We present recent results of an extensive aging test, performed at the CERN
Gamma Irradiation Facility on two single--gap RPC prototypes, developed for the
LHCb Muon System. With a method based on a model describing the behaviour of an
RPC under high particle flux conditions, we have periodically measured the
electrode resistance R of the two RPC prototypes over three years: we observe a
large spontaneous increase of R with time, from the initial value of about 2
MOhm to more than 250 MOhm. A corresponding degradation of the RPC rate
capabilities, from more than 3 kHz/cm2 to less than 0.15 kHz/cm2 is also found.Comment: 6 pages, 7 figures, presented at Siena 2002, 8th Topical Seminar on
  Innovative Particle and Radiation Detectors 21-24 October 2002, Siena, Ital

Adinolfi

Agosteo

Arnaldi

Bacci

Bizzeti

Cardarelli

Crotty

D. Domenici

E. Santovetti

G. Carboni

G. Collazuol

G. Ganis

G. Passaleva

Ganis

M. Veltri

Passaleva

R. Messi

S. De Capua

Ćwiok

English

arXiv

We present recent results of an extensive aging test, performed at the CERN Gamma Irradiation Facility on two single-gap RPC prototypes, developed for the LHCb Muon System. With a method based on a model describing the behaviour of an RPC under high particle flux conditions, we have periodically measured the electrode resistance R of the two RPC prototypes over three years: we observe a large spontaneous increase of R with time, from the initial value of about 2 MΩ to more than 250 MΩ. A corresponding degradation of the RPC rate capabilities, from more than 3 kHz/cm2 to less than 0.15 kHz/cm2 is also found.</p

Carboni, G.

Collazoul, G.

De Capua, S.

Domenici, D.

Ganis, G.

Messi, R.

Passaleva, G.

Santovetti, E.

Veltri, M.

The University of Manchester - Institutional Repository

New results from an extensive aging test on bakelite Resistive Plate Chambers

We present recent results of an extensive aging test, performed at the CERN Gamma Irradiation Facility on two single-gap RPC prototypes, developed for the LHCb Muon System. With a method based on a model describing the behaviour of an RPC under high particle flux conditions, we have periodically measured the electrode resistance R of the two RPC prototypes over three years: we observe a large spontaneous increase of R with time, from the initial value of about 2 MOhm to more than 250 MOhm. A corresponding degradation of the RPC rate capabilities, from more than 3 kHz/cm^2 to less than 0.15 kHz/cm^2 is also found

Carboni, G

Collazuol, Gianmaria

De Capua, Stefano

Domenici, Danilo

Ganis, Gerardo

Messi, Roberto

Passaleva, Giovanni

Santovetti, Emanuele

Veltri, Michele

CERN Document Server

1New results from an extensive aging test on bakelite Resistive PlateChambersG. CarboniaROM2]Universita di Roma \Tor Vergata" and INFN { Roma II, Via della RicercaScientica 1, I{00133 Roma, Italy, G. CollazuolbFIRE]Universita di Firenze and INFN { Firenze, ViaG. Sansone 1, I{50019, Sesto F.no, Firenze, Italy, S. De Capuac[ROM2], D. Domenicid[ROM2], G.Ganise[ROM2], R. Messif [ROM2], G. Passalevag[FIRE], E. Santovettih[ROM2], M.VeltriiURBI]INFN{Firenze and Universita di Urbino, V. S. Chiara 27, I{61029 Urbino, Italya[b[i[We present recent results of an extensive aging test, performed at the CERN Gamma Irradiation Facilityon two single{gap RPC prototypes, developed for the LHCb Muon System. With a method based on a modeldescribing the behaviour of an RPC under high particle flux conditions, we have periodically measured theelectrode resistance R of the two RPC prototypes over three years: we observe a large spontaneous increase of Rwith time, from the initial value of about 2 MΩ to more than 250 MΩ. A corresponding degradation of the RPCrate capabilities, from more than 3 kHz/cm2 to less than 0.15 kHz/cm2 is also found.1. INTRODUCTIONResistive Plate Chamber (RPC) detectors havebeen proposed to cover a large fraction (about48%) of the Muon System [1,2] of the LHCb ex-periment [3]. The LHCb apparatus covers theforward part of the solid angle and is thereforesubject to a very large particle flux. In particu-lar, the particle rates expected in the Muon Sys-tem are signicantly larger than those expectedin the corresponding sub{detectors in ATLAS [4]and CMS experiments [5]. In the regions cov-ered by RPCs, the maximum particle rate is ex-pected to vary between 0.25 and 0.75 kHz/cm2,depending mainly on the polar angle. To copewith such high rates, LHCb RPCs are operatedin avalanche mode [6{8], allowing to obtain a ratecapability up to some kHz/cm2 [9{12]. In theseconditions, the rate capability of an RPC is deter-mined by the volume resistivity ρ of the electrodesand scales roughly as 1/ρ. For this reason, RPCsused in experiments working at high rates are gen-∗corresponding authorerally built with bakelite electrodes, that can beproduced with resistivities as low as 109 Ωcm.Variations of the electrode resistivity aect di-rectly the rate capability of the RPCs: it is there-fore very important to be able to monitor thisparameter during the chamber operation. Thesevariations can be due to changes in environmentalparameters like temperature and humidity [13,14]or to possible aging eects due to operating con-ditions.To study these eects, we have devised, in theframework of the aging studies for the LHCbmuon chambers, an extensive series of tests whichstarted in January 2001 and is expected to lastuntil December 2002, exploiting the large CERNGamma Irradiation Facility [15].In this paper we present the latest results fromthese studies. Results from a systematic seriesof measurements of the resistivity of the bakeliteelectrodes of two RPC prototypes are presented.A study of the RPC rate capabilities is also dis-cussed.22. SETUP OF THE AGING TESTThe aging tests have been performed at theGamma Irradiation Facility (GIF) at CERN. TheGIF is a test area where particle detectors areexposed to an adjustable photon flux from an in-tense 137Cs radioactive source with an activity ofabout 655 GBq. A muon beam from the SPS ac-celerator traverses the test area, so that variousmeasurements can be performed on detectors inpresence of the high background flux of photonsfrom the source. In our case the source was alsoused to perform an accelerated aging test of thedetectors [16]. A system of remotely controlledlters allows us to vary the photon rate by fourorders of magnitude. The photon rate has beenfound to depend from the lter absorption factorAbs as  = 1/Abs0.7X5 beamS1 S2 S3Hodoscope137CsSource filters312ConcreteshieldFigure 1. Schematic view of the test setup (not toscale). The positions of the RPCs correspondingto the various measurements are indicated (1-3).The distance of the RPC from the source in posi-tion 1 was about 55 cm, and in position 3 about140 cm. The scintillator counters (S1-S3 and theHodoscope) were used for measuring the RPC ef-ciency with the particle beam.In our tests, two equal RPC detectors wereoperated in the GIF area. The detectors (50 50 cm2 with a 2 mm gas gap) were built usingbakelite plates (2 mm thick) of nominal resistivity 1010 Ωcm, and treated internally with linseedoil.Both detectors were operated with thesame gas mixture, normally 95% C2H2F4, 4%i− C4H10 and 1% SF6. High-voltage V0, gas mix-ture composition, currents and temperature werecontinuously monitored and recorded during allthe test period.The test setup is schematically shown in Fig-ure 1. Normally during the aging test one detec-tor (RPC A) was placed in position 1, very closeto the source and almost continuously exposedto radiation, whereas the second (RPC B) wasplaced far from the source (position 2), to serveas a reference.Position 3 was used to perform eciency mea-surements with the particle beam. In this casethe signals from the RPCs were read out on 3 cmwide strips using fast electronics. A telescope ofscintillator counters, also shown in Figure 1, pro-vided the trigger and a hodoscope measured theparticle position (x− y) with an accuracy betterthan 1 cm. At the minimum source attenuationthe measured flux density in Position 3 was about1 kHz/cm2 [17].3. BEHAVIOUR OF RPCs UNDERHIGH FLUX CONDITIONSWe have observed that the current drawn byRPC detectors subject to a high particle flux,shows a characteristic behaviour: it depends linearly on the applied voltage,above a certain threshold; it saturates with increasing flux values; it depends exponentially from the tempera-ture at a xed applied voltage.We have interpreted these eects with a model,described in details in [18]. In this model we as-sume that the onset of the avalanche in the detec-tor arises only above a threshold voltage VT . Due3to space charge eects, the avalanche charge de-pends linearly on the eective voltage Vgap acrossthe gas gap: q / Vgap − VT . Due to the volt-age drop across the RPC electrodes, we have:Vgap = V0 − IR, where V0 is the nominal ap-plied voltage, I is the current drawn by the RPCand R is the total volume resistance of the twoelectrodes. The current I depends linearly on qthrough the incident flux : I = q(Vgap). Inthe limit of innite flux, in order to keep the cur-rent nite, we should have Vgap = VT and in theseconditions we get a saturation value for the cur-rent Imax = (V0 − VT )/R. At nite flux values,one obtains [18]:I =(X1 + X)V0 − VTR=V0 − VTReff, (1)where X / R and Reff = R(1 + X)/X . Equa-tion 1 is the central equation of the model: itshows that the current depends indeed linearlyfrom V0 and that it saturates at high flux valuesand incorporates the temperature dependence ofthe current through the electrode resistance R.Typical I−V0 curves of the two detectors undertest, taken for Abs = 1 are shown in Figure 2. Aperfect linearity is observed above the thresholdvoltage VT (about 8500 V in this case). The re-sistance R can thus be measured by tting thesecurves with Equation 1 and obtaining the param-eter X by measuring the RPC current at xed V0for dierent flux values. In this way, the electroderesistance can be measured in a non destructiveway and easily monitored during detector opera-tion.4. RESULTS ON RPC RESISTANCEMEASUREMENTSIn the framework of the RPC aging tests,we have performed a systematic set of measure-ments of the RPC electrode resistances using themethod described above.In order to compare measurements taken at dif-ferent ambient temperatures T , we have rescaledthe values of R to 20C assuming an exponen-tial dependence from T [18]. Figure 3 shows themeasurement of R for the two RPCs at dier-ent temperatures. An exponential t gives an av-Run  453RPC ARA =  110 MΩ Abs = 1RPC BRB =  89 MΩV0 (V)I (µA)051015208000 9000 10000Figure 2. Current vs. V0 for RPC A (solid cir-cles) and B (open circles) for absorption factor1. The eective resistances Reff as obtained fromthe slopes are also shown. In this case we hadX = 49.8 8.9.erage temperature coecient of hαi = 0.126 0.008 C−1 in nice agreement with that obtainedfor the bakelite slabs themselves. This conrmsthat R is indeed the volume resistance of the RPCelectrodes.The measurements of R rescaled to 20C areshown in Figure 4 and 5 for RPC A and RPC Brespectively. The time scale starts at the begin-ning of the aging test in January 2001. In therst 200 days, RPC A was subject to an intensephoton flux and integrated a charge of about 0.4C/cm2 which corresponds to about 10 years ofoperation in LHCb [16]. As shown in Figure 6The current drawn by the detector decreased bya factor of six in the same period. This behaviourwas interpreted as a corresponding increase of theelectrode resistance due to the current flowingthrough the electrodes themselves. This is in-deed clearly visible in Figure 4. In the remain-ing time, the chamber was again operated at theGIF but, because of the large resistance, the cur-rent drawn by the detector was negligible. Still,415/05/200214/06/2002R20A = (154.6 ± 37.4) MΩαA = 0.134 ± 0.036R20B = ( 79.2 ± 1.6) MΩαB = 0.121 ± 0.012R20B = ( 97.3 ± 2.2) MΩαB = 0.126 ± 0.003T (oC)R (MΩ) 103050709011016 18 20 22 24 26 28 30103050709011016 18 20 22 24 26 28 30Figure 3. Resistances of RPC A and B plottedversus temperature. The temperature coecientα is tted using an exponential dependence [18].a large increase of R by another factor four is ob-served. This eect is conrmed by the measure-ments performed on the prototype B (Figure 5)that was exposed to the intense photon flux onlyfor a short time. These results seem to indicatethat the resistivity of the bakelite electrodes tendsto spontaneously increase at a rate much largerthan that observed when the RPCs are subjectto the particle flux expected in the LHCb exper-iment. It is clear that this would be by far themain aging eect over 10 years of operation inLHCb.We don’t have yet a quantitative interpretationof these phenomena, although we believe that thisis probably related to a decrease of water contentin the bakelite plates. While water evaporationfrom the plates is always present, it is probablyenhanced both by the current flowing in the elec-trodes and by the flux of dry gas in the chambers.To cross{check this interpretation we are going tostart a series of measurements, flushing our RPCswith a gas mixture containing water vapour.time (days)R20 (MΩ)0701402102803500 100 200 300 400 500 600Figure 4. Measurements of R for RPC A versustime. The time scale starts at the beginning ofthe aging test in January 20015. ANALYSIS OF RATE CAPABILITIESThe maximum incident particle rate that anRPC can stand is roughly proportional to theinverse of the electrode volume resistivity. It istherefore essential to check what is the eect ofthe increase of the bakelite resistivity describedabove on the detector rate capability. To studyquantitatively this eect, we dene the rate ca-pability rmax of our prototypes as the maximumrate where their eciency is at least 95%, at amaximum operating voltage of 10.6 kV (this guar-antees a plateau of about 400 V below the thresh-old of streamer regime). The results of threebeam tests performed over a three years periodare summarised in Table 1. In the 1999 test, themaximum available particle rate at the GIF wasabout 3 kHz/cm2 but, at this rate, the chambereciency was well above 95%; the rate capabilitywas therefore much larger than 3 kHz/cm2. Wecan clearly see that the rate capability is droppingroughly as 1/R as expected. The rate capability5Table 1Resistance R and rate capability rmax (see text) measured for RPC A in three dierent beam tests atthe GIF. The values at 20 oC are obtained with a temperature coecient α = 0.126 and scaling rmaxlike 1/R.Test T (C) R (MΩ) R(20oC) (MΩ) rmax (kHz/cm2) rmax(20oC) (kHz/cm2)Oct. 1999 23.0 1.8 2.6 > 3 > 3Aug. 2001 25.1 31.6 58.3 1.0 0.6Jul. 2002 24.5 102.4 175.7 0.3 0.2time (days)R20 (MΩ)0701402102803500 100 200 300 400 500 600Figure 5. Measurements of R for RPC B versustime. The time scale starts at the beginning ofthe aging test in January 2001extrapolated to the latest measurements of R istherefore about 0.15 kHz/cm2 which is well belowthe rates foreseen in LHCb (> 0.25 kHz/cm2).These results have recently brought the LHCbcollaboration to abandon the RPC technology forthe Muon System of the experiment.6. CONCLUSIONSWe have developed and applied a method tomeasure RPC electrode resistance R, on{line,during chamber operations, in an easy and nondestructive way. Using this method we have stud-ied extensively the aging eects on bakelite RPCs.We have observed an increase of R by two ordersof magnitude in about three years. Pure spon-taneous aging is the dominant eect. The RPCrate capability dropped correspondingly from afew kHz/cm2 to less than 0.15 kHz/cm2. Theseresults brought the LHCb collaboration to aban-don the RPC technology for the Muon System ofthe experiment.time (days)I 20 (µA)20304050607080901002003000 50 100 150Figure 6. Current for RPC A corrected for tem-perature (see text) plotted versus time. The de-tector was placed at about 55 cm from the source.6rate (Hz/cm2)ε/εoff 9800 V10000 V10200 V10400 V10600 V0.650.70.750.80.850.90.9510 200 400 600 800 1000 1200 1400Figure 7. Eciency vs. rate for RPC A, at dif-ferent V0 values, measured in August 2001.REFERENCES1. The LHCb Collaboration, LHCb Muon Sys-tem Technical design Report, CERN/LHCC2001-010, 2001.2. A. Bizzeti et al., Design and Construction ofthe RPC Detector for the LHCb Muon sys-tem, Note LHCb 2001-027, June 20013. The LHCb Collaboration, LHCb TechnicalProposal, CERN/LHCC 98-4, 1998.4. The ATLAS Collaboration, ATLAS MuonSpectrometer Technical Design Report,CERN/LHCC 97-22, 1997.5. The CMS Collaboration, CMS Muon Tech-nical Design Report, CERN/LHCC 97-32,1997.6. R. Cardarelli, A. Di Ciaccio and R. Santonico,Nucl. Instr. and Meth. A333 (1993) 399.7. C. Bacci et al., Nucl. Instr. and Meth. A352(1995) 552.8. R. Cardarelli, V. Makeev, R. Santonico, Nucl.Instr. and Meth. A382 (1996) 470.9. R. Cardarelli, Scientica Acta VIII (Univ.Pavia), 3 (1993) 159.10. M. Adinol et al., Nucl. Instr. and Meth.A456 (2000) 95.11. R. Arnaldi et al., Nucl. Instr. and Meth. A456(2000) 72.12. M. Cwiok et al., Nucl. Instr. and Meth. A456(2000) 87.13. R. Arnaldi et al., Nucl. Instr. and Meth. A456(2000) 140.14. I. Crotty et al., Nucl. Instr. and Meth. A346(1994) 107.15. S. Agosteo et al., Nucl. Instr. and Meth. A452(2000) 94.16. G. Passaleva et al., Proceedings of the \In-ternational Workshop on Aging Phenomenain Gaseous Detectors", DESY, October 2001;physics/0302029. Note LHCb 2003-013.17. G. Ganis et al., Proceedings of the \VI Work-shop on RPC and Related Detectors", Coim-bra, November 2001; physics/021004518. G. Carboni et al. , \A model for RPC detec-tors operating at high rate", to be publishedin Nucl. Instr. and Meth. A. Note LHCb-2002-069.

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We present recent results of an extensive aging test, performed at the CERN Gamma Irradiation Facility on two single-gap RPC prototypes, developed for the LHCb Muon System. With a method based on a model describing the behaviour of an RPC under high particle flux conditions, we have periodically measured the electrode resistance R of the two RPC prototypes over three years: we observe a large spontaneous increase of R with time, from the initial value of about 2 MOmega to more than 250 MOmega. A corresponding degradation of the RPC rate capabilities, from more than 3 kHz/cm(2) to less than 0.15 kHz/cm(2) is also found

New results from an extensive aging test on bakelite Resistive Plate Chambers

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