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Study of the counting rate capability of MRPC detectors built with soda lime glass
Nuclear Instruments and Methods in Physics Research A 830 (2016) 182–190
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the counting rate capability of MRPC detectors built with soda lime glass R. Forster a, O. Margoto Rodríguez a,f, W. Park a, A. Rodríguez Rodríguez a,e,n,1, M.C.S. Williams c,d, A. Zichichi b,c,d, R. Zuyeuski a,b a
ICSC World Laboratory, Geneva, Switzerland Museo Storico della Fisica e Centro Studi e Ricerche E.Fermi, Roma, Italy c CERN, Geneva, Switzerland d INFN and Dipartimento di Fisica e Astronomia, Universit di Bologna, Italy e Center for Technological Applications and Nuclear Development, La Habana, Cuba f University Hermanoz Saiz, Pinar del Rio, Cuba b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 May 2016 Accepted 20 May 2016 Available online 24 May 2016
We report the results of three MRPC detectors built with soda lime glass and tested in the T10 beam line at CERN. The detectors consist of a stack of 280 μm thick glass sheets with 6 gaps of 220 μm . We built two identical MRPCs, except one had the edges of glass treated with resistive paint. A third detector was built with one HV electrode painted as strips. The detectors' efficiency and time resolution were studied at different particle flux in a pulsed beam environment. The results do not show any improvement with the painted edge technique at higher particle flux. We heated the MRPCs up to 40 °C to evaluate the influence of temperature in the rate capability. Results from this warming has indicated an improvement on the rate capability. The dark count rates show a significant dependence with the temperature. & 2016 Elsevier B.V. All rights reserved.
Keywords: MRPC Rate capability Temperature Efficiency Time resolution
1. Introduction The Multigap Resistive Plate Chamber (MRPC) is a detector with excellent time resolution ( < 100 ps), high efficiency ( > 95%), insensitive to magnetic fields and a relatively low cost. These characteristics, as well as the easy segmentation have made it a reliable and proven tool for Time of Flight detectors (TOF) in different experiments such as ALICE [1], STAR [2], HADES [3] and FOPI [4]. Several MRPC designs built with soda lime glass ( ρ ∼ 1013 Ω cm ) have been already tested with successful results at rates under 1 kHz/cm2 [5–7]. Nowadays, there is a strong R&D effort to improve the MRPC's rate capabilities and to install these devices in experiments such as the CBM TOF wall [8]. The rate capabilities of the MRPC detectors is determined by the bulk resistivity of the glass plates and therefore thinner and lower resistivity materials are needed. New materials with resistivity in the order to 1010 Ω cm are being investigated [9,8]. An alternative way to enhance the rate capabilities of MRPCs is to increase the temperature of the glass plates. However, it is n Correspondence to: GSI Helmholtzzentrum für Schwerionenforschung, GmbH Planckstraße 1, 64291 Darmstadt, Germany. E-mail address: [email protected] (A. Rodríguez Rodríguez). 1 Current Institute: Goethe University, Frankfurt am Main, Germany.
http://dx.doi.org/10.1016/j.nima.2016.05.080 0168-9002/& 2016 Elsevier B.V. All rights reserved.
necessary to evaluate many factors (e.g. dark currents, rate capability) before applying this technique to large detector arrays. We present the results obtained with new prototypes of MRPC built with soda lime glass. The tests were carried out in the T10 beam line at CERN. We have studied the detectors performance at different particle flux and evaluate the influence of a moderate warming on the rate capabilities. 1.1. Rate analysis If we consider a small area, A, of the glass sheet, there is a capacitance (c = Aε0 εr /d ) and resistance (R = ρd/A) between the surfaces. Thus any charge deposited on the surface will decay with a relaxation time τ = ρε0 εr . For float glass ρ ∼ 5 × 1012Ω cm and the relative dielectric constant (εr ∼ 8), τ is ∼3 s. When a charged particle passes through the MRPC, charge is produced by the gas avalanches and deposited onto the surface of the resistive plates. This charge will create a voltage drop across the resistive plate. In the steady state (constant flux of particles over the whole surface of the MRPC), the voltage drop is just Q A·Rate·RMRPC , where QA is the total charge produced inside the MRPC for a through-going charged particle and RMRPC is the total resistance (of all plates) per unit area. Thus in a 6 gas gaps detector made with glass sheets of 280 μm thickness, a particle rate of 1 kHz/cm2 causes a voltage drop of approximately 2000 V, if we
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Layer dopped with holes Resistive paint Charge movement
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Avalanche
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Fig. 1. Inner glass plate with painted edge. The rate capability is limited by the time needed for a localized discharge to pass through the glass electrode. However if the surface resistivity of the glass is low, charge can flow on the surface of the glass. The painted edges of the glass sheet form a low resistive bridge between the two surfaces.
assume that 2 pC charge is generated by each through-going particle. Given this we should not expect the MRPC to work at fluxes above 1 kHz/cm2. The current flow through a glass sheet is a complex process; the glass can be considered as an amorphous semiconductor. In a high electric field, the electrons will move within the plate until there is no electric field inside the glass sheet. Thus one of the glass surfaces will be charged with electrons and the other will present a lack of electrons (i.e. holes). This process could be understood as doping the glass surface with a thin layer of electrons and holes. Based on this assumption two MRPCs prototypes, with major changes on the inner glasses and HV electrodes have been designed and tested. In MRPC2, the edges of the glass sheets have been coated with resistive paint from both sides, connecting the surfaces of the plates. The resistive paint should act as a bridge for the produced charges, allowing a fast evacuation of the charge on the surfaces (Fig. 1). The detector MRPC3 was designed with a HV electrode painted as strips. A detailed explanation of these devices is given in the next section.
2. Design and construction of MRPCs We present the results obtained with three MRPCs. They were built with the same materials and geometrical design; however for the MRPC2, the edges of the glass plates have been treated differently. In Fig. 2 left is shown a schematic of the cross section of the Standard MRPC (MRPC1). The detectors consist of six gas gaps of 220 μm width. The glass plates are manufactured by AGC Glass and have a thickness of 280 μm and a bulk resistivity of
( ρ ∼ 5 × 1012Ω cm ). The outer surface of the outermost glasses has been covered with a resistive paint; this coating acts as the electrodes to apply the high voltage. The surface resistivity of all painted layers are approximately (∼5 MΩ/square). Two printed circuit board (PCB) of 1.5 mm thickness insulates 24 readout strips from the anode and cathode. Each readout strip is 0.7 20.5 cm2, separated by 1 mm from its neighbor. The detectors active area is 19.6 19.6 cm2. The spacers are mono-filament commercial fishing line of 220 μm stretched across the surface of the glass from one side to the other and wound around plastic screws fixed at both sides of the chamber. The anode and cathode boards are connected by pins; thus a differential signal is sent to the NINO ASIC [10] front end. These pins also mechanically attach the pcbs together. A honeycomb panel of 5 mm thickness is attached to the anode pcb sheet using double-sided adhesive tape. The finished chambers are mounted in gas-tight aluminum boxes. 2.1. MRPC2 The painted edge MRPC (MRPC2) has a modification at the edges of the glass sheets as shown in Fig. 2 right. The external plates, were covered with resistive paint in the outermost surface as the standard procedure. Two opposite edges of the inner surface were painted with a strip of 1.5 cm width. Both surfaces were thus connected around the edge with the resistive paint. The inner glass sheets were painted in the same way with a band 0.5 cm width. Spacers placed at the corners and between the painted edges kept the gaps size and also protect from sparks between two neighboring layers; these spacers were made from 50 μm Mylar and covered with 85 μm thick double-sided adhesive tape.
Mylar (0.175 mm)
Fishing line (0.22 mm)
PCB with cathode read out strips
Outer glass (0.28 mm)
Mylar (0.05 mm)
Adhesive tape Inner glass (0.28 mm)
Resistive Paint
Front End Card connector
Mylar (0.175 mm)
PCB with anode read out strips
Connector pin PCB with read out strips
Plastic screw Honeycomb panel
Fig. 2. Left. Standard MRPC (6/220). Right. Details of the painted edges on MRPC2.
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Fig. 3. Schematic of the HV strips electrode in the MRPC3.
2.2. MRPC3 The HV strips MRPC (MRPC3) consists of a standard detector where the coating of one of the HV electrodes was applied as strips. Fig. 3 shows a schematic of this electrode. To cover the detector active area, 8 strips of 2.1 cm width separated at 0.4 cm from its neighbor were applied. Each strip was connected to a lemo connector and grounded by 1 MΩ resistor. These HV strips should not be confused with the standards readout strips; they run in orthogonal direction, see Fig. 3.
3. Beam test set-up The MRPCs were mounted separately and tested in the T10 test beam facility at CERN. The beam was mainly negative pions of 5 GeV/c momentum. There were 4–5 spills (400 ms long) per supercycle that had a period of approximately 22 s. The gas mixture was 95% of C2H2 F4 and 5% SF6 with a continuous flow of 5 l/h. To provide the trigger three set of scintillators–photomultipliers were used. Two pairs of crossed scintillators (P1–P2 and P3–P4) were placed on each side of the MRPC along the beam line. The upstream pair (P1–P2) has an active area of 1 1 cm2. The downstream pair (P3–P4) of 2 2 cm2 area. In addition special timing scintillators were incorporated, consisting of two orthogonal
scintillator bars of (2 2 10 cm3), read at each end by photomultiplier tubes (S1–S2 and S3–S4). This pair were placed upstream the MRPC and provided an accurate time reference. The time difference between both bars ((S1 + S 2) /2 − (S 3 + S 4) /2) had a sigma of 80 ps, and hence the expected time resolution from the average value ((S1 + S 2 + S 3 + S 4) /4) is 40 ps. The schematic of the experimental set-up in the beam line is shown in Fig. 4. The MRPCs were read out from both side using the 24 channels NINO ASIC cards designed for the ALICE TOF [10]. The threshold was set at 160 mV (∼40 fC) and remained the same during all measurements. The data was taken using two CAEN HPTDCs (V1290 A). This system has a time resolution of 30–40 ps. The OR signal from the front end card was also connected to a four channel digital oscilloscope Lecroy WaveRunner 104MXi-A (1 GHz) to check the MRPC signals during the run.
4. Results and discussion 4.1. Efficiency and time resolution at low rate The first measurements were taken at 1.5 kHz/cm2. This rate was monitored using the scintillator bars ( S1 − S4 ) defining a 2 2 cm2 active area. The beam was centered in the middle of one strip and the efficiency and time resolution were measured as a MRPC
Trigger fingers (P3-P4)
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Fig. 4. Schematic of the experimental setup.
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Fig. 5. Efficiency and time resolution measured at 1.5 kHz/cm2. The lines are to guide the eye.
function of the applied voltage; the results are shown in Fig. 5. The detectors have reached the efficiency plateau (over 95%) above 15 kV, which extends more than 1 kV. A comparison between detectors' results reveals that at 16.0 kV, the MRPC1 has a time resolution of (73.4 71.1) ps and the MRPC2 (85.6 71.2) ps. Since the strip was read at each end, the mean time ( (tEND1 + tEND2 ) /2) is independent of the hit position along the strip direction. In Fig. 6 left a typical histogram of the time difference measured between the MRPC2 (TMRPC ) and the reference
scintillators (TRef ) is shown. The use of a fixed threshold introduces a time shift (slewing) depending of the amplitude of the signal. The NINO chip measures the input charge and encodes it into the width of the LVDS pulse; this information is used to correct for slewing. The spectrum of the fast charge at 16 kV measured with the time-over-threshold (TOT) technique is presented in Fig. 6 right. As can be seen multiple peaks appear in the charge distribution. The reason of this shape are the multiple reflection of the signal due to the imperfect matching impedance between the transmission line and the NINO ASIC. Fig. 7 left shows a scatter plot
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Fig. 6. Left. Time difference between MRPC2 (TMRPC) and reference scintillators ( TRef = (S1 + S2 + S3 + S4 )/4 ). The distribution is presented before slewing correction. The measured time in MRPCs is the average time between both ends, and thus independent of the hit position. Right. Distribution of the average Time over Threshold (TOT) in MRPC2.
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Fig. 7. Left. Time difference as a function of the average pulse width in MRPC2; the pulse width is defined by the input charge. The profile is fitted using a fourth order polynomial. Right. Time distribution after the slewing correction.
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the beam line defined the beam intensity. The measurements were taken at 4 different rates (1.5 kHz/cm2, 16.0 kHz/cm2, 28.0 kHz/cm2 and 35.0 kHz/cm2). The results are presented in Fig. 8. The dark count rate is also displayed in the efficiency plots. The noise level was measured using a CAEN visual scaler. The dark count rate increases with HV in the detectors. MRPC1 has lower level of noise (below 4 Hz/cm2) than the MRPC2; this is related to minor assembly differences. The time resolution at different rates is also plotted as a function of the high voltage in Fig. 9. These results are presented after applying the slewing correction. The results have shown that there is no improvement in the rate capability by using the MRPC2. We
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Dark Count Rate [Hz/cm2]
of the time difference as a function of the average width of the signal. This profile has been fitted with a fourth order polynomial and the correction applied to the time difference. The distribution after correction for time slewing is shown in Fig. 7 right. To finally quote the time resolution of the MRPC2 we quadratically subtracted the jitter of the scintillators (40 ps) ( (85.6)2 − (40)2 ) resulting in a time resolution of (75.7 ± 1.4) ps.
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expected that the painted edges would help to decrease the charge density on the surface of the glasses. When the avalanche charge spreads over the glass surface, charge should migrate to the edges where there is an easy path to the other side. A summary of the efficiency and time resolution values at different rates is displayed in Fig. 10. Values correspond to several measurements taken at 16 kV on each MRPC. We have also included two measurements at 3.5 kHz/cm2 and 6.0 kHz/cm2; these measurements were taken under the same experimental conditions. The plots show the clear dependence of the efficiency and
4.3. Movement of charges in a glass surface The MRPC3 detector was devoted to test the movement of charges on the glass surface. It was biased at 16.5 kV by the cathode continuous electrode; while the anode strips were
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time resolution with the particle flux on both detectors. It should be noted that the rates we quote in this paper are for a pulsed beam (400 ms spill every 5 s) and only a small area of the chamber was illuminated by the beam.
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1
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Fig. 11. Oscilloscope pictures of the measured signals on the HV strips of the MRPC3. The beam intensity was monitored at 16 kHz/cm2. Top. Beam spot in the strip number 4 (edge). Bottom. Beam spot on strip number 1 (central).
connected to ground with 1 MΩ resistors. The beam intensity was monitored to be 16 kHz/cm2. The signals of four neighboring strips were readout by a Tektronix TDS7154B oscilloscope; the input impedance of the oscilloscope was set to 1 MΩ. Fig. 11 shows the voltage on each strip during one spill in two different states: left, the beam was centered on the edge HV strip (number 4 in oscilloscope picture) and right, the beam was centered on the central strip (number 1). The signal width is 400 ms which corresponds to the spill length. The induced charge is mainly observed on the strips 4 and 1 respectively; however a very small signal can also be distinguished on the neighbor strips. Since the average amplitude of the signal on the neighbor strips is very small, we can consider this is not related to movement of the induced charge on the glass surface, but to the size of the beam spot and its halo.
4.4. Temperature and rate capability The MRPCs were tested at different temperatures. For insulator and semiconductor materials, an increase of temperature results in a reduction of bulk resistivity. This process is described by Arrhenius law. For commercial glasses, an increase of approximately 25 °C represents a change of one order in the bulk resistivity. The MRPCs, placed on the beam line, were heated using two infrared lamps of 250 W of power. The temperature was monitored using a probe placed inside the aluminum boxes. The set of measurements were taken at 20 (room temperature, see Fig. 8), 31 and 40 °C. Fig. 12 shows the efficiency plots for detectors at the rates mentioned above and 31 °C. As can be noticed, a small improvement (approximately 5%) can be achieved in efficiency at high rates by increasing the temperature of the inner glasses. The dark count rates are also displayed in the plots. There is a significant
R. Forster et al. / Nuclear Instruments and Methods in Physics Research A 830 (2016) 182–190
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increase in the level of noise of the MRPC1 with the temperature. At 16 kV the dark rate for MRPC1 is the double of MRPC2. The high level of dark rate and dark currents in the MRPC1 made impossible to test it at 40 °C. Fig. 13 presents the time resolution plots at the same temperature and rates. The MRPC2 performance was also evaluated at 40 °C. Efficiency, dark count rate and time resolution measured at different particle fluxes are summarised in Fig. 14. The time resolution does not change in comparison to 31 °C. The efficiency shows a significant improvement for high rates in comparison with results obtained at
room temperature. A moderate warming allows an efficiency value over 80% at rates up to 35 kHz/cm2. A clear efficiency plateau was achieved at different particle fluxes above 15 kV. Reminder: these results were achieved irradiating a single point of the detector's active area in pulsed beam environment. The dark rate shows an exponential increment with the HV, up to approximately 40 Hz/cm2 at 16 kV. There is also an increase of the dark rate related to the increase of temperature in the detector. The MRPC2 have shown a lower level of noise than the MRPC1 during the warming process.
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stable performance at low counting rates (1.5 kHz/cm2) in a pulsed beam with an efficiency plateau reached after 15 kV and over 95%; the average time resolution was 80 ps. At higher rates the efficiency and time resolution suffer an important degradation. The results obtained with the MRPC2 does not show any improvement at higher particle fluxes. The performance of the MRPC3 was studied at a fixed voltage and particle flux. The current has been measured in four neighboring strips. Results have shown that there is not a noticeable movement of charges on the glass surface that could explain the good performance of detectors in pulsed beam environments. It was demonstrated that a moderate warming (up to 40 °C) can enhance the detector rate capability. The dark count rate in the MRPC1 has shown a stronger dependence with the temperature in comparison to the MRPC2.
Acknowledgments The results presented here were obtained at the T10 test beam in the East Hall at CERN. The authors acknowledge the support received by the operators of the PS.
References
Fig. 14. Efficiency, dark count rate and time resolution at 40 °C for the MRPC2.
5. Conclusions The performance of three different MRPCs has been tested in the test beam facility T10 at CERN. We have painted the edges of the glass sheets of one chamber (MRPC2) to study if this is a possible route to improve the rate capability. Results have shown a
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the counting rate capability of MRPC detectors built with soda lime glass R. Forster a, O. Margoto Rodríguez a,f, W. Park a, A. Rodríguez Rodríguez a,e,n,1, M.C.S. Williams c,d, A. Zichichi b,c,d, R. Zuyeuski a,b a
ICSC World Laboratory, Geneva, Switzerland Museo Storico della Fisica e Centro Studi e Ricerche E.Fermi, Roma, Italy c CERN, Geneva, Switzerland d INFN and Dipartimento di Fisica e Astronomia, Universit di Bologna, Italy e Center for Technological Applications and Nuclear Development, La Habana, Cuba f University Hermanoz Saiz, Pinar del Rio, Cuba b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 May 2016 Accepted 20 May 2016 Available online 24 May 2016
We report the results of three MRPC detectors built with soda lime glass and tested in the T10 beam line at CERN. The detectors consist of a stack of 280 μm thick glass sheets with 6 gaps of 220 μm . We built two identical MRPCs, except one had the edges of glass treated with resistive paint. A third detector was built with one HV electrode painted as strips. The detectors' efficiency and time resolution were studied at different particle flux in a pulsed beam environment. The results do not show any improvement with the painted edge technique at higher particle flux. We heated the MRPCs up to 40 °C to evaluate the influence of temperature in the rate capability. Results from this warming has indicated an improvement on the rate capability. The dark count rates show a significant dependence with the temperature. & 2016 Elsevier B.V. All rights reserved.
Keywords: MRPC Rate capability Temperature Efficiency Time resolution
1. Introduction The Multigap Resistive Plate Chamber (MRPC) is a detector with excellent time resolution ( < 100 ps), high efficiency ( > 95%), insensitive to magnetic fields and a relatively low cost. These characteristics, as well as the easy segmentation have made it a reliable and proven tool for Time of Flight detectors (TOF) in different experiments such as ALICE [1], STAR [2], HADES [3] and FOPI [4]. Several MRPC designs built with soda lime glass ( ρ ∼ 1013 Ω cm ) have been already tested with successful results at rates under 1 kHz/cm2 [5–7]. Nowadays, there is a strong R&D effort to improve the MRPC's rate capabilities and to install these devices in experiments such as the CBM TOF wall [8]. The rate capabilities of the MRPC detectors is determined by the bulk resistivity of the glass plates and therefore thinner and lower resistivity materials are needed. New materials with resistivity in the order to 1010 Ω cm are being investigated [9,8]. An alternative way to enhance the rate capabilities of MRPCs is to increase the temperature of the glass plates. However, it is n Correspondence to: GSI Helmholtzzentrum für Schwerionenforschung, GmbH Planckstraße 1, 64291 Darmstadt, Germany. E-mail address: [email protected] (A. Rodríguez Rodríguez). 1 Current Institute: Goethe University, Frankfurt am Main, Germany.
http://dx.doi.org/10.1016/j.nima.2016.05.080 0168-9002/& 2016 Elsevier B.V. All rights reserved.
necessary to evaluate many factors (e.g. dark currents, rate capability) before applying this technique to large detector arrays. We present the results obtained with new prototypes of MRPC built with soda lime glass. The tests were carried out in the T10 beam line at CERN. We have studied the detectors performance at different particle flux and evaluate the influence of a moderate warming on the rate capabilities. 1.1. Rate analysis If we consider a small area, A, of the glass sheet, there is a capacitance (c = Aε0 εr /d ) and resistance (R = ρd/A) between the surfaces. Thus any charge deposited on the surface will decay with a relaxation time τ = ρε0 εr . For float glass ρ ∼ 5 × 1012Ω cm and the relative dielectric constant (εr ∼ 8), τ is ∼3 s. When a charged particle passes through the MRPC, charge is produced by the gas avalanches and deposited onto the surface of the resistive plates. This charge will create a voltage drop across the resistive plate. In the steady state (constant flux of particles over the whole surface of the MRPC), the voltage drop is just Q A·Rate·RMRPC , where QA is the total charge produced inside the MRPC for a through-going charged particle and RMRPC is the total resistance (of all plates) per unit area. Thus in a 6 gas gaps detector made with glass sheets of 280 μm thickness, a particle rate of 1 kHz/cm2 causes a voltage drop of approximately 2000 V, if we
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Layer dopped with holes Resistive paint Charge movement
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Fig. 1. Inner glass plate with painted edge. The rate capability is limited by the time needed for a localized discharge to pass through the glass electrode. However if the surface resistivity of the glass is low, charge can flow on the surface of the glass. The painted edges of the glass sheet form a low resistive bridge between the two surfaces.
assume that 2 pC charge is generated by each through-going particle. Given this we should not expect the MRPC to work at fluxes above 1 kHz/cm2. The current flow through a glass sheet is a complex process; the glass can be considered as an amorphous semiconductor. In a high electric field, the electrons will move within the plate until there is no electric field inside the glass sheet. Thus one of the glass surfaces will be charged with electrons and the other will present a lack of electrons (i.e. holes). This process could be understood as doping the glass surface with a thin layer of electrons and holes. Based on this assumption two MRPCs prototypes, with major changes on the inner glasses and HV electrodes have been designed and tested. In MRPC2, the edges of the glass sheets have been coated with resistive paint from both sides, connecting the surfaces of the plates. The resistive paint should act as a bridge for the produced charges, allowing a fast evacuation of the charge on the surfaces (Fig. 1). The detector MRPC3 was designed with a HV electrode painted as strips. A detailed explanation of these devices is given in the next section.
2. Design and construction of MRPCs We present the results obtained with three MRPCs. They were built with the same materials and geometrical design; however for the MRPC2, the edges of the glass plates have been treated differently. In Fig. 2 left is shown a schematic of the cross section of the Standard MRPC (MRPC1). The detectors consist of six gas gaps of 220 μm width. The glass plates are manufactured by AGC Glass and have a thickness of 280 μm and a bulk resistivity of
( ρ ∼ 5 × 1012Ω cm ). The outer surface of the outermost glasses has been covered with a resistive paint; this coating acts as the electrodes to apply the high voltage. The surface resistivity of all painted layers are approximately (∼5 MΩ/square). Two printed circuit board (PCB) of 1.5 mm thickness insulates 24 readout strips from the anode and cathode. Each readout strip is 0.7 20.5 cm2, separated by 1 mm from its neighbor. The detectors active area is 19.6 19.6 cm2. The spacers are mono-filament commercial fishing line of 220 μm stretched across the surface of the glass from one side to the other and wound around plastic screws fixed at both sides of the chamber. The anode and cathode boards are connected by pins; thus a differential signal is sent to the NINO ASIC [10] front end. These pins also mechanically attach the pcbs together. A honeycomb panel of 5 mm thickness is attached to the anode pcb sheet using double-sided adhesive tape. The finished chambers are mounted in gas-tight aluminum boxes. 2.1. MRPC2 The painted edge MRPC (MRPC2) has a modification at the edges of the glass sheets as shown in Fig. 2 right. The external plates, were covered with resistive paint in the outermost surface as the standard procedure. Two opposite edges of the inner surface were painted with a strip of 1.5 cm width. Both surfaces were thus connected around the edge with the resistive paint. The inner glass sheets were painted in the same way with a band 0.5 cm width. Spacers placed at the corners and between the painted edges kept the gaps size and also protect from sparks between two neighboring layers; these spacers were made from 50 μm Mylar and covered with 85 μm thick double-sided adhesive tape.
Mylar (0.175 mm)
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Fig. 2. Left. Standard MRPC (6/220). Right. Details of the painted edges on MRPC2.
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Fig. 3. Schematic of the HV strips electrode in the MRPC3.
2.2. MRPC3 The HV strips MRPC (MRPC3) consists of a standard detector where the coating of one of the HV electrodes was applied as strips. Fig. 3 shows a schematic of this electrode. To cover the detector active area, 8 strips of 2.1 cm width separated at 0.4 cm from its neighbor were applied. Each strip was connected to a lemo connector and grounded by 1 MΩ resistor. These HV strips should not be confused with the standards readout strips; they run in orthogonal direction, see Fig. 3.
3. Beam test set-up The MRPCs were mounted separately and tested in the T10 test beam facility at CERN. The beam was mainly negative pions of 5 GeV/c momentum. There were 4–5 spills (400 ms long) per supercycle that had a period of approximately 22 s. The gas mixture was 95% of C2H2 F4 and 5% SF6 with a continuous flow of 5 l/h. To provide the trigger three set of scintillators–photomultipliers were used. Two pairs of crossed scintillators (P1–P2 and P3–P4) were placed on each side of the MRPC along the beam line. The upstream pair (P1–P2) has an active area of 1 1 cm2. The downstream pair (P3–P4) of 2 2 cm2 area. In addition special timing scintillators were incorporated, consisting of two orthogonal
scintillator bars of (2 2 10 cm3), read at each end by photomultiplier tubes (S1–S2 and S3–S4). This pair were placed upstream the MRPC and provided an accurate time reference. The time difference between both bars ((S1 + S 2) /2 − (S 3 + S 4) /2) had a sigma of 80 ps, and hence the expected time resolution from the average value ((S1 + S 2 + S 3 + S 4) /4) is 40 ps. The schematic of the experimental set-up in the beam line is shown in Fig. 4. The MRPCs were read out from both side using the 24 channels NINO ASIC cards designed for the ALICE TOF [10]. The threshold was set at 160 mV (∼40 fC) and remained the same during all measurements. The data was taken using two CAEN HPTDCs (V1290 A). This system has a time resolution of 30–40 ps. The OR signal from the front end card was also connected to a four channel digital oscilloscope Lecroy WaveRunner 104MXi-A (1 GHz) to check the MRPC signals during the run.
4. Results and discussion 4.1. Efficiency and time resolution at low rate The first measurements were taken at 1.5 kHz/cm2. This rate was monitored using the scintillator bars ( S1 − S4 ) defining a 2 2 cm2 active area. The beam was centered in the middle of one strip and the efficiency and time resolution were measured as a MRPC
Trigger fingers (P3-P4)
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Fig. 4. Schematic of the experimental setup.
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Fig. 5. Efficiency and time resolution measured at 1.5 kHz/cm2. The lines are to guide the eye.
function of the applied voltage; the results are shown in Fig. 5. The detectors have reached the efficiency plateau (over 95%) above 15 kV, which extends more than 1 kV. A comparison between detectors' results reveals that at 16.0 kV, the MRPC1 has a time resolution of (73.4 71.1) ps and the MRPC2 (85.6 71.2) ps. Since the strip was read at each end, the mean time ( (tEND1 + tEND2 ) /2) is independent of the hit position along the strip direction. In Fig. 6 left a typical histogram of the time difference measured between the MRPC2 (TMRPC ) and the reference
scintillators (TRef ) is shown. The use of a fixed threshold introduces a time shift (slewing) depending of the amplitude of the signal. The NINO chip measures the input charge and encodes it into the width of the LVDS pulse; this information is used to correct for slewing. The spectrum of the fast charge at 16 kV measured with the time-over-threshold (TOT) technique is presented in Fig. 6 right. As can be seen multiple peaks appear in the charge distribution. The reason of this shape are the multiple reflection of the signal due to the imperfect matching impedance between the transmission line and the NINO ASIC. Fig. 7 left shows a scatter plot
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Time spectrum after correction
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the beam line defined the beam intensity. The measurements were taken at 4 different rates (1.5 kHz/cm2, 16.0 kHz/cm2, 28.0 kHz/cm2 and 35.0 kHz/cm2). The results are presented in Fig. 8. The dark count rate is also displayed in the efficiency plots. The noise level was measured using a CAEN visual scaler. The dark count rate increases with HV in the detectors. MRPC1 has lower level of noise (below 4 Hz/cm2) than the MRPC2; this is related to minor assembly differences. The time resolution at different rates is also plotted as a function of the high voltage in Fig. 9. These results are presented after applying the slewing correction. The results have shown that there is no improvement in the rate capability by using the MRPC2. We
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of the time difference as a function of the average width of the signal. This profile has been fitted with a fourth order polynomial and the correction applied to the time difference. The distribution after correction for time slewing is shown in Fig. 7 right. To finally quote the time resolution of the MRPC2 we quadratically subtracted the jitter of the scintillators (40 ps) ( (85.6)2 − (40)2 ) resulting in a time resolution of (75.7 ± 1.4) ps.
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expected that the painted edges would help to decrease the charge density on the surface of the glasses. When the avalanche charge spreads over the glass surface, charge should migrate to the edges where there is an easy path to the other side. A summary of the efficiency and time resolution values at different rates is displayed in Fig. 10. Values correspond to several measurements taken at 16 kV on each MRPC. We have also included two measurements at 3.5 kHz/cm2 and 6.0 kHz/cm2; these measurements were taken under the same experimental conditions. The plots show the clear dependence of the efficiency and
4.3. Movement of charges in a glass surface The MRPC3 detector was devoted to test the movement of charges on the glass surface. It was biased at 16.5 kV by the cathode continuous electrode; while the anode strips were
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time resolution with the particle flux on both detectors. It should be noted that the rates we quote in this paper are for a pulsed beam (400 ms spill every 5 s) and only a small area of the chamber was illuminated by the beam.
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Fig. 11. Oscilloscope pictures of the measured signals on the HV strips of the MRPC3. The beam intensity was monitored at 16 kHz/cm2. Top. Beam spot in the strip number 4 (edge). Bottom. Beam spot on strip number 1 (central).
connected to ground with 1 MΩ resistors. The beam intensity was monitored to be 16 kHz/cm2. The signals of four neighboring strips were readout by a Tektronix TDS7154B oscilloscope; the input impedance of the oscilloscope was set to 1 MΩ. Fig. 11 shows the voltage on each strip during one spill in two different states: left, the beam was centered on the edge HV strip (number 4 in oscilloscope picture) and right, the beam was centered on the central strip (number 1). The signal width is 400 ms which corresponds to the spill length. The induced charge is mainly observed on the strips 4 and 1 respectively; however a very small signal can also be distinguished on the neighbor strips. Since the average amplitude of the signal on the neighbor strips is very small, we can consider this is not related to movement of the induced charge on the glass surface, but to the size of the beam spot and its halo.
4.4. Temperature and rate capability The MRPCs were tested at different temperatures. For insulator and semiconductor materials, an increase of temperature results in a reduction of bulk resistivity. This process is described by Arrhenius law. For commercial glasses, an increase of approximately 25 °C represents a change of one order in the bulk resistivity. The MRPCs, placed on the beam line, were heated using two infrared lamps of 250 W of power. The temperature was monitored using a probe placed inside the aluminum boxes. The set of measurements were taken at 20 (room temperature, see Fig. 8), 31 and 40 °C. Fig. 12 shows the efficiency plots for detectors at the rates mentioned above and 31 °C. As can be noticed, a small improvement (approximately 5%) can be achieved in efficiency at high rates by increasing the temperature of the inner glasses. The dark count rates are also displayed in the plots. There is a significant
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increase in the level of noise of the MRPC1 with the temperature. At 16 kV the dark rate for MRPC1 is the double of MRPC2. The high level of dark rate and dark currents in the MRPC1 made impossible to test it at 40 °C. Fig. 13 presents the time resolution plots at the same temperature and rates. The MRPC2 performance was also evaluated at 40 °C. Efficiency, dark count rate and time resolution measured at different particle fluxes are summarised in Fig. 14. The time resolution does not change in comparison to 31 °C. The efficiency shows a significant improvement for high rates in comparison with results obtained at
room temperature. A moderate warming allows an efficiency value over 80% at rates up to 35 kHz/cm2. A clear efficiency plateau was achieved at different particle fluxes above 15 kV. Reminder: these results were achieved irradiating a single point of the detector's active area in pulsed beam environment. The dark rate shows an exponential increment with the HV, up to approximately 40 Hz/cm2 at 16 kV. There is also an increase of the dark rate related to the increase of temperature in the detector. The MRPC2 have shown a lower level of noise than the MRPC1 during the warming process.
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stable performance at low counting rates (1.5 kHz/cm2) in a pulsed beam with an efficiency plateau reached after 15 kV and over 95%; the average time resolution was 80 ps. At higher rates the efficiency and time resolution suffer an important degradation. The results obtained with the MRPC2 does not show any improvement at higher particle fluxes. The performance of the MRPC3 was studied at a fixed voltage and particle flux. The current has been measured in four neighboring strips. Results have shown that there is not a noticeable movement of charges on the glass surface that could explain the good performance of detectors in pulsed beam environments. It was demonstrated that a moderate warming (up to 40 °C) can enhance the detector rate capability. The dark count rate in the MRPC1 has shown a stronger dependence with the temperature in comparison to the MRPC2.
Acknowledgments The results presented here were obtained at the T10 test beam in the East Hall at CERN. The authors acknowledge the support received by the operators of the PS.
References
Fig. 14. Efficiency, dark count rate and time resolution at 40 °C for the MRPC2.
5. Conclusions The performance of three different MRPCs has been tested in the test beam facility T10 at CERN. We have painted the edges of the glass sheets of one chamber (MRPC2) to study if this is a possible route to improve the rate capability. Results have shown a
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