 2 mmgas gap has initially been developed to operate in streamer mode at very low flux.  Howeverit has been shown that one can operate it with a high fraction of freon (e.g. 85% freon13B1) in avalanche mode.  Another approach is to have a wide gap RPC and operate it inavalanche mode with more conventional freon-free gas mixtures.  We have discussed therelative differences in performance previously [1].  One finds a smaller dynamic range ofgain for the wide gap, thus one can operate it with a lower average avalanche charge for agiven threshold.  This leads to a higher rate capability and lower power dissipation in thegas volume; however it is easier to get good timing with narrow gap RPC.  In this paper wediscuss a new development of the RPC: the multigap RPC.  We show that with this detectorone can keep the advantages of the wide gap RPC, but have substantially improved timing.2. THE REASON FOR THE MULTI-GAPIn a parallel plate chamber, the gas gap is used both for the creation of the primaryionisation clusters and also for the gas gain.  The signal generated is by avalanchemultiplication across the gap.  The number of electrons, N, in an avalanche is given by:N=Noeαx, where α is the first Townsend coefficient, x the distance the avalanche hasprogressed from its initial position and No the initial number of electrons.  Thus thegenerated signal depends on the position of the primary ionisation clusters.  Withconventional non-flammable gas mixtures used in gaseous detectors, the number of primaryionisation clusters is 3-4 clusters/mm.  Primary ionisation follows Poisson statistics; thus towork at efficiencies close to 100%, a detectable avalanche signal needs to be produced froma single electron cluster located anywhere within the closest 1-1.5 mm to the cathode.  Thishas been previously discussed in our paper concerning the wide gap RPC [2].  Thisvariation of the position of the initial cluster generates a time jitter.  Typically, at theelectric fields we work with in the wide gap RPC, the drift speed of electrons is ~10 ns/mm;thus we would expect time resolutions with a full width at base (FWAB) in the order of 15ns.  This is what we observe [1].  The 2 mm gap RPC has been operated in avalanche modewith a high fraction of Freon 13B1 by us [1] and other researchers in this field [3,4].  SinceFreon 13B1 is heavy, the number of primary ionisation clusters should be ~10 clusters/mm.3Thus variations of the position of primary ionisation within the first 300-400 µm will definethe time resolution.  It is no surprise that the 2 mm gap chamber has a better timeresolution.Our goal is to improve the time resolution of the wide gap RPC without unduesacrifice of its other good qualities.  The principle advantage is the lower dynamic range ofsignal; this leads to smaller current flowing through the gas gap and resistive plates.  Thisallows operation at a higher rate and also lower power dissipated in the gas gap.  Anotheradvantage is that the larger gap is less sensitive to unavoidable variations of the gap width;thus mass production of large areas becomes easier.One way around this problem of the distribution of primary ionisation over 1.5 mm(i.e. the cause of the large time jitter) is to divide up this 1.5 mm into smaller slices.  Forexample in figure 1 we show (schematically) a 9 mm gap divided into 3 x 3 mm gaps.  Thevoltage is still applied on electrodes mounted on the outside surfaces of the chamber.  Theinternal plates are electrically floating, taking a voltage due to electrostatics.  In this casedetectable avalanches have to be producd from primary ionisation within 0.5 mm of any ofthe three cathode surfaces, and thus one could expect a threefold reduction in time jitter.Obviously it is necessary to operate these two devices with a different Townsendcoefficient, α.  For example, we consider an RPC with one 9 mm gap filled with a gas thatproduces an average of 3 primary ionisation clusters per mm for a minimum ionisingparticle.  99% of through-going charged particles produce ionisation within the 1.5 mmclosest to the cathode.  If we set the gas gain to be 106 over the remaining 7.5 mm, then asingle electron avalanching over this distance would produce a signal of 10 fC (only 7% ofthe total charge generates the fast signal due to the movement of the electrons).  This 10 fCsignal appears to be a reasonable lower limit for the threshold for electronics in use todayon large area detectors.  The gas gain of a single electron avalanching over the full 9 mmwould be 1.6 107.  We refer to this ratio of gain as the ‘dynamic range of gain’ (=16 in thiscase).  In order to have this gas gain, the value of the Townsend coefficient has to be α/p =0.024.  In the case of 3 x 3 mm gas gaps, 99% of through-going charged particles produceionisation within any of 0.5 mm regions closest to the cathode plate of all sub-gaps (asshown in figure 1).  The minimum signal  is when only one gap has a detectable avalanche,with a single electron avalanching over 2.5 mm.  Since the avalanche forms over a factor 3shorter distance, the induced signal is a factor 3 smaller; thus to produce a 10 fC signal weneed an avalanche that generates 3 106 electrons.  An electron that avalanches over the full3 mm would produce an avalanche of 5.9 107 electrons.  In this case the dynamic range ofgain is 20, but we need to operate with an α/p of 0.078.  In general α/p increasesexponentially with electric field [5]; thus a higher α/p implies also a larger dα/dE.  Sharmaand Sauli [5] were unable to measure values of α/p much above 0.04 due to the onset of4spark breakdown in their test cell.  Thus we find that this triple gap chamber has a similardynamic range of gain to the 9 mm single gap, but a much increased Townsend coefficient.Our concern is that for such a high value of the Townsend coefficient, it may be difficult tofind a gas that gives a long stable efficiency plateau.The two LHC experiments, ATLAS and CMS, propose using double gap RPCs in‘OR’ [6,7].  Is the principle of operation of these double gap RPCs different from themultigap RPCs described in this paper?  Obviously the multigap allows a greater number ofsub-gaps; we are now building a 6 gap version (3 sub-gaps on each side of a read out stripboard in a similar manner to the device proposed for LHC).  We have also built a multigapRPC consisting of 2 x 4 mm gas gaps.  In a comparison with the 3 x 3 mm multigap, onewould expect the 2 x 4 mm chamber to have a degraded time resolution, but maybe animproved efficiency plateau (since one works at a lower Townsend coefficient) and also animproved rate capability (since there is one less resistive plate in the chamber).  Howeverthe 3 x 3 mm has a longer efficiency plateau, especially at high fluxes.  This suggests theseintermediate plates have an unexpected beneficial effect on the performance of the chamber.Some reasons are suggested in section 5; obviously a more in-depth study is needed.3. CONSTRUCTION OF THE MULTI-GAP RPC.We use a similar construction technique as for our more standard wide-gap RPCs.The outer resistive plates are made from melamine-phenolic foils 0.8 mm thick.  Thesurface facing the gas is melamine; the electrodes are fixed to the phenolic surfaces.  Theintermediate dividing plates are melamine-phenolic-melamine foils 0.8 mm thick.  Aprinted-circuit board with strips forms the anode; the strips are on a 15 mm pitch with 0.25mm inter-strip gaps.  This is glued to the melamine-phenolic resistive plate with an epoxy*with a volume resistivity, ρ = 1011-13 Ω cm.  The cathode is a pad of conductive nickelpaint (1 Ω/   ).  The intermediate plates were held in place with spacer bars 6 mm wide and3 mm in height.  These spacer bars run the length of the chamber with a small gap at theend to allow for gas flow.  These spacer bars and the wider (3 cm wide) bar that made theframe around the edge of the chamber were glued to the melamine surfaces with double-sided tape.  The glue layer of this tape had a thickness of 100 µm.  When the chamber hadbeen fully assembled we potted the outside edges with dilute CAF-4#; this made thechamber gas tight and reduced any corona from the edge of the melamine sheets.  These                                               *  CY1311-HY1300, Ciba-Geigy Araldite Electronics Resins, Ciba-Geigy Plastics,Duxford, Cambridge, England#  Dow-Corning CAF-4 diluted with cellulose thinners.5spacer bars were offset from layer to layer so that any through-going particle would onlymiss one layer due to spacers.  The active area of the chamber is 24 x 24 cm2.4. TEST BEAM RESULTS.This chamber was tested in the T9 test beam in the East-hall at CERN.  The polarityof the final quadrupole was reversed in order to defocus the beam so that the whole activearea of the chamber was exposed to a uniform flux (i.e. flood illumination).  The totallength of the spill was 400 ms with a 250 ms flat top.  The electronics used was similar tothe electronics used for our previous RPC tests [8].  The threshold is ~10-50 fC and peakingtime 5 ns.  The efficiency versus voltage is shown in figure 2; we also show the darkcurrent.  The gas mixture used was 86% Argon, 8.5% CO2, 0.5% C4F10 and 5 % DME.We did not obtain a good efficiency plateau for gas mixtures without the addition of 0.5%C4F10.  We have worked with C4F10 previously (with a single wide gap chamber) [8],obtaining a better plateau but losing in rate capability.  Previously we had also found thatthe shape of the plateau changed with rate.  For this multigap chamber we see no change inplateau shape, just an overall reduction at the highest rate.  The rate capability is more thanequal to the best of the wide gap chambers [1].The time spectra of the average of the leading and trailing edge of the signal (similarto what one would expect from a constant fraction discriminator) is shown in figure 3 for anapplied voltage of 16 kV (200 V above the knee of the efficiency plateau).  The chargespectrum of the fast signal is shown in figure 4, also for a voltage of 16 kV.5. DISCUSSIONWe have tested a new type of RPC: the multigap RPC.  Basically we have taken aconventional wide gap RPC and added some inner resistive plates to divide the wide gapinto sub-gaps.  In this example, we have tested a 3 x 3 mm gap RPC.  One worry is thatthese inner resistive plates are electrically floating; how can we ensure that they take thevoltages as shown in figure 1?  We could add some electrode within the frame and apply avoltage to these inner resistive plates (around the edge of the chamber); however we do notthink that this is necessary.  The reason is that these inner resistive plates are in a strongelectric field and therefore take a voltage due to electrostatics.  If for any reason the voltageon one plate deviates, this will cause an increase of electric field in one sub gap and adecrease in the other; the gap with the higher field will produce larger avalanches (i.e. morecharge) compared to the other sub-gap producing smaller avalanches.  We see that this willrestore the voltage to its proper value so as to equalise the field in the two sub-gaps.  Wehave now tested this chamber with a variety of gases and rates and find the behaviour6consistently good.  We therefore suspect that leaving these inner plates electrically floatingis a stable solution.  Obviously we will test larger chambers to verify this concept.The multigap technique allows us to add many internal spacers, since we canarrange to have through-going particles passing through a maximum of one spacer.  Alsosince the spacer has a higher relative dielectric constant (ε~4) than gas, the electrostaticcoupling between the avalanche and the pick-up strips is enhanced.  This will help alleviateany reduction in efficiency around a spacer.  However it should be noted that these internalspacer bars will help eliminate the need for external large flat plates, originally thoughtnecessary for constructional purposes.We can build a multigap RPC on either side of a pick-up strip board (similar inconcept to those proposed for the CMS experiment).  Thus the timing will further improveand the efficiency plateau extend to lower voltages.  We intend to test this concept.  Theonly disadvantage (compared to the conventional wide gap RPC) is an increase of the stripmultiplicity.  Indeed we observe an increase, but it is small (the average increases from 1.5to 2 strips per cluster).  Since we suspect that some of our cluster size is caused by cross-talk within the pre-amplifiers mounted on the chamber, we avoid giving a precise numberuntil we have studied this problem in more detail.One problem with the conventional RPC is that extra large avalanches charge a localspot on the surface which results in local inefficiency.  If these internal plates are thinenough, a local deposition of charge on a surface of one of these will indeed cause a localdecrease in electric field (and thus efficiency) in one sub-gap, but the neighbouring sub-gapwill have an increase in field.  This compensation helps to produce a longer efficiencyplateau.  It should be noted that all these internal resistive plates have electrons fromavalanches fed into one side, and positive ions into the other.  On average there should beno net flow of charge into or out of these plates.6. CONCLUSIONWe have developed a new type of RPC.  Our initial tests are extremely promising.The time resolution is equal to the best we have obtained with the 2 mm gap chamber(operating in pure avalanche mode).  The rate capability is equal to the best we evermanaged with a conventional wide gap RPC.  In addition the dark current is very low andthe efficiency plateau long and stable.7AcknowledgementsIn all our work we are indebted to the service of the PS division, which providessuch a useful facility of test beams in the CERN East hall.  In addition, we have had toparasite on the beam allocation to other groups in order to test this chamber.  We thankeveryone that has helped make this such a pleasant possibility.References1. A comparison of the wide gap and narrow gap Resistive Plate Chamber.E. Cerron Zeballos, I. Crotty, D. Hatzifotiadou, J. Lamas Valverde, S. Neupane,V. Peskov, S. Singh, M.C.S. Williams and A. Zichichi, CERN PPE/95-146,CERN/LAA-MC 95-21 and subm. to Nucl. Instr. and Meth.2. The Wide Gap Resistive Plate Chamber, I. Crotty, E. Cerron Zeballos,J. Lamas Valverde, D. Hatzifotiadou, M.C.S. Williams and A. Zichichi, Nucl. Instr.and Meth. A 306(1995)512.3. R. Cardarelli, A. Di Ciaccio, R. Santonico, Nucl. Instr. and Meth. A333(1993)399.4. C. Bacci et al. Nucl. Instr. and Meth. A352(1995)552.5. See for example: A. Sharma and F. Sauli,  Nucl. Instr. and Meth. A 334(1993)420.6. ATLAS Technical Proposal, CERN/LHCC/94-43, LHCC/P2, Dec. 15, 1994.7. The CMS Technical Proposal, CERN/LHCC 94-38, LHCC/P1, Dec. 15, 1994.8. Latest Results of the Wide Gap RPC,  E. Cerron Zeballos, I. Crotty,D. Hatzifotiadou, J. Lamas Valverde, S. Neupane, V. Peskov, S. Singh,M.C.S. Williams and A. Zichichi, presented at the 3rd International Workshop onResistive Plate Chambers and Related Detectors, Pavia, Italy.  11-12 Oct. 1995.9. High Rate Resistive Plate Chambers, E. Cerron Zeballos, I. Crotty, D. Hatzifotiadou,J. Lamas Valverde, S. Neupane, S. Singh, M.C.S. Williams and A. Zichichi, CERNPPE/95-93, CERN/LAA-MC 95-16, presented at Vienna Wire Chamber Conference,Feb. 13, 1995.  Proceedings to be published in Nuclear Instruments and Methods.8FIGURE CAPTIONS1. Schematic diagram and principle of operation of multi-gap RPC compared to aconventional 9 mm single gap RPC.2. Efficiency versus high voltage for various fluxes.  The beam was defocused, thus thewhole active area of the chamber exposed.  The gas mixture was 86% Argon, 8.5%CO2, 0.5% C4F10 and 5% DME.3. Time spectra of the average of the leading and trailing edge timing at 16 kV (200 Vabove the knee of the efficiency plateau).4. Charge spectrum of the fast signal taken at 16 kV (200 V above the knee of theefficiency plateau).9AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACONVENTIONAL WIDE GAP RPCPrimary ionisation produced in the 1.5 mm closest to the cathode generates detectable avalanches Gas gap (5-9 mm)Pick-up strips (0 V)Cathode (-15 kV)MULTI-GAP RPCAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACathode (-15 kV)Pick-up strips (0 V)Intermediate Anode-Cathode plates (melamine resistive plates 0.8 mm thick) (takes voltage due to electrostatics) Primary ionisation produced in the 0.5 mm closest to the cathode generates detectable avalanches (-10 kV)(-5 kV)Figure 1AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA3 mm3 mm3 mm1002040608010000.511.5214.5 15 15.5 16 16.5 17 17.580 Hz/cm2400 Hz/cm24 kHz/cm2Dark current [µA]µVoltage [kV]Figure 23 x 3 mm Multi Gap RPC   Flood illumination110 50 100 150150010005000Time [ns] →→FWHM = 4 nsTime resolution  3 x 3 mm gapMULTI-GAP RPCFigure 316 kV86 %  Argon 8.5% CO2 0.5%  C4F10   5% DMEFlood illumination 150 Hz/cm2 120 5 10 15 pC0 100 200 300 400 500ADC bins0100020003000Average 2 pCFigure 416 kV  86%  Argon 8.5% CO2 0.5% C4F10 5% DMEMULTI-GAP RPCFlood illumination 150 Hz/cm2

A new type of resistive plate chamber: the multigap RPC

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