- Email: [email protected]
Study of gas mixtures and ageing of the multigap resistive plate chamber used for the Alice TOF
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97 www.elsevier.com/locate/nima
Study of gas mixtures and ageing of the multigap resistive plate chamber used for the Alice TOF A.V. Akindinova, A. Alicib,c, F. Anselmoc, P. Antoniolic, M. Basileb,c, G. Cara Romeoc, L. Cifarellib,c, F. Cindoloc, F. Cosenzad, I. D’ Antonec, A. De Carod, S. De Pasqualed, A. Di Bartolomeod, M. Fusco Girardd, V. Golovinea, M. Guerzonic, M. Guidad, D. Hatzifotiadouc, A.B. Kaidalova, D.H. Kime, D.W. Kimf, S.M. Kisseleva, G. Laurentic, E. Lioubleva, K. Leef, S.C. Leef, M.L. Luvisettoc, A. Margottic, A.N. Martemiyanova, F. Masserac, S. Meneghinic, R. Michinellic, R. Naniac, P. Otiougovae, G. Pancaldic, A. Pescic, R. Pilastrinic, O. Pinazzac, P.A. Polozova, M. Rizzic, E. Scapparonec,, G. Sciolib,c, S.B. Sellittod, F. Semeriac, S. Serrac, A.V. Smirnitskia, M.M. Tchoumakova, E. Ugolinic, E. Usenkog, G. Valentic, K.G. Voloshina, M.C.S. Williamsc, B.V. Zagreeva, C. Zampollib,c, A. Zichichib,c, A. Zucchinic, M. Zuffac a
Institute for Theoretical and Experimental Physics, Moscow, Russia b Dipartimento di Fisica dell’Universita`, Bologna, Italy c Sezione INFN-Bologna, Via Irnerio 46, Bologna 40129, Italy d Dipartimento di Fisica dell’Universita` and INFN, Salerno, Italy e World Laboratory, Lausanne, Switzerland f Department of Physics, Kangnung National University, Kangnung, South Korea g Institute for High Energy Physics, Protvino, Russia Available online 22 July 2004
Abstract We present in this paper a study of the Alice-TOF Multigap Resistive Plate Chamber (MRPC) performance by using several gas mixtures. We also present a search for possible ageing effects, by studying two MRPCs irradiated at the CERN Gamma Irradiation Facility. r 2004 Elsevier B.V. All rights reserved.
Corresponding author. Tel.: +39-051-2091009; fax: +39-051-247244.
E-mail address: [email protected] (E. Scapparone). 0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.007
ARTICLE IN PRESS 94
A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
PACS: 82.80.Rt; 29.40Cs Keywords: Time resolution; Gas mixture; Time of flight
2.1. SF 6
Glass Multigap Resistive Plate Chamber (MRPC) have been chosen as optimal detector for the AliceTOF system. It consists of a stack of resistive plates, spaced one from the other with a 250 mm diameter fishing line, creating a series of gas gaps. This device, developed during a long R&D started in the framework of LAA project, shows a very good efficiency and an excellent time resolution. During the last couple of years the performance of the MRPC has dramatically improved. The impressive results [1] have been obtained through better construction techniques, the use of a stateof-the-art front-end ASIC [2] and the implementation of a high resolution TDC (HPTDC) system [3]. A time resolution between 40 and 50 ps was reached with a long streamer-free plateau, with efficiency larger than 99.5%. Our baseline mixture has been C2 F4 H2 (90%)–C4 H10 (5%)– SF6 (5%). Using MagBoltz [4] we find [5] that at the knee of the efficiency plateau (E field ’ 96 kV=cm) the effective Townsend coefficient aeff is ’ 150 mm1 . This implies that aD is 37, well above the threshold for transition to streamers (aD ’ 20Þ. However, due to space charge effects [6], avalanches are limited in size at ’ 2 107 electrons. This leads to a long streamer-free plateau and to a small dependence of gas gain on applied electric field. In this paper we present measurements of the efficiency and time resolution for a variety of gas mixtures. In addition we have studied the possible ageing effects using this baseline mixture.
As a first step we studied the MRPC performance as a function of the SF6 fraction. Fig. 1 shows the efficiency as a function of the applied voltage for a fraction of SF6 ranging from 0 to 10%. By increasing the fraction of SF6 , the efficiency plateau shifts to higher voltages. Full efficiency (X99%) is reached above ’ 12 kV for SF6 ¼ 10%, in comparison to 11 kV if SF6 is cut to 0%. Two competing processes affect the time resolution with increasing SF6 concentration. Large fractions of SF6 require higher electric fields. As a consequence, according to MagBoltz, a higher drift velocity is expected, which results in an improved time resolution. On the other hand the SF6 , due to the large capture cross-section for low-energy electrons, can capture all electrons in a cluster of ionization. Thus, by increasing the concentration of SF6 , the number of clusters of
2. Gas mixture study The data used for this study were collected at the CERN PS-T10, using a 6 GeV=c pion beam. A detailed description of the detector and set-up may be found in Refs. [1,7].
Efficiency
1. Introduction
100 95 90 85 80 75 70 65 60
No SF6 Standard mixture (SF6=5 %) SF6=2 % SF6=10 %
10
10.5
11
11.5
12
12.5
13
13.5
14
13.5
14
σ(ps)
HV(kV) 90 85 80 75 70 65 60 55 50 45 40
NO SF6 SF6 2 % Standard mixture(SF6=5 %) SF6 10 %
10
10.5
11
11.5
12
12.5
13
HV(kV)
Fig. 1. Efficiency (top) and time resolution (bottom) for different SF6 fraction used in the gas mixture.
ARTICLE IN PRESS A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
2.2. CO2 CO2 is a non-flammable, cheap gas. We tried to substitute SF6 and C4 H10 with CO2 , by using a C2 F4 H2 (97.5%)–CO2 (2.5%) mixture. The efficiency looks unchanged for HVX11 kV, but a deterioration of ’ 15 ps is found for the time resolution in the interval 11 kVpHVp12:5 kV. Such mixture does not fulfill the TOF MRPC requirements. 2.3. C 2 F 5 H C2 F5 H is an alternative to the standard C2 F4 H2 as the main mixture component. In a previous paper [8] we found that this gas is very effective at suppressing streamers, so we wanted to test it without SF6 . We used two different mixtures: the first one is made of C2 F5 H(95%)–C4 H10 (5%), while the second one is made of C2 F5 H(97.5%)–C4 H10 (2.5%). In both cases the efficiency is almost unchanged while the time resolution is degraded of ’ 20–25 ps. 2.4. C 4 H 10 A two-component (C2 F4 H2 –SF6 ) gas mixture, would be simpler and safer, since C4 H10 is a flammable gas. Fig. 2 shows the efficiency and the time resolution for two mixtures (C2 F4 H2 (94.7%)–SF6 %(5.3%) and C2 F4 H2 (93%)–SF6 %(7%)), compared with the
100
100
80
Standard mixture C2F4H2 = 94.7 % SF6= 5.3 % C2F4H2 = 93 % SF6= 7 %
80
60
60
40
40
20
20
σ(ps)
Efficiency
ionization that generate an avalanche (and therefore take part in signal generation) is reduced leading to a degradation of time resolution [9]. Given these two competing processes the best resolution is obtained at SF6 ¼ 5%, when a fraction of C4 H10 ¼ 5% is also used. The degradation of the time resolution at 0% SF6 is made worse by the presence of streamers. The large charge produced by streamers upsets the delicate balance of voltage on all the intermediate plates. It also makes the time slewing correction less effective, since for the same leading time value, a large variety of pulse charge is detected. In conclusion, a fraction of 5%, gives the best results, while worst results were obtained for smaller fractions.
95
0
0 10
10.5
11
11.5
12
12.5
13
13.5
14
HV(V)
Fig. 2. Efficiency and time resolution for different twocomponent gas mixture (C2 F4 H2 –SF6 ), compared with the standard one.
standard one. As far as the efficiency is concerned, results are similar for HVX12 kV. By using a fraction of SF6 ¼ 7% similar performance with respect to the standard mixture are obtained for the time resolution too. In addition the plateau where we get the best time resolution looks larger compared with that obtained with the standard mixture. We prefer this simpler mixture, avoiding the C4 H10 ; although no ageing effects are expected for this mixture, we will shortly repeat all the ageing tests.
3. Ageing study Ageing is one of the main concerns in RPC longterm run. Evidences for ageing were reported both from bakelite RPC [10] and glass-based RPC [11]. Aged chambers may show a loss of efficiency, an increase of dark current and, for bakelite RPC, an increase of resistivity of the electrodes, the latter compromising the rate capability. Compared to the devices quoted above, the MRPCs destined for the ALICE TOF, are operated in a very pure avalanche mode with no streamer contamination. In addition the average total charge generated through going charged particle is only 2 pC (compared to 20 pC and above for the 2 mm gap
ARTICLE IN PRESS 96
A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
RPC); this effect will lessen any ageing problems. Finally since ageing is usually a degradation of the surface of the electrode, the MRPC with 10 gaps has a very large area of electrode and thus the surface degradation is diluted. To study the MRPC ageing, a dedicated exposure to the CERN Gamma Irradiation Facility (GIF) was performed, using two test chambers (CH1,CH2) filled with the standard gas mixture, at a flow of 1:4 cm3 =s. During a 190 day exposure the current drawn by the two chambers was continuously monitored. The gas going to the second chamber (CH2) was bubbled through water for some time of the exposure. When 1% of water vapour was added to the gas mixture, the dark current increased to 0:18 mA=m2 , returning to be less than 10 nA=m2 when the water vapour was removed. The total collected charge was Qtot ¼ 14 mC=cm2 (Fig. 3). Since the average total charge produced within the MRPC by a through going particle is 2 pC, this corresponds to 7 109 particles/cm2 . Considering that the charged particle rate at the TOF position (3.7 m from the beam pipe) is 50 Hz=cm2 , the total collected charge Qtot corresponds to a live time of 1:4 108 s, i.e. 1620 days. A search for HF in the outgoing gas of the MRPCs, was made by the CERN EST/SM-CP
100 90 80 70 60 50 40 30 before irradiation
20
Efficiency [%]
Efficiency [%]
Resolution [ps]
Resolution [ps]
10 0 10
after irradiation
11 12 13 Voltage applied across 5 gas gaps [kV]
14
Fig. 4. Efficiency (top) and time resolution (bottom) of the strip before and after the exposure at the CERN GIF.
group, using gas chromatography. The Fluorine concentration was measured for both CH1 and CH2(CH2 was being supplied with 1% water vapour during this measurement). No HF(below 0.02 ppm) was found. The two chambers were mounted in aluminium gas boxes with dimensions ð14 16:7 125Þ cm3 ; the active detector volume is about 0.7% of the total volume. Another chamber was exposed to the GIF and collected a charge of 10 mC=cm2 . This was tested in the PS-T10 beam before and after the GIF exposure. The efficiency and time resolution were carefully analyzed to search for a possible performance degradation (Fig. 4). Time resolutions well below 50 ps were measured and efficiencies exceeding 99.5% were obtained.
4. Conclusion
Fig. 3. Total charge collected at the CERN GIF as a function of the running time.
We presented in this paper a study of the MRPC efficiency and time resolution obtained with several gas mixtures. CO2 is not a good candidate: its use results in a spoiling of time resolution. The same conclusion is obtained when the main mixture component, C2 F4 H2 , is replaced by C2 F5 H. The optimal SF6 fraction appears to be 5%, if the SF6 is used in combination with C4 H10 . The isobutane can be completely removed, and a
ARTICLE IN PRESS A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
similar performance can be obtained by increasing the SF6 fraction to 7%. As far as ageing is concerned, two strips were irradiated at CERN GIF, collecting a total amount of 14 mC=cm2 . No sign of degradation nor increase of dark current was observed. A search for HF in the outgoing gas gave negative results. A chamber that collected a charge of 10 mC=cm2 was carefully measured before and after irradiation. We did not observe any efficiency and time resolution degradation. References [1] See D. Hatzifotiadou, Nucl. Instr. and Meth. A, (2004), these Proceedings.
97
[2] See C. Williams, Nucl. Instr. and Meth. A, (2004), these Proceedings. [3] See P. Antonioli, Nucl. Instr. and Meth. A, (2004), these Proceedings. [4] S. Biagi, Nucl. Instr. and Meth. A 421 (1999) 234. [5] See C. Williams at Fourth International Symposium on LHC Physics and Detectors, Fermilab, May 1–2, 2003, Proceedings in press. [6] See C. Lippman, Nucl. Instr. and Meth. A, (2004), these Proceedings. [7] A. Akindinov, et al., Nucl. Instr. and Meth. A 490 (2002) 58. [8] E.C. Zeballon, et al., Nucl. Instr. and Meth. A 396 (1997) 93. [9] S. Biagi, private communication. [10] J. Lu, Nucl. Instr. and Meth. A 508 (2003) 128. [11] A. Abashian, et al., Nucl. Instr. and Meth. A 449 (2000) 112.
Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97 www.elsevier.com/locate/nima
Study of gas mixtures and ageing of the multigap resistive plate chamber used for the Alice TOF A.V. Akindinova, A. Alicib,c, F. Anselmoc, P. Antoniolic, M. Basileb,c, G. Cara Romeoc, L. Cifarellib,c, F. Cindoloc, F. Cosenzad, I. D’ Antonec, A. De Carod, S. De Pasqualed, A. Di Bartolomeod, M. Fusco Girardd, V. Golovinea, M. Guerzonic, M. Guidad, D. Hatzifotiadouc, A.B. Kaidalova, D.H. Kime, D.W. Kimf, S.M. Kisseleva, G. Laurentic, E. Lioubleva, K. Leef, S.C. Leef, M.L. Luvisettoc, A. Margottic, A.N. Martemiyanova, F. Masserac, S. Meneghinic, R. Michinellic, R. Naniac, P. Otiougovae, G. Pancaldic, A. Pescic, R. Pilastrinic, O. Pinazzac, P.A. Polozova, M. Rizzic, E. Scapparonec,, G. Sciolib,c, S.B. Sellittod, F. Semeriac, S. Serrac, A.V. Smirnitskia, M.M. Tchoumakova, E. Ugolinic, E. Usenkog, G. Valentic, K.G. Voloshina, M.C.S. Williamsc, B.V. Zagreeva, C. Zampollib,c, A. Zichichib,c, A. Zucchinic, M. Zuffac a
Institute for Theoretical and Experimental Physics, Moscow, Russia b Dipartimento di Fisica dell’Universita`, Bologna, Italy c Sezione INFN-Bologna, Via Irnerio 46, Bologna 40129, Italy d Dipartimento di Fisica dell’Universita` and INFN, Salerno, Italy e World Laboratory, Lausanne, Switzerland f Department of Physics, Kangnung National University, Kangnung, South Korea g Institute for High Energy Physics, Protvino, Russia Available online 22 July 2004
Abstract We present in this paper a study of the Alice-TOF Multigap Resistive Plate Chamber (MRPC) performance by using several gas mixtures. We also present a search for possible ageing effects, by studying two MRPCs irradiated at the CERN Gamma Irradiation Facility. r 2004 Elsevier B.V. All rights reserved.
Corresponding author. Tel.: +39-051-2091009; fax: +39-051-247244.
E-mail address: [email protected] (E. Scapparone). 0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.007
ARTICLE IN PRESS 94
A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
PACS: 82.80.Rt; 29.40Cs Keywords: Time resolution; Gas mixture; Time of flight
2.1. SF 6
Glass Multigap Resistive Plate Chamber (MRPC) have been chosen as optimal detector for the AliceTOF system. It consists of a stack of resistive plates, spaced one from the other with a 250 mm diameter fishing line, creating a series of gas gaps. This device, developed during a long R&D started in the framework of LAA project, shows a very good efficiency and an excellent time resolution. During the last couple of years the performance of the MRPC has dramatically improved. The impressive results [1] have been obtained through better construction techniques, the use of a stateof-the-art front-end ASIC [2] and the implementation of a high resolution TDC (HPTDC) system [3]. A time resolution between 40 and 50 ps was reached with a long streamer-free plateau, with efficiency larger than 99.5%. Our baseline mixture has been C2 F4 H2 (90%)–C4 H10 (5%)– SF6 (5%). Using MagBoltz [4] we find [5] that at the knee of the efficiency plateau (E field ’ 96 kV=cm) the effective Townsend coefficient aeff is ’ 150 mm1 . This implies that aD is 37, well above the threshold for transition to streamers (aD ’ 20Þ. However, due to space charge effects [6], avalanches are limited in size at ’ 2 107 electrons. This leads to a long streamer-free plateau and to a small dependence of gas gain on applied electric field. In this paper we present measurements of the efficiency and time resolution for a variety of gas mixtures. In addition we have studied the possible ageing effects using this baseline mixture.
As a first step we studied the MRPC performance as a function of the SF6 fraction. Fig. 1 shows the efficiency as a function of the applied voltage for a fraction of SF6 ranging from 0 to 10%. By increasing the fraction of SF6 , the efficiency plateau shifts to higher voltages. Full efficiency (X99%) is reached above ’ 12 kV for SF6 ¼ 10%, in comparison to 11 kV if SF6 is cut to 0%. Two competing processes affect the time resolution with increasing SF6 concentration. Large fractions of SF6 require higher electric fields. As a consequence, according to MagBoltz, a higher drift velocity is expected, which results in an improved time resolution. On the other hand the SF6 , due to the large capture cross-section for low-energy electrons, can capture all electrons in a cluster of ionization. Thus, by increasing the concentration of SF6 , the number of clusters of
2. Gas mixture study The data used for this study were collected at the CERN PS-T10, using a 6 GeV=c pion beam. A detailed description of the detector and set-up may be found in Refs. [1,7].
Efficiency
1. Introduction
100 95 90 85 80 75 70 65 60
No SF6 Standard mixture (SF6=5 %) SF6=2 % SF6=10 %
10
10.5
11
11.5
12
12.5
13
13.5
14
13.5
14
σ(ps)
HV(kV) 90 85 80 75 70 65 60 55 50 45 40
NO SF6 SF6 2 % Standard mixture(SF6=5 %) SF6 10 %
10
10.5
11
11.5
12
12.5
13
HV(kV)
Fig. 1. Efficiency (top) and time resolution (bottom) for different SF6 fraction used in the gas mixture.
ARTICLE IN PRESS A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
2.2. CO2 CO2 is a non-flammable, cheap gas. We tried to substitute SF6 and C4 H10 with CO2 , by using a C2 F4 H2 (97.5%)–CO2 (2.5%) mixture. The efficiency looks unchanged for HVX11 kV, but a deterioration of ’ 15 ps is found for the time resolution in the interval 11 kVpHVp12:5 kV. Such mixture does not fulfill the TOF MRPC requirements. 2.3. C 2 F 5 H C2 F5 H is an alternative to the standard C2 F4 H2 as the main mixture component. In a previous paper [8] we found that this gas is very effective at suppressing streamers, so we wanted to test it without SF6 . We used two different mixtures: the first one is made of C2 F5 H(95%)–C4 H10 (5%), while the second one is made of C2 F5 H(97.5%)–C4 H10 (2.5%). In both cases the efficiency is almost unchanged while the time resolution is degraded of ’ 20–25 ps. 2.4. C 4 H 10 A two-component (C2 F4 H2 –SF6 ) gas mixture, would be simpler and safer, since C4 H10 is a flammable gas. Fig. 2 shows the efficiency and the time resolution for two mixtures (C2 F4 H2 (94.7%)–SF6 %(5.3%) and C2 F4 H2 (93%)–SF6 %(7%)), compared with the
100
100
80
Standard mixture C2F4H2 = 94.7 % SF6= 5.3 % C2F4H2 = 93 % SF6= 7 %
80
60
60
40
40
20
20
σ(ps)
Efficiency
ionization that generate an avalanche (and therefore take part in signal generation) is reduced leading to a degradation of time resolution [9]. Given these two competing processes the best resolution is obtained at SF6 ¼ 5%, when a fraction of C4 H10 ¼ 5% is also used. The degradation of the time resolution at 0% SF6 is made worse by the presence of streamers. The large charge produced by streamers upsets the delicate balance of voltage on all the intermediate plates. It also makes the time slewing correction less effective, since for the same leading time value, a large variety of pulse charge is detected. In conclusion, a fraction of 5%, gives the best results, while worst results were obtained for smaller fractions.
95
0
0 10
10.5
11
11.5
12
12.5
13
13.5
14
HV(V)
Fig. 2. Efficiency and time resolution for different twocomponent gas mixture (C2 F4 H2 –SF6 ), compared with the standard one.
standard one. As far as the efficiency is concerned, results are similar for HVX12 kV. By using a fraction of SF6 ¼ 7% similar performance with respect to the standard mixture are obtained for the time resolution too. In addition the plateau where we get the best time resolution looks larger compared with that obtained with the standard mixture. We prefer this simpler mixture, avoiding the C4 H10 ; although no ageing effects are expected for this mixture, we will shortly repeat all the ageing tests.
3. Ageing study Ageing is one of the main concerns in RPC longterm run. Evidences for ageing were reported both from bakelite RPC [10] and glass-based RPC [11]. Aged chambers may show a loss of efficiency, an increase of dark current and, for bakelite RPC, an increase of resistivity of the electrodes, the latter compromising the rate capability. Compared to the devices quoted above, the MRPCs destined for the ALICE TOF, are operated in a very pure avalanche mode with no streamer contamination. In addition the average total charge generated through going charged particle is only 2 pC (compared to 20 pC and above for the 2 mm gap
ARTICLE IN PRESS 96
A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
RPC); this effect will lessen any ageing problems. Finally since ageing is usually a degradation of the surface of the electrode, the MRPC with 10 gaps has a very large area of electrode and thus the surface degradation is diluted. To study the MRPC ageing, a dedicated exposure to the CERN Gamma Irradiation Facility (GIF) was performed, using two test chambers (CH1,CH2) filled with the standard gas mixture, at a flow of 1:4 cm3 =s. During a 190 day exposure the current drawn by the two chambers was continuously monitored. The gas going to the second chamber (CH2) was bubbled through water for some time of the exposure. When 1% of water vapour was added to the gas mixture, the dark current increased to 0:18 mA=m2 , returning to be less than 10 nA=m2 when the water vapour was removed. The total collected charge was Qtot ¼ 14 mC=cm2 (Fig. 3). Since the average total charge produced within the MRPC by a through going particle is 2 pC, this corresponds to 7 109 particles/cm2 . Considering that the charged particle rate at the TOF position (3.7 m from the beam pipe) is 50 Hz=cm2 , the total collected charge Qtot corresponds to a live time of 1:4 108 s, i.e. 1620 days. A search for HF in the outgoing gas of the MRPCs, was made by the CERN EST/SM-CP
100 90 80 70 60 50 40 30 before irradiation
20
Efficiency [%]
Efficiency [%]
Resolution [ps]
Resolution [ps]
10 0 10
after irradiation
11 12 13 Voltage applied across 5 gas gaps [kV]
14
Fig. 4. Efficiency (top) and time resolution (bottom) of the strip before and after the exposure at the CERN GIF.
group, using gas chromatography. The Fluorine concentration was measured for both CH1 and CH2(CH2 was being supplied with 1% water vapour during this measurement). No HF(below 0.02 ppm) was found. The two chambers were mounted in aluminium gas boxes with dimensions ð14 16:7 125Þ cm3 ; the active detector volume is about 0.7% of the total volume. Another chamber was exposed to the GIF and collected a charge of 10 mC=cm2 . This was tested in the PS-T10 beam before and after the GIF exposure. The efficiency and time resolution were carefully analyzed to search for a possible performance degradation (Fig. 4). Time resolutions well below 50 ps were measured and efficiencies exceeding 99.5% were obtained.
4. Conclusion
Fig. 3. Total charge collected at the CERN GIF as a function of the running time.
We presented in this paper a study of the MRPC efficiency and time resolution obtained with several gas mixtures. CO2 is not a good candidate: its use results in a spoiling of time resolution. The same conclusion is obtained when the main mixture component, C2 F4 H2 , is replaced by C2 F5 H. The optimal SF6 fraction appears to be 5%, if the SF6 is used in combination with C4 H10 . The isobutane can be completely removed, and a
ARTICLE IN PRESS A.V. Akindinov et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 93–97
similar performance can be obtained by increasing the SF6 fraction to 7%. As far as ageing is concerned, two strips were irradiated at CERN GIF, collecting a total amount of 14 mC=cm2 . No sign of degradation nor increase of dark current was observed. A search for HF in the outgoing gas gave negative results. A chamber that collected a charge of 10 mC=cm2 was carefully measured before and after irradiation. We did not observe any efficiency and time resolution degradation. References [1] See D. Hatzifotiadou, Nucl. Instr. and Meth. A, (2004), these Proceedings.
97
[2] See C. Williams, Nucl. Instr. and Meth. A, (2004), these Proceedings. [3] See P. Antonioli, Nucl. Instr. and Meth. A, (2004), these Proceedings. [4] S. Biagi, Nucl. Instr. and Meth. A 421 (1999) 234. [5] See C. Williams at Fourth International Symposium on LHC Physics and Detectors, Fermilab, May 1–2, 2003, Proceedings in press. [6] See C. Lippman, Nucl. Instr. and Meth. A, (2004), these Proceedings. [7] A. Akindinov, et al., Nucl. Instr. and Meth. A 490 (2002) 58. [8] E.C. Zeballon, et al., Nucl. Instr. and Meth. A 396 (1997) 93. [9] S. Biagi, private communication. [10] J. Lu, Nucl. Instr. and Meth. A 508 (2003) 128. [11] A. Abashian, et al., Nucl. Instr. and Meth. A 449 (2000) 112.