- Email: [email protected]
Production and quality control of STAR-TOF MRPC
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Production and quality control of STAR-TOF MRPC Yi Wang , Jingbo Wang, Jianping Cheng, Yuanjing Li, Qian Yue, Huangshan Chen, Jin Li Department of Engineering Physics, Tsinghua University, Beijing 10084, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 July 2009 Received in revised form 10 November 2009 Accepted 10 November 2009 Available online 27 November 2009
Multi-gap resistive plate chamber (MRPC) has been adopted to construct the full barrel time-of-flight (TOF) detector for the STAR experiment at RHIC. The cylindrical TOF covers a total area of approximately 64 m2 and uses 4032 MRPC modules. 70% of the modules were produced in Tsinghua University and the rest were fabricated in University of Science and Technology of China (USTC). In this paper, the production facilities, manufacturing procedures, quality control methods and test results for the Tsinghua MRPC modules are described. & 2009 Elsevier B.V. All rights reserved.
Keywords: MRPC Production Quality control
1. Introduction The multi-gap resistive plate chamber (MRPC) has good time resolution (less than 100 ps) and high detection efficiency (higher than 95%). This technology has been adopted to construct the full barrel time-of-flight (TOF) detector for the STAR experiment at RHIC [1–3]. The cylindrical TOF detector covers a total area of approximately 64 m2. The whole TOF system consists of 120 trays and each tray consists of 32 MRPC modules. Another six trays are produced as standbys. Thus the total number of modules is 4032. Tsinghua University and University of Science and Technology of China (USTC) are responsible for the STAR-TOF MRPC production. Approximately 30% of the modules have been built at USTC [4], and the rest were fabricated at the Miyun production facility of Tsinghua University. The structure of MRPC module for STAR is shown in Fig. 1. Each module consists of six read-out pads. The size of each pad is 3.1 6.0 cm2. There is a 3-mm gap between each pad. The total active area of one module is 18.6 6 cm2 and the working electronegative gas consists of 95% F134a and 5% iso-butane. The electric field in the gas gap is close to 100 kV/cm. The detector works in avalanche mode and has an excellent time response. The STAR-TOF barrel was fitted in the space initially used for the scintillator trigger counters (CTB trays). 32 MRPC modules and the corresponding electronics are all assembled in one tray that has dimensions of 241 cm 21.6 cm 11.43 cm. Therefore the limited space left for MRPC modules makes the mechanical dimensions of the MRPC’s very tight. The requirements for the mechanical dimensions are summarized in Table 1. Corresponding author. Tel.: + 86 106 277 1960; fax: + 86 106 278 2658.
E-mail address: [email protected] (Y. Wang). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.11.045
To assure the quality of MRPC modules, strict production techniques and quality control methods are necessary. A complete set of instructions and procedures for manufacturing techniques and quality assurance (QA) system were set up in Tsinghua University. Our professional engineers also developed several production tools and machines. Two sets of cosmic ray test systems ensure that all modules can be tested.
2. MRPC production All MRPC modules had to be produced within about two and a half years. The production began in the spring of 2006 and ended in August, 2008. An MRPC workshop was established at Miyun manufacturing base of Nuclear Technology Company (NUCTECH). The 200 m2 workshop consists of a material processing room, a washing room, a clean room and a testing laboratory. The mass production of MRPC modules is a systematic project. All steps, from material preparation to products shipment, are very important. 2.1. Material preparation and test All materials have to be tested. The mechanical dimensions of MRPC modules have to be controlled strictly; thus the dimension precision of component material has to be assured. Table 2 shows the specifications of main materials. For glass and graphite tape, resistivity is important. Visual inspection is essential for all materials. For example, a piece of glass with a scratch on its surface has to be rejected. The four corners of glass have to be sanded off to avoid sparks in the MRPC module. Fig. 2 shows the machine designed to sand
ARTICLE IN PRESS Y. Wang et al. / Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
201
Fig. 1. Structure of the MRPC module for STAR-TOF. (a) Long side view and (b) short side view.
Table 1 Requirements for MRPC’s mechanical dimensions.
Length (mm) Width (mm) Thickness between two PCBs (mm) HV lead length (cm) Signal lead length (cm)
Nominal
Minimum
Maximum
212 94 9.7 18 22.5
211.5 93.5 9.4 17.7 22
212.5 94.5 10 18.5 23
Table 2 Specifications of main materials. Material
Dimension (mm)
Tolerance (mm)
Outer glass
207 78 0.7
Inner glass
200 61 0.54
Graphite tape PCB
7 0.2, 7 0.2, 4 1012 O cm 7 0.01 7 0.2, 7 0.2, 4 1012 O cm 7 0.01 0.5 200 kO/& 7 0.2
202 74 0.13 210 94 1.5 6 pads, 31.5 63/pad 212 84 0.35 7 0.1
Not broken No scratch
208 84 4.0
7 1.0, 7 0.2
No scratch, not broken No damage
Height: 3.8
0.05
Diameter 0.22 18 cm 22.5 cm
7 0.005 7 0.5 7 0.5
Mylar film Honeycomb board L-shaped and cylinder supporter Nylon wire HV lead Signal lead
Resistivity
Visual inspection No scratch No scratch
Not broken
Fig. 2. The machine was designed to sand off the corners of glass.
off the corners of glass. Washing is also an important process for glass. 0.2% HS-G20 liquid1 is used for ultrasonic washing. The liquid is kept at 60 1C and the washing process lasts about 20 min. Then the glass has to be rinsed and dried.
1 A special product for the washing of glass produced by Beijing Hang Yida clean technology Co. Ltd.
ARTICLE IN PRESS 202
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Fig. 3. Technicians are assembling MRPC modules.
Table 3 Specifications of a qualified MRPC under testing conditions. Testing conditions
Specifications
Working gas: 95% F134A + 5% isobutane HV: 14 kV FEE threshold: 80 mV
Leakage current: o 2 nA Noise rate: o50 Hz for each pad Avalanche ratio: 4 80% of ADC spectrum Efficiency: 490% Timing resolution of 90% channels o120 ps Crosstalk of two pads: o 0.4
80 70 60 Fig. 4. The silicone sealant dispensing machine.
Counts
50 Avalanche
40
Streamer
30 20 10 0 400
600
800 1000 1200 Amplitude (*0.2pC)
1400
1600
Fig. 6. A typical charge spectrum of MRPC.
Fig. 5. MRPC assembling workbench.
2.2. MRPC assembling MRPC assembling was carried out in a clean room. The temperature and moisture of the clean room can be controlled and its cleanliness reaches 100 K. The temperature is 22 75 1C and
the moisture is maintained below 40%. Fig. 3 shows the assembling workshop in Tsinghua. Eight technicians are working on the production. We have designed several assembling tools to improve the production efficiency and ensure the quality of the products. Fig. 4 shows the machine to apply silicon sealant around the edge of the outer glass sheets. Fig. 5 shows the assembling workbench. Other tools, such as the electrode stamping workbench (which controls the application of the carbon tape to the glass) and the electrode production workbench, are also developed and used in the production.
ARTICLE IN PRESS Y. Wang et al. / Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
We have established a complete set of instructions for all manufacturing procedures. A video introducing manufacture and QA of MRPC was composed to train technicians and workers.
3. Performance testing MRPC modules have to be tested comprehensively. Dark current, noise rate, amplitude and time resolution are important specifications for a module. Some of them are correlated with each other to some extent. After thorough R&D and verification with small batch production, we established the specifications for a qualified MRPC module [5,6]. Table 3 shows these required specifications under definite testing conditions. In Table 3 we specify that 80% of the signals from the MRPC should be purely avalanche type at the voltage of 14 kV with this gas mixture (95% F13A+5% SF6). We measure this fraction from the ADC spectrum; an example is shown in Fig. 6. We associate large signals (above 800 counts) with streamers. This limit of 800 counts corresponds to a signal of 2 pC from the MRPC. A high fraction of streamers will adversely affect the time resolution. The crosstalk is represented by correlation coefficient. This coefficient COV(a,b) between pads a and b is defined as [7] /XðaÞXðbÞS/XðaÞS/XðbÞS COVða; bÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi /XðaÞ2 S/XðaÞS2 /XðbÞ2 S/XðbÞS2
Fig. 7. Flow chart of MRPC performance testing.
Fig. 8. Distribution of time resolution and avalanche ratio of 198 pads.
203
ð1Þ
where X(a) =1 if pad a has time output, otherwise X(a) =0. /X(a)S represents the average of X(a). The testing flow chart is summarized in Fig. 7. It is too time consuming to measure the time resolution for each module. According to our studies [6], the time resolution of one module is strongly correlated to the avalanche ratio. In previous small batch production, 33 modules were produced. The relation between the time resolution and avalanche ratio of 198 pads was analyzed. Fig. 8 shows the distribution of time resolution and avalanche ratio of 198 pads. It can be seen that the higher the avalanche ratio, the better the time resolution. If the ratio of avalanche signals is larger than 80%, the probability that its time resolution is better than 120 ps is about 93%. It can be seen that the time resolution of one module is nearly 240 ps, but its charge spectrum is still reasonable. In reality, this module is not qualified because its leakage current is about 10 nA and its noise is about 120 Hz. As we know, the time resolution of MRPC modules run in STAR is about 80 ps, which is obtained after Z direction correction and particle energy selections [8]. These corrections cannot be done in cosmic ray test. Thus the time resolution in cosmic ray test can only reach 100 ps. Considering the 20% relative error, 120 ps is determined as the time resolution in cosmic ray test [6]. The mass test method was established based on this performance.
Fig. 9. Cosmic ray amplitude testing system.
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Two sets of cosmic ray testing system were set up in testing laboratory. Figs. 9 and 10 show cosmic ray amplitude testing system and time testing system, respectively. As shown in Fig. 9,
the cosmic ray telescope consists of two 56 cm 5 cm 5 cm plastic scintillation counters with photo-multiplier tubes. The coincidence of PMT2 and PMT4 gives a cosmic trigger signal for ADC and TDC crates. The width of gate is 100 ns. Four MRPC modules in two gas boxes can be tested simultaneously. Differential signals are sent to the FEE used in STAR-TOF [8]. For a trigger event, 24 channels of charge and 27 channels of time (including times of PMT1, 3 and 4) signals are recorded. Charge spectrum of each channel of four modules can be obtained from the recorded data and then we can determine which module is qualified. 500 events are sufficient for amplitude analysis for each channel, and 10 h is needed to get this number of events. Therefore at least 8 modules can be tested per day. The cosmic ray telescope of time testing system consists of three 21 cm 5 cm 5 cm plastic scintillation counters with photo-multiplier tubes. The coincidence of PMT1 and PMT5 provides the cosmic trigger for the ADC (CAEN V792N) gate and the stop signal for TDC (CAEN V775E) converter. The width of gate is also 100 ns. The time resolution of four PMT (PMT2-5), (t2+ t3 t4 t5)/4, is about 80 ps, which is used to eliminate the jitter of the stop signal. Two MRPC modules in one gas box can be tested at the same time. Eq. (1) is used to eliminate the jitter 0 ¼ tm ðt2 þt3 þ t4 þt5 Þ=4 tm
ð2Þ
0 tm
Fig. 10. Cosmic ray time testing system.
where tm and are the time of MRPC, t2 t5 are the time of PMT2–PMT5, respectively. A sixth-order polynomial is used for the slewing correction. As an example, we show the time resolution of each step for module TM2001 in Fig. 11. The raw time resolution as seen by the TDC is 598 ps. After removing the time jitter of the stop signal by subtracting (t2+t3+ t4+ t5)/4 (see above for details) the time
Fig. 11. The time resolution of TM2001 in each analysis step. (a) The time spectrum before any processing, (b) the time resolution after subtracting time jitter of stop signal, (c) the time spectrum after slewing correction and (d) the time spectrum of four PMTs. The histogram (t2+ t3 t4 t5)/4 is used because its width is the same as the time resolution of our reference counters given by (t2 + t3+ t4+ t5)/4.
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Fig. 12. Statistics of time resolution of 604 modules. (a) Time resolution distribution of all channels and (b)–(g) time distribution of each pad.
spectrum has a sigma of 234 ps. After slewing correction this is reduced to 143 ps. We then subtract quadratically the time jitter of our reference counters (80 ps) to obtain a final resolution of 118 ps. We estimate the time resolution of our reference counters with the following: we assume that the time resolution of each pair (t2 +t3)/2 and (t4+ t5)/2 is the same. Thus histogramming the time difference between these pairs, (t2+ t3)/2 (t4 +t5)/2 should have a width sqrt(2) larger than an individual pair. In
our measurements we generate a time reference from the average of all four counters: (t2 +t3+ t4+t5)/4; the time jitter of this average is sqrt(2) smaller than an individual pair. Thus the time jitter of our reference counters should be a factor 2 smaller than we observe in the histogram of the time difference between the pairs. Thus we choose to histogram (t2 +t3 t4 t5)/4 so that this width gives directly the time resolution of our reference counters given by (t2+ t3+t4+ t5)/4. This is shown in plot (d) of Fig. 11.
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700
5. Conclusions
600
MRPC mass production is a systematic project, comprehensive construction flow and strict quality assurance methods are essential. In our manufacturing base, different kinds of detectors such as ionization chambers and scintillator detectors used in Xray scanning system are produced, and NUCTECH exports its products to countries all over the world. ISO 9000 and ISO 14000 are the essential criteria that have to be carried out. MRPC is also a kind of gas detector and these criteria are also used to instruct our production. Through two and half years of hard efforts, 3100 MRPCs were produced, and the yield reaches 95%. About 90 trays have been mounted on STAR and have been running in STAR for 2 months. The results [9] show that our modules work very well. The MRPC production facility will also play an important role in the successive R&D and production of STAR MTD (Muon Track Detector) and FAIR–CBM high rate TOF.
Count
500 400 300 200 100 0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Correlation coefficient of neighboring pads
Fig. 13. Crosstalk distribution of neighboring pads, average being 0.2.
Acknowledgements
4. Production status 3100 MRPCs were produced in Tsinghua University, 95% (2951) passed QA and 2824 were shipped to University of Texas—Austin. Most of them have been assembled in trays and mounted on STAR. Fig. 12 shows the time resolution statistics of 604 modules. Crosstalk distribution of neighboring pads can be seen in Fig. 13. It can be concluded that our products have excellent performances. This can be obtained from the following: 1) The mechanical dimensions of modules were controlled very well. 2) The average noise is about 6 Hz/pad, which is somewhat lower than the required value. 3) The average time resolution of all pads is about 95 ps, and nearly uniformly distributed among six pads. 4) Crosstalk between pads is small, i.e. the signal is very clean.
The authors would like to thank the STAR-TOF collaboration group. This work was supported by the National Natural Science Foundation of China under Grant nos. 10620210287, 10610285, 10675072 and 10775082. This study is also supported by the Ministry of Science and Technology under Grant no. 2008CB817707.
References [1] The STAR TOF collaboration, Technical design update to proposal for a large area time of flight system for STAR, August 16, 2005. [2] B. Bonner, et al., Nucl. Instr. and Meth. A 478 (2002) 176. [3] Yi Wang, et al., Nucl. Instr. and Meth. A 538 (2005) 425. [4] Tao Zou, et al., Nucl. Instr. and Meth. A 605 (2009) 282. [5] Yi Wang, et al., Nucl. Instr. and Meth. A 537 (2005) 698. [6] Yi Wang, et al., High Energy Phys. Nucl. Phys. 30 (7) (2006) 655. [7] /http://www.rhip.utexas.edu/ tofp/documents/overview.htmlS. [8] F. Geurts, et al., Nucl. Instr. and Meth. A 533 (2004) 60. [9] Geary Eppley, STAR large-area TOF project, Workshop on RHIC-STAR full TOF detector and related physics in China, April 27–29, Hangzhou, China.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Production and quality control of STAR-TOF MRPC Yi Wang , Jingbo Wang, Jianping Cheng, Yuanjing Li, Qian Yue, Huangshan Chen, Jin Li Department of Engineering Physics, Tsinghua University, Beijing 10084, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 July 2009 Received in revised form 10 November 2009 Accepted 10 November 2009 Available online 27 November 2009
Multi-gap resistive plate chamber (MRPC) has been adopted to construct the full barrel time-of-flight (TOF) detector for the STAR experiment at RHIC. The cylindrical TOF covers a total area of approximately 64 m2 and uses 4032 MRPC modules. 70% of the modules were produced in Tsinghua University and the rest were fabricated in University of Science and Technology of China (USTC). In this paper, the production facilities, manufacturing procedures, quality control methods and test results for the Tsinghua MRPC modules are described. & 2009 Elsevier B.V. All rights reserved.
Keywords: MRPC Production Quality control
1. Introduction The multi-gap resistive plate chamber (MRPC) has good time resolution (less than 100 ps) and high detection efficiency (higher than 95%). This technology has been adopted to construct the full barrel time-of-flight (TOF) detector for the STAR experiment at RHIC [1–3]. The cylindrical TOF detector covers a total area of approximately 64 m2. The whole TOF system consists of 120 trays and each tray consists of 32 MRPC modules. Another six trays are produced as standbys. Thus the total number of modules is 4032. Tsinghua University and University of Science and Technology of China (USTC) are responsible for the STAR-TOF MRPC production. Approximately 30% of the modules have been built at USTC [4], and the rest were fabricated at the Miyun production facility of Tsinghua University. The structure of MRPC module for STAR is shown in Fig. 1. Each module consists of six read-out pads. The size of each pad is 3.1 6.0 cm2. There is a 3-mm gap between each pad. The total active area of one module is 18.6 6 cm2 and the working electronegative gas consists of 95% F134a and 5% iso-butane. The electric field in the gas gap is close to 100 kV/cm. The detector works in avalanche mode and has an excellent time response. The STAR-TOF barrel was fitted in the space initially used for the scintillator trigger counters (CTB trays). 32 MRPC modules and the corresponding electronics are all assembled in one tray that has dimensions of 241 cm 21.6 cm 11.43 cm. Therefore the limited space left for MRPC modules makes the mechanical dimensions of the MRPC’s very tight. The requirements for the mechanical dimensions are summarized in Table 1. Corresponding author. Tel.: + 86 106 277 1960; fax: + 86 106 278 2658.
E-mail address: [email protected] (Y. Wang). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.11.045
To assure the quality of MRPC modules, strict production techniques and quality control methods are necessary. A complete set of instructions and procedures for manufacturing techniques and quality assurance (QA) system were set up in Tsinghua University. Our professional engineers also developed several production tools and machines. Two sets of cosmic ray test systems ensure that all modules can be tested.
2. MRPC production All MRPC modules had to be produced within about two and a half years. The production began in the spring of 2006 and ended in August, 2008. An MRPC workshop was established at Miyun manufacturing base of Nuclear Technology Company (NUCTECH). The 200 m2 workshop consists of a material processing room, a washing room, a clean room and a testing laboratory. The mass production of MRPC modules is a systematic project. All steps, from material preparation to products shipment, are very important. 2.1. Material preparation and test All materials have to be tested. The mechanical dimensions of MRPC modules have to be controlled strictly; thus the dimension precision of component material has to be assured. Table 2 shows the specifications of main materials. For glass and graphite tape, resistivity is important. Visual inspection is essential for all materials. For example, a piece of glass with a scratch on its surface has to be rejected. The four corners of glass have to be sanded off to avoid sparks in the MRPC module. Fig. 2 shows the machine designed to sand
ARTICLE IN PRESS Y. Wang et al. / Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
201
Fig. 1. Structure of the MRPC module for STAR-TOF. (a) Long side view and (b) short side view.
Table 1 Requirements for MRPC’s mechanical dimensions.
Length (mm) Width (mm) Thickness between two PCBs (mm) HV lead length (cm) Signal lead length (cm)
Nominal
Minimum
Maximum
212 94 9.7 18 22.5
211.5 93.5 9.4 17.7 22
212.5 94.5 10 18.5 23
Table 2 Specifications of main materials. Material
Dimension (mm)
Tolerance (mm)
Outer glass
207 78 0.7
Inner glass
200 61 0.54
Graphite tape PCB
7 0.2, 7 0.2, 4 1012 O cm 7 0.01 7 0.2, 7 0.2, 4 1012 O cm 7 0.01 0.5 200 kO/& 7 0.2
202 74 0.13 210 94 1.5 6 pads, 31.5 63/pad 212 84 0.35 7 0.1
Not broken No scratch
208 84 4.0
7 1.0, 7 0.2
No scratch, not broken No damage
Height: 3.8
0.05
Diameter 0.22 18 cm 22.5 cm
7 0.005 7 0.5 7 0.5
Mylar film Honeycomb board L-shaped and cylinder supporter Nylon wire HV lead Signal lead
Resistivity
Visual inspection No scratch No scratch
Not broken
Fig. 2. The machine was designed to sand off the corners of glass.
off the corners of glass. Washing is also an important process for glass. 0.2% HS-G20 liquid1 is used for ultrasonic washing. The liquid is kept at 60 1C and the washing process lasts about 20 min. Then the glass has to be rinsed and dried.
1 A special product for the washing of glass produced by Beijing Hang Yida clean technology Co. Ltd.
ARTICLE IN PRESS 202
Y. Wang et al. / Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
Fig. 3. Technicians are assembling MRPC modules.
Table 3 Specifications of a qualified MRPC under testing conditions. Testing conditions
Specifications
Working gas: 95% F134A + 5% isobutane HV: 14 kV FEE threshold: 80 mV
Leakage current: o 2 nA Noise rate: o50 Hz for each pad Avalanche ratio: 4 80% of ADC spectrum Efficiency: 490% Timing resolution of 90% channels o120 ps Crosstalk of two pads: o 0.4
80 70 60 Fig. 4. The silicone sealant dispensing machine.
Counts
50 Avalanche
40
Streamer
30 20 10 0 400
600
800 1000 1200 Amplitude (*0.2pC)
1400
1600
Fig. 6. A typical charge spectrum of MRPC.
Fig. 5. MRPC assembling workbench.
2.2. MRPC assembling MRPC assembling was carried out in a clean room. The temperature and moisture of the clean room can be controlled and its cleanliness reaches 100 K. The temperature is 22 75 1C and
the moisture is maintained below 40%. Fig. 3 shows the assembling workshop in Tsinghua. Eight technicians are working on the production. We have designed several assembling tools to improve the production efficiency and ensure the quality of the products. Fig. 4 shows the machine to apply silicon sealant around the edge of the outer glass sheets. Fig. 5 shows the assembling workbench. Other tools, such as the electrode stamping workbench (which controls the application of the carbon tape to the glass) and the electrode production workbench, are also developed and used in the production.
ARTICLE IN PRESS Y. Wang et al. / Nuclear Instruments and Methods in Physics Research A 613 (2010) 200–206
We have established a complete set of instructions for all manufacturing procedures. A video introducing manufacture and QA of MRPC was composed to train technicians and workers.
3. Performance testing MRPC modules have to be tested comprehensively. Dark current, noise rate, amplitude and time resolution are important specifications for a module. Some of them are correlated with each other to some extent. After thorough R&D and verification with small batch production, we established the specifications for a qualified MRPC module [5,6]. Table 3 shows these required specifications under definite testing conditions. In Table 3 we specify that 80% of the signals from the MRPC should be purely avalanche type at the voltage of 14 kV with this gas mixture (95% F13A+5% SF6). We measure this fraction from the ADC spectrum; an example is shown in Fig. 6. We associate large signals (above 800 counts) with streamers. This limit of 800 counts corresponds to a signal of 2 pC from the MRPC. A high fraction of streamers will adversely affect the time resolution. The crosstalk is represented by correlation coefficient. This coefficient COV(a,b) between pads a and b is defined as [7] /XðaÞXðbÞS/XðaÞS/XðbÞS COVða; bÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi /XðaÞ2 S/XðaÞS2 /XðbÞ2 S/XðbÞS2
Fig. 7. Flow chart of MRPC performance testing.
Fig. 8. Distribution of time resolution and avalanche ratio of 198 pads.
203
ð1Þ
where X(a) =1 if pad a has time output, otherwise X(a) =0. /X(a)S represents the average of X(a). The testing flow chart is summarized in Fig. 7. It is too time consuming to measure the time resolution for each module. According to our studies [6], the time resolution of one module is strongly correlated to the avalanche ratio. In previous small batch production, 33 modules were produced. The relation between the time resolution and avalanche ratio of 198 pads was analyzed. Fig. 8 shows the distribution of time resolution and avalanche ratio of 198 pads. It can be seen that the higher the avalanche ratio, the better the time resolution. If the ratio of avalanche signals is larger than 80%, the probability that its time resolution is better than 120 ps is about 93%. It can be seen that the time resolution of one module is nearly 240 ps, but its charge spectrum is still reasonable. In reality, this module is not qualified because its leakage current is about 10 nA and its noise is about 120 Hz. As we know, the time resolution of MRPC modules run in STAR is about 80 ps, which is obtained after Z direction correction and particle energy selections [8]. These corrections cannot be done in cosmic ray test. Thus the time resolution in cosmic ray test can only reach 100 ps. Considering the 20% relative error, 120 ps is determined as the time resolution in cosmic ray test [6]. The mass test method was established based on this performance.
Fig. 9. Cosmic ray amplitude testing system.
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Two sets of cosmic ray testing system were set up in testing laboratory. Figs. 9 and 10 show cosmic ray amplitude testing system and time testing system, respectively. As shown in Fig. 9,
the cosmic ray telescope consists of two 56 cm 5 cm 5 cm plastic scintillation counters with photo-multiplier tubes. The coincidence of PMT2 and PMT4 gives a cosmic trigger signal for ADC and TDC crates. The width of gate is 100 ns. Four MRPC modules in two gas boxes can be tested simultaneously. Differential signals are sent to the FEE used in STAR-TOF [8]. For a trigger event, 24 channels of charge and 27 channels of time (including times of PMT1, 3 and 4) signals are recorded. Charge spectrum of each channel of four modules can be obtained from the recorded data and then we can determine which module is qualified. 500 events are sufficient for amplitude analysis for each channel, and 10 h is needed to get this number of events. Therefore at least 8 modules can be tested per day. The cosmic ray telescope of time testing system consists of three 21 cm 5 cm 5 cm plastic scintillation counters with photo-multiplier tubes. The coincidence of PMT1 and PMT5 provides the cosmic trigger for the ADC (CAEN V792N) gate and the stop signal for TDC (CAEN V775E) converter. The width of gate is also 100 ns. The time resolution of four PMT (PMT2-5), (t2+ t3 t4 t5)/4, is about 80 ps, which is used to eliminate the jitter of the stop signal. Two MRPC modules in one gas box can be tested at the same time. Eq. (1) is used to eliminate the jitter 0 ¼ tm ðt2 þt3 þ t4 þt5 Þ=4 tm
ð2Þ
0 tm
Fig. 10. Cosmic ray time testing system.
where tm and are the time of MRPC, t2 t5 are the time of PMT2–PMT5, respectively. A sixth-order polynomial is used for the slewing correction. As an example, we show the time resolution of each step for module TM2001 in Fig. 11. The raw time resolution as seen by the TDC is 598 ps. After removing the time jitter of the stop signal by subtracting (t2+t3+ t4+ t5)/4 (see above for details) the time
Fig. 11. The time resolution of TM2001 in each analysis step. (a) The time spectrum before any processing, (b) the time resolution after subtracting time jitter of stop signal, (c) the time spectrum after slewing correction and (d) the time spectrum of four PMTs. The histogram (t2+ t3 t4 t5)/4 is used because its width is the same as the time resolution of our reference counters given by (t2 + t3+ t4+ t5)/4.
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Fig. 12. Statistics of time resolution of 604 modules. (a) Time resolution distribution of all channels and (b)–(g) time distribution of each pad.
spectrum has a sigma of 234 ps. After slewing correction this is reduced to 143 ps. We then subtract quadratically the time jitter of our reference counters (80 ps) to obtain a final resolution of 118 ps. We estimate the time resolution of our reference counters with the following: we assume that the time resolution of each pair (t2 +t3)/2 and (t4+ t5)/2 is the same. Thus histogramming the time difference between these pairs, (t2+ t3)/2 (t4 +t5)/2 should have a width sqrt(2) larger than an individual pair. In
our measurements we generate a time reference from the average of all four counters: (t2 +t3+ t4+t5)/4; the time jitter of this average is sqrt(2) smaller than an individual pair. Thus the time jitter of our reference counters should be a factor 2 smaller than we observe in the histogram of the time difference between the pairs. Thus we choose to histogram (t2 +t3 t4 t5)/4 so that this width gives directly the time resolution of our reference counters given by (t2+ t3+t4+ t5)/4. This is shown in plot (d) of Fig. 11.
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MRPC mass production is a systematic project, comprehensive construction flow and strict quality assurance methods are essential. In our manufacturing base, different kinds of detectors such as ionization chambers and scintillator detectors used in Xray scanning system are produced, and NUCTECH exports its products to countries all over the world. ISO 9000 and ISO 14000 are the essential criteria that have to be carried out. MRPC is also a kind of gas detector and these criteria are also used to instruct our production. Through two and half years of hard efforts, 3100 MRPCs were produced, and the yield reaches 95%. About 90 trays have been mounted on STAR and have been running in STAR for 2 months. The results [9] show that our modules work very well. The MRPC production facility will also play an important role in the successive R&D and production of STAR MTD (Muon Track Detector) and FAIR–CBM high rate TOF.
Count
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0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Correlation coefficient of neighboring pads
Fig. 13. Crosstalk distribution of neighboring pads, average being 0.2.
Acknowledgements
4. Production status 3100 MRPCs were produced in Tsinghua University, 95% (2951) passed QA and 2824 were shipped to University of Texas—Austin. Most of them have been assembled in trays and mounted on STAR. Fig. 12 shows the time resolution statistics of 604 modules. Crosstalk distribution of neighboring pads can be seen in Fig. 13. It can be concluded that our products have excellent performances. This can be obtained from the following: 1) The mechanical dimensions of modules were controlled very well. 2) The average noise is about 6 Hz/pad, which is somewhat lower than the required value. 3) The average time resolution of all pads is about 95 ps, and nearly uniformly distributed among six pads. 4) Crosstalk between pads is small, i.e. the signal is very clean.
The authors would like to thank the STAR-TOF collaboration group. This work was supported by the National Natural Science Foundation of China under Grant nos. 10620210287, 10610285, 10675072 and 10775082. This study is also supported by the Ministry of Science and Technology under Grant no. 2008CB817707.
References [1] The STAR TOF collaboration, Technical design update to proposal for a large area time of flight system for STAR, August 16, 2005. [2] B. Bonner, et al., Nucl. Instr. and Meth. A 478 (2002) 176. [3] Yi Wang, et al., Nucl. Instr. and Meth. A 538 (2005) 425. [4] Tao Zou, et al., Nucl. Instr. and Meth. A 605 (2009) 282. [5] Yi Wang, et al., Nucl. Instr. and Meth. A 537 (2005) 698. [6] Yi Wang, et al., High Energy Phys. Nucl. Phys. 30 (7) (2006) 655. [7] /http://www.rhip.utexas.edu/ tofp/documents/overview.htmlS. [8] F. Geurts, et al., Nucl. Instr. and Meth. A 533 (2004) 60. [9] Geary Eppley, STAR large-area TOF project, Workshop on RHIC-STAR full TOF detector and related physics in China, April 27–29, Hangzhou, China.