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Studies of an MSGC equipped with a GEM grid as a tracking device
Nuclear Instruments and Methods in Physics Research A 419 (1998) 394—399
Studies of an MSGC equipped with a GEM grid as a tracking device W. Beaumont , T. Beckers *, J. De Troy , C. Van Dyck , O. Bouhali, F. Udo, C. Vander Velde, W. Van Doninck, P. Vanlaer , V. Zhukov University of Antwerp (UIA), Physics Department, Universititatsplein 1, B-2610 Antwerp, Belgium IIHE ULB-VUB, Brussels, Belgium
Abstract The performance of a Micro Strip Gas Counter (MSGC) equipped with a Gas Electron Multiplier (GEM) has been studied in a cosmic hodoscope. Using Ne/DME gas mixtures, measurements have been performed to study gain, transparency and spatial resolution. 1998 Elsevier Science B.V. All rights reserved. Keywords: MSGC; GEM; Ne/DME gas mixture
1. Introduction The high particle rates in present and future high energy physics experiments put severe demands on their tracking detectors. Micro Strip Gas Counters (MSGCs) provide a cost efficient way of tackling these demands offering full efficiency for minimum ionizing particles, fast response and appropriate spatial resolution. However, to obtain stable and safe operation in this harsh environment, the gain of the MSGC is limited to a few thousand. Therefore, it could be useful to provide a preamplification stage in the form of a Gas Electron Multiplier
* Corresponding author. Work supported by a grant from the BOF-UA. Financially supported by FNRS. On leave of absence from MSU Moscow, Russia.
(GEM) allowing the MSGC to work at very stable low gain. The GEM [1—3] is a polymer foil coated on both sides with metal and perforated with regularly spaced holes. Mounted between the drift plane and the MSGC substrate, a potential difference between the two metal layers creates a dipole field in the holes, inducing charge amplification for electrons drifting from the top gas volume to the substrate. Before this device can be used in tracking applications, it has to be proven that there are no negative consequences to the introduction of the GEM foil in terms of gain, efficiency and spatial resolution.
2. Experimental setup and analysis procedure The measurements were performed using a cosmic hodoscope described in detail in Ref. [4]. Two
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 8 1 3 - 4
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
scintillators interleaved with 10 cm of lead selected muons with momentum above 300 MeV/c. Drift chambers allowed to reconstruct the track of selected particles. Precise tracking is accomplished by means of a stack of three MSGCs with parallel strips. After the alignment procedure, the prediction of the coordinate of the impact point of the particle in the detector under test was done with an accuracy of &25 lm. The detectors consisted of an MSGC substrate, the same as above, with a GEM [5] placed 3 mm above the substrate. The MSGC substrates, 10;10 cm, have aluminium strips printed on 300 lm thick D263 glass. The strips have an anode
395
to anode pitch of 200 lm, the anodes and cathodes are 7 and 90 lm wide, respectively. Two different GEMs were used, GEM2 and GEM3. They were produced on a 50 lm thick copper coated Kapton foil. The distance between the holes is 200 lm (140 lm), their diameter is 100 lm (80 lm) in the copper and 80 lm (50 lm) in the Kapton for GEM2 (GEM3). A metallized Kapton drift plane was placed 3 mm above the GEM. A voltage » was applied to the cathode strips, » to the GEM plane facing the MSGC, » to the opposite GEM plane and » to the drift plane. The detector signals were always normalized with the signals of a reference MSGC running at fixed
Fig. 1. Cluster size and cluster charge fitted with a Landau function for (a) MSGC mode: » "575 V, » "» "» "1.5 kV; (b) GEM3: » "200 V, » "1.5 kV, » "1.92 kV, » "2.5 kV and (c) GEM2: » "500 V, » "2 kV, » "2.3 kV, » "3.5 kV. Measurements were performed in a Ne/DME 2/1 gas mixture.
IV. MICROSTRUCTURE DEVICES
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W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
voltages to eliminate temperature and pressure influences. To compare the results obtained with the GEM detectors to normal MSGC results, measurements were done with the GEM2 detector in the so-called MSGC mode i.e. running with voltages » " » "» . 3. GEM amplification The charge collected on the MSGC was spread over several strips forming a cluster. Fig. 1 compares the cluster size and cluster charge distribution of the two detectors and GEM2 in MSGC
mode. The voltage settings for MSGC mode provided an amplification of &1600. In GEM mode the voltages on GEM2 (GEM3) resulted in a GEM amplification of &3 (&1500) and MSGC amplification of &1000 (&2). The increase in cluster size in GEM mode is attributed to the diffusion of electrons over a larger drift distance. The GEM amplification factor was studied in Ne/DME 2/1 as a function of the voltage difference between the GEM electrodes. In Fig. 2 the result is shown for GEM2 and GEM3 together with a reference measurement in Ar/CO [3]. The figure clearly shows the lower performance of GEM2 that was of older design. To avoid saturation of the preamplifiers an increase in GEM gain was
Fig. 2. The GEM amplification factor as a function of the voltage difference on the GEM electrodes. Measurements with GEM2 and GEM3 were performed in a Ne/DME 2/1 gas mixture. Results from a reference measurement in Ar/CO [3] were added.
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
compensated by a decrease of the cathode voltage. High drift fields were used between the MSGC and the GEM to obtain a high transparency. This combination might have biased the electron collection
Fig. 3. Dependence on »
397
efficiency on the anodes resulting in lower GEM amplification values. The dependence of the collected charge on » at fixed » and » is plotted in Fig. 3. Changing
of the amplitude of the anode signal for GEM2.
Fig. 4. Angular dependence of the spatial resolution for the detectors in GEM mode as compared to that for MSGC mode.
IV. MICROSTRUCTURE DEVICES
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W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
Fig. 5. Uniformity of the spatial resolution across the MSGC strips for the detectors in GEM mode as compared to that for MSGC mode.
» changes the amplification near the anodestrips. To eliminate this bias the amplitudes were normalized to those of the MSGC mode. The maximum amplitude in Fig. 3 corresponds to a ratio of drift fields in upper and lower gaps E /E & 0.7. This could be explained by the geometry of the field lines in the detector, resulting in a better transparency of the GEM.
that in MSGC mode. The asymmetry in the figure is due to the hodoscope acceptance. The spatial resolution as a function of the position of the incident particle along a line perpendicular to the strips is shown in Fig. 5. The alignment error on the data was &25 lm. Although the cluster size increases for the detectors in GEM mode (see Fig. 1) no significant changes in the resolution of the detectors were observed.
4. Spatial resolution 5. Conclusion Since the spatial resolution of the detectors has a strong dependence on the angle of the incident particle, the selection of tracks for the spatial resolution study was dependent on two angles. The angle was defined as the projection of the polar angle in a plane parallel to the strips while the angle h was defined as the projection of the polar angle in a plane perpendicular to the strips. Tracks with angle 46 were selected for this resolution study. The spatial resolution for GEM2 and GEM3 as a function of the angle h is compared in Fig. 4 to
An MSGC detector equipped with a GEM grid was shown to have a GEM amplification factor well above 2000. This feature allows a lower MSGC gain, providing a solution to possible discharge problems in normal MSGCs. Stable operation of GEM foils over several months in a gas mixture with high DME content was demonstrated. It was shown that the spatial resolution for a detector equipped with a GEM, operating in a Ne/DME 2/1 gas mixture, with a GEM gain &1500 and an
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
MSGC gain &2, is similar to the resolution of a detector running in MSGC mode. References
399
[3] J. Benlloch et al., Development of the gas electron multiplier (GEM), IEEE Nucl. Sci. Symp., Albuquerque, 1997, CERN-PPE/97-146. [4] O. Bouhali et al., Nucl. Instr. and Meth. A 378 (1996) 432. [5] Kindly provided by Prof. F. Sauli, CERN.
[1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [2] R. Bouclier et al., Nucl. Instr. and Meth. A 396 (1997) 50.
IV. MICROSTRUCTURE DEVICES
Studies of an MSGC equipped with a GEM grid as a tracking device W. Beaumont , T. Beckers *, J. De Troy , C. Van Dyck , O. Bouhali, F. Udo, C. Vander Velde, W. Van Doninck, P. Vanlaer , V. Zhukov University of Antwerp (UIA), Physics Department, Universititatsplein 1, B-2610 Antwerp, Belgium IIHE ULB-VUB, Brussels, Belgium
Abstract The performance of a Micro Strip Gas Counter (MSGC) equipped with a Gas Electron Multiplier (GEM) has been studied in a cosmic hodoscope. Using Ne/DME gas mixtures, measurements have been performed to study gain, transparency and spatial resolution. 1998 Elsevier Science B.V. All rights reserved. Keywords: MSGC; GEM; Ne/DME gas mixture
1. Introduction The high particle rates in present and future high energy physics experiments put severe demands on their tracking detectors. Micro Strip Gas Counters (MSGCs) provide a cost efficient way of tackling these demands offering full efficiency for minimum ionizing particles, fast response and appropriate spatial resolution. However, to obtain stable and safe operation in this harsh environment, the gain of the MSGC is limited to a few thousand. Therefore, it could be useful to provide a preamplification stage in the form of a Gas Electron Multiplier
* Corresponding author. Work supported by a grant from the BOF-UA. Financially supported by FNRS. On leave of absence from MSU Moscow, Russia.
(GEM) allowing the MSGC to work at very stable low gain. The GEM [1—3] is a polymer foil coated on both sides with metal and perforated with regularly spaced holes. Mounted between the drift plane and the MSGC substrate, a potential difference between the two metal layers creates a dipole field in the holes, inducing charge amplification for electrons drifting from the top gas volume to the substrate. Before this device can be used in tracking applications, it has to be proven that there are no negative consequences to the introduction of the GEM foil in terms of gain, efficiency and spatial resolution.
2. Experimental setup and analysis procedure The measurements were performed using a cosmic hodoscope described in detail in Ref. [4]. Two
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 8 1 3 - 4
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
scintillators interleaved with 10 cm of lead selected muons with momentum above 300 MeV/c. Drift chambers allowed to reconstruct the track of selected particles. Precise tracking is accomplished by means of a stack of three MSGCs with parallel strips. After the alignment procedure, the prediction of the coordinate of the impact point of the particle in the detector under test was done with an accuracy of &25 lm. The detectors consisted of an MSGC substrate, the same as above, with a GEM [5] placed 3 mm above the substrate. The MSGC substrates, 10;10 cm, have aluminium strips printed on 300 lm thick D263 glass. The strips have an anode
395
to anode pitch of 200 lm, the anodes and cathodes are 7 and 90 lm wide, respectively. Two different GEMs were used, GEM2 and GEM3. They were produced on a 50 lm thick copper coated Kapton foil. The distance between the holes is 200 lm (140 lm), their diameter is 100 lm (80 lm) in the copper and 80 lm (50 lm) in the Kapton for GEM2 (GEM3). A metallized Kapton drift plane was placed 3 mm above the GEM. A voltage » was applied to the cathode strips, » to the GEM plane facing the MSGC, » to the opposite GEM plane and » to the drift plane. The detector signals were always normalized with the signals of a reference MSGC running at fixed
Fig. 1. Cluster size and cluster charge fitted with a Landau function for (a) MSGC mode: » "575 V, » "» "» "1.5 kV; (b) GEM3: » "200 V, » "1.5 kV, » "1.92 kV, » "2.5 kV and (c) GEM2: » "500 V, » "2 kV, » "2.3 kV, » "3.5 kV. Measurements were performed in a Ne/DME 2/1 gas mixture.
IV. MICROSTRUCTURE DEVICES
396
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
voltages to eliminate temperature and pressure influences. To compare the results obtained with the GEM detectors to normal MSGC results, measurements were done with the GEM2 detector in the so-called MSGC mode i.e. running with voltages » " » "» . 3. GEM amplification The charge collected on the MSGC was spread over several strips forming a cluster. Fig. 1 compares the cluster size and cluster charge distribution of the two detectors and GEM2 in MSGC
mode. The voltage settings for MSGC mode provided an amplification of &1600. In GEM mode the voltages on GEM2 (GEM3) resulted in a GEM amplification of &3 (&1500) and MSGC amplification of &1000 (&2). The increase in cluster size in GEM mode is attributed to the diffusion of electrons over a larger drift distance. The GEM amplification factor was studied in Ne/DME 2/1 as a function of the voltage difference between the GEM electrodes. In Fig. 2 the result is shown for GEM2 and GEM3 together with a reference measurement in Ar/CO [3]. The figure clearly shows the lower performance of GEM2 that was of older design. To avoid saturation of the preamplifiers an increase in GEM gain was
Fig. 2. The GEM amplification factor as a function of the voltage difference on the GEM electrodes. Measurements with GEM2 and GEM3 were performed in a Ne/DME 2/1 gas mixture. Results from a reference measurement in Ar/CO [3] were added.
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
compensated by a decrease of the cathode voltage. High drift fields were used between the MSGC and the GEM to obtain a high transparency. This combination might have biased the electron collection
Fig. 3. Dependence on »
397
efficiency on the anodes resulting in lower GEM amplification values. The dependence of the collected charge on » at fixed » and » is plotted in Fig. 3. Changing
of the amplitude of the anode signal for GEM2.
Fig. 4. Angular dependence of the spatial resolution for the detectors in GEM mode as compared to that for MSGC mode.
IV. MICROSTRUCTURE DEVICES
398
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
Fig. 5. Uniformity of the spatial resolution across the MSGC strips for the detectors in GEM mode as compared to that for MSGC mode.
» changes the amplification near the anodestrips. To eliminate this bias the amplitudes were normalized to those of the MSGC mode. The maximum amplitude in Fig. 3 corresponds to a ratio of drift fields in upper and lower gaps E /E & 0.7. This could be explained by the geometry of the field lines in the detector, resulting in a better transparency of the GEM.
that in MSGC mode. The asymmetry in the figure is due to the hodoscope acceptance. The spatial resolution as a function of the position of the incident particle along a line perpendicular to the strips is shown in Fig. 5. The alignment error on the data was &25 lm. Although the cluster size increases for the detectors in GEM mode (see Fig. 1) no significant changes in the resolution of the detectors were observed.
4. Spatial resolution 5. Conclusion Since the spatial resolution of the detectors has a strong dependence on the angle of the incident particle, the selection of tracks for the spatial resolution study was dependent on two angles. The angle was defined as the projection of the polar angle in a plane parallel to the strips while the angle h was defined as the projection of the polar angle in a plane perpendicular to the strips. Tracks with angle 46 were selected for this resolution study. The spatial resolution for GEM2 and GEM3 as a function of the angle h is compared in Fig. 4 to
An MSGC detector equipped with a GEM grid was shown to have a GEM amplification factor well above 2000. This feature allows a lower MSGC gain, providing a solution to possible discharge problems in normal MSGCs. Stable operation of GEM foils over several months in a gas mixture with high DME content was demonstrated. It was shown that the spatial resolution for a detector equipped with a GEM, operating in a Ne/DME 2/1 gas mixture, with a GEM gain &1500 and an
W. Beaumont et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 394—399
MSGC gain &2, is similar to the resolution of a detector running in MSGC mode. References
399
[3] J. Benlloch et al., Development of the gas electron multiplier (GEM), IEEE Nucl. Sci. Symp., Albuquerque, 1997, CERN-PPE/97-146. [4] O. Bouhali et al., Nucl. Instr. and Meth. A 378 (1996) 432. [5] Kindly provided by Prof. F. Sauli, CERN.
[1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [2] R. Bouclier et al., Nucl. Instr. and Meth. A 396 (1997) 50.
IV. MICROSTRUCTURE DEVICES