Recent results of an aging test performed at the CERN Gamma Irradiation
Facility on a single--gap RPC prototype developed for the LHCb Muon System are
presented. The results are based on an accumulated charge of about 0.45
C/cm$^2$, corresponding to about 4 years of LHCb running at the highest
background rate. The performance of the chamber has been studied under several
photon flux values exploiting a muon beam. A degradation of the rate capability
above 1 kHz/cm$^2$ is observed, which can be correlated to a sizeable increase
of resistivity of the chamber plates. An increase of the chamber dark current
is also observed. The chamber performance is found to fulfill the LHCb
operation requirements.Comment: 6 pages, 9 figures, presented at the International Workshop on Aging
  Phenomena in Gaseous Detectors'', DESY-Hamburg (Germany), October 200

A. Bizzeti

Agosteo

Aielli

D. Domenici

E. Santovetti

G. Carboni

G. Collazuol

G. Ganis

G. Passaleva

M. Veltri

R. Messi

S. De Capua

English

arXiv

Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single--gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm$^2$, corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied under several photon flux values exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm$^2$ is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements.Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single--gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm$^2$, corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied under several photon flux values exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm$^2$ is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements

Bizzeti, Andrea

Carboni, Giovanni

Collazuol, Gianmaria

De Capua, Stefano

Domenici, Danilo

Ganis, Gerardo

Messi, Roberto

Passaleva, Giovanni

Santovetti, Emanuele

Veltri, Michele

CERN Document Server

1First results from an aging test of a prototype RPC for the LHCb MuonSystemA. BizzetiaFIRE]University of Florence and INFN { Florence, Via G. Sansone 1, I{50019, Sesto F.no,Florence, Italy, G. CarbonibROM2]University of Rome \Tor Vergata" and INFN { Rome II, Via dellaRicerca Scientica 1, I{00133 Rome Italy, G. Collazuolc[FIRE], S. De Capuad[ROM2], D.Domenicie[ROM2], G. Ganisf [ROM2], R. Messig[ROM2],G. Passalevah[FIRE]∗, E. Santovettii[ROM2], M. Veltrij[FIRE]a[b[Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single{gap RPCprototype developed for the LHCb Muon System are presented. The results are based on an accumulated chargeof about 0.45 C/cm2, corresponding to about 4 years of LHCb running at the highest background rate. Theperformance of the chamber has been studied under several photon flux values exploiting a muon beam. Adegradation of the rate capability above 1 kHz/cm2 is observed, which can be correlated to a sizeable increaseof resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamberperformance is found to fulll the LHCb operation requirements.1. IntroductionResistive Plate Chamber (RPC) detectors havebeen adopted by the LHCb Collaboration to covera large fraction (about 48%) of the Muon Sys-tem [1,2]. The LHCb experiment [3] 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 expectedby ATLAS [4] and CMS experiments [5]. In theregions covered by RPCs, the maximum particlerate is expected to vary between 0.25 and 0.75kHz/cm2, depending mainly on the polar angle.These large rates are potentially dangerous fromthe point of view of the RPC aging, which mustbe carefully studied and evaluated.The main aging eect for RPCs is produced bythe current flowing through the resistive plates;It has been demonstrated, indeed, that the irra-diation of bakelite slabs with photons up to anintegrated dose of 20 kGy does not produce anydegradation in the bakelite properties [6]. The∗corresponding authormost important parameter to evaluate the agingeects is given therefore by the total charge flowaccross the detector during its lifetime . Assum-ing an average avalanche charge of 30 pC [6], thetotal charge integrated by LHCb RPCs over 10years of operation range from 0.35 C/cm2, in theregions of the Muon System where the particleflux is smaller, to about 1.1 C/cm2 in the highrate regions.Aging tests performed by the RPC groups ofATLAS and CMS have shown that their detec-tors can withstand the radiation doses expectedin those experiments. However, these tests arebased on an integrated charge of at most 0.3C/cm2 [7,8] which is four times smaller than themaximum expected in LHCb RPCs.To cope with the LHCb requirements, an ex-tensive aging programme has been devised, whichstarted in January 2001 and is expected to last,at a rst stage, until December 2001, exploit-ing the large CERN Gamma Irradiation Facility(GIF) [9], where a 137Cs gamma source of about655 GBq is available.22. Setup of the aging testThe aging test consists in irradiating for aboutone year a single gap RPC (irradiated RPC) atthe GIF facility. The position closest to thesource available for the chamber is at about 1meter, where the photon flux is, roughly, 1.5kHz/cm2. Another chamber (reference RPC) isinstalled just outside the irradiation area. Bothchambers share the same gas and high voltagelines so that they are operated in the same con-ditions. In this way it is possible to disentanglethe eects due to variations in the environmentparameters, such as temperature, pressure or gasquality, from those due to the irradiation.Irradiated RPCReference RPCIrradiation time (days)I (µA)T (o C)I (µA) 1002000 50 100 150 200 25020300 50 100 150 200 2500.115.05300 50 100 150 200 250Figure 1. Current drawn by the irradiated RPCas a function of irradiation time (top), monitoredtemperature (middle) and current drawn by thereference chamber (bottom).The two RPCs prepared for the aging test haveresistive plates made of phenolic bakelite with re-sistivity ρ = 9  109 Ω cm. The plates are oiledwith linseed oil. The size of both chambers is50x50 cm2. The readout strips of the irradiatedRPC are 3 cm wide and have been cut in twoalong their length, so that the nal strip areais 3x25 cm2. The strips of the reference cham-ber, instead, are 3 cm wide and 50 cm long. TheRPCs are operated with a gas mixture consist-ing of 95% C2H2F4, 4% iC4H10, 1% SF6. Tosimulate the LHCb conditions and perform thetest in a reasonable amount of time, the appliedvoltage was chosen in such a way to have an av-erage avalanche charge of about 50 pC, that, atthe rate quoted above, yields a current densityof about 80 nA/cm2. This, with a realistic dutyfactor of 30%, gives an integrated charge of about0.8 C/cm2 in one year of irradiation, which corre-sponds to about 7 LHCb years in the worst back-ground conditions.∼ 4 LHCb years at 750 Hz/cm2Irradiated RPCReference RPCirradiation time (days)Q int(C/cm2 )00.10.20.30.40.50 50 100 150 200 250Figure 2. Integrated currents versus time fromthe beginning of the aging testAll the relevant parameters of the test, temper-ature, pressure, high voltage and currents drawnby the chambers, are continuously monitored andrecorded on a PC. In Figure 1 the current drawnby the irradiated RPC is shown as a function oftime (top plot) along with the temperature (mid-3dle plot). In the bottom plot the current drawnby the reference RPC is also shown. The latteris reasonably stable and constant over the wholeaging test, showing that the chambers have beenoperated rather smoothly and no large systematiceects are expected to aect the performance ofthe two RPCs. In Figure 2, the integrated chargeas a function of time is shown. The irradiatedRPC has accumulated about 0.45 C/cm2 corre-sponding to about 4 LHCb years in the regionswith the highest background rate and to morethan 10 LHCb years in the low background re-gions.To check the performance of the irradiatedchamber at this intermediate aging stage, a beamtest was performed, using the muon beam avail-able at the GIF. In the next sections the resultsof this test will be described.80 cm 170 cm 60 cm 15 cm120 cmS1 S2 S3HodoscopeX5 beamCs137Source filtersCB AFigure 3. Setup of the test at the GIF.A=irradiated RPC; B=reference RPC; C=1010cm2 RPC.3. Beam test resultsThe irradiated and the reference RPCs havebeen carefully tested exploiting the muon beamavailable at the GIF. The test setup is sketched inFigure 3. The combined use of the gamma sourceand the beam allows a test of the chamber per-formance parameters, like eciency, cluster sizeand rate capability, under dierent photon fluxes,ranging from zero (source o) up to a maximumthat, in this test setup, was about 1 kHz/cm2.The chambers were operated in the same condi-tions as in the aging test from the point of viewof the gas and high voltage supplies. The cham-ber eciency was evaluated by tracking the beamparticles with a scintillator hodoscope and withan additional small 1010 cm2 RPC. The photonfluxes were estimated from the chamber countingrates measured during dedicated o-spill gates.The measurements described in the next sectionswere obtained at three dierent source attenua-tion values, namely attenuation 1, 2 and 5 cor-responding roughly to 1 kHz/cm2, 0.7 kHz/cm2and 0.4 kHz/cm2 photon fluxes respectively.3.1. Analysis of currentsThe current drawn by the irradiated chamberwith the source on, was found to be about a factorof two smaller than that of the reference RPC ascan be seen from Figure 4.Irradiated RPCReference RPCAtt = 1Irradiated RPCReference RPCAtt = 2Irradiated RPCReference RPCAtt = 5applied voltage (V)current (µA)0204060801007500 8000 8500 9000 9500 10000 10500 110000204060801007500 8000 8500 9000 9500 10000 10500 110000204060801007500 8000 8500 9000 9500 10000 10500 11000Figure 4. RPC currents versus the applied volt-age at dierent photon fluxes4Att=1Att=2Att=5Irradiated RPCAtt=1Att=2Att=5Reference RPCapplied voltage (V)rate (Hz/cm2)0200400600800100012007500 8000 8500 9000 9500 10000 10500 110000200400600800100012007500 8000 8500 9000 9500 10000 10500 11000Figure 5. RPC counting rates as a function of theapplied voltageThis eect can be interpreted within a modelintroduced in [10]. In this simple model one as-sumes that under a high particle flux, the RPCworking point is determined by an eective volt-age Vgas = V − IR where V is the applied volt-age, I is the current drawn by the chamber andR is the resistance of the resistive plates. Inthis framework, quantities such as the averageavalanche charge Qγ or the chamber eciency εare only functions of the gas properties and ofVgas. This means that once the correct value ofthe resistance R has been determined, the func-tions ε(Vgas) and Qγ(Vgas) must be universalfunctions independent on the particle flux and onthe plate characteristics. This method allows anon destructive estimation of the plate resistivity,that can be monitored on{line during the agingtest.Qγ is measured by normalising the currentdrawn by the chambers to the rate of convertedphotons, which can be obtained from the chambercounting rate at the plateau, corrected for the ef-ciency. The measured counting rates are plottedin Figure 5.Irradiated RPCAtt=1Att=2Att=5R≈23.1 MΩ, ρ≈1.4*1011ΩcmReference RPCAtt=1Att=2Att=5R≈6.6 MΩ, ρ≈4.1*1010ΩcmVgas (V)Q γ (pC)0204060801007500 8000 8500 9000 9500 10000 10500 11000Figure 6. Distribution of Qγ as a function of Vgasfor the two RPCs at dierent photon fluxesThe distributions of Qγ for the two RPCs, atdierent photon fluxes, are plotted as a func-tion of Vgas in Figure 6; they are fairly consis-tent with a single curve. The resulting values ofthe plate resistance, for the irradiated and thereference RPCs are Rirr = 23.1MΩ and Rref =6.6MΩ respectively corresponding to equivalentbakelite resistivities ρirr = 1.4  1011Ωcm andρref = 4.1  1010Ωcm. Compared to the originalresistivity value and to the reference chamber, alarge increase in the plate resistivity for the irradi-ated RPC can be observed, which can be ascribedto the radiation eect and that accounts for thecurrent drop mentioned above.The eciencies of the two RPCs are plotted asa function of Vgas in Figure 7. Again, the curvesare consistent with an universal function ε(Vgas).3.2. Efficiencies and rate capabilitiesBecause of the stringent requirements imposedby the LHCb muon trigger [1], the RPCs are re-quired to have an eciency above 95% up to themaximum expected background rate. In orderto test the ability of the irradiated RPC to work5no irradiationAtt=1Att=2Att=5Irradiated RPCno irradiationAtt=1Att=2Att=5Reference RPCVgas (V)efficiency00.20.40.60.817500 8000 8500 9000 9500 10000 10500 1100000.20.40.60.817500 8000 8500 9000 9500 10000 10500 11000Figure 7. Eciencies versus Vgas at dierent pho-ton fluxes.within the LHCb operational parameters, the ratecapability of the two RPCs has been carefullytested. In Figure 8 the RPC eciency as a func-tion of the photon flux for three dierent highvoltage values is shown. The eciency values arenormalised to those obtained with the source o.At the nominal applied voltage of 10.2 kV, theeciency exceeds 95% for both chambers up to aphoton flux of about 0.8 kHz/cm2. However, assoon as the high voltage is raised by a few hun-dreds volts, the RPCs recover their full eciencyup to a flux of more than 1 kHz/cm2. Moreoverthere is no evidence of a dierent behaviour be-tween the irradiated and the reference RPC. Fromthis test it is possible to conclude that the RPCsunder test can be safely operated, from the agingpoint of view, at least for 4 LHCb years in theworst expected background conditions.3.3. Dark currentsIn the design of RPCs for LHCb, the level ofdark currents drawn by the chambers is of con-cern, especially from the aging point of view. Thedark currents will contribute an aging eect thatIrradiated RPCReference RPCIrradiated RPCReference RPCIrradiated RPCReference RPCRate (Hz/cm2)ε/ε(0)0.850.90.9510 200 400 600 800 10000.850.90.9510 200 400 600 800 10000.850.90.9510 200 400 600 800 1000Figure 8. Eciency versus rate for the two RPCsat three dierent high voltage values. The ef-ciencies are normalised to those obtained withthe source omust be kept well below the aging caused by theparticle flux. Considering as acceptable an extra-aging of 25%, the dark current density must bekept below 3 nA/cm2, over the experiment life-time.To check if these constraints are fullled, thebehaviour of the dark currents of the chambersunder test has been studied. In Figure 9 the darkcurrents drawn by the two RPCs as a function ofthe applied voltage, measured at dierent inter-mediate times during the aging test are shown. Alarge increase of the dark current drawn by the ir-radiated RPC can be clearly observed. The eectof the irradiation amounts to about 2 nA/cm2.This behaviour is still under investigation and nointerpretation can be worked out yet. Anyhow, itis worth to stress that the dark current observedin the irradiated chamber is still below the max-imum allowed value for the LHCb Muon System.601234568250 8500 8750 9000 9250 9500 9750 10000 10250Aug 2001Apr 2001Nov 2000Applied voltage (V)Dark Current (µA)Irradiated RPC00.10.20.30.40.50.60.70.80.918250 8500 8750 9000 9250 9500 9750 10000 10250Aug 2001Apr 2001Nov 2000Applied voltage (V)Dark Current (µA)Reference RPCFigure 9. Dark currents, measured at dierentintermediated times of the aging test, versus theapplied voltage.4. Summary and conclusionsThe results of an aging test of an RPC pro-totype for the LHCb muon system have been de-scribed. The irradiated prototype has been testedwith a muon beam at the GIF after a charge of0.45 C/cm2 has been integrated, correspondingto about 4 years of LHCb at the highest expectedbackground rate and to more than 10 LHCb yearsin the lower rate regions of the Muon System.The performance of the irradiated RPC has beencompared to that of a reference chamber. Asan eect of the irradiation, a large increase ofthe resistivity of the chamber plates in the irra-diated RPC has been observed; a clear increaseof the dark current is also observed. The irradi-ated RPC prototype is still well ecient up to themaximum rate expected in the LHCb muon sys-tem and the dark current level is under control.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. M. Adinol et al., Proposal for the RPC muondetector, LHCb Note 2000-053, May 2000.7. G. Aielli et al., Further Advances in AgingStudies for RPCs, these proceedings.8. G. Pugliese et al., Aging Studies for the Resis-tive Plate Chambers of the CMS Muon Trig-ger Detectors, these proceedings.9. S. Agosteo et al., CERN-EP-2000-031, 2000,submitted to Nucl. Instr. and Methods A.10. G. Aielli et al., Nucl. Instr. and Meth. A 456,(2000), 126.

First results from an aging test of a prototype RPC for the LHCb Muon System

Crossref

Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm2, corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied, under several photon flux values, exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm2 is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements.</p

Bizzeti, A.

Carboni, G.

Collazuol, G.

De Capua, S.

Domenici, D.

Ganis, G.

Messi, R.

Passaleva, G.

Santovetti, E.

Veltri, M.

The University of Manchester - Institutional Repository

Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm2 ; corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied, under several photon flux values, exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm2 is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements

BIZZETI, Andrea

Archivio istituzionale della ricerca - Università di Modena e Reggio Emilia

Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm(2), corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied, under several photon flux values, exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm(2) is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements. (C) 2003 Published by Elsevier B.V

S. D. Capua

COLLAZUOL, GIANMARIA

Archivio istituzionale della ricerca - Università di Padova

Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm(2), corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied, under several photon flux values, exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm(2) is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements

A. BIZZETI

G. CARBONI

G. COLLAZUOL

S. DE CAPUA

D. DOMENICI

G. GANIS

R. MESSI

G. PASSALEVA

E. SANTOVETTI

VELTRI, MICHELE

Archivio istituzionale della ricerca - Università di Urbino

First results from an aging test of a prototype RPC for the LHCb muon system

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.262.3400

First results from an aging test of a prototype RPC for the LHCb Muon
  System

First results from an aging test of a prototype RPC for the LHCb Muon System

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The University of Manchester - Institutional Repository

Archivio istituzionale della ricerca - Università di Modena e Reggio Emilia

Archivio istituzionale della ricerca - Università di Padova

The University of Manchester - Institutional Repository

Archivio istituzionale della ricerca - Università di Urbino