- Email: [email protected]
Freonless gas mixtures for glass RPC operated in streamer mode
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
Freonless gas mixtures for glass RPC operated in streamer mode Y. Hoshia, Y. Mikamib, T. Nagamineb, K. Watanabeb, A. Yamaguchib,*, Y. Yusab a
Department of Physics, Tohoku University, Sendai 980-8578, Japan b Tohoku-Gakuin University, Tagajo 985-8537, Japan
Abstract We have operated successfully glass Resistive Plate Counters (RPC) at the BELLE experiment by using a nonflammable gas mixture of 30% argon, 8% butane-silver and 62% freon, HFC-134a. This freon is nonozone depleting freon, but a freon has a high global warming potential in general. Thus a gas mixture for RPC might be desired to be freonless in the near future. We have tested the performance of glass RPC operated in streamer mode with a nonfreon gas mixture of argon, butane, CO2 and/or O2 gases, and found an efficiency of about 90%. r 2003 Elsevier Science B.V. All rights reserved. Keywords: RPC; Streamer mode; Nonfreon
1. Introduction A gas mixture for the RPC operated in the streamer mode consists of argon (Ar), butane ðC4 H10 Þ and freon gas [1]. The butane and freon prevent a secondary streamer and the streamer size from spreading transversely, respectively. So both are generally needed to provide stable operation with high efficiency. We have used a nonflammable and nonozone depleting gas mixture for the large area RPCs system of the BELLE experiment at KEK. The gas mixture, ‘‘HFC-134a mixture’’ consists of 30% Ar, 8% butane-silver and 62% freon, HFC-134a ðCH2 FCF3 Þ [2,3]. The RPC system with the HFC-134a mixture is successfully operated with highly efficient and stable perfor*Corresponding author. Tel.: +81-22-217-6721; fax: +8122-217-6728. E-mail address: [email protected] (A. Yamaguchi).
mance in the streamer mode [3–7]. The single RPC efficiency is 93%, and the superlayer module has been checked to achieve the design value of 99%. As alternatives to freon, we consider oxygen ðO2 Þ and sulfur hexafluoride ðSF6 Þ: They have high electron affinity of 0:44 eV for O2 and 1:05 eV for SF6 [8]. For adding a small fraction of SF6 ; 1–2%, we found the efficiency to be 92% for the glass RPC with the mixture of 90% Ar, 8–9% C4 H10 [9]. However, as SF6 has much higher global warming potential, 24,900 times as large as CO2 : Oxygen has lower electron affinity than that of freon, but it does not influence global warming at all. We have tested O2 gas as an alternative gas candidate for RPC gas [10]. The performance of bakelite RPC has been studied with various freonless gas mixtures, comparising of Ar, methane ðCH4 Þ and iso-butane [11]. The efficiency of more than 90% was found for the mixtures of 54–80% Ar and 46–20%
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01277-4
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
iso-butane. However, these mixtures are flammable as the nonflammable limit of butane is about 10% [12]. In this paper, we report on the performance of the glass RPC with freonless and nonflammable gas mixtures.
2. Experimental setup 2.1. Freonless gas mixtures A RPC gas mixture could be desirable to be environmentally friendly and to be nonflammable for use in a large-scale experiment. The following properties are required of alternative gas mixtures to the HFC-134a mixture: *
* *
nonozone depleting and less global warming potential, nonflammable, having performance similar to the ‘‘HFC 134a mixture’’.
A C4 H10 fraction for the nonflammable limit depends on O2 concentration in the freonless mixtures of C4 H10 ; O2 and CO2 inert gas [10]. The limit is B5% C4 H10 for the mixture with 10% O2 : We measured the performance of glass RPC with the gas mixtures listed in Table 1. The concentrations of Ar, C4 H10 ; CO2 and O2 gas are controlled by mass flow controllers. The flow rate was set at 10 cm3 =min in total. 2.2. Glass RPC and setup We constructed test glass-RPCs of 30 cm 30 cm in size using 2 mm thick float glasses.
57
Fig. 1 shows (a) top view and (b) side view of RPC with 2 mm gas gap defined by Noryl spacers. A volume resistivity of the float glass was measured to be 7:4 1012 O cm at 25 C: Fig. 2 shows the temperature dependence of the average volume resistivity obtained from the measurement of 5 float glasses, 30 cm 30 cm in area and 2 mm is thickness. The surface resistivity of the float glass is measured to be B1 1012 O=& for the ‘‘air surface’’ (the side facing the air when the float glass was produced), while the ‘‘tin face’’ varies from B5 109 to B7 1010 O=&: The test RPCs are constructed with the ‘‘air surface’’ facing inside. Fig. 3 shows charge cluster size induced on readout strips. It depends on surface resistivity of electrode for supplying the high voltage (HV) on the PRC with a freonless gas mixture. The cluster size is almost constant at the resistivity of more than B106 O=&: A carbon tape, SHINTRON STR-9140 with a surface resistivity of B5 106 O=& was used for this test. The experimental setup and the logic of the readout system for testing the RPC by using cosmic rays are shown elsewhere [9]. Cosmic rays are triggered by 3-fold coincidence of scintillation counters. A ‘‘finger’’ counter of 1 cm width is set at the center of the RPC in order to trigger with a cosmic ray passing through the central part. A 25 cm 25 cm copper readout pad and 18 readout strips are placed at the bottom and top of the RPC to read out the RPC signals. The signals are divided into an ADC, a TDC, and a scaler through a discriminator. RPC signal counts for the efficiency measurement are gated by a 100 ns signal generator by cosmic ray trigger. The time
Table 1 Composition of the gas mixtures used in this test HFC-134a
C4 H10 =Ar
C4 H10 ¼ 8
C4 H10 =O2 ¼ 6=5
C4 H10 =O2 ¼ 4=10
134a/C4 H10 =Ar 8/30/62
C4 H10 =Ar 70/30 50/50 30/70
C4 H10 =Ar=CO2 8/30/62 8/20/72 8/10/82 8/5/87
C4 H10 =O2 =Ar=CO2 6/5/30/59 6/5/20/69 6/5/10/79 6/5/ 5/84
C4 H10 =O2 =Ar=CO2 4/10/30/56 4/10/20/66 4/10/10/76 4/10/ 5/81
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Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
10
3
2
Volume resistivity (10
12
Ω cm)
10
10
1
10
-1
-10
0
10
20
30
40
50
60
o
Temperature ( C) Fig. 2. Temperature dependence of glass volume resistivity.
Fig. 1. Schematic view of glass RPC. (a) top view, (b) side view.
resolution of the TDC-start is estimated to be less than 1 ns with the 3-fold coincidence.
Cluster size (cm)
10
1
3. Performance Using cosmic rays, the plateau curves of efficiency vs HV are measured with the gas mixtures shown in Table 1. The plateau curve is shifted toward lower HV when increasing the fraction of Ar in the freonless mixture because it does not make the streamer quench strongly. To quantify the operating HV for comparison of the RPC performance tested with various gas mixtures a plateau HV was defined as a 0:50 kV above the knee of the efficiency plateau curve. The knee voltage is defined as the cross point of the extrapolated rising edge of the efficiency curve
10
-1
1
10
10
2
10
3
10
4
Surface resistivity (kΩ /sq.) Fig. 3. Electrode resistivity dependence of induced charge cluster size.
and the efficiency plateau. The plateau HVs are 7:027:8 kV for 30–5% Ar in the freonless mixtures, 6.0, 7.4, 9:0 kV for 70%, 50% and 30% Ar in the Ar-butane mixtures, respectively, and 8:22 kV for 62% HFC-134a mixture. Fig. 4 shows
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100 95 90 85 80 75 70 65 60 55 50 0
C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62 10
20
(a)
30
40
50
60
0.8 0.6 0.4 0.2 0 0
0.05 0.04 0.03
C 4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C 4H 10/O 2 = 4/10 Flammable C 4H 10/134a = 8/62
0.02 0.01 10
20
30
40
50
60
Time resolution (ns)
Single count (Hz/cm2)
0.06
30
40
50
60
70
Ar (%) 3 2.5 2 1.5
0.5
250 200 C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62 10
20
30
40
50
60
Cluster size (mm)
300
50
20
Ar (%)
(f)
40
50
60
70
(f)
10 8 6 C4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62
4 2 0 0
70
30
Ar (%) 12
400
100
10
(d)
Ar (%)
150
C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62
1
0 0
70
350
Mean charge (pC)
20
3.5
(c)
(e)
10
(b)
0.07
0 0
C 4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C 4H 10/O 2 = 4/10 Flammable C 4H 10/134a = 8/62
1
70
Ar (%)
0 0
59
1.2
Dark current ( µA/m2)
Efficiency (%)
Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
10
20
30
40
50
60
70
Ar (%)
Fig. 4. Performance at plateau HV. (a) efficiency, (b) dark current, (c) single count rate, (d) time resolution, (e) mean charge, and (f) cluster size.
the RPC performance at plateau HV: (a) efficiency, (b) dark current, (c) single count rate, (d) time resolution, (e) mean charge and (f) cluster size. The solid circles show for the 62% HFC-134a gas mixture as a reference, the open squares for the Ar and butane mixture, the solid squares for the 8% C4 H10 and 0% O2 ; the triangles for the 6% C4 H10 and 5% O2 and the reversed triangles for the 4% C4 H10 and 10% O2 mixtures. The efficiencies measured with the freonless mixtures are a bit less than 90% as shown in Fig. 4(a). An efficiency loss from geometrical inefficiency of spacer is estimated to be B3%: The other loss may be caused by an increase of insensitive time/area due to large discharge and large cluster size.
The dark current is about the same, B0:8 mA=m2 ; at the plateau HV for each freonless mixture and B3:5 times higher than that of the HFC-134a mixture. The single counts have been measured by counting a 500 ns wide output pulse from the discriminator in order not to count afterpulses. The single count rate is about the same, B0:05 Hz=cm2 for each freonless mixture and B1:2 times higher than that of HFC-134a mixture. Fig. 5 shows the ADC distributions at the plateau HV for freonless (10% Ar), flammable and freon gas mixture. The distributions for the freonless mixtures have distinct second peaks while that for HFC-134a mixture has a major first peak and small second one. The first peak fractions for
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
60
Nent = 882 Mean = 313.4 RMS = 120.9
30
Nent = 889 Mean = 286.8 RMS = 115.8
50
25 40 20 30 15 20
10
10
5 0 0
100
200
300
400
500
600
0 0
100
200
ADC (pC)
300
400
500
600
ADC (pC)
(a) Ar/C 4H10/CO2/O2 = 10/8/82/0
(b) Ar/C 4H10 /CO2/O2 = 10/6/79/5
Nent = 899 Mean = 297 RMS = 112.7
45
Nent = 922 Mean = 203.1 RMS = 75.63
140
40 120 35 100
30
80
25 20
60
15 40 10 20
5 0 0
100
200
300
400
500
600
0 0
100
200
ADC (pC)
300
400
500
600
ADC (pC)
(c) Ar/C 4H10/CO2/O2 = 10/4/76/10
(d) Ar/C 4H10/CO2/O2 = 50/50/0/0
350
Nent == 964 964 Nent Mean Mean == 98.28 98.28 RMS RMS == 44.84 44.84
300 250 200 150 100 50 0
0
100
200
300
400
500
600
ADC (pC)
(e) Ar/C4H10 /134a = 30/8/62 Fig. 5. ADC distributions at the plateau HV for (a)–(c) freonless (10% Ar), (d) frammable, and (e) 62% HFC-134a gas mixture.
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62 120
120
Nent = 882 Mean = 69.5 RMS = 3.853
100
80
Mean
= 68.63 ± 0.08397
Sigma
= 2.026 ± 0.07602
60
40
40
20
20
55
60
65
70 75 80 TDC (ns)
85
0 50
95 100
90
(a) Ar/C 4H10/CO2/O2 = 10/8/82/0 120
Mean
= 67.14 ± 0.07886
Sigma
= 1.709 ± 0.06377
40
40
20
20
60
65
70 75 80 TDC (ns)
85
65
90
Mean
= 68.47 ± 0.08915
Sigma
= 1.994 ± 0.07766
70 75 80 TDC (ns)
85
90
95 100
Nent = 922 Mean = 77.56 RMS = 3.986 Chi2 / ndf = 61.44 / 51 Constant = 53.65 ± 2.555 Mean = 77.03 ± 0.1158 Sigma = 3.202 ± 0.1092
80
60
55
60
100
Chi2 / ndf = 125.7 / 43 Constant = 89.97 ± 4.625
60
0 50
55
120
RMS = 3.478
80
Chi2 / ndf = 110.6 / 46 Constant = 77.86 ± 4.085
(b) Ar/C 4H10/CO2/O2 = 10/6/79/5
Nent = 899 Mean = 67.92
100
RMS = 3.68
80
60
0 50
Nent = 889 Mean = 69.24
100
Chi2 / ndf = 116 / 46 Constant = 75 ± 3.902
61
0 50
95 100
(c) Ar/C 4H10 /CO2 /O2 = 10/4/76/10
55
60
65
70 75 80 TDC (ns)
85
90
95 100
(d) Ar/C 4H10 /CO2 /O2 = 50/50/0/0
120
Nent = 964 Mean = 73.18 RMS = 3.824 Chi2 / ndf = 86.63 / 54 Constant = 66.77 ± 2.888 Mean = 72.71 ± 0.09427 Sigma = 2.602 ± 0.0702
100
80
60
40
20
0 50
55
60
65
70
75
80
85
90
95 100
TDC (ns)
(e) Ar/C 4H10/134a = 30/8/62 Fig. 6. TDC distributions at the plateau HV for (a)–(c) freonless (10% Ar), (d) frammable, and (e) 62% HFC-134a gas mixture. Solid curves are Gaussian fits to the distributions.
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Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
the freonless mixtures are 20–40% for Ar concentration of 5–30%, while that for the HFC-134a mixture is 82%. The first ADC peaks vary from 170 to 240 pC with increasing Ar fraction, and the first peak value for the HFC-134a mixture is 80 pC: The HV dependence of the charge distributions shows that the second peak becomes bigger and broader above the plateau HV for the freonless mixtures, but remains of the same shape for the HFC-134a mixture. Fig. 6 shows the TDC distributions at the plateau HV for freonless (10% Ar), flammable and freon gas mixture. The distributions for the freonless mixtures are significantly narrower than that of the HFC-134a mixture. The time resolutions obtained at the plateau HVs are about 2 ns for the freonless mixtures and 2:6 ns for the HFC134a mixture. The intrinsic induced-charge spread sintr: ; cluster size, is calculated by s2meas: ¼ s2hit-pos: þ s2intr: ; where smeas: is the standard deviation of a Gaussian fit to the distribution of the charge sum induced on each of 18 strips over all collected events, and shit-pos: is the standard deviation of the hit position distribution of cosmic rays. The charge cluster size for C4 H10 =O2 ¼ 4=10 mixture in the freonless mixtures is B6 mm that is slightly larger than 5 mm obtained for the HFC-134a mixture. This difference, however, is not so critical for a RPC with readout strips greater than 1 cm width.
4. Summary We have tested the performance of a glass RPC operated in the streamer mode with CO2 and O2 gases as alternatives to HFC-134a. The performance of gas mixture of 4% C4 H10 ; 10% O2 ; 10% Ar and 76% CO2 shows an efficiency of 90%, a time resolution of 1:7 ns and a cluster size of 6 mm: It is better than that of the other freonless mixtures. However, the RPC performance of the freonless gas mixtures is worse than of the HFC134a gas mixture as a whole. The difference of the
performance can be explained by the low capability of electron capture and the insufficient capability of absorbing UV photons of the freonless mixtures. The low-quenching capability also reduces the efficiency slightly and, in addition, it raises the signal pulse height and the mean induced charge due to multiple streamers. It also causes better time resolution. On the other hand, the insufficient UV photon-absorption and electron affinity results in more multipulses, so that the efficiency is lower than that of freon gas mixture. The efficiency of 90% seems to be enough for combining the two RPC layers into a superlayer, because the superlayer efficiency is estimated to be more than 98%. For the glass RPC used in low rate background such as BELLE KL0 =m subdetector, the freonless and nonflammable mixture can be useful. The gas mixture being environmentally safe will be a great advantage for the experiments using large area RPC system.
References [1] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. A 187 (1981) 377. [2] K. Abe, et al., Proceedings of the RPC97, Naples, Italy 1997, Sci. Acta XIII-2 (1998) 281. [3] A. Abashian, et al., Nucl. Instr. and Meth. A 449 (2000) 112. [4] BELLE Collaboration, KEK Progress Report 2000-4, November 2000. [5] K. Abe, et al., IEEE Trans. Nucl. Sci. NS-46-6 (1999) 2017. [6] K. Abe, et al., IEEE Trans. Nucl. Sci. NS-47-6 (2000) 1784. [7] M. Yamaga, et al., Nucl. Instr. and Meth. A 456 (2000) 109. [8] D.R. Lind, CRC Handbook of Radiation Chemistry, CRC Press, Boca Raton, FL, 1991. [9] K. Abe, et al., Nucl. Instr. and Meth. A 455 (2000) 397. [10] S. Narita, et al., IEEE Trans. Nucl. Sci. NS-48-3 (2001) 893. [11] S. Ganter, et al., Nucl. Instr. and Meth. A 414 (1998) 182. [12] B. Lewis, G. Elbe, Combustion, Flames and Explosions of Gases, Cambridge University Press, Cambridge, 1938.
Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
Freonless gas mixtures for glass RPC operated in streamer mode Y. Hoshia, Y. Mikamib, T. Nagamineb, K. Watanabeb, A. Yamaguchib,*, Y. Yusab a
Department of Physics, Tohoku University, Sendai 980-8578, Japan b Tohoku-Gakuin University, Tagajo 985-8537, Japan
Abstract We have operated successfully glass Resistive Plate Counters (RPC) at the BELLE experiment by using a nonflammable gas mixture of 30% argon, 8% butane-silver and 62% freon, HFC-134a. This freon is nonozone depleting freon, but a freon has a high global warming potential in general. Thus a gas mixture for RPC might be desired to be freonless in the near future. We have tested the performance of glass RPC operated in streamer mode with a nonfreon gas mixture of argon, butane, CO2 and/or O2 gases, and found an efficiency of about 90%. r 2003 Elsevier Science B.V. All rights reserved. Keywords: RPC; Streamer mode; Nonfreon
1. Introduction A gas mixture for the RPC operated in the streamer mode consists of argon (Ar), butane ðC4 H10 Þ and freon gas [1]. The butane and freon prevent a secondary streamer and the streamer size from spreading transversely, respectively. So both are generally needed to provide stable operation with high efficiency. We have used a nonflammable and nonozone depleting gas mixture for the large area RPCs system of the BELLE experiment at KEK. The gas mixture, ‘‘HFC-134a mixture’’ consists of 30% Ar, 8% butane-silver and 62% freon, HFC-134a ðCH2 FCF3 Þ [2,3]. The RPC system with the HFC-134a mixture is successfully operated with highly efficient and stable perfor*Corresponding author. Tel.: +81-22-217-6721; fax: +8122-217-6728. E-mail address: [email protected] (A. Yamaguchi).
mance in the streamer mode [3–7]. The single RPC efficiency is 93%, and the superlayer module has been checked to achieve the design value of 99%. As alternatives to freon, we consider oxygen ðO2 Þ and sulfur hexafluoride ðSF6 Þ: They have high electron affinity of 0:44 eV for O2 and 1:05 eV for SF6 [8]. For adding a small fraction of SF6 ; 1–2%, we found the efficiency to be 92% for the glass RPC with the mixture of 90% Ar, 8–9% C4 H10 [9]. However, as SF6 has much higher global warming potential, 24,900 times as large as CO2 : Oxygen has lower electron affinity than that of freon, but it does not influence global warming at all. We have tested O2 gas as an alternative gas candidate for RPC gas [10]. The performance of bakelite RPC has been studied with various freonless gas mixtures, comparising of Ar, methane ðCH4 Þ and iso-butane [11]. The efficiency of more than 90% was found for the mixtures of 54–80% Ar and 46–20%
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01277-4
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
iso-butane. However, these mixtures are flammable as the nonflammable limit of butane is about 10% [12]. In this paper, we report on the performance of the glass RPC with freonless and nonflammable gas mixtures.
2. Experimental setup 2.1. Freonless gas mixtures A RPC gas mixture could be desirable to be environmentally friendly and to be nonflammable for use in a large-scale experiment. The following properties are required of alternative gas mixtures to the HFC-134a mixture: *
* *
nonozone depleting and less global warming potential, nonflammable, having performance similar to the ‘‘HFC 134a mixture’’.
A C4 H10 fraction for the nonflammable limit depends on O2 concentration in the freonless mixtures of C4 H10 ; O2 and CO2 inert gas [10]. The limit is B5% C4 H10 for the mixture with 10% O2 : We measured the performance of glass RPC with the gas mixtures listed in Table 1. The concentrations of Ar, C4 H10 ; CO2 and O2 gas are controlled by mass flow controllers. The flow rate was set at 10 cm3 =min in total. 2.2. Glass RPC and setup We constructed test glass-RPCs of 30 cm 30 cm in size using 2 mm thick float glasses.
57
Fig. 1 shows (a) top view and (b) side view of RPC with 2 mm gas gap defined by Noryl spacers. A volume resistivity of the float glass was measured to be 7:4 1012 O cm at 25 C: Fig. 2 shows the temperature dependence of the average volume resistivity obtained from the measurement of 5 float glasses, 30 cm 30 cm in area and 2 mm is thickness. The surface resistivity of the float glass is measured to be B1 1012 O=& for the ‘‘air surface’’ (the side facing the air when the float glass was produced), while the ‘‘tin face’’ varies from B5 109 to B7 1010 O=&: The test RPCs are constructed with the ‘‘air surface’’ facing inside. Fig. 3 shows charge cluster size induced on readout strips. It depends on surface resistivity of electrode for supplying the high voltage (HV) on the PRC with a freonless gas mixture. The cluster size is almost constant at the resistivity of more than B106 O=&: A carbon tape, SHINTRON STR-9140 with a surface resistivity of B5 106 O=& was used for this test. The experimental setup and the logic of the readout system for testing the RPC by using cosmic rays are shown elsewhere [9]. Cosmic rays are triggered by 3-fold coincidence of scintillation counters. A ‘‘finger’’ counter of 1 cm width is set at the center of the RPC in order to trigger with a cosmic ray passing through the central part. A 25 cm 25 cm copper readout pad and 18 readout strips are placed at the bottom and top of the RPC to read out the RPC signals. The signals are divided into an ADC, a TDC, and a scaler through a discriminator. RPC signal counts for the efficiency measurement are gated by a 100 ns signal generator by cosmic ray trigger. The time
Table 1 Composition of the gas mixtures used in this test HFC-134a
C4 H10 =Ar
C4 H10 ¼ 8
C4 H10 =O2 ¼ 6=5
C4 H10 =O2 ¼ 4=10
134a/C4 H10 =Ar 8/30/62
C4 H10 =Ar 70/30 50/50 30/70
C4 H10 =Ar=CO2 8/30/62 8/20/72 8/10/82 8/5/87
C4 H10 =O2 =Ar=CO2 6/5/30/59 6/5/20/69 6/5/10/79 6/5/ 5/84
C4 H10 =O2 =Ar=CO2 4/10/30/56 4/10/20/66 4/10/10/76 4/10/ 5/81
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Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
10
3
2
Volume resistivity (10
12
Ω cm)
10
10
1
10
-1
-10
0
10
20
30
40
50
60
o
Temperature ( C) Fig. 2. Temperature dependence of glass volume resistivity.
Fig. 1. Schematic view of glass RPC. (a) top view, (b) side view.
resolution of the TDC-start is estimated to be less than 1 ns with the 3-fold coincidence.
Cluster size (cm)
10
1
3. Performance Using cosmic rays, the plateau curves of efficiency vs HV are measured with the gas mixtures shown in Table 1. The plateau curve is shifted toward lower HV when increasing the fraction of Ar in the freonless mixture because it does not make the streamer quench strongly. To quantify the operating HV for comparison of the RPC performance tested with various gas mixtures a plateau HV was defined as a 0:50 kV above the knee of the efficiency plateau curve. The knee voltage is defined as the cross point of the extrapolated rising edge of the efficiency curve
10
-1
1
10
10
2
10
3
10
4
Surface resistivity (kΩ /sq.) Fig. 3. Electrode resistivity dependence of induced charge cluster size.
and the efficiency plateau. The plateau HVs are 7:027:8 kV for 30–5% Ar in the freonless mixtures, 6.0, 7.4, 9:0 kV for 70%, 50% and 30% Ar in the Ar-butane mixtures, respectively, and 8:22 kV for 62% HFC-134a mixture. Fig. 4 shows
ARTICLE IN PRESS
100 95 90 85 80 75 70 65 60 55 50 0
C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62 10
20
(a)
30
40
50
60
0.8 0.6 0.4 0.2 0 0
0.05 0.04 0.03
C 4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C 4H 10/O 2 = 4/10 Flammable C 4H 10/134a = 8/62
0.02 0.01 10
20
30
40
50
60
Time resolution (ns)
Single count (Hz/cm2)
0.06
30
40
50
60
70
Ar (%) 3 2.5 2 1.5
0.5
250 200 C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62 10
20
30
40
50
60
Cluster size (mm)
300
50
20
Ar (%)
(f)
40
50
60
70
(f)
10 8 6 C4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62
4 2 0 0
70
30
Ar (%) 12
400
100
10
(d)
Ar (%)
150
C4H 10/O 2 = 8/0 C4H 10/O 2 = 6/5 C4H 10/O 2 = 4/10 Flammable C4H 10/134a = 8/62
1
0 0
70
350
Mean charge (pC)
20
3.5
(c)
(e)
10
(b)
0.07
0 0
C 4H 10/O 2 = 8/0 C 4H 10/O 2 = 6/5 C 4H 10/O 2 = 4/10 Flammable C 4H 10/134a = 8/62
1
70
Ar (%)
0 0
59
1.2
Dark current ( µA/m2)
Efficiency (%)
Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
10
20
30
40
50
60
70
Ar (%)
Fig. 4. Performance at plateau HV. (a) efficiency, (b) dark current, (c) single count rate, (d) time resolution, (e) mean charge, and (f) cluster size.
the RPC performance at plateau HV: (a) efficiency, (b) dark current, (c) single count rate, (d) time resolution, (e) mean charge and (f) cluster size. The solid circles show for the 62% HFC-134a gas mixture as a reference, the open squares for the Ar and butane mixture, the solid squares for the 8% C4 H10 and 0% O2 ; the triangles for the 6% C4 H10 and 5% O2 and the reversed triangles for the 4% C4 H10 and 10% O2 mixtures. The efficiencies measured with the freonless mixtures are a bit less than 90% as shown in Fig. 4(a). An efficiency loss from geometrical inefficiency of spacer is estimated to be B3%: The other loss may be caused by an increase of insensitive time/area due to large discharge and large cluster size.
The dark current is about the same, B0:8 mA=m2 ; at the plateau HV for each freonless mixture and B3:5 times higher than that of the HFC-134a mixture. The single counts have been measured by counting a 500 ns wide output pulse from the discriminator in order not to count afterpulses. The single count rate is about the same, B0:05 Hz=cm2 for each freonless mixture and B1:2 times higher than that of HFC-134a mixture. Fig. 5 shows the ADC distributions at the plateau HV for freonless (10% Ar), flammable and freon gas mixture. The distributions for the freonless mixtures have distinct second peaks while that for HFC-134a mixture has a major first peak and small second one. The first peak fractions for
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
60
Nent = 882 Mean = 313.4 RMS = 120.9
30
Nent = 889 Mean = 286.8 RMS = 115.8
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25 40 20 30 15 20
10
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5 0 0
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ADC (pC)
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400
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ADC (pC)
(a) Ar/C 4H10/CO2/O2 = 10/8/82/0
(b) Ar/C 4H10 /CO2/O2 = 10/6/79/5
Nent = 899 Mean = 297 RMS = 112.7
45
Nent = 922 Mean = 203.1 RMS = 75.63
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40 120 35 100
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80
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60
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5 0 0
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ADC (pC)
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400
500
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ADC (pC)
(c) Ar/C 4H10/CO2/O2 = 10/4/76/10
(d) Ar/C 4H10/CO2/O2 = 50/50/0/0
350
Nent == 964 964 Nent Mean Mean == 98.28 98.28 RMS RMS == 44.84 44.84
300 250 200 150 100 50 0
0
100
200
300
400
500
600
ADC (pC)
(e) Ar/C4H10 /134a = 30/8/62 Fig. 5. ADC distributions at the plateau HV for (a)–(c) freonless (10% Ar), (d) frammable, and (e) 62% HFC-134a gas mixture.
ARTICLE IN PRESS Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62 120
120
Nent = 882 Mean = 69.5 RMS = 3.853
100
80
Mean
= 68.63 ± 0.08397
Sigma
= 2.026 ± 0.07602
60
40
40
20
20
55
60
65
70 75 80 TDC (ns)
85
0 50
95 100
90
(a) Ar/C 4H10/CO2/O2 = 10/8/82/0 120
Mean
= 67.14 ± 0.07886
Sigma
= 1.709 ± 0.06377
40
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20
20
60
65
70 75 80 TDC (ns)
85
65
90
Mean
= 68.47 ± 0.08915
Sigma
= 1.994 ± 0.07766
70 75 80 TDC (ns)
85
90
95 100
Nent = 922 Mean = 77.56 RMS = 3.986 Chi2 / ndf = 61.44 / 51 Constant = 53.65 ± 2.555 Mean = 77.03 ± 0.1158 Sigma = 3.202 ± 0.1092
80
60
55
60
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Chi2 / ndf = 125.7 / 43 Constant = 89.97 ± 4.625
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0 50
55
120
RMS = 3.478
80
Chi2 / ndf = 110.6 / 46 Constant = 77.86 ± 4.085
(b) Ar/C 4H10/CO2/O2 = 10/6/79/5
Nent = 899 Mean = 67.92
100
RMS = 3.68
80
60
0 50
Nent = 889 Mean = 69.24
100
Chi2 / ndf = 116 / 46 Constant = 75 ± 3.902
61
0 50
95 100
(c) Ar/C 4H10 /CO2 /O2 = 10/4/76/10
55
60
65
70 75 80 TDC (ns)
85
90
95 100
(d) Ar/C 4H10 /CO2 /O2 = 50/50/0/0
120
Nent = 964 Mean = 73.18 RMS = 3.824 Chi2 / ndf = 86.63 / 54 Constant = 66.77 ± 2.888 Mean = 72.71 ± 0.09427 Sigma = 2.602 ± 0.0702
100
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40
20
0 50
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75
80
85
90
95 100
TDC (ns)
(e) Ar/C 4H10/134a = 30/8/62 Fig. 6. TDC distributions at the plateau HV for (a)–(c) freonless (10% Ar), (d) frammable, and (e) 62% HFC-134a gas mixture. Solid curves are Gaussian fits to the distributions.
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Y. Hoshi et al. / Nuclear Instruments and Methods in Physics Research A 508 (2003) 56–62
the freonless mixtures are 20–40% for Ar concentration of 5–30%, while that for the HFC-134a mixture is 82%. The first ADC peaks vary from 170 to 240 pC with increasing Ar fraction, and the first peak value for the HFC-134a mixture is 80 pC: The HV dependence of the charge distributions shows that the second peak becomes bigger and broader above the plateau HV for the freonless mixtures, but remains of the same shape for the HFC-134a mixture. Fig. 6 shows the TDC distributions at the plateau HV for freonless (10% Ar), flammable and freon gas mixture. The distributions for the freonless mixtures are significantly narrower than that of the HFC-134a mixture. The time resolutions obtained at the plateau HVs are about 2 ns for the freonless mixtures and 2:6 ns for the HFC134a mixture. The intrinsic induced-charge spread sintr: ; cluster size, is calculated by s2meas: ¼ s2hit-pos: þ s2intr: ; where smeas: is the standard deviation of a Gaussian fit to the distribution of the charge sum induced on each of 18 strips over all collected events, and shit-pos: is the standard deviation of the hit position distribution of cosmic rays. The charge cluster size for C4 H10 =O2 ¼ 4=10 mixture in the freonless mixtures is B6 mm that is slightly larger than 5 mm obtained for the HFC-134a mixture. This difference, however, is not so critical for a RPC with readout strips greater than 1 cm width.
4. Summary We have tested the performance of a glass RPC operated in the streamer mode with CO2 and O2 gases as alternatives to HFC-134a. The performance of gas mixture of 4% C4 H10 ; 10% O2 ; 10% Ar and 76% CO2 shows an efficiency of 90%, a time resolution of 1:7 ns and a cluster size of 6 mm: It is better than that of the other freonless mixtures. However, the RPC performance of the freonless gas mixtures is worse than of the HFC134a gas mixture as a whole. The difference of the
performance can be explained by the low capability of electron capture and the insufficient capability of absorbing UV photons of the freonless mixtures. The low-quenching capability also reduces the efficiency slightly and, in addition, it raises the signal pulse height and the mean induced charge due to multiple streamers. It also causes better time resolution. On the other hand, the insufficient UV photon-absorption and electron affinity results in more multipulses, so that the efficiency is lower than that of freon gas mixture. The efficiency of 90% seems to be enough for combining the two RPC layers into a superlayer, because the superlayer efficiency is estimated to be more than 98%. For the glass RPC used in low rate background such as BELLE KL0 =m subdetector, the freonless and nonflammable mixture can be useful. The gas mixture being environmentally safe will be a great advantage for the experiments using large area RPC system.
References [1] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. A 187 (1981) 377. [2] K. Abe, et al., Proceedings of the RPC97, Naples, Italy 1997, Sci. Acta XIII-2 (1998) 281. [3] A. Abashian, et al., Nucl. Instr. and Meth. A 449 (2000) 112. [4] BELLE Collaboration, KEK Progress Report 2000-4, November 2000. [5] K. Abe, et al., IEEE Trans. Nucl. Sci. NS-46-6 (1999) 2017. [6] K. Abe, et al., IEEE Trans. Nucl. Sci. NS-47-6 (2000) 1784. [7] M. Yamaga, et al., Nucl. Instr. and Meth. A 456 (2000) 109. [8] D.R. Lind, CRC Handbook of Radiation Chemistry, CRC Press, Boca Raton, FL, 1991. [9] K. Abe, et al., Nucl. Instr. and Meth. A 455 (2000) 397. [10] S. Narita, et al., IEEE Trans. Nucl. Sci. NS-48-3 (2001) 893. [11] S. Ganter, et al., Nucl. Instr. and Meth. A 414 (1998) 182. [12] B. Lewis, G. Elbe, Combustion, Flames and Explosions of Gases, Cambridge University Press, Cambridge, 1938.