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Study on the position resolution of resistive plate chamber
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 591 (2008) 411–416 www.elsevier.com/locate/nima
Study on the position resolution of resistive plate chamber Ye Jin, Cheng Jianping, Yue Qian, Li Yuanjing, Li Jin, Wang Yi Department of Engineering Physics, Tsinghua University, Beijing 100084, China Received 23 December 2007; received in revised form 20 February 2008; accepted 28 February 2008 Available online 15 March 2008
Abstract A prototype RPC detector with a 2-mm readout strip pitch has been built for research into its position resolution. After determining the optimal threshold and working voltage, the efficiency and position resolution of this prototype RPC detector have been studied carefully. Using cosmic-ray muons with different directions and incident positions, we have obtained less than 0.5-mm average statistical errors of position resolution for two different RPC readout structures: 1- and 1.7-mm readout strip widths. The results show that the RPC can also give high position resolution with very little variation from its traditional structure. This will lead to possible applications of the RPC in medical imaging and other domains. r 2008 Published by Elsevier B.V. PACS: 29.40.n; 29.40.Gx; 87.57.U Keywords: Resistive plate chamber (RPC); Efficiency; Position resolution
1. Introduction After being developed by Santonico in the early 1980s [1], the RPC has been used widely for the detection of highenergy charged particles, especially muons with a large-scale spectrometer. The RPC is valued in the domain of highenergy physics for its superior time resolution, high efficiency, moderate position resolution and, more importantly, low cost. Now different high-energy particle spectrometers all over the world have been coupled with the RPC, using it as a trigger system and/or muon detector such as BELLE at KEK-B [2,3], BaBar at SLAC [4], CMS and ATLAS at LHC [5,6] and BESIII at IHEP of China [7,8]. As a gas detector, the RPC can achieve time resolution on the nanosecond scale when detecting charged particles in the streamer mode. This number can be lowered to tens of picoseconds if the avalanche mode has been chosen and a thinner gas layer is used such as a multi-layer resistive plate chamber (MRPC) [9]. Following its large-scale application in particle physics experiment, many new designs have been developed and studied for different requirements such as a large-area array detector system in astroparticle physics [10] Corresponding author. Tel.: +86 10 62772821; fax: +86 10 62782658.
E-mail address: [email protected] (Q. Yue). 0168-9002/$ - see front matter r 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.02.102
and a high position resolution imager in the medical imaging field [11]. The development of the RPC, which can serve as a gamma or neutron imaging detector with suitable changes to its structure and materials, for high position resolution has been a new direction of research, but its basis is still the studies of the RPC with high position resolution for charged particles [12]. To achieve good spatial resolution, the RPC should be run strictly in avalanche mode, and it will be necessary to use a narrow, high-density readout strip or pad, which will require a tremendous number of electronic channels. In this work, we have designed a readout structure based on the method of charge dispersion in order to achieve good spatial resolution with a moderate number of readout channels. This method has shown good experimental performance and excellent position resolution, and the readout channels have been limited to a reasonable number at the same time. 2. The structure of RPC A single-gap RPC has been designed and constructed with plate glass, which has a resistance of about 1012 O cm, as the resistive layer. The depth of the resistive glass is
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700 mm in our prototype RPC. Two 150-mm-thick carbon films are attached to the outside surfaces of two parallel resistive glasses and serve as high-voltage providers. The surface resistance of the carbon film is about 4 105 O/&. The detailed structure of this prototype RPC is shown in Fig. 1. In the beginning of this study, a 1D readout strip has been designed as the pickup for anode-induced charge for the sake of simplicity. If necessary, it can easily be changed to a 2D signal readout structure. The width of a readout strip is 1 mm and the gap between strips is 1 mm as well. Another readout structure with a 1.7-mm strip and a 0.3-mm gap has also been designed for contrastive study. The 2-mm gas gap is maintained by poles made of polycarbonate with a diameter of 2 mm. A whole copper film has been designed to act as a readout structure for cathode-induced charge pickup. The signals coming from the cathode readout film were connected to ground and not used in this work. A 350-mm-thick layer of Mylar film has been used as insulation between the carbon film (highvoltage provider) and the readout structure. In order to maintain the rigidity of the RPC structure, a 5-mm-thick honeycomb plate is stuck to the outside surface of each readout PCB of the RPC detector. High voltage was supplied to the carbon film via a small copper flake stuck to carbon film with conductive glue. The total sensitive area of the RPC detector is about 100 mm 100 mm. A non-flammable gas mixture has been used as a working gas, which contains two ingredients: tetra-fluoro-ethane (more than 90%) and iso-butane. For our experiment, varying iso-butane contents ranging from 9.2% to 6% have been tested in the performance study of the prototype RPC detector. The working gas flow was maintained at a suitable speed when the detectors were tested.
then must be coincident in order to get the trigger signal of the testing system for RPC signal selection. The signals from the RPC anode readout strips were fed into homemade current-sensitive preamplifiers that have eight channels per unit. The outputs of the preamplifiers were sent to the main amplifiers for shaping and stretching from a typical FWTM of 50 ns to about 1 ms width. Then the stretched signals were sent to a 40-MHz 10-bit Flash ADC to digitize them, and the total charge of each current pulse was obtained by integrating the pulse with an onboard FPGA chip. The integrating time window for FPGA can be adjusted and is 1.2 ms in this case. The 32-ch main amplifiers with corresponding FADC channels and FPGA chips were integrated into a set of homemade PCB units, which were inserted into a VME64 bus crate for data transfer. The data from the VME64 bus were read out by a PowerPC (MVE5100, Motorola) interface and sent to DAQ system by net line. The DAQ system and PowerPC were run with the Linux operation system. The allowed trigger rate is more than 1 kHz, although our cosmic-ray trigger rate is less than 1 Hz.
3. Experimental setup Cosmic-ray muons were used for the performance measurement of this prototype RPC detector. A coincident telescope of cosmic-ray was set up in order to get the muons to track one by one with two plastic scintillaters (5 5 25 cm3), whose light was collected by two PMTs (CR105, Hamamatsu). The experimental setup is shown in Fig. 2. The signals from the PMTs are discriminated and
Fig. 2. Schematic layout of the experimental setup for RPC testing.
Honeycomb Plate PCB Board Cathode Plate Resistive Glass Polycarbonate Pole Carbon Film Mylar Film Readout Strip
Fig. 1. The structure of prototype RPC and detailed sub-structure(not in scale).
ARTICLE IN PRESS J. Ye et al. / Nuclear Instruments and Methods in Physics Research A 591 (2008) 411–416
Two RPC detectors were fixed side by side in a gas box to be tested at the same time. After triggered by the coincident system of plastic scintillaters, the DAQ system records the charge information from each electronic channel (64 total channels) of the two detectors, as shown in Fig. 2. The charge dispersion method was used to find the center of the avalanche signal of the RPC detector using two-Gaussian fitting. The detailed information will be mentioned in Section 4.3. We have proved that this experimental setup is stable and gives us an efficient tool for the measurement of the prototype RPC performance.
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under different working voltages of the RPC detector from 74.5 kV to 75 kV (74.5 kV means that we provide anode with a high voltage of +4.5kV and 4.5 kV for the cathode, and so on). The ratio of F134a to iso-butane in the working gas is 90.8:9.2 in Fig. 2. Other gas ratios give almost the same results. From the curves, a threshold value of 50 can be reasonably chosen in order to get rid of the most noise signal. At the same time we obtain a maximum efficiency of 95% with 75-kV high voltage. This threshold value will be used for all data collection process later in this paper. It should be pointed out that we include both unsaturated and saturated events when efficiency was calculated.
4. Experimental results The system ran for two months and data were collected for different experimental conditions. The optimal high voltage and working gas can be selected offline when the data are processed. Several running parameters including the threshold of the FADC and the width of the integrated window of the FPGA can be set online via the DAQ program each time before the system starts to take the data. The real optimal threshold can be obtained through offline data analysis. We get the output charge value of each channel, which has been subtracted from the corresponding FADC pedestal with hardware. The channels with charges that do not pass through the threshold will be given zero charge. There should be at least two channels with a Q charge bigger than the hardware threshold for an effective event, and thus it is possible to provide a better position resolution using the method of charge dispersion. 4.1. Detector threshold At the beginning of the data analysis, we have to obtain the proper FADC threshold for real signal selection. Fig. 3 shows the relationship between efficiency and the FADC threshold
4.2. Detector efficiency For the measurement of position resolution, we have to get rid of the streamer (and/or saturated) signals. Fig. 4 shows the total charge spectra for all events. The total charge for each event can be achieved by summing the charge values from all 32 readout strips. Fig. 4(a) gives the spectrum of unsaturated events and Fig. 4(b) gives the spectrum of saturated events. One can see that the unsaturated events have a typical landau spectrum. In Fig. 4(b), the spectrum of saturated events has two parts. For events with a small Q value (less than 0.5E6), the number of channels with non-zero Q charge output is usually bigger than five, and the corresponding saturated channels consist of the central one or two channels. It is possible to reconstruct these current pulses in order to make them usable [13]. We will report this result in another paper. The events with a larger Q value (larger than 0.5E6) are caused by the streamer mode and can be cut off. Fig. 5 shows the curves of efficiency versus high voltages. The dotted and solid lines show results including and excluding the saturated events, respectively. The efficiency that includes the saturated events can be about 95%, and the
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Fig. 3. The relationship between efficiency and different threshold values for different high voltages (working gas: F134a:iso-butane ¼ 90.8:9.2).
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efficiency with a saturated events cutoff has a maximum value of 80% that declines rapidly beyond a certain high voltage. The optimal voltage is 74700 V for the situation with just unsaturated events when the working gas ratio is 92% tetra-fluoro-ethane and 8% iso-butane. Though the saturated signals can be decreased by changing the dynamic range of amplifier or/and FADC, we do not consider this situation in this work because the position resolution can be obtained with these unsaturated events.
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Fig. 4. The spectra of total charge of all events with working gas F134a:iso-butane ¼ 90.8:9.2 and 4.7-kV voltage. (a) Spectrum of unsaturated events and (b) spectrum of saturated events.
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For one avalanche signal of the RPC, there are two reasons for the charge distribution at the readout strips. One is due to the evolvement of the avalanche electron multiplication and the other is the charge expansion when the induced charge passes through the carbon film, which has a surface resistance of about 4 105 O/&. Many studies have shown that these two processes will both give a Gaussian expansion for the total charge distribution of an event [14,15]. Fig. 6 shows the charge distribution collected by FADC channels for different events with 1-mm readout strips (a) and 1.7-mm strips (b). It is reasonable to get the real incident position of a cosmic-ray muon by fitting the charge dispersion with a two-Gaussian function. The fitting results of several random events from the RPC with 1- and a 1.7-mm readout strips have also been given in Fig. 6(a) and (b). The two-Gaussian function was used to do the fitting. The fitting results are very nice for most of the events. Here we conclude that the Gaussian function with a narrow width describes the charge distribution resulting from avalanche evolvement, and another Gaussian function describes the charge distribution from charge dispersion when it passes through the carbon film with a relative resistance much less than resistive glass. For the position resolution in this paper, we just consider the statistical error of the Gaussian function with the narrow width.
F134a : iso-butane 90.8 : 9.2 92 : 8 94 : 6 96 : 4 90.8 : 9.2 92 : 8 94 : 6 96 : 4
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Fig. 5. The relationship between efficiency of RPC detector and its high voltage with different gas ratios. The open markers come from the total events (saturated and unsaturated events) and the solid markers are just for unsaturated events.
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Fig. 6. The dispersion of induced charge collected by readout strips of two RPCs with different widths: (a) 1 mm and (b) 1.7 mm. The fitting results by two-Gaussian function have been shown for different events.
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It is difficult to get very precise incident position and direction of the cosmic-ray muons with our plastic scintillater telescope due to its large size. As such, we did not measure the real position resolution at each position of the RPC detector, and we just gave the statistical error for each event from different positions of the RPC detector and different incident directions of the cosmic-ray muons. The distribution of statistical errors for all the unsaturated events is shown in Fig. 7. From this measurement, we can obtain the average statistical error, which is less than 0.5 mm for both readout strips. This is a relatively good
F134a : iso-butane = 92.8
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Fig. 7. The distribution of statistical errors obtained by two-Gaussian fitting of all unsaturated events from RPC detectors with 1-mm (RPC1) and 1.7-mm (RPC2) readout strips, respectively. The gas ratio is 92% F134a and 8% iso-butane and the voltage is 4700 V. Most of the events have a statistical position resolution less than 0.5 mm.
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Table 1 The average statistic position resolution and the percentage of events with less than 0.5-mm resolution for different gas and readout structure for high voltage of 74700 V RPC readout structure (mm) Gas (F134a content) Average statistic position resolution (mm) Percent below 0.5-mm resolution (%)
1+1 90.8 0.309 83.86
1.7+0.3 92 0.341 80.00
94 0.313 83.42
position resolution for this type of RPC. One can see that the same trend exists for the distributions of statistical errors of different RPC readout strips with 1- and 1.7-mm widths with some subtle differences. The RPC detector with a 1-mm strip has a less average statistical error than that of 1.7-mm readout strip (Table 1). 5. Summary We have successfully produced an RPC detector for the study of its position resolution. After determining the optimal threshold and working voltage, the efficiency and position resolution of this prototype RPC detector have been studied carefully. Using cosmic-rays with different directions and incident positions, we have obtained less than 0.5-mm average statistical errors for both of the two RPC readout structures with strip widths of 1 and 1.7 mm, respectively. This result is very good compared with traditional RPCs used for time measurement. This result shows that an RPC can also make position measurements with higher precision. In fact, this kind of RPC needs to be calibrated by charged particles with precise incident position and direction if one wants to know its real position resolution. A cosmic-ray muon coincident system with less than 1-mm precision is under construction for our further study of RPC performance. Also the dependence of position resolution of the RPC on the width of its readout strip, the surface resistance of carbon film and the width of gas gap are important for RPC application in the medical imaging field. This dependence will be under consideration in our future experiment. Acknowledgments We would like to thank Mr. Shaofeng Wang for his help in the set up of the electronics system at the very beginning
96 0.325 82.42
90.8 0.380 77.58
92 0.432 72.12
94 0.386 76.77
96 0.381 76.46
of this study and Mr. Shaoji Mao for his detailed instruction and warm help in selecting materials when producing these RPC detectors. Thanks are also due to Ms. Chunling Hui and her working group for their hard and careful work in producing these RPC detectors. Professor Yulan Li and Dr. Huirong Qi helped us in the set up of the electronics and DAQ system. This research was supported by the basic research funding of Engineering Physics Department of Tsinghua University (Contract No. 110041704).
References [1] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. A. 187 (1981) 377. [2] G. Bruno, Eur. Phys. J. C 33 (2004) s1032. [3] Belle Collaboration, Nucl. Instr. and Meth. A 479 (2002) 117. [4] BaBar Collaboration, Technical Design Report, SLAC-R-95-457, March 1995. [5] CMS Collaboration, Technical Design Report, CERN/LHCC/200038, 2000. [6] ATLAS Collaboration, Technical Design Report, CERN/LHCC/ 98-14, 1998. [7] J. Zhang, et al., Nucl. Instr. and Meth. A 540 (2005) 102. [8] The BESIII Detector, IHEP-BEPCII-SB-13, IHEP, Beijing. [9] B. Bonner, G. Eppley, J. Lamas-Valverde, et al., Nucl. Instr. and Meth. A 478 (2002) 176. [10] Q. Gou, Nucl. Instr. and Meth. A 560 (2006) 617. [11] A. Blanco, et al., Nucl. Instr. and Meth. A 533 (2004) 139. [12] M.S. Dixit, J. Dubeau, et al., Nucl. Instr. and Meth. A 518 (2004) 721. [13] Q. Yue, W.P. Lai, W.C. Chang, et al., Nucl. Instr. and Meth. A 511 (2003) 408. [14] Werner Riegler, Christian Lippmann, Nucl. Instr. and Meth. A 518 (2004) 86. [15] M.S. Dixit, J. Dubeau, J.-P. Martin, et al., Nucl. Instr. and Meth. A 518 (2004) 721.
Nuclear Instruments and Methods in Physics Research A 591 (2008) 411–416 www.elsevier.com/locate/nima
Study on the position resolution of resistive plate chamber Ye Jin, Cheng Jianping, Yue Qian, Li Yuanjing, Li Jin, Wang Yi Department of Engineering Physics, Tsinghua University, Beijing 100084, China Received 23 December 2007; received in revised form 20 February 2008; accepted 28 February 2008 Available online 15 March 2008
Abstract A prototype RPC detector with a 2-mm readout strip pitch has been built for research into its position resolution. After determining the optimal threshold and working voltage, the efficiency and position resolution of this prototype RPC detector have been studied carefully. Using cosmic-ray muons with different directions and incident positions, we have obtained less than 0.5-mm average statistical errors of position resolution for two different RPC readout structures: 1- and 1.7-mm readout strip widths. The results show that the RPC can also give high position resolution with very little variation from its traditional structure. This will lead to possible applications of the RPC in medical imaging and other domains. r 2008 Published by Elsevier B.V. PACS: 29.40.n; 29.40.Gx; 87.57.U Keywords: Resistive plate chamber (RPC); Efficiency; Position resolution
1. Introduction After being developed by Santonico in the early 1980s [1], the RPC has been used widely for the detection of highenergy charged particles, especially muons with a large-scale spectrometer. The RPC is valued in the domain of highenergy physics for its superior time resolution, high efficiency, moderate position resolution and, more importantly, low cost. Now different high-energy particle spectrometers all over the world have been coupled with the RPC, using it as a trigger system and/or muon detector such as BELLE at KEK-B [2,3], BaBar at SLAC [4], CMS and ATLAS at LHC [5,6] and BESIII at IHEP of China [7,8]. As a gas detector, the RPC can achieve time resolution on the nanosecond scale when detecting charged particles in the streamer mode. This number can be lowered to tens of picoseconds if the avalanche mode has been chosen and a thinner gas layer is used such as a multi-layer resistive plate chamber (MRPC) [9]. Following its large-scale application in particle physics experiment, many new designs have been developed and studied for different requirements such as a large-area array detector system in astroparticle physics [10] Corresponding author. Tel.: +86 10 62772821; fax: +86 10 62782658.
E-mail address: [email protected] (Q. Yue). 0168-9002/$ - see front matter r 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.02.102
and a high position resolution imager in the medical imaging field [11]. The development of the RPC, which can serve as a gamma or neutron imaging detector with suitable changes to its structure and materials, for high position resolution has been a new direction of research, but its basis is still the studies of the RPC with high position resolution for charged particles [12]. To achieve good spatial resolution, the RPC should be run strictly in avalanche mode, and it will be necessary to use a narrow, high-density readout strip or pad, which will require a tremendous number of electronic channels. In this work, we have designed a readout structure based on the method of charge dispersion in order to achieve good spatial resolution with a moderate number of readout channels. This method has shown good experimental performance and excellent position resolution, and the readout channels have been limited to a reasonable number at the same time. 2. The structure of RPC A single-gap RPC has been designed and constructed with plate glass, which has a resistance of about 1012 O cm, as the resistive layer. The depth of the resistive glass is
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700 mm in our prototype RPC. Two 150-mm-thick carbon films are attached to the outside surfaces of two parallel resistive glasses and serve as high-voltage providers. The surface resistance of the carbon film is about 4 105 O/&. The detailed structure of this prototype RPC is shown in Fig. 1. In the beginning of this study, a 1D readout strip has been designed as the pickup for anode-induced charge for the sake of simplicity. If necessary, it can easily be changed to a 2D signal readout structure. The width of a readout strip is 1 mm and the gap between strips is 1 mm as well. Another readout structure with a 1.7-mm strip and a 0.3-mm gap has also been designed for contrastive study. The 2-mm gas gap is maintained by poles made of polycarbonate with a diameter of 2 mm. A whole copper film has been designed to act as a readout structure for cathode-induced charge pickup. The signals coming from the cathode readout film were connected to ground and not used in this work. A 350-mm-thick layer of Mylar film has been used as insulation between the carbon film (highvoltage provider) and the readout structure. In order to maintain the rigidity of the RPC structure, a 5-mm-thick honeycomb plate is stuck to the outside surface of each readout PCB of the RPC detector. High voltage was supplied to the carbon film via a small copper flake stuck to carbon film with conductive glue. The total sensitive area of the RPC detector is about 100 mm 100 mm. A non-flammable gas mixture has been used as a working gas, which contains two ingredients: tetra-fluoro-ethane (more than 90%) and iso-butane. For our experiment, varying iso-butane contents ranging from 9.2% to 6% have been tested in the performance study of the prototype RPC detector. The working gas flow was maintained at a suitable speed when the detectors were tested.
then must be coincident in order to get the trigger signal of the testing system for RPC signal selection. The signals from the RPC anode readout strips were fed into homemade current-sensitive preamplifiers that have eight channels per unit. The outputs of the preamplifiers were sent to the main amplifiers for shaping and stretching from a typical FWTM of 50 ns to about 1 ms width. Then the stretched signals were sent to a 40-MHz 10-bit Flash ADC to digitize them, and the total charge of each current pulse was obtained by integrating the pulse with an onboard FPGA chip. The integrating time window for FPGA can be adjusted and is 1.2 ms in this case. The 32-ch main amplifiers with corresponding FADC channels and FPGA chips were integrated into a set of homemade PCB units, which were inserted into a VME64 bus crate for data transfer. The data from the VME64 bus were read out by a PowerPC (MVE5100, Motorola) interface and sent to DAQ system by net line. The DAQ system and PowerPC were run with the Linux operation system. The allowed trigger rate is more than 1 kHz, although our cosmic-ray trigger rate is less than 1 Hz.
3. Experimental setup Cosmic-ray muons were used for the performance measurement of this prototype RPC detector. A coincident telescope of cosmic-ray was set up in order to get the muons to track one by one with two plastic scintillaters (5 5 25 cm3), whose light was collected by two PMTs (CR105, Hamamatsu). The experimental setup is shown in Fig. 2. The signals from the PMTs are discriminated and
Fig. 2. Schematic layout of the experimental setup for RPC testing.
Honeycomb Plate PCB Board Cathode Plate Resistive Glass Polycarbonate Pole Carbon Film Mylar Film Readout Strip
Fig. 1. The structure of prototype RPC and detailed sub-structure(not in scale).
ARTICLE IN PRESS J. Ye et al. / Nuclear Instruments and Methods in Physics Research A 591 (2008) 411–416
Two RPC detectors were fixed side by side in a gas box to be tested at the same time. After triggered by the coincident system of plastic scintillaters, the DAQ system records the charge information from each electronic channel (64 total channels) of the two detectors, as shown in Fig. 2. The charge dispersion method was used to find the center of the avalanche signal of the RPC detector using two-Gaussian fitting. The detailed information will be mentioned in Section 4.3. We have proved that this experimental setup is stable and gives us an efficient tool for the measurement of the prototype RPC performance.
413
under different working voltages of the RPC detector from 74.5 kV to 75 kV (74.5 kV means that we provide anode with a high voltage of +4.5kV and 4.5 kV for the cathode, and so on). The ratio of F134a to iso-butane in the working gas is 90.8:9.2 in Fig. 2. Other gas ratios give almost the same results. From the curves, a threshold value of 50 can be reasonably chosen in order to get rid of the most noise signal. At the same time we obtain a maximum efficiency of 95% with 75-kV high voltage. This threshold value will be used for all data collection process later in this paper. It should be pointed out that we include both unsaturated and saturated events when efficiency was calculated.
4. Experimental results The system ran for two months and data were collected for different experimental conditions. The optimal high voltage and working gas can be selected offline when the data are processed. Several running parameters including the threshold of the FADC and the width of the integrated window of the FPGA can be set online via the DAQ program each time before the system starts to take the data. The real optimal threshold can be obtained through offline data analysis. We get the output charge value of each channel, which has been subtracted from the corresponding FADC pedestal with hardware. The channels with charges that do not pass through the threshold will be given zero charge. There should be at least two channels with a Q charge bigger than the hardware threshold for an effective event, and thus it is possible to provide a better position resolution using the method of charge dispersion. 4.1. Detector threshold At the beginning of the data analysis, we have to obtain the proper FADC threshold for real signal selection. Fig. 3 shows the relationship between efficiency and the FADC threshold
4.2. Detector efficiency For the measurement of position resolution, we have to get rid of the streamer (and/or saturated) signals. Fig. 4 shows the total charge spectra for all events. The total charge for each event can be achieved by summing the charge values from all 32 readout strips. Fig. 4(a) gives the spectrum of unsaturated events and Fig. 4(b) gives the spectrum of saturated events. One can see that the unsaturated events have a typical landau spectrum. In Fig. 4(b), the spectrum of saturated events has two parts. For events with a small Q value (less than 0.5E6), the number of channels with non-zero Q charge output is usually bigger than five, and the corresponding saturated channels consist of the central one or two channels. It is possible to reconstruct these current pulses in order to make them usable [13]. We will report this result in another paper. The events with a larger Q value (larger than 0.5E6) are caused by the streamer mode and can be cut off. Fig. 5 shows the curves of efficiency versus high voltages. The dotted and solid lines show results including and excluding the saturated events, respectively. The efficiency that includes the saturated events can be about 95%, and the
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Fig. 3. The relationship between efficiency and different threshold values for different high voltages (working gas: F134a:iso-butane ¼ 90.8:9.2).
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efficiency with a saturated events cutoff has a maximum value of 80% that declines rapidly beyond a certain high voltage. The optimal voltage is 74700 V for the situation with just unsaturated events when the working gas ratio is 92% tetra-fluoro-ethane and 8% iso-butane. Though the saturated signals can be decreased by changing the dynamic range of amplifier or/and FADC, we do not consider this situation in this work because the position resolution can be obtained with these unsaturated events.
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Fig. 4. The spectra of total charge of all events with working gas F134a:iso-butane ¼ 90.8:9.2 and 4.7-kV voltage. (a) Spectrum of unsaturated events and (b) spectrum of saturated events.
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Efficiency (%)
80 70 60 50
For one avalanche signal of the RPC, there are two reasons for the charge distribution at the readout strips. One is due to the evolvement of the avalanche electron multiplication and the other is the charge expansion when the induced charge passes through the carbon film, which has a surface resistance of about 4 105 O/&. Many studies have shown that these two processes will both give a Gaussian expansion for the total charge distribution of an event [14,15]. Fig. 6 shows the charge distribution collected by FADC channels for different events with 1-mm readout strips (a) and 1.7-mm strips (b). It is reasonable to get the real incident position of a cosmic-ray muon by fitting the charge dispersion with a two-Gaussian function. The fitting results of several random events from the RPC with 1- and a 1.7-mm readout strips have also been given in Fig. 6(a) and (b). The two-Gaussian function was used to do the fitting. The fitting results are very nice for most of the events. Here we conclude that the Gaussian function with a narrow width describes the charge distribution resulting from avalanche evolvement, and another Gaussian function describes the charge distribution from charge dispersion when it passes through the carbon film with a relative resistance much less than resistive glass. For the position resolution in this paper, we just consider the statistical error of the Gaussian function with the narrow width.
F134a : iso-butane 90.8 : 9.2 92 : 8 94 : 6 96 : 4 90.8 : 9.2 92 : 8 94 : 6 96 : 4
40 30 20 10 0 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 High Voltage (V)
5100
5200
Fig. 5. The relationship between efficiency of RPC detector and its high voltage with different gas ratios. The open markers come from the total events (saturated and unsaturated events) and the solid markers are just for unsaturated events.
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Fig. 6. The dispersion of induced charge collected by readout strips of two RPCs with different widths: (a) 1 mm and (b) 1.7 mm. The fitting results by two-Gaussian function have been shown for different events.
RPC 1
1200
RPC 2
1000 800 Counts
It is difficult to get very precise incident position and direction of the cosmic-ray muons with our plastic scintillater telescope due to its large size. As such, we did not measure the real position resolution at each position of the RPC detector, and we just gave the statistical error for each event from different positions of the RPC detector and different incident directions of the cosmic-ray muons. The distribution of statistical errors for all the unsaturated events is shown in Fig. 7. From this measurement, we can obtain the average statistical error, which is less than 0.5 mm for both readout strips. This is a relatively good
F134a : iso-butane = 92.8
600 400 200
Fig. 7. The distribution of statistical errors obtained by two-Gaussian fitting of all unsaturated events from RPC detectors with 1-mm (RPC1) and 1.7-mm (RPC2) readout strips, respectively. The gas ratio is 92% F134a and 8% iso-butane and the voltage is 4700 V. Most of the events have a statistical position resolution less than 0.5 mm.
0 0
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1 1.5 Position resolution (mm)
2
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J. Ye et al. / Nuclear Instruments and Methods in Physics Research A 591 (2008) 411–416
Table 1 The average statistic position resolution and the percentage of events with less than 0.5-mm resolution for different gas and readout structure for high voltage of 74700 V RPC readout structure (mm) Gas (F134a content) Average statistic position resolution (mm) Percent below 0.5-mm resolution (%)
1+1 90.8 0.309 83.86
1.7+0.3 92 0.341 80.00
94 0.313 83.42
position resolution for this type of RPC. One can see that the same trend exists for the distributions of statistical errors of different RPC readout strips with 1- and 1.7-mm widths with some subtle differences. The RPC detector with a 1-mm strip has a less average statistical error than that of 1.7-mm readout strip (Table 1). 5. Summary We have successfully produced an RPC detector for the study of its position resolution. After determining the optimal threshold and working voltage, the efficiency and position resolution of this prototype RPC detector have been studied carefully. Using cosmic-rays with different directions and incident positions, we have obtained less than 0.5-mm average statistical errors for both of the two RPC readout structures with strip widths of 1 and 1.7 mm, respectively. This result is very good compared with traditional RPCs used for time measurement. This result shows that an RPC can also make position measurements with higher precision. In fact, this kind of RPC needs to be calibrated by charged particles with precise incident position and direction if one wants to know its real position resolution. A cosmic-ray muon coincident system with less than 1-mm precision is under construction for our further study of RPC performance. Also the dependence of position resolution of the RPC on the width of its readout strip, the surface resistance of carbon film and the width of gas gap are important for RPC application in the medical imaging field. This dependence will be under consideration in our future experiment. Acknowledgments We would like to thank Mr. Shaofeng Wang for his help in the set up of the electronics system at the very beginning
96 0.325 82.42
90.8 0.380 77.58
92 0.432 72.12
94 0.386 76.77
96 0.381 76.46
of this study and Mr. Shaoji Mao for his detailed instruction and warm help in selecting materials when producing these RPC detectors. Thanks are also due to Ms. Chunling Hui and her working group for their hard and careful work in producing these RPC detectors. Professor Yulan Li and Dr. Huirong Qi helped us in the set up of the electronics and DAQ system. This research was supported by the basic research funding of Engineering Physics Department of Tsinghua University (Contract No. 110041704).
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