- Email: [email protected]
Electron drift velocity measurements at high electric fields
Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
Electron drift velocity measurements at high electric fields P. Colasa, A. Delbarta, J. Derre! a, I. Giomatarisa,*, F. Jeanneaua, V. Lepeltierb, I. Papadopoulosa, Ph. Rebourgearda b
a CEA/DSM/DAPNIA/CE Saclay, F-91191 Gif-sur-Yvette, France LAL, IN2P3-CNRS et Universite! Paris-Sud, BP 34, 91898 Orsay Cedex, France
Abstract A method to measure the electron drift velocity is presented. A pulsed UV nitrogen laser is used to excite both the drift and cathode nickel micromeshes of a Micromegas detector. The signals induced on the anode are then readout by a fast current amplifier. Several results have been obtained for various gas mixtures and electric fields from 10 V=cm to 14 kV=cm: Relevant applications with low (TPCs mode) and high (pre-amplification mode) electric fields will be discussed. r 2002 Elsevier Science B.V. All rights reserved.
1. Introduction In several applications of gaseous detectors it is important to know the electron drift velocity in gas. With a very simple device based on the Micromegas principle [1], measurements are performed in different gas mixtures. Results are presented for a wide range of electric fields.
2. Setup description and measurement principle The setup is based on a standard Micromegas detector used in a double photodetector mode [2]. The cathode micromesh is the first photon converter and on top of it another mesh is used as a second photon converter and drift electrode. A scheme of this setup is given in Fig. 1. *Corresponding author. Tel.: +33-1-690-82298; fax: +33-1690-83024. E-mail address: [email protected] (I. Giomataris).
The photon source is a UV ðl ¼ 337 nmÞ pulsed laser ðN2 Þ; focalized through a lens in order to increase the photo-electric effect on the two meshes. Two signals are induced on the grounded anode (see Fig. 2) which is read through a fast preamplifier ð1 GHzÞ: one for the photo-electrons created on the drift electrode and one for the photo-electrons created on the cathode mesh. Since the distance between the two electrodes is known, the time delay between these two signals provides the electron drift velocity in the conversion gap. This measurement has been made for several conversion field values in different gas mixtures: argon, neon or helium with isobutane or CF4 as quencher.
3. Calibration measurements A first set of measurements were made in well known gas mixtures to check the method. Some results obtained in helium mixture, P10 (argon + 10% CH4 ) and pure CH4 are represented in
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 6 0 - 0
216
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
Laser UV
Backfield electrode (HV1) E = 0 kV/cm
Drift electrode (HV1)
3 mm
100 µm
E = 0-10 kV/cm
S2
S1
Micromesh (HV2) E = 50 kV/cm
Anode plane (Grounded)
Fig. 1. Experimental setup.
Drift velocity (cm/µs)
He + 20% iC4H10
5
4.5 4
3.5 3
2.5
Micromegas U. Becker et. al., 07/97 U. Becker et. al., 06/96
2 1.5 1
Fig. 2. Signal measurement.
0.5 0
Figs. 3–5 and compared with published measurements [3–5] and with Monte-Carlo calculations performed with Magboltz [6]. The agreement is quite good.
4. Results in several gas mixtures and for different purposes 4.1. Pre-amplification mode in Micromegas The pre-amplification mode is a nice solution for Micromegas to cope with hadron fluxes as it
0
1
2
3
4
5
6
7
8
9
10
E/P (kV . cm-1 . atm-1)
Fig. 3. Calibration in helium mixture.
has been shown in Ref. [7]. Different gases (argon, neon, helium) with small amount of quencher (isobutane or CF4 ), which can provide a comfortable pre-amplification gain, are studied here in order to find gas mixtures which allow a high enough electron drift velocity. Results are presented in Figs. 6–8. Neon based mixtures are attractive, both for their pre-amplification factor and for the high drift velocity they allow to reach (greater than 5 cm=ms). For instance in Ne+5%
217
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219 Neon mixtures
Drift velocity (cm/µs)
Drift velocity (cm/µs)
P10
6 6
5.5
5.5
5
5 4.5
4.5
4 3.5
4
6
5
4
3
3.5
7
2.5 2
3
0
0.2
0.4
0.6
0.8
1
1.2
3
1.4
2.5
2% iC4H10 4% iC4H10 5% iC4H10 7.5% iC4H10
2
2
Micromegas 1.5
1
U. Becker et. al., 07/97
Magboltz 1
0
1
2
5
4
3
0
7
6
E/P (kV . cm-1 . atm-1)
0
1
2
5
4
6
E/P (kV . cm-1 . atm-1)
Fig. 6. Drift velocity in neon mixtures.
Fig. 4. Calibration in P10.
Argon mixtures
Pure CH4
11
Drift velocity (cm/µs)
Drift velocity (cm/µs)
3
10 9 8 7
7 4.5 4
6
3.5
2% iso 3 5% iso 10% iso 2.5
5
2 1.5
4
6
10
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
5
3
4
Micromegas
3
F. Fulda-Quenzer et al., NIM A 235 (1985) 517
2
BJ-marie et al, 1979
1
2
0
2
4
6
8
10
12
14
E/P (kV . cm-1 . atm-1)
Fig. 5. Calibration in pure CH4 :
iC4 H10 the pre-amplification mode starts at 2:5 kV=cm and the drift velocity reaches 5 cm=mm for 3:6 kV=cm while in Ar + 5% iC4 H10 it starts at 6 kV=cm and the higher velocity is about 4 cm=ms:
1
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 7. Drift velocity in argon mixtures.
4.2. Interest for time projection chamber Gas mixtures for TPC should allow a high electron drift velocity at very low electric fields. A drift velocity higher than 5 cm=ms can be reached
218
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
in CF4 mixtures for fields as low as 200 V=cm (Fig. 9). The advantage of such a gas is obvious compared to quenchers like CO2 (Fig. 10). As a comparison, the drift velocity reaches a maximum
of 4 cm=ms at 400 V=cm in Ar + 5% CO2 while it reaches a maximum of 9 cm=ms at 250 V=cm in Ar + 5% CF4 :
Argon mixtures --- CO2
5
Drift velocity (cm/µs)
Drift velocity (cm/µs)
Helium mixtures
4.5 4
3.5 3
5
4
3
2.5 2
2% iC4H10 5% iC4H10 10% iC4H10 5% iC4H10+5%CF4 5%iC4H10+10%CF4 2% iC4H10+5%CF4
1.5 1 0.5 0
0
1
2
3
5
4
6
10% CO2
2
5% CO2 5% CO2 + 5% CH4
1
5% CO2 + 10% CH4 0
7
E/P (kV . cm-1 . atm-1)
Fig. 8. Drift velocity in helium mixtures.
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 10. Drift velocity in argon with CO2 fractions.
Drift velocity (cm/µs)
Neon mixtures
Drift velocity (cm/µs)
Argon mixtures --- CF4
12
10% CF4 5% CF4 2% CF4
10
2%CF4+5%CH4
10
2%CF4+10%CH4 8 6
8
9 8 7 6
4
5
2
6
00
0.2
0.4
0.6
0.8
1
1.2
4
1.4
4
5% 5% 2% 2%
3 2
0
2
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 9. Drift velocity in argon with CF4 fractions.
1
0
1
2
3
4
iC4H10 + 5% CF4 iC4H10 + 10% CF4 iC4H10 + 5% CF4 iC4H10 + 10% CF4 5
6
7
8
E/P (kV . cm-1 . atm-1)
Fig. 11. Neon mixtures with isobutane and CF4 :
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
Drift velocity (cm/µs)
Neon mixture --- COMPASS
219
5. Conclusion
9
The feasibility of a simple device to measure electron drift velocities in a wide range of electric fields, based on the Micromegas detection principle, has been demonstrated. There is a good agreement with previous published results and simulations. Drift velocity measurements are presented in various gas mixtures. The experimental results at very high field are of great interest for some applications and could be used to improve the accuracy of Monte-Carlo simulations.
8 7 6 5 4 3 2
11% C2H6 11% C2H6 + 10% CF4
1 0
0
1
2
3
7 6 E/P (kV . cm-1 . atm-1)
4
5
Acknowledgements We would like to thank A. Giganon and E. Mahe for the technical support and S. Herlant for his precious assistance during the measurements.
Fig. 12. COMPASS gas mixture.
References 4.3. Small drift and high flux Some measurements have been made to find the best gas mixture in order to cope with high particle fluxes and allow high gain and good time resolution. Adding a small fraction of CF4 in Neon leads to a dramatic increase of the electron velocity (Fig. 11). The results obtained with the gas mixture used for the Micromegas tracker of the COMPASS experiment [8] are also represented on the Fig. 12.
[1] Y. Giomataris, et al., Nucl. Instr. and Meth. A 376 (1996) 29. [2] J. Derr!e, et al., Nucl. Instr. and Meth. A 449 (2000) 314. [3] U. Becker, et al., Nucl. Instr. and Meth. A 306 (1991) 94. [4] U. Becker, et al., MIT LNS rep. # LNS-TR-94-05. [5] F. Fulda-Quenzer, et al., Nucl. Instr. and Meth. A 235 (1985) 517. [6] S.F. Biagi, Magboltz Source Code Version 2.01, CERN. [7] A. Delbart, et al., Performance of Micromegas with Preamplification Mode at High Intensity Hadron Beams, 9th Vienna Conference on Instrumentation, Vienna/Austria, February 19–23, 2001. [8] CERN/SPSLC 96-14 SPLSLC/P297 and CERN/SPSLC 9630 SPLSLC/P297 add.1.
Electron drift velocity measurements at high electric fields P. Colasa, A. Delbarta, J. Derre! a, I. Giomatarisa,*, F. Jeanneaua, V. Lepeltierb, I. Papadopoulosa, Ph. Rebourgearda b
a CEA/DSM/DAPNIA/CE Saclay, F-91191 Gif-sur-Yvette, France LAL, IN2P3-CNRS et Universite! Paris-Sud, BP 34, 91898 Orsay Cedex, France
Abstract A method to measure the electron drift velocity is presented. A pulsed UV nitrogen laser is used to excite both the drift and cathode nickel micromeshes of a Micromegas detector. The signals induced on the anode are then readout by a fast current amplifier. Several results have been obtained for various gas mixtures and electric fields from 10 V=cm to 14 kV=cm: Relevant applications with low (TPCs mode) and high (pre-amplification mode) electric fields will be discussed. r 2002 Elsevier Science B.V. All rights reserved.
1. Introduction In several applications of gaseous detectors it is important to know the electron drift velocity in gas. With a very simple device based on the Micromegas principle [1], measurements are performed in different gas mixtures. Results are presented for a wide range of electric fields.
2. Setup description and measurement principle The setup is based on a standard Micromegas detector used in a double photodetector mode [2]. The cathode micromesh is the first photon converter and on top of it another mesh is used as a second photon converter and drift electrode. A scheme of this setup is given in Fig. 1. *Corresponding author. Tel.: +33-1-690-82298; fax: +33-1690-83024. E-mail address: [email protected] (I. Giomataris).
The photon source is a UV ðl ¼ 337 nmÞ pulsed laser ðN2 Þ; focalized through a lens in order to increase the photo-electric effect on the two meshes. Two signals are induced on the grounded anode (see Fig. 2) which is read through a fast preamplifier ð1 GHzÞ: one for the photo-electrons created on the drift electrode and one for the photo-electrons created on the cathode mesh. Since the distance between the two electrodes is known, the time delay between these two signals provides the electron drift velocity in the conversion gap. This measurement has been made for several conversion field values in different gas mixtures: argon, neon or helium with isobutane or CF4 as quencher.
3. Calibration measurements A first set of measurements were made in well known gas mixtures to check the method. Some results obtained in helium mixture, P10 (argon + 10% CH4 ) and pure CH4 are represented in
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 6 0 - 0
216
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
Laser UV
Backfield electrode (HV1) E = 0 kV/cm
Drift electrode (HV1)
3 mm
100 µm
E = 0-10 kV/cm
S2
S1
Micromesh (HV2) E = 50 kV/cm
Anode plane (Grounded)
Fig. 1. Experimental setup.
Drift velocity (cm/µs)
He + 20% iC4H10
5
4.5 4
3.5 3
2.5
Micromegas U. Becker et. al., 07/97 U. Becker et. al., 06/96
2 1.5 1
Fig. 2. Signal measurement.
0.5 0
Figs. 3–5 and compared with published measurements [3–5] and with Monte-Carlo calculations performed with Magboltz [6]. The agreement is quite good.
4. Results in several gas mixtures and for different purposes 4.1. Pre-amplification mode in Micromegas The pre-amplification mode is a nice solution for Micromegas to cope with hadron fluxes as it
0
1
2
3
4
5
6
7
8
9
10
E/P (kV . cm-1 . atm-1)
Fig. 3. Calibration in helium mixture.
has been shown in Ref. [7]. Different gases (argon, neon, helium) with small amount of quencher (isobutane or CF4 ), which can provide a comfortable pre-amplification gain, are studied here in order to find gas mixtures which allow a high enough electron drift velocity. Results are presented in Figs. 6–8. Neon based mixtures are attractive, both for their pre-amplification factor and for the high drift velocity they allow to reach (greater than 5 cm=ms). For instance in Ne+5%
217
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219 Neon mixtures
Drift velocity (cm/µs)
Drift velocity (cm/µs)
P10
6 6
5.5
5.5
5
5 4.5
4.5
4 3.5
4
6
5
4
3
3.5
7
2.5 2
3
0
0.2
0.4
0.6
0.8
1
1.2
3
1.4
2.5
2% iC4H10 4% iC4H10 5% iC4H10 7.5% iC4H10
2
2
Micromegas 1.5
1
U. Becker et. al., 07/97
Magboltz 1
0
1
2
5
4
3
0
7
6
E/P (kV . cm-1 . atm-1)
0
1
2
5
4
6
E/P (kV . cm-1 . atm-1)
Fig. 6. Drift velocity in neon mixtures.
Fig. 4. Calibration in P10.
Argon mixtures
Pure CH4
11
Drift velocity (cm/µs)
Drift velocity (cm/µs)
3
10 9 8 7
7 4.5 4
6
3.5
2% iso 3 5% iso 10% iso 2.5
5
2 1.5
4
6
10
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
5
3
4
Micromegas
3
F. Fulda-Quenzer et al., NIM A 235 (1985) 517
2
BJ-marie et al, 1979
1
2
0
2
4
6
8
10
12
14
E/P (kV . cm-1 . atm-1)
Fig. 5. Calibration in pure CH4 :
iC4 H10 the pre-amplification mode starts at 2:5 kV=cm and the drift velocity reaches 5 cm=mm for 3:6 kV=cm while in Ar + 5% iC4 H10 it starts at 6 kV=cm and the higher velocity is about 4 cm=ms:
1
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 7. Drift velocity in argon mixtures.
4.2. Interest for time projection chamber Gas mixtures for TPC should allow a high electron drift velocity at very low electric fields. A drift velocity higher than 5 cm=ms can be reached
218
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
in CF4 mixtures for fields as low as 200 V=cm (Fig. 9). The advantage of such a gas is obvious compared to quenchers like CO2 (Fig. 10). As a comparison, the drift velocity reaches a maximum
of 4 cm=ms at 400 V=cm in Ar + 5% CO2 while it reaches a maximum of 9 cm=ms at 250 V=cm in Ar + 5% CF4 :
Argon mixtures --- CO2
5
Drift velocity (cm/µs)
Drift velocity (cm/µs)
Helium mixtures
4.5 4
3.5 3
5
4
3
2.5 2
2% iC4H10 5% iC4H10 10% iC4H10 5% iC4H10+5%CF4 5%iC4H10+10%CF4 2% iC4H10+5%CF4
1.5 1 0.5 0
0
1
2
3
5
4
6
10% CO2
2
5% CO2 5% CO2 + 5% CH4
1
5% CO2 + 10% CH4 0
7
E/P (kV . cm-1 . atm-1)
Fig. 8. Drift velocity in helium mixtures.
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 10. Drift velocity in argon with CO2 fractions.
Drift velocity (cm/µs)
Neon mixtures
Drift velocity (cm/µs)
Argon mixtures --- CF4
12
10% CF4 5% CF4 2% CF4
10
2%CF4+5%CH4
10
2%CF4+10%CH4 8 6
8
9 8 7 6
4
5
2
6
00
0.2
0.4
0.6
0.8
1
1.2
4
1.4
4
5% 5% 2% 2%
3 2
0
2
0
1
2
3
4
5
6
7
8
9
E/P (kV . cm-1 . atm-1)
Fig. 9. Drift velocity in argon with CF4 fractions.
1
0
1
2
3
4
iC4H10 + 5% CF4 iC4H10 + 10% CF4 iC4H10 + 5% CF4 iC4H10 + 10% CF4 5
6
7
8
E/P (kV . cm-1 . atm-1)
Fig. 11. Neon mixtures with isobutane and CF4 :
P. Colas et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 215–219
Drift velocity (cm/µs)
Neon mixture --- COMPASS
219
5. Conclusion
9
The feasibility of a simple device to measure electron drift velocities in a wide range of electric fields, based on the Micromegas detection principle, has been demonstrated. There is a good agreement with previous published results and simulations. Drift velocity measurements are presented in various gas mixtures. The experimental results at very high field are of great interest for some applications and could be used to improve the accuracy of Monte-Carlo simulations.
8 7 6 5 4 3 2
11% C2H6 11% C2H6 + 10% CF4
1 0
0
1
2
3
7 6 E/P (kV . cm-1 . atm-1)
4
5
Acknowledgements We would like to thank A. Giganon and E. Mahe for the technical support and S. Herlant for his precious assistance during the measurements.
Fig. 12. COMPASS gas mixture.
References 4.3. Small drift and high flux Some measurements have been made to find the best gas mixture in order to cope with high particle fluxes and allow high gain and good time resolution. Adding a small fraction of CF4 in Neon leads to a dramatic increase of the electron velocity (Fig. 11). The results obtained with the gas mixture used for the Micromegas tracker of the COMPASS experiment [8] are also represented on the Fig. 12.
[1] Y. Giomataris, et al., Nucl. Instr. and Meth. A 376 (1996) 29. [2] J. Derr!e, et al., Nucl. Instr. and Meth. A 449 (2000) 314. [3] U. Becker, et al., Nucl. Instr. and Meth. A 306 (1991) 94. [4] U. Becker, et al., MIT LNS rep. # LNS-TR-94-05. [5] F. Fulda-Quenzer, et al., Nucl. Instr. and Meth. A 235 (1985) 517. [6] S.F. Biagi, Magboltz Source Code Version 2.01, CERN. [7] A. Delbart, et al., Performance of Micromegas with Preamplification Mode at High Intensity Hadron Beams, 9th Vienna Conference on Instrumentation, Vienna/Austria, February 19–23, 2001. [8] CERN/SPSLC 96-14 SPLSLC/P297 and CERN/SPSLC 9630 SPLSLC/P297 add.1.