- Email: [email protected]
Progress with the gas electron multiplier
Nuclear Instruments and Methods in Physics Research A 409 (1998) 79—83
Progress with the gas electron multiplier C. Bu¨ttner!, M. Capea´ns", W. Dominik#, M. Hoch", J.C. Labbe´", G. Manzin$, G. Million", L. Ropelewski", F. Sauli",*, A. Sharma% ! Inst. Exp. Kernphysik, Univ. Karlsruhe, Germany " CERN, CH-1211,Geneva, Switzerland # Inst. Exp. Physics, University Warsaw, Poland $ Laboratori Nazionali INFN, Legnaro, Italy % GRPHE University Haute Alsace, Mulhouse, France
Abstract The Gas Electron Multiplier (GEM) amplifies electrons released in a gas and transfers them to a second element of detection, multiwire or microstrip chamber. We describe recent developments realized with two GEM meshes in series, followed by a simple printed circuit as read-out electrode. Full detection efficiency for minimum ionizing electrons has been obtained, with good energy and time resolution; bi-dimensional localization can be achieved recording the induced signals on orthogonal strips printed on a double-sided thin circuit. While with a single, 50-lm thick GEM mesh amplification factors around one hundred are attained, with a twin mesh having the facing electrodes in electrical contact (emulating a thicker GEM) we have obtained gains in excess of a thousand, large enough for efficient detection of radiation. We believe that this can lead to the development of a new generation of simple, reliable, cheap detectors for high-rate tracking and bi-dimensional imaging of radiation. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Gas; Electron; Multiplier; GEM; Detector; Amplification
The recently introduced Gas Electron Multiplier (GEM) [1,2] allows to amplify electrons released in a gas by ionizing radiation; amplifications above a hundred have been demonstrated in a wide range of gases [3]. Coupled to a second element of detection (multiwire or microstrip chamber), a GEM mesh provides large gains and hence greatly improves the reliability of the device that can be operated at substantially lower voltages. Addition of GEM pre-amplifier meshes for safer operation to
* Corresponding author: E-mail: [email protected].
a set of problematic large size (27]25 cm2) microstrip gas chambers (MSGCs) has been adopted already to implement the tracker in a major experiment (HERA-B at DESY) and is being considered by others. The gain provided by one GEM at the upper end of its operating range is sufficient for the detection of radiation; a safer operation is however obtained with two meshes in cascade. This paper describes measurements realized with a chamber consisting of a double GEM and a printed circuit as read-out electrode, as well as preliminary results obtained with an emulated thicker mesh (SuperGEM) that allows to reach gains above one thousand.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 4 0 - 0
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C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
The double GEM detector is shown schematically in Fig. 1. It consists of an upper electrode, delimiting a 3 mm thick conversion and drift region, of two GEM meshes 2 mm apart, and of an induction gap 1.5 mm thick terminating with a printed circuit board with parallel pick-up strips 300 lm wide at 400 lm pitch. The GEM meshes, with an active area of 10]10 cm2, are manufactured on copper clad kapton foils 50 lm thick,
Fig. 1. Schematics of the Double-GEM detector with printed circuit readout.
etched with a high density of holes (typically 120 lm in diameter at 200 lm pitch) using the photo-lithographic process described in the references. With the application of suitable potentials, electrons released by ionization in the upper gas layer drift into the open channels of the first GEM, multiply in avalanche in the high field, transfer to the second GEM and multiply again. No alignment is necessary between the two meshes for proper operation. Signals are induced on the pick-up strips by the electron swarm leaving the second GEM, with an amplitude corresponding to the total drifting charge. As discussed in Ref. [3], use of the ionization mode for signal induction and absence of cancellations due to the presence of opposite polarity pulses result in larger effective signals, given the total gain, than in a conventional avalanche mode. Fig. 2 shows the combined charge gain for the double GEM detector, measured on the pick-up strips with an 55Fe source, and a 90—10 gas filling of argon—dimethylether (DME). Even with a single GEM operating (*» "0), GEM2 signals are large enough for detection. Exposing the
Fig. 2. Combined gain characteristics of the double-GEM detector.
C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
detector to a 90Sr source, and triggering on external scintillation counters to select minimum ionizing electrons, we have measured the pulse height spectrum shown in Fig. 3, and the efficiency plateau given in Fig. 4 as a function of one potential, the other being fixed. Measurements have been realized on 16 adjacent strips equipped with a fast multiple amplifier circuit [4] followed by ADCs; the signal to-noise ratio in the middle of the plateau is around 100. The rise-time of the signal in the ionization mode, corresponding to the drift time of electrons across the last gap (about 30 ns) is a priori slower than in the avalanche mode; both are, however, faster than the rise-time of the amplifiers (45 ns) and are therefore indistinguishable. Moreover, as there is no slow ion component in the first case, signals are effectively shorter in duration implying a higher intrinsic rate capability. Connecting the amplifiers to individual discriminators and to a logical OR, we have measured the time resolution of the detector for minimum ionizing electrons, perpendicular to the chamber: the distribution is quasi-Gaussian with 30 ns FWHM. The value depends on
81
the gas, and suffers from the moderate drift field used for the measurement (3 kV/cm); it can probably be improved by a different choice of operating conditions. In each event, signals are induced on two or three adjacent strips; for X-rays the FWHM of the distribution corresponds to the strip width (400 lm). Although the position resolution has not yet been measured, it is expected to be comparable with the one obtained with micro-strip chambers, dominated by energy loss statistics and diffusion. A thin, double-sided printed circuit with perpendicular strips for the read-out achieves two-dimensional localization; work is in progress using this geometry. The lower GEM electrode facing the readout strips detects a signal equal in charge and opposite in sign; despite the large capacitance of the source (&500 pF) we have achieved a decent energy resolution, 25% FWHM at 6 keV, see Fig. 5, sufficient for discrimination and triggering. This opens the possibility of using, for the localization of neutral radiation, cheap, high-density analogue multiplexers such as those developed for particle physics, that require a gate to be operated.
Fig. 3. Total and single strip charge distribution for fast electrons.
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C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
Fig. 4. Efficiency of the double-GEM for fast electrons.
Fig. 5. Pulse height spectrum for 55Fe detected on the lower GEM electrode.
C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
83
Fig. 6. Pre-amplification factor of the super-GEM mesh.
The maximum gain of a GEM mesh depends on its thickness. On the first prototypes, manufactured on 25 lm thick kapton foils, a maximum gain just above ten was observed. On the present 50 lm thick GEMs, gains between one and two hundred are obtained; an extrapolation suggests that a 100 lm thick GEM should allow amplification factors well above a thousand. Limitations in the present etching technology and availability of materials have not yet allowed to directly confirm this conjecture; we have however emulated a thicker mesh overlapping, in electrical contact, two standard grids: precise facing of the holes is allowed by the accuracy of manufacturing (&5 lm), necessary to align holes on the two sides of the mesh. With this device, named SuperGEM, we have obtained the amplification curve shown in Fig. 6; for the measurement, the two meshes received identical potentials, symmetric with respect to the central electrodes. The gain is largely sufficient for the detection of tracks on a passive printed circuit electrode. Pending the development of thicker devices, the double GEM and Super-GEM meshes with printed
circuit read-out represent a very promising step towards the realization of simple, reliable and cheap gaseous detectors capable of two-dimensional localization at high rates; the absence of high voltages applied to the readout electrodes completely eliminates possibile damages of the electronics on discharges. Flexibility and operation in non-flammable gases [3] makes the use of these devices for applications in particle physics and in other fields particularly attractive.
References [1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [2] R. Bouclier, M. Capea´ns, W. Dominik, M. Hoch, J.-C. Labbe´, G. Million, L. Ropelewski, F. Sauli, A. Sharma, IEEE Trans. Nucl. Sci. NS-44 (1997) 646. [3] R. Bouclier, W. Dominik, M. Hoch, J.-C. Labbe´, G. Million, L. Ropelewski, F. Sauli, A. Sharma, G. Manzin, Nucl. Instr. and Meth. A 396 (1997) 50. [4] P. Jarron, F. Anghinolfi, E. Delagne, W. Dabrowski, L. Scharfetter, Nucl. Instr. and Meth. A 377 (1996) 435.
I. TRACKING (GAS)
Progress with the gas electron multiplier C. Bu¨ttner!, M. Capea´ns", W. Dominik#, M. Hoch", J.C. Labbe´", G. Manzin$, G. Million", L. Ropelewski", F. Sauli",*, A. Sharma% ! Inst. Exp. Kernphysik, Univ. Karlsruhe, Germany " CERN, CH-1211,Geneva, Switzerland # Inst. Exp. Physics, University Warsaw, Poland $ Laboratori Nazionali INFN, Legnaro, Italy % GRPHE University Haute Alsace, Mulhouse, France
Abstract The Gas Electron Multiplier (GEM) amplifies electrons released in a gas and transfers them to a second element of detection, multiwire or microstrip chamber. We describe recent developments realized with two GEM meshes in series, followed by a simple printed circuit as read-out electrode. Full detection efficiency for minimum ionizing electrons has been obtained, with good energy and time resolution; bi-dimensional localization can be achieved recording the induced signals on orthogonal strips printed on a double-sided thin circuit. While with a single, 50-lm thick GEM mesh amplification factors around one hundred are attained, with a twin mesh having the facing electrodes in electrical contact (emulating a thicker GEM) we have obtained gains in excess of a thousand, large enough for efficient detection of radiation. We believe that this can lead to the development of a new generation of simple, reliable, cheap detectors for high-rate tracking and bi-dimensional imaging of radiation. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Gas; Electron; Multiplier; GEM; Detector; Amplification
The recently introduced Gas Electron Multiplier (GEM) [1,2] allows to amplify electrons released in a gas by ionizing radiation; amplifications above a hundred have been demonstrated in a wide range of gases [3]. Coupled to a second element of detection (multiwire or microstrip chamber), a GEM mesh provides large gains and hence greatly improves the reliability of the device that can be operated at substantially lower voltages. Addition of GEM pre-amplifier meshes for safer operation to
* Corresponding author: E-mail: [email protected].
a set of problematic large size (27]25 cm2) microstrip gas chambers (MSGCs) has been adopted already to implement the tracker in a major experiment (HERA-B at DESY) and is being considered by others. The gain provided by one GEM at the upper end of its operating range is sufficient for the detection of radiation; a safer operation is however obtained with two meshes in cascade. This paper describes measurements realized with a chamber consisting of a double GEM and a printed circuit as read-out electrode, as well as preliminary results obtained with an emulated thicker mesh (SuperGEM) that allows to reach gains above one thousand.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 4 0 - 0
I. TRACKING (GAS)
80
C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
The double GEM detector is shown schematically in Fig. 1. It consists of an upper electrode, delimiting a 3 mm thick conversion and drift region, of two GEM meshes 2 mm apart, and of an induction gap 1.5 mm thick terminating with a printed circuit board with parallel pick-up strips 300 lm wide at 400 lm pitch. The GEM meshes, with an active area of 10]10 cm2, are manufactured on copper clad kapton foils 50 lm thick,
Fig. 1. Schematics of the Double-GEM detector with printed circuit readout.
etched with a high density of holes (typically 120 lm in diameter at 200 lm pitch) using the photo-lithographic process described in the references. With the application of suitable potentials, electrons released by ionization in the upper gas layer drift into the open channels of the first GEM, multiply in avalanche in the high field, transfer to the second GEM and multiply again. No alignment is necessary between the two meshes for proper operation. Signals are induced on the pick-up strips by the electron swarm leaving the second GEM, with an amplitude corresponding to the total drifting charge. As discussed in Ref. [3], use of the ionization mode for signal induction and absence of cancellations due to the presence of opposite polarity pulses result in larger effective signals, given the total gain, than in a conventional avalanche mode. Fig. 2 shows the combined charge gain for the double GEM detector, measured on the pick-up strips with an 55Fe source, and a 90—10 gas filling of argon—dimethylether (DME). Even with a single GEM operating (*» "0), GEM2 signals are large enough for detection. Exposing the
Fig. 2. Combined gain characteristics of the double-GEM detector.
C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
detector to a 90Sr source, and triggering on external scintillation counters to select minimum ionizing electrons, we have measured the pulse height spectrum shown in Fig. 3, and the efficiency plateau given in Fig. 4 as a function of one potential, the other being fixed. Measurements have been realized on 16 adjacent strips equipped with a fast multiple amplifier circuit [4] followed by ADCs; the signal to-noise ratio in the middle of the plateau is around 100. The rise-time of the signal in the ionization mode, corresponding to the drift time of electrons across the last gap (about 30 ns) is a priori slower than in the avalanche mode; both are, however, faster than the rise-time of the amplifiers (45 ns) and are therefore indistinguishable. Moreover, as there is no slow ion component in the first case, signals are effectively shorter in duration implying a higher intrinsic rate capability. Connecting the amplifiers to individual discriminators and to a logical OR, we have measured the time resolution of the detector for minimum ionizing electrons, perpendicular to the chamber: the distribution is quasi-Gaussian with 30 ns FWHM. The value depends on
81
the gas, and suffers from the moderate drift field used for the measurement (3 kV/cm); it can probably be improved by a different choice of operating conditions. In each event, signals are induced on two or three adjacent strips; for X-rays the FWHM of the distribution corresponds to the strip width (400 lm). Although the position resolution has not yet been measured, it is expected to be comparable with the one obtained with micro-strip chambers, dominated by energy loss statistics and diffusion. A thin, double-sided printed circuit with perpendicular strips for the read-out achieves two-dimensional localization; work is in progress using this geometry. The lower GEM electrode facing the readout strips detects a signal equal in charge and opposite in sign; despite the large capacitance of the source (&500 pF) we have achieved a decent energy resolution, 25% FWHM at 6 keV, see Fig. 5, sufficient for discrimination and triggering. This opens the possibility of using, for the localization of neutral radiation, cheap, high-density analogue multiplexers such as those developed for particle physics, that require a gate to be operated.
Fig. 3. Total and single strip charge distribution for fast electrons.
I. TRACKING (GAS)
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C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
Fig. 4. Efficiency of the double-GEM for fast electrons.
Fig. 5. Pulse height spectrum for 55Fe detected on the lower GEM electrode.
C. Bu~ ttner et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 79—83
83
Fig. 6. Pre-amplification factor of the super-GEM mesh.
The maximum gain of a GEM mesh depends on its thickness. On the first prototypes, manufactured on 25 lm thick kapton foils, a maximum gain just above ten was observed. On the present 50 lm thick GEMs, gains between one and two hundred are obtained; an extrapolation suggests that a 100 lm thick GEM should allow amplification factors well above a thousand. Limitations in the present etching technology and availability of materials have not yet allowed to directly confirm this conjecture; we have however emulated a thicker mesh overlapping, in electrical contact, two standard grids: precise facing of the holes is allowed by the accuracy of manufacturing (&5 lm), necessary to align holes on the two sides of the mesh. With this device, named SuperGEM, we have obtained the amplification curve shown in Fig. 6; for the measurement, the two meshes received identical potentials, symmetric with respect to the central electrodes. The gain is largely sufficient for the detection of tracks on a passive printed circuit electrode. Pending the development of thicker devices, the double GEM and Super-GEM meshes with printed
circuit read-out represent a very promising step towards the realization of simple, reliable and cheap gaseous detectors capable of two-dimensional localization at high rates; the absence of high voltages applied to the readout electrodes completely eliminates possibile damages of the electronics on discharges. Flexibility and operation in non-flammable gases [3] makes the use of these devices for applications in particle physics and in other fields particularly attractive.
References [1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [2] R. Bouclier, M. Capea´ns, W. Dominik, M. Hoch, J.-C. Labbe´, G. Million, L. Ropelewski, F. Sauli, A. Sharma, IEEE Trans. Nucl. Sci. NS-44 (1997) 646. [3] R. Bouclier, W. Dominik, M. Hoch, J.-C. Labbe´, G. Million, L. Ropelewski, F. Sauli, A. Sharma, G. Manzin, Nucl. Instr. and Meth. A 396 (1997) 50. [4] P. Jarron, F. Anghinolfi, E. Delagne, W. Dabrowski, L. Scharfetter, Nucl. Instr. and Meth. A 377 (1996) 435.
I. TRACKING (GAS)