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A 20 ps timing device—A Multigap Resistive Plate Chamber with 24 gas gaps
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 594 (2008) 39– 43
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A 20 ps timing device—A Multigap Resistive Plate Chamber with 24 gas gaps S. An a, Y.K. Jo a, J.S. Kim a, M.M. Kim a, D. Hatzifotiadou b, M.C.S. Williams b,, A. Zichichi c,d, R. Zuyeuski a a
World Laboratory, Geneva, Switzerland Sezione INFN, Bologna, Italy `, Bologna, Italy Dipartimento di Fisica dell’Universita d PH Department, CERN, Geneva, Switzerland b c
a r t i c l e in fo
abstract
Article history: Received 7 April 2008 Received in revised form 22 May 2008 Accepted 16 June 2008 Available online 21 June 2008
The Multigap Resistive Plate Chamber (MRPC) is unique in its power of high precision time measurements; this is why it has been used for time-of-flight purposes at various experiments. We report on tests of a new configuration consisting of 24 gaps where a time resolution of 20 ps has been obtained. & 2008 Elsevier B.V. All rights reserved.
Keywords: Multigap RPC MRPC Timing TOF
1. Introduction We report on the performance of a Multigap Resistive Plate Chamber (MRPC) with 24 gaps. The time resolution measured with this device is 20 ps. The tests have been performed at CERN using the T10 beam and with cosmic rays.
2. The MRPC The MRPC has been used as a timing device in various experiments. For example, the ALICE TOF [1] uses a 10-gap MRPC with gaps of 250 mm width in a double stack configuration. The time resolution of this MRPC (10/250) is 40 ps; the thickness of the detector, defined as the distance between the two external pickup plates, is 12 mm. The MRPC reported in this paper has 24 gas gaps with a width of 160 mm; this MRPC (24/160) has 20 ps time resolution. The thickness (as defined above) is 22 mm. The MRPC consists of a stack of resistive plates, each separated from its neighbour by very precise spacers, so a series of gaps with well controlled widths are created. The outer surfaces of the outermost plates are coated with a resistive coating where a high voltage is applied; this creates a high electric field in each gas gap. A through-going charged particle creates clusters of ionisation (positive ions and electrons). If the electric field is sufficiently Corresponding author.
E-mail address: [email protected] (M.C.S. Williams). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.06.013
strong the electrons will create an avalanche as they move towards the anode. The movement of charge in any of the gas gaps induces a signal on the external pick up electrodes. In Fig. 1a schematic of the cross section of the MRPC (24/160) is shown. It has been constructed as four stacks, each with six gas gaps. There are pickup electrodes between each stack; thus in this case, one has three anode readout electrodes and two cathode readout electrodes. These electrodes are divided into strips and read out at both ends using differential front end electronics. This MRPC (24/160) has resistive plates made of ‘soda-lime’ glass sheets of 400 mm thickness. The spacers are monofilament fishing line and run across the surface of the glass from one side to the other as shown in Fig. 2. The length of the printed circuit board (pcb) was 610 mm; this length was chosen such that the pcb containing the strips could be fabricated at the CERN pcb facility. At each end, 10 mm was reserved to make the high voltage connection to the resistive coating (as shown in Fig. 2); the remaining 590 mm was divided into 24 readout strips on a 24.5 mm pitch with 2 mm separating each strip from its neighbour. The width of 74 mm was defined by the glass sheet that we had on hand. Thus the effective area of each pickup electrode is 2:45 7:4 cm2 . We use five pcbs in this design: a central anode pcb, two intermediate cathode pcbs and two outer anode pcbs. The shape of these electrodes is shown in Fig. 2. The anode and cathode signals generated on these boards are connected by pins to the central pcb and thus a differential signal is collected and is sent to the NINO [2] asic (application-specific integrated circuit) front end. These pins also mechanically attach the pcbs together. The NINO asic was mounted as close as possible
ARTICLE IN PRESS 40
S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
POSITIVE HIGH VOLTAGE LAYER
ANODE PICKUP ELECTRODE
NEGATIVE HIGH VOLTAGE LAYER CATHODE PICKUP ELECTRODE ANODE PICKUP ELECTRODE
POSITIVE HIGH VOLTAGE LAYER
NINO
NINO
NEGATIVE HIGH VOLTAGE LAYER
CATHODE PICKUP ELECTRODE
POSITIVE HIGH VOLTAGE LAYER
ANODE PICKUP ELECTRODE
Fig. 1. Schematic representation of the 24 gap MRPC. It is divided into four stacks, each with six gas gaps of 160 mm.
Hole for plastic screw fishing line was wound around these screws
Outer glass 85 mm wide Limit of resistive coating and edge of internal glass 74 mm wide
fishing line spacer 1 cm at end used to make hv connection to resistive coating
Readout pads
2 mm gap between pads Fig. 2. View looking down at readout strips.
to the MRPC inside the gas volume and at each end of the strip. Two such MRPC devices were mounted 7 cm apart in a gas tight box through which a mixture of gas was flowed.
3. Chamber readout The NINO chip [2] outputs a LVDS signal with the leading edge corresponding to the time when the input signal crosses a simple threshold; the input charge is encoded into the width of this LVDS
pulse. This width would be in the range of 2–10 ns for signals from this MRPC (24/160). However, the NINO asic has an additional function where the width of all output signals can be increased by a fixed amount of time; we enabled this function to add an additional 14 ns to the pulse width. Nowadays, TDCs can typically measure the time of both edges; thus the time of the signal is that of the leading edge and the time difference (time(trailing edge)time(leading edge)) is the pulse width, which is a measurement of the input charge. For small signals a time delay is produced due to the finite rise time; this is known as
ARTICLE IN PRESS S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
power for NINO + LVDS buffer
NINO NINO
5 m cables (SkewClear Amphenol) LVDS to NIM converters 10 GS oscilloscope
41
supercycle that had a period of 25 s. Since we were running parasitically we only tested the performance of one strip at a single position. We selected a 1 cm2 spot of a negative 5 GeV/c (mostly pion) beam using scintillators just upstream of the MRPCs; there was no hit requirement concerning the MRPC. The gas was a mixture of 95% C2 F4 H2 and 5% SF6 . The chamber was flushed with this mixture in the lab and then was placed in the test beam and operated without a flow of gas. The data presented here was taken some hours after placing the chamber in the test beam. Leaving the chamber without flushing the gas produces a steady degradation of time resolution (20 ps degraded to 55 ps after 5 days of no gas flow); the reasons for this degradation are being investigated and the results will be published at a later date. In any case the 20 ps time resolution has been obtained with many hours of no gas flow. The efficiency versus applied voltage is shown in Fig. 4. The efficiency was calculated using an OR signal (for eight readout strips) generated on the front-end card. The coincidences of this
Fig. 3. A sketch of the setup.
100 90
Efficiency [%]
80 70
Efficiency MRPC1
60
Efficiency MRPC2
50 40 30 20 10 0 10
90 80
Applied Voltage 12.5 kV across 6 gaps
The gas box containing the two MRPC devices was mounted in the T10 test beam; the test beam was set at negative 5 GeV/c momentum with an intensity of 400 particles in a 1 cm 1 cm beam spot per spill (300 ms long). There was only one spill per
UNCORRECTED DATA 5 GeV/c beam
70 60
Sigma = 112.5 ps
50
Time Resolution of each MRPC is 112.5/√2 = 79.5 ps
40 30 20 10 0 -1500
4. Beam test
11 12 13 Applied Voltage across 6 gas gaps [kV]
Fig. 4. The efficiency versus applied voltage across six gas gaps measured in the T10 test beam at CERN. The error bars are contained in the size of the symbols; the line is just to guide the eye.
Events / 10 ps
time slewing. Since we measure the input charge we can correct for this. The NINO is designed to work at low thresholds (down to 10 fC) however, we had a problem with the LVDS output signals. Depending on the routing of the cable carrying these signals we could have some kind of feedback leading to oscillations. For these tests we set the thresholds at 100 fC to avoid any oscillation/ feedback issues. For most of the measurements presented here we have used a four channel digital oscilloscope. Nowadays oscilloscopes can make time measurements with an accuracy of some picoseconds. The oscilloscopes that we have used for this measurement, the LeCroy WM8300 and the Tektronix DPO7254, both quote a jitter floor of 1 ps. We verified this jitter floor by passively splitting a signal and sending these to two channels of the oscilloscope; we then measured the jitter of time difference between these two signals. This resulted in jitter measurements between 1.5 and 2 ps. However, this is rather idealised. When we started to use the LVDS to NIM converter so that we could supply single ended signals to the oscilloscope we had a minimum level of 10 ps jitter. A major cause was cross talk in the ground between neighbouring channels; any noise on the ground obviously introduces jitter. Other data was obtained using the HPTDC system built for ALICE TOF [3]. This system has a time resolution per channel between 20 and 30 ps; obviously this will be a major contribution to the measured time resolution. The oscilloscopes use the WINDOWS operating system; thus the WINDOWS environment allows us to run a LabVIEW program internally on the oscilloscope. We used this program to read out information about the signals on receipt of a trigger; thus the oscilloscope was used as a data acquisition system. Initially we used a Tektronix DPO7250 oscilloscope but changed to a LeCroy WM8300 during these measurements. Although both oscilloscopes had comparable accuracy, the LeCroy oscilloscope had a readout speed 10 times faster and this became critically important when we tested the MRPCs in a test beam. A sketch of our setup is shown in Fig. 3. For some tests, discussed later, we used the data acquisition system of the ALICE TOF that read out a VME unit equipped with the HPTDC asic [3].
-1000
-500 0 Timediff [ps]
500
1000
Fig. 5. Time difference between measured time in MRPC1 and MRPC2. The measured time is the mean of the times measured at each end of the strip, thus independent of the position of the hit. Since this is the timepdifference between ffiffiffi two MRPC devices, the individual time resolution is a factor 2 smaller than the sigma of the distribution shown here. This distribution is for the raw data with no slewing corrections applied.
ARTICLE IN PRESS 42
S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
0 -100
Timediff [ps]
Polynomial Fit
-200 -300 -400 -500 -600 31 32 33 34 35 (Width of pulse)end1 + (Width of pulse)end2 [ns]
Fig. 6. Time difference between the two MRPCs shown as a function of the pulse width measured in one chamber. The input charge defines the width of the output pulse; thus this plot shows what is known as time slewing.
250
Applied Voltage 13 kV across 6 gaps
SLEWING CORRECTIONS APPLIED
5. Cosmic ray test The gas box with the detectors was transported from the test beam to the cosmic ray test area. At this location we flowed the gas (95% C2 F4 H2 and 5% SF6 ) continuously. The trigger was derived from the MRPC itself, together with a scintillator mounted below
5 GeV/c beam
200
120
Sigma = 30 ps
150
100
Cosmic ray data ALICE-TOF DAQ
Time Resolution of MRPC
Events / 10 ps
Events / 10 ps
OR with the scintillators were counted and compared to counts in the scintillator. The strips are read out at each side; the sum of these two times is independent of the hit position: we define the mean time to be T mean ¼ ðT end1 þ T end2 Þ=2. The time resolution can be obtained from the distribution of Timediff ¼ ðT mean ÞMRPC1 ðT mean ÞMRPC2 ; this is shown in Fig. 5. The variance of this distribution is the sum of the variances of pffiffiffithe time resolution of each MRPC; thus the sigma is divided by 2 and is quoted as the time resolution of each MRPC with the assumption that the two MRPCs have the same time resolution. Slewing corrections can be applied; the width of the output pulse is governed by the amount of input charge; thus the variation of Timediff with the pulse width is a measure of slewing as shown in Fig. 6. Timediff , after correction for slewing, is shown in Fig. 7. The time resolution versus applied voltage is shown in Fig. 8, for both cases: with and without slewing corrections.
is 30/√2 = 21.2 ps
100
50
80
sigma = 41.7 ps
60
Time resolution of each MRPC is 41.7/√2 = 29.5 ps
40 0 -400
-200
0 Timediff [ps]
200
20
400
0 Fig. 7. Time difference between the two MRPCs after correcting for time slewing.
- 400
- 200
0
200
400
Timediff [ps]
50
Cosmic ray data Oscilloscope DAQ
160 TEST BEAM DATA
40 Time resolution - no correction
120
Time resolution after slewing correction
100 80
Events / 10 ps
Time Resolution [ps]
140
sigma = 39.2 ps
30 Time resolution of each MRPC is 39.2 /√2 = 27.7 ps
20
60
10
40 20
0 - 400
0 11.5
12.0 12.5 13.0 Applied voltage across 6 gas gaps [kV]
13.5
Fig. 8. The time resolution versus applied voltage across six gas gaps measured in the T10 test beam at CERN. The error bars are contained in the size of the symbols; the lines are just to guide the eye.
- 200
0
200
400
Timediff [ps] Fig. 9. The time difference between the two MRPCs using the cosmic ray trigger. Two data acquisition systems were employed: the oscilloscope (LeCroy WM8300) and the ALICE-TOF HPTDC system. The applied voltage was 12.5 kV across six gas gaps.
ARTICLE IN PRESS S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
signal in the strip is 1 cm/50 ps. Thus a variation of time difference ðT end1 T end2 Þ of 100 ps corresponds to a change of 1 cm in the hit position. The full length of the strip (7.4 cm) is displayed in these plots. We see a change of time of 30 ps between hits at the centre of the strips compared to the ends. This is due to the geometry of the strips. Correcting for this effect improves the time resolution by 1 ps (i.e. improves from 30 to 29 ps). In general the time resolution measured using the cosmic ray trigger was between 5 and 10 ps worse than the time resolution measured in the test beam. The following factors explain this degradation. In the test beam the trigger determines a 1 1 cm2 spot near the center of the strip, while the cosmic ray data is for a hit anywhere in the 2:5 cm 7:4 cm strip. The cosmic ray particles have a distribution of incident angles, which can be measured in the strip direction using the time difference from both ends of the strips. We do make a correction related to the calculated path length of the particles between the two MRPCs; however, the hit position is not measured across the strip width (2.5 cm) and this adds to the measured time resolution. In addition the cosmic ray data was collected over a long period of time ð24 hÞ compared to the test beam run that lasted 10 min; obviously the long runs place strong demands on the stability of the time measurement system. Finally the momentum of the test beam was set to be 5 GeV/c. However, for the cosmic ray data, all particles that could penetrate 5 cm lead were accepted; the very low momenta particles will broaden the time resolution since the MRPCs were 7 cm apart.
60 MRPC1
50
Timediff [ps]
40 30 20 10 0 -10 -20 -30 0
1
2 3 4 5 6 Distance along strip of MRPC1 [cm]
7
MRPC2
30 20 Timediff [ps]
43
10 0
-10
6. Discussion
-20
Clearly when measuring time resolution, the time resolution of the measuring device is very important. In this case this is 20 ps for the HPTDC system and at best 10 ps using the oscilloscope since jitter introduced by the front end electronics must be included. There is also a contribution from the transit time of the signal in the strip; we have attempted to eliminate this by reading out both ends of the strip and using the mean time. However there is still a contribution from the width of the strip (2.5 cm). The intrinsic time resolution of the MRPC itself also contributes and there are many factors that affect this; however, by making the gaps smaller should make the time resolution better since the avalanches grow faster (they have to, since the gap is smaller). As discussed earlier we were forced to set the thresholds at around 100 fC. Lower values showed some oscillations due to feed back from the cables carrying the output pulse. Even though this higher threshold allowed us to test this device, still there must be noise injected into the front end and noise produces jitter. We believe that the measured time resolution of 20 ps will be improved with better electronics.
-30
0
1
2 3 4 5 Distance along strip of MRPC2 [cm]
6
7
Fig. 10. Time variation along the strip direction: (a) distance along strip of MRPC1 [cm], (b) distance along strip of MRPC2 [cm].
the MRPCs after a 5 cm thick lead block. When we used the oscilloscope for data acquisition, the trigger was derived from the strip under study with the additional requirement of no hit in the neighbouring strips. We also used the ALICE TOF TDC system, built using the HPTDC chips. Each MRPC had eight strips (i.e. 16 channels) equipped with NINO front end electronics; the HPTDC system allowed us to read out the 32 channels. In this case we used the OR signal from the front end electronics as the trigger. In Fig. 9 we show typical timing distributions for cosmic ray data using both the ALICE-TOF HPTDC system and the DAQ system based on an oscilloscope (LeCroy WM8300 in this case). In general the time distributions measured with the oscilloscope were always 1 or 2 ps better. The time resolution of an individual HPTDC channel is, at best, 20 ps. Since we read out both sides of the strip and take the average of these two times, we could expect the contribution to the measured time resolution to be pffiffiffi 20 ps= 2 ¼ 14 ps in the best case. Obviously this has an impact on the final time resolution. In Fig. 10 we show the time variation plotted against the hit position; we find the hit position from the time difference of the hit time measured at the ends of the strip. The velocity of the
Acknowledgements Some of the results presented here were obtained at the T10 test beam in the East Hall at CERN. As always we have been well served by the competent operators of the PS. We thank them. References [1] A.N. Akindinov, et al., Nucl. Instr. and Meth. A 533 (2004) 74. [2] F. Anghinolfi, P. Jarron, A.N. Martemiyanov, E. Usenko, H. Wenninger, M.C.S. Williams, A. Zichichi, Nucl. Instr. and Meth. A 533 (2004) 183. [3] A. Akindinov, et al., Nucl. Instr. and Meth. A 533 (2004) 178.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A 20 ps timing device—A Multigap Resistive Plate Chamber with 24 gas gaps S. An a, Y.K. Jo a, J.S. Kim a, M.M. Kim a, D. Hatzifotiadou b, M.C.S. Williams b,, A. Zichichi c,d, R. Zuyeuski a a
World Laboratory, Geneva, Switzerland Sezione INFN, Bologna, Italy `, Bologna, Italy Dipartimento di Fisica dell’Universita d PH Department, CERN, Geneva, Switzerland b c
a r t i c l e in fo
abstract
Article history: Received 7 April 2008 Received in revised form 22 May 2008 Accepted 16 June 2008 Available online 21 June 2008
The Multigap Resistive Plate Chamber (MRPC) is unique in its power of high precision time measurements; this is why it has been used for time-of-flight purposes at various experiments. We report on tests of a new configuration consisting of 24 gaps where a time resolution of 20 ps has been obtained. & 2008 Elsevier B.V. All rights reserved.
Keywords: Multigap RPC MRPC Timing TOF
1. Introduction We report on the performance of a Multigap Resistive Plate Chamber (MRPC) with 24 gaps. The time resolution measured with this device is 20 ps. The tests have been performed at CERN using the T10 beam and with cosmic rays.
2. The MRPC The MRPC has been used as a timing device in various experiments. For example, the ALICE TOF [1] uses a 10-gap MRPC with gaps of 250 mm width in a double stack configuration. The time resolution of this MRPC (10/250) is 40 ps; the thickness of the detector, defined as the distance between the two external pickup plates, is 12 mm. The MRPC reported in this paper has 24 gas gaps with a width of 160 mm; this MRPC (24/160) has 20 ps time resolution. The thickness (as defined above) is 22 mm. The MRPC consists of a stack of resistive plates, each separated from its neighbour by very precise spacers, so a series of gaps with well controlled widths are created. The outer surfaces of the outermost plates are coated with a resistive coating where a high voltage is applied; this creates a high electric field in each gas gap. A through-going charged particle creates clusters of ionisation (positive ions and electrons). If the electric field is sufficiently Corresponding author.
E-mail address: [email protected] (M.C.S. Williams). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.06.013
strong the electrons will create an avalanche as they move towards the anode. The movement of charge in any of the gas gaps induces a signal on the external pick up electrodes. In Fig. 1a schematic of the cross section of the MRPC (24/160) is shown. It has been constructed as four stacks, each with six gas gaps. There are pickup electrodes between each stack; thus in this case, one has three anode readout electrodes and two cathode readout electrodes. These electrodes are divided into strips and read out at both ends using differential front end electronics. This MRPC (24/160) has resistive plates made of ‘soda-lime’ glass sheets of 400 mm thickness. The spacers are monofilament fishing line and run across the surface of the glass from one side to the other as shown in Fig. 2. The length of the printed circuit board (pcb) was 610 mm; this length was chosen such that the pcb containing the strips could be fabricated at the CERN pcb facility. At each end, 10 mm was reserved to make the high voltage connection to the resistive coating (as shown in Fig. 2); the remaining 590 mm was divided into 24 readout strips on a 24.5 mm pitch with 2 mm separating each strip from its neighbour. The width of 74 mm was defined by the glass sheet that we had on hand. Thus the effective area of each pickup electrode is 2:45 7:4 cm2 . We use five pcbs in this design: a central anode pcb, two intermediate cathode pcbs and two outer anode pcbs. The shape of these electrodes is shown in Fig. 2. The anode and cathode signals generated on these boards are connected by pins to the central pcb and thus a differential signal is collected and is sent to the NINO [2] asic (application-specific integrated circuit) front end. These pins also mechanically attach the pcbs together. The NINO asic was mounted as close as possible
ARTICLE IN PRESS 40
S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
POSITIVE HIGH VOLTAGE LAYER
ANODE PICKUP ELECTRODE
NEGATIVE HIGH VOLTAGE LAYER CATHODE PICKUP ELECTRODE ANODE PICKUP ELECTRODE
POSITIVE HIGH VOLTAGE LAYER
NINO
NINO
NEGATIVE HIGH VOLTAGE LAYER
CATHODE PICKUP ELECTRODE
POSITIVE HIGH VOLTAGE LAYER
ANODE PICKUP ELECTRODE
Fig. 1. Schematic representation of the 24 gap MRPC. It is divided into four stacks, each with six gas gaps of 160 mm.
Hole for plastic screw fishing line was wound around these screws
Outer glass 85 mm wide Limit of resistive coating and edge of internal glass 74 mm wide
fishing line spacer 1 cm at end used to make hv connection to resistive coating
Readout pads
2 mm gap between pads Fig. 2. View looking down at readout strips.
to the MRPC inside the gas volume and at each end of the strip. Two such MRPC devices were mounted 7 cm apart in a gas tight box through which a mixture of gas was flowed.
3. Chamber readout The NINO chip [2] outputs a LVDS signal with the leading edge corresponding to the time when the input signal crosses a simple threshold; the input charge is encoded into the width of this LVDS
pulse. This width would be in the range of 2–10 ns for signals from this MRPC (24/160). However, the NINO asic has an additional function where the width of all output signals can be increased by a fixed amount of time; we enabled this function to add an additional 14 ns to the pulse width. Nowadays, TDCs can typically measure the time of both edges; thus the time of the signal is that of the leading edge and the time difference (time(trailing edge)time(leading edge)) is the pulse width, which is a measurement of the input charge. For small signals a time delay is produced due to the finite rise time; this is known as
ARTICLE IN PRESS S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
power for NINO + LVDS buffer
NINO NINO
5 m cables (SkewClear Amphenol) LVDS to NIM converters 10 GS oscilloscope
41
supercycle that had a period of 25 s. Since we were running parasitically we only tested the performance of one strip at a single position. We selected a 1 cm2 spot of a negative 5 GeV/c (mostly pion) beam using scintillators just upstream of the MRPCs; there was no hit requirement concerning the MRPC. The gas was a mixture of 95% C2 F4 H2 and 5% SF6 . The chamber was flushed with this mixture in the lab and then was placed in the test beam and operated without a flow of gas. The data presented here was taken some hours after placing the chamber in the test beam. Leaving the chamber without flushing the gas produces a steady degradation of time resolution (20 ps degraded to 55 ps after 5 days of no gas flow); the reasons for this degradation are being investigated and the results will be published at a later date. In any case the 20 ps time resolution has been obtained with many hours of no gas flow. The efficiency versus applied voltage is shown in Fig. 4. The efficiency was calculated using an OR signal (for eight readout strips) generated on the front-end card. The coincidences of this
Fig. 3. A sketch of the setup.
100 90
Efficiency [%]
80 70
Efficiency MRPC1
60
Efficiency MRPC2
50 40 30 20 10 0 10
90 80
Applied Voltage 12.5 kV across 6 gaps
The gas box containing the two MRPC devices was mounted in the T10 test beam; the test beam was set at negative 5 GeV/c momentum with an intensity of 400 particles in a 1 cm 1 cm beam spot per spill (300 ms long). There was only one spill per
UNCORRECTED DATA 5 GeV/c beam
70 60
Sigma = 112.5 ps
50
Time Resolution of each MRPC is 112.5/√2 = 79.5 ps
40 30 20 10 0 -1500
4. Beam test
11 12 13 Applied Voltage across 6 gas gaps [kV]
Fig. 4. The efficiency versus applied voltage across six gas gaps measured in the T10 test beam at CERN. The error bars are contained in the size of the symbols; the line is just to guide the eye.
Events / 10 ps
time slewing. Since we measure the input charge we can correct for this. The NINO is designed to work at low thresholds (down to 10 fC) however, we had a problem with the LVDS output signals. Depending on the routing of the cable carrying these signals we could have some kind of feedback leading to oscillations. For these tests we set the thresholds at 100 fC to avoid any oscillation/ feedback issues. For most of the measurements presented here we have used a four channel digital oscilloscope. Nowadays oscilloscopes can make time measurements with an accuracy of some picoseconds. The oscilloscopes that we have used for this measurement, the LeCroy WM8300 and the Tektronix DPO7254, both quote a jitter floor of 1 ps. We verified this jitter floor by passively splitting a signal and sending these to two channels of the oscilloscope; we then measured the jitter of time difference between these two signals. This resulted in jitter measurements between 1.5 and 2 ps. However, this is rather idealised. When we started to use the LVDS to NIM converter so that we could supply single ended signals to the oscilloscope we had a minimum level of 10 ps jitter. A major cause was cross talk in the ground between neighbouring channels; any noise on the ground obviously introduces jitter. Other data was obtained using the HPTDC system built for ALICE TOF [3]. This system has a time resolution per channel between 20 and 30 ps; obviously this will be a major contribution to the measured time resolution. The oscilloscopes use the WINDOWS operating system; thus the WINDOWS environment allows us to run a LabVIEW program internally on the oscilloscope. We used this program to read out information about the signals on receipt of a trigger; thus the oscilloscope was used as a data acquisition system. Initially we used a Tektronix DPO7250 oscilloscope but changed to a LeCroy WM8300 during these measurements. Although both oscilloscopes had comparable accuracy, the LeCroy oscilloscope had a readout speed 10 times faster and this became critically important when we tested the MRPCs in a test beam. A sketch of our setup is shown in Fig. 3. For some tests, discussed later, we used the data acquisition system of the ALICE TOF that read out a VME unit equipped with the HPTDC asic [3].
-1000
-500 0 Timediff [ps]
500
1000
Fig. 5. Time difference between measured time in MRPC1 and MRPC2. The measured time is the mean of the times measured at each end of the strip, thus independent of the position of the hit. Since this is the timepdifference between ffiffiffi two MRPC devices, the individual time resolution is a factor 2 smaller than the sigma of the distribution shown here. This distribution is for the raw data with no slewing corrections applied.
ARTICLE IN PRESS 42
S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
0 -100
Timediff [ps]
Polynomial Fit
-200 -300 -400 -500 -600 31 32 33 34 35 (Width of pulse)end1 + (Width of pulse)end2 [ns]
Fig. 6. Time difference between the two MRPCs shown as a function of the pulse width measured in one chamber. The input charge defines the width of the output pulse; thus this plot shows what is known as time slewing.
250
Applied Voltage 13 kV across 6 gaps
SLEWING CORRECTIONS APPLIED
5. Cosmic ray test The gas box with the detectors was transported from the test beam to the cosmic ray test area. At this location we flowed the gas (95% C2 F4 H2 and 5% SF6 ) continuously. The trigger was derived from the MRPC itself, together with a scintillator mounted below
5 GeV/c beam
200
120
Sigma = 30 ps
150
100
Cosmic ray data ALICE-TOF DAQ
Time Resolution of MRPC
Events / 10 ps
Events / 10 ps
OR with the scintillators were counted and compared to counts in the scintillator. The strips are read out at each side; the sum of these two times is independent of the hit position: we define the mean time to be T mean ¼ ðT end1 þ T end2 Þ=2. The time resolution can be obtained from the distribution of Timediff ¼ ðT mean ÞMRPC1 ðT mean ÞMRPC2 ; this is shown in Fig. 5. The variance of this distribution is the sum of the variances of pffiffiffithe time resolution of each MRPC; thus the sigma is divided by 2 and is quoted as the time resolution of each MRPC with the assumption that the two MRPCs have the same time resolution. Slewing corrections can be applied; the width of the output pulse is governed by the amount of input charge; thus the variation of Timediff with the pulse width is a measure of slewing as shown in Fig. 6. Timediff , after correction for slewing, is shown in Fig. 7. The time resolution versus applied voltage is shown in Fig. 8, for both cases: with and without slewing corrections.
is 30/√2 = 21.2 ps
100
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sigma = 41.7 ps
60
Time resolution of each MRPC is 41.7/√2 = 29.5 ps
40 0 -400
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0 Timediff [ps]
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0 Fig. 7. Time difference between the two MRPCs after correcting for time slewing.
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Cosmic ray data Oscilloscope DAQ
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Time resolution after slewing correction
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Time Resolution [ps]
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sigma = 39.2 ps
30 Time resolution of each MRPC is 39.2 /√2 = 27.7 ps
20
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0 11.5
12.0 12.5 13.0 Applied voltage across 6 gas gaps [kV]
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Fig. 8. The time resolution versus applied voltage across six gas gaps measured in the T10 test beam at CERN. The error bars are contained in the size of the symbols; the lines are just to guide the eye.
- 200
0
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Timediff [ps] Fig. 9. The time difference between the two MRPCs using the cosmic ray trigger. Two data acquisition systems were employed: the oscilloscope (LeCroy WM8300) and the ALICE-TOF HPTDC system. The applied voltage was 12.5 kV across six gas gaps.
ARTICLE IN PRESS S. An et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 39–43
signal in the strip is 1 cm/50 ps. Thus a variation of time difference ðT end1 T end2 Þ of 100 ps corresponds to a change of 1 cm in the hit position. The full length of the strip (7.4 cm) is displayed in these plots. We see a change of time of 30 ps between hits at the centre of the strips compared to the ends. This is due to the geometry of the strips. Correcting for this effect improves the time resolution by 1 ps (i.e. improves from 30 to 29 ps). In general the time resolution measured using the cosmic ray trigger was between 5 and 10 ps worse than the time resolution measured in the test beam. The following factors explain this degradation. In the test beam the trigger determines a 1 1 cm2 spot near the center of the strip, while the cosmic ray data is for a hit anywhere in the 2:5 cm 7:4 cm strip. The cosmic ray particles have a distribution of incident angles, which can be measured in the strip direction using the time difference from both ends of the strips. We do make a correction related to the calculated path length of the particles between the two MRPCs; however, the hit position is not measured across the strip width (2.5 cm) and this adds to the measured time resolution. In addition the cosmic ray data was collected over a long period of time ð24 hÞ compared to the test beam run that lasted 10 min; obviously the long runs place strong demands on the stability of the time measurement system. Finally the momentum of the test beam was set to be 5 GeV/c. However, for the cosmic ray data, all particles that could penetrate 5 cm lead were accepted; the very low momenta particles will broaden the time resolution since the MRPCs were 7 cm apart.
60 MRPC1
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6. Discussion
-20
Clearly when measuring time resolution, the time resolution of the measuring device is very important. In this case this is 20 ps for the HPTDC system and at best 10 ps using the oscilloscope since jitter introduced by the front end electronics must be included. There is also a contribution from the transit time of the signal in the strip; we have attempted to eliminate this by reading out both ends of the strip and using the mean time. However there is still a contribution from the width of the strip (2.5 cm). The intrinsic time resolution of the MRPC itself also contributes and there are many factors that affect this; however, by making the gaps smaller should make the time resolution better since the avalanches grow faster (they have to, since the gap is smaller). As discussed earlier we were forced to set the thresholds at around 100 fC. Lower values showed some oscillations due to feed back from the cables carrying the output pulse. Even though this higher threshold allowed us to test this device, still there must be noise injected into the front end and noise produces jitter. We believe that the measured time resolution of 20 ps will be improved with better electronics.
-30
0
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2 3 4 5 Distance along strip of MRPC2 [cm]
6
7
Fig. 10. Time variation along the strip direction: (a) distance along strip of MRPC1 [cm], (b) distance along strip of MRPC2 [cm].
the MRPCs after a 5 cm thick lead block. When we used the oscilloscope for data acquisition, the trigger was derived from the strip under study with the additional requirement of no hit in the neighbouring strips. We also used the ALICE TOF TDC system, built using the HPTDC chips. Each MRPC had eight strips (i.e. 16 channels) equipped with NINO front end electronics; the HPTDC system allowed us to read out the 32 channels. In this case we used the OR signal from the front end electronics as the trigger. In Fig. 9 we show typical timing distributions for cosmic ray data using both the ALICE-TOF HPTDC system and the DAQ system based on an oscilloscope (LeCroy WM8300 in this case). In general the time distributions measured with the oscilloscope were always 1 or 2 ps better. The time resolution of an individual HPTDC channel is, at best, 20 ps. Since we read out both sides of the strip and take the average of these two times, we could expect the contribution to the measured time resolution to be pffiffiffi 20 ps= 2 ¼ 14 ps in the best case. Obviously this has an impact on the final time resolution. In Fig. 10 we show the time variation plotted against the hit position; we find the hit position from the time difference of the hit time measured at the ends of the strip. The velocity of the
Acknowledgements Some of the results presented here were obtained at the T10 test beam in the East Hall at CERN. As always we have been well served by the competent operators of the PS. We thank them. References [1] A.N. Akindinov, et al., Nucl. Instr. and Meth. A 533 (2004) 74. [2] F. Anghinolfi, P. Jarron, A.N. Martemiyanov, E. Usenko, H. Wenninger, M.C.S. Williams, A. Zichichi, Nucl. Instr. and Meth. A 533 (2004) 183. [3] A. Akindinov, et al., Nucl. Instr. and Meth. A 533 (2004) 178.