- Email: [email protected]
Micromegas, a microstrip detector for Compass
Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
Micromegas, a microstrip detector for Compass Ph. Abbona, J. Balla, Y. Bedfera, C. Carascoa, E. Delagnesa, D. Duranda, J.-C. Faivrea, H. Fonvieilleb, A. Giganona, F. Kunne*,a, J.-M. Le Goffa, F. Lehara, A. Magnona, D. Neyreta, E. Pasquettoa, H. Pereiraa, S. Platchkova, E. Poissona, Ph. Rebourgearda, D. Thersa a
b
DAPNIA/Sphn-Bat 703, CEA Saclay, F91191 Gif/Yvette, France LPC IN2P3/CNRS, Univ.Blaise Pascal Clermont II, F69177 Aubiere, France
Abstract Recent results obtained with a gaseous microstrip detector Micromegas developed for the tracking in the high-rate environment of the COMPASS experiment at CERN are presented. A 26 36 cm2 prototype equipped with the low-noise preamplifier SFE16 was tested in a high-energy hadron beam at CERN. With a gas mixture based on neon, the full efficiency of the Micromegas prototype is obtained at a gain of 6400; the spatial resolution is 50 mm and the time jitter 8.5 ns. We have studied the problem of discharges that affect this kind of microstrip detector, and found that, at fixed gain, the probability of discharge is higher for heavier gas mixtures in the detector. In the conditions of the COMPASS experiment, discharges are kept at a low rate. # 2001 Elsevier Science B.V. All rights reserved.
1. The Micromegas detector and the front-end electronics Micromegas is a gaseous detector with a parallel plate electrode structure and microstrips [1]. A micromesh separates the detector into two regions: the conversion gap (2.5 mm thick, 1 kV/cm) where the ionization charges are collected, and the amplification gap (100 mm thick, 50 kV/cm) where the avalanche takes place. The prototype, described in detail in [2], has an active area of 26 36 cm2 , with a central dead zone of 5 cm in diameter. The strip pitch is 317 mm. In the COMPASS experiment the Micromegas chambers *Corresponding author. Tel.: +33-1-69084345. E-mail addresses: [email protected] (F. Kunne).
will see a maximum flux density of 270 kHz/cm2 , corresponding to a particle rate per strip of 90 kHz. The strips are equipped with the SFE16 circuit [3], a low-noise (825 e ) preamplifier with a peaking time of 85 ns, developed to provide a full detector efficiency at gains of a few thousands in order to minimize the discharge probability. The read-out is logical. We record only the leading and trailing edge times ðTlead and Ttrail Þ of the signal on the strips, and not the amplitude (logical read-out). These values are used to calculate the Time over Threshold TOT ¼ Tlead Ttrail which is correlated to the amplitude, and the mean time of the signal Tmean ¼ Tlead þ aTtrail . The weighting coefficient a depends on the TOT value, and allows to suppress the effect of the time walk on Tmean .
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 1 5 8 - X
SECTION II.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
2. Performances of the detector The efficiency, the timing and spatial resolutions of the Micromegas detector equipped with the SFE16 and filled with various gas mixtures have been measured in the 10 GeV T9 hadron beam at CERN at fluxes between 15 and 50 kHz [2]. The results obtained with the Ar2iC4 H10 (89–11) and the Ne2C2 H6 CF4 (79–11–10) mixtures are summarized here. The detector efficiency reaches 99% at a mesh voltage value of 425 Vð 440 VÞ, corresponding to a gain of 3700 (6400) with the argon (neon) mixture (Fig. 1). A comfortable plateau is obtained with the two mixtures; it extends over more than 50 V with neon. At the beginning of the efficiency plateau, we measure a mean cluster size of 2.4 (2.1) strips, a time over threshold of 195 (170) ns, a position resolution of 62 (50) mm and a time jitter of 12.3 (8.8) ns with the argon (neon) mixture. Measurements of the efficiency were also performed at fluxes varying between 1 and 120 kHz per strip. At the highest rate, we observe the expected drop of efficiency of 2% due to the occupancy of the electronics.
3. Discharges study Micromegas is subject to discharges similarly to other micro-strip detectors. This effect is observed as a spurious increase in the mesh current which occurs at a low rate compared to the rate of incident particles. The discharges do not appear to alter the properties of the detector, but generate a dead-time and require a special protection for the front-end electronics. We have measured the rates of discharges in the T9 p/proton beam at incident particle rates between 0.1 and 2 MHz for various gas mixtures. The rates of discharges, when normalized to the incident flux, do not depend on the flux. The probability of discharge per incident particle are shown as a function of the gain for various gas mixtures in Fig. 2. We observe that the probability is a power function of the gain. An important observation is that the probability depends on the
Fig. 1. Efficiency versus mesh voltage for two gas mixtures.
mean atomic number hZi or the mean weight hAi of the gas mixture. Indeed the data points are grouped in three differents sets corresponding to comparable values of hZi. In our experimental conditions, the probability of discharges does not depend at first order, on the parameters which characterize the detection efficiency of the MIP’s: the drift velocity of the electrons, the drift velocity of the ions and the size of the transverse diffusion do not influence much the rate of discharges. Finally, for hadron energies between 3 and 15 GeV, the discharge probabilities are insensitive to the energy of the incident particles. In the COMPASS experiment, Micromegas will be used for the tracking of charged particles in the vicinity of the high-energy and high-intensity muon beam, downstream of the fixed polarized target. In this hot region, Micromegas (even with a 5 cm diameter dead zone in the center), will see an integrated flux of 3 107 particles/s. These particles come from the beam halo and from interactions of the 200 GeV muon beam within the target. We have measured the discharge rates in conditions similar to those of the COMPASS experiment using successively the Ar2iC4 H10 (89– 11) and the Ne2C2 H6 2CF4 (79–11–10) mixtures. In order to disentangle the contributions from the beam halo and from the particles produced in the target, measurements were performed with and without a 1.3 m polyethylene target (2.7 rad length) in total.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
Fig. 2. Left: Discharge probability versus detector gain in a 15 GeV hadron beam for various gas mixtures. Right: Number of discharges per spill in the conditions of COMPASS. Full points (squares) correspond to data taken with the target, with the gas mixture based on argon (neon). Empty points (squares) correspond to data taken without the target.
Fig. 3. A large Micromegas detector with deported electronics: strip length 70 cm, active area 40 40 cm2 .
The number of discharges per spill of 2 108 incident muons, is shown in Fig. 2. The rate increases in presence of the target. At fixed gain, the rates are about one order of magnitude lower with the neon mixture than with the argon one. At full efficiency, we count 1.3 discharges per 2 s spill
with the argon mixture without the target. With the neon mixture, the number of discharges per spill drops to 0.1. This corresponds to a probability of discharge per incident particle of 5 109 for the beam halo alone, two orders of magnitude less than for the hadron beam. Since SECTION II.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
the dead time associated to a discharge was measured to be 3 ms [2], the above rates are acceptable. In order to reduce the quantity of matter introduced by the detector in the acceptance of the COMPASS spectrometer, we have built a new prototype for which the read-out (SFE16 chips) is deported away: the strips are 70 cm long, while the active area fixed by the size of the mesh is 40 40 cm2 (Fig. 3).
thousands with a spatial resolution of 50 mm and a time jitter of 8.5 ns. We have shown that the discharge probability depends on the mean value of the atomic number of the gas mixture. It is proportional to the incident flux of particle, and about two orders of magnitude lower with muons than with hadrons. In the conditions of the COMPASS experiment there is about 0.2 discharge per spill.
4. Summary References 2
A 2636 cm Micromegas microstrip detector was tested in a pion beam using a logical read-out with a low-noise preamplifier chip, the SFE16. With a Ne2C2 H6 2CF4 mixture the detector operates at full efficiency at a gain of a few
[1] Y. Giomataris et al., Nucl. Instr. and Meth. A 376 (1996) 29. [2] D. Thers et al., Nucl. Instr. and Meth., to be published in Nucl. Instr. and Meth. A. [3] E. Delagnes et al., IEEE Trans. Nucl. Sci. 47 (2000) 1447.
Micromegas, a microstrip detector for Compass Ph. Abbona, J. Balla, Y. Bedfera, C. Carascoa, E. Delagnesa, D. Duranda, J.-C. Faivrea, H. Fonvieilleb, A. Giganona, F. Kunne*,a, J.-M. Le Goffa, F. Lehara, A. Magnona, D. Neyreta, E. Pasquettoa, H. Pereiraa, S. Platchkova, E. Poissona, Ph. Rebourgearda, D. Thersa a
b
DAPNIA/Sphn-Bat 703, CEA Saclay, F91191 Gif/Yvette, France LPC IN2P3/CNRS, Univ.Blaise Pascal Clermont II, F69177 Aubiere, France
Abstract Recent results obtained with a gaseous microstrip detector Micromegas developed for the tracking in the high-rate environment of the COMPASS experiment at CERN are presented. A 26 36 cm2 prototype equipped with the low-noise preamplifier SFE16 was tested in a high-energy hadron beam at CERN. With a gas mixture based on neon, the full efficiency of the Micromegas prototype is obtained at a gain of 6400; the spatial resolution is 50 mm and the time jitter 8.5 ns. We have studied the problem of discharges that affect this kind of microstrip detector, and found that, at fixed gain, the probability of discharge is higher for heavier gas mixtures in the detector. In the conditions of the COMPASS experiment, discharges are kept at a low rate. # 2001 Elsevier Science B.V. All rights reserved.
1. The Micromegas detector and the front-end electronics Micromegas is a gaseous detector with a parallel plate electrode structure and microstrips [1]. A micromesh separates the detector into two regions: the conversion gap (2.5 mm thick, 1 kV/cm) where the ionization charges are collected, and the amplification gap (100 mm thick, 50 kV/cm) where the avalanche takes place. The prototype, described in detail in [2], has an active area of 26 36 cm2 , with a central dead zone of 5 cm in diameter. The strip pitch is 317 mm. In the COMPASS experiment the Micromegas chambers *Corresponding author. Tel.: +33-1-69084345. E-mail addresses: [email protected] (F. Kunne).
will see a maximum flux density of 270 kHz/cm2 , corresponding to a particle rate per strip of 90 kHz. The strips are equipped with the SFE16 circuit [3], a low-noise (825 e ) preamplifier with a peaking time of 85 ns, developed to provide a full detector efficiency at gains of a few thousands in order to minimize the discharge probability. The read-out is logical. We record only the leading and trailing edge times ðTlead and Ttrail Þ of the signal on the strips, and not the amplitude (logical read-out). These values are used to calculate the Time over Threshold TOT ¼ Tlead Ttrail which is correlated to the amplitude, and the mean time of the signal Tmean ¼ Tlead þ aTtrail . The weighting coefficient a depends on the TOT value, and allows to suppress the effect of the time walk on Tmean .
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 1 5 8 - X
SECTION II.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
2. Performances of the detector The efficiency, the timing and spatial resolutions of the Micromegas detector equipped with the SFE16 and filled with various gas mixtures have been measured in the 10 GeV T9 hadron beam at CERN at fluxes between 15 and 50 kHz [2]. The results obtained with the Ar2iC4 H10 (89–11) and the Ne2C2 H6 CF4 (79–11–10) mixtures are summarized here. The detector efficiency reaches 99% at a mesh voltage value of 425 Vð 440 VÞ, corresponding to a gain of 3700 (6400) with the argon (neon) mixture (Fig. 1). A comfortable plateau is obtained with the two mixtures; it extends over more than 50 V with neon. At the beginning of the efficiency plateau, we measure a mean cluster size of 2.4 (2.1) strips, a time over threshold of 195 (170) ns, a position resolution of 62 (50) mm and a time jitter of 12.3 (8.8) ns with the argon (neon) mixture. Measurements of the efficiency were also performed at fluxes varying between 1 and 120 kHz per strip. At the highest rate, we observe the expected drop of efficiency of 2% due to the occupancy of the electronics.
3. Discharges study Micromegas is subject to discharges similarly to other micro-strip detectors. This effect is observed as a spurious increase in the mesh current which occurs at a low rate compared to the rate of incident particles. The discharges do not appear to alter the properties of the detector, but generate a dead-time and require a special protection for the front-end electronics. We have measured the rates of discharges in the T9 p/proton beam at incident particle rates between 0.1 and 2 MHz for various gas mixtures. The rates of discharges, when normalized to the incident flux, do not depend on the flux. The probability of discharge per incident particle are shown as a function of the gain for various gas mixtures in Fig. 2. We observe that the probability is a power function of the gain. An important observation is that the probability depends on the
Fig. 1. Efficiency versus mesh voltage for two gas mixtures.
mean atomic number hZi or the mean weight hAi of the gas mixture. Indeed the data points are grouped in three differents sets corresponding to comparable values of hZi. In our experimental conditions, the probability of discharges does not depend at first order, on the parameters which characterize the detection efficiency of the MIP’s: the drift velocity of the electrons, the drift velocity of the ions and the size of the transverse diffusion do not influence much the rate of discharges. Finally, for hadron energies between 3 and 15 GeV, the discharge probabilities are insensitive to the energy of the incident particles. In the COMPASS experiment, Micromegas will be used for the tracking of charged particles in the vicinity of the high-energy and high-intensity muon beam, downstream of the fixed polarized target. In this hot region, Micromegas (even with a 5 cm diameter dead zone in the center), will see an integrated flux of 3 107 particles/s. These particles come from the beam halo and from interactions of the 200 GeV muon beam within the target. We have measured the discharge rates in conditions similar to those of the COMPASS experiment using successively the Ar2iC4 H10 (89– 11) and the Ne2C2 H6 2CF4 (79–11–10) mixtures. In order to disentangle the contributions from the beam halo and from the particles produced in the target, measurements were performed with and without a 1.3 m polyethylene target (2.7 rad length) in total.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
Fig. 2. Left: Discharge probability versus detector gain in a 15 GeV hadron beam for various gas mixtures. Right: Number of discharges per spill in the conditions of COMPASS. Full points (squares) correspond to data taken with the target, with the gas mixture based on argon (neon). Empty points (squares) correspond to data taken without the target.
Fig. 3. A large Micromegas detector with deported electronics: strip length 70 cm, active area 40 40 cm2 .
The number of discharges per spill of 2 108 incident muons, is shown in Fig. 2. The rate increases in presence of the target. At fixed gain, the rates are about one order of magnitude lower with the neon mixture than with the argon one. At full efficiency, we count 1.3 discharges per 2 s spill
with the argon mixture without the target. With the neon mixture, the number of discharges per spill drops to 0.1. This corresponds to a probability of discharge per incident particle of 5 109 for the beam halo alone, two orders of magnitude less than for the hadron beam. Since SECTION II.
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Ph. Abbon et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 29–32
the dead time associated to a discharge was measured to be 3 ms [2], the above rates are acceptable. In order to reduce the quantity of matter introduced by the detector in the acceptance of the COMPASS spectrometer, we have built a new prototype for which the read-out (SFE16 chips) is deported away: the strips are 70 cm long, while the active area fixed by the size of the mesh is 40 40 cm2 (Fig. 3).
thousands with a spatial resolution of 50 mm and a time jitter of 8.5 ns. We have shown that the discharge probability depends on the mean value of the atomic number of the gas mixture. It is proportional to the incident flux of particle, and about two orders of magnitude lower with muons than with hadrons. In the conditions of the COMPASS experiment there is about 0.2 discharge per spill.
4. Summary References 2
A 2636 cm Micromegas microstrip detector was tested in a pion beam using a logical read-out with a low-noise preamplifier chip, the SFE16. With a Ne2C2 H6 2CF4 mixture the detector operates at full efficiency at a gain of a few
[1] Y. Giomataris et al., Nucl. Instr. and Meth. A 376 (1996) 29. [2] D. Thers et al., Nucl. Instr. and Meth., to be published in Nucl. Instr. and Meth. A. [3] E. Delagnes et al., IEEE Trans. Nucl. Sci. 47 (2000) 1447.