A systematic study of the performance of the Resistive Plate Chambers as a function of the bakelite temperature is presented. The current, the rate and the efficiency were measured in the temperature range 22-40degreesC. The values of the relative humidity during the data taking were in the range 40-60%. Measurements show a strong dependence of the efficiency on bakelite temperature. (C) 2003 Elsevier Science B.V. All rights reserved

G. Hull

Gabriele Pellizzoni

M. De Vincenzi

Stefano Maria Mari

English

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Nuclear Instruments and Methods in Physics Research A 508 (2003) 94–97Study of the performance of standard RPC chambersas a function of bakelite temperatureM. De Vincenzi, G. Hull, S.M. Mari*, G. PellizzoniDipartimento di Fisica, Universit "a di Roma TRE and INFN Sezione di Roma TRE,Via della Vasca Navale 84, 00146 Roma, ItalyAbstractA systematic study of the performance of the Resistive Plate Chambers as a function of the bakelite temperature ispresented. The current, the rate and the efficiency were measured in the temperature range 22–40C: The values of therelative humidity during the data taking were in the range 40–60%. Measurements show a strong dependence of theefficiency on bakelite temperature.r 2003 Elsevier Science B.V. All rights reserved.Keywords: Gaseous detector; RPC chambers; Temperature dependence; Resistivity of bakelite; RPC efficiency; RPC performances1. IntroductionThe Resistive Plate Counters (RPC) are widelyused in many accelerator and Cosmic Ray experi-ments because of their good time and spatialresolution and because of their mechanical simpli-city. In the next LHC experiments the producedparticles rate will reach 1 KHz=cm2; at this highrate it is important to know the RPC performanceas a function of the bakelite properties. The use ofRPC in extensive air shower experiments [1] forthe detection of the space–time shower profile alsorequires high performance which can be affectedby the temperature of the bakelite. The tempera-ture behavior of the RPC performance is expectedto be related to the gas density and to the bakeliteelectrical properties. The change in the gas densitydue to small temperature variation can beaccounted for by re-scaling the applied highvoltage ðV Þ according to the relationship V ¼V0ðP0=PÞðT=T0Þ where P0 and T0 are arbitrarystandard values. However for wide temperaturevariation it should be considered that in the idealgas approximation the parameter which fixesthe high voltage is given by gap pressure=temperature: The effective voltage acting on thegap depends on the bakelite electrical properties(resistivity r; dielectric constant e) which areaffected by temperature variation also. In thefollowing the performance of a standard RPCchamber as a function of the temperature ispresented. The RPC working parameters (current,time resolution, efficiency) were measured in thetemperature range 22–40C; in the RH range 40–60% and in the atmospheric pressure range 998–1016 mb: The bakelite resistivity of the RPCchambers used in this test is rB1012 O cm:ARTICLE IN PRESS*Corresponding author.E-mail address: mari@fis.uniroma3.it (S.M. Mari).0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0168-9002(03)01284-12. Experimental setupThe test was carried out using two RPCchambers of the type used by the Argo-YBJexperiment, described elsewhere [2]. Each RPC,which covers an area of 50 50 cm2; is realized bytwo bakelite plates of high resistivityðrB1012 O cmÞ where a PVC frame and a 1010 cm lattice of B8 mm diameter spacers define agap of 2 mm: The gap was filled with the gasmixture used in the Argo-YBJ experiment: Argon(15%), Isobutane (10%), tetrafluorethane (75%),with an accuracy of about 1%, and the RPCs wereoperated in streamer mode. The RPC signals arepicked up by means of an aluminum pad of 4646 cm2 glued on 0:2 mm thick film of Poly-Ethylen-Tereftalate (PET), a grounded aluminumfoil was used to shield the RPC and a rigidaluminum structure was used to support the RPC.The pressure on the RPC gap, due to the rigidstructure, was 5 g=cm2: One of the two RPCs(RPC1) was mounted horizontally in an isother-mal box where stable temperature condition wasobtained and relative humidity has been mon-itored. The air in the box was continuouslycirculated in order to avoid local gradients, thetemperature in the box was measured by meansof four digital thermometers, with an accuracy of0:1C; and five transistor with an accuracy of0:4C: The temperature was continuously mea-sured during all the tests. A worm-wheel of tubes isused for thermalizing the gas mixture beforeentering in the RPC1. The temperature of thegas flowing out from the RPCs was also measured.Another RPC (RPC2) was mounted outside theisothermal box and was used as a referencechamber. The trigger of the experiment is definedby the coincidence of three plastic scintillators; inthis way muons with a momentum bigger than0:4 GeV=c are selected and the time accuracy isbetter than 1 ns: In Fig. 1 a schematic view of theexperimental setup is reported.3. RPC performanceThe measurements were taken at the tempera-ture: T ¼ 22C; 30C; 35C; and 40C: For eachtemperature about 1000 events were collected invarious data sets. The chambers were continuouslyoperated and monitored during the data taking.Measurements of current, efficiency, single rateand time resolution were performed for all thetested temperatures. In Fig. 2 the RPC currents areshown as a function of the applied high voltage forARTICLE IN PRESSFig. 1. Schematic view of the experimental setup.HV (V)I (microA)Fig. 2. RPC current as a function of the applied high voltagefor all the tested temperatures.M. De Vincenzi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 94–97 95all the tested temperatures, the current at T ¼40C was reported for the first data set taken atthis temperature. Comparing data taken at T ¼22C with those taken at TX30C; the currentbehavior is according to a negative thermalcoefficient of the resistivity of the bakelite [3].The RPC single rate curves for all the testedtemperatures are reported in Fig. 3, the data atT ¼ 40C is related to the first data set. The singlerate curves start to rise at the same high voltagevalues as the current curves do showing theentering in the streamer regime of the chamber.The RPC efficiency was measured for all the testedtemperatures by means of three-fold scintillatorsARTICLE IN PRESSHV (V)Rate (Hz)Fig. 3. RPC single rate as a function of the applied voltage forall the tested temperatures.HV (V)EfficiencyFig. 4. RPC efficiency as a function of the applied high voltagefor all the tested temperatures.HV (V)I (microA)Fig. 5. RPC current as a function of the applied voltage for thedata at T ¼ 40C:HV (V)Rate (Hz)Fig. 6. RPC single rate as a function of the applied voltage forthe data at T ¼ 40C:M. De Vincenzi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 94–9796coincidence. In Fig. 4 the efficiency vs. voltagecurves are reported, all the curves show anefficiency value in the plateau region of about98% which is the nominal limit due to thegeometrical acceptance. In Figs. 5–7 the RPCcurrent, single rate and efficiency are reported forthe data sets taken at T ¼ 40C: The plots showthat the current, the single rate and the efficiencymeasured at a fixed voltage are clearly decreasingduring the operating period at this temperaturevalue. At the beginning of the data taking at T ¼40C the efficiency reaches a plateau value of 98%as for the T ¼ 22C; 30C; and 35C (see Fig. 4);on the contrary the efficiency curves, related to thedata taken during the 400 h of the operationat T ¼ 40C; show a drop down of about 40%(see Fig. 7). The efficiency was partially recoveredincreasing the high-voltage up to 12; 000 V; veryclose to the break down voltage of the materialused to built the RPC. The RPC1 was againoperated at T ¼ 30C but the measured efficiencywas still 40% demonstrating a permanent effi-ciency loss. The second RPC (RPC2) was warmedin the isothermal box for 170 h at T ¼ 40C butwith the high-voltage switched off. After thisheating cycle, the efficiency was again measured,at T ¼ 30C reaching 98%. The results onmeasured efficiency of RPC1 and RPC2 show thatthe loss of the efficiency of the RPC1 is due to theaction of both high voltage and temperature.4. ConclusionAn extensive study of the RPC performance as afunction of the bakelite temperature was per-formed. The RPC current, single rate and effi-ciency were measured at temperature valuesT ¼ 22C; 30C; 35C; and 40C: The efficiencyfor Tp35C reaches the nominal value of 98%.At T ¼ 40C data show clearly a permanentefficiency loss: after about 400 h at T ¼ 40C;the RPC efficiency drops down to about 40%.References[1] C. Bacci, et al., Nucl. Phys. B 85 (2000) 338.[2] C. Bacci, et al., Nucl. Instr. and Meth. A 443 (2000)342.[3] M. Abbrescia, et al., Nucl. Instr. and Meth. A 359 (1995)603.ARTICLE IN PRESSHV (V)EfficiencyFig. 7. RPC efficiency as a function of the applied voltage forthe data at T ¼ 40C:M. De Vincenzi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 94–97 97

Study of the performance of standard RPC chambers as a function of bakelite temperature

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Study of the performance of standard RPC chambers as a function of bakelite temperature

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