- Email: [email protected]
The ion trap: a new approach to gaseous microstructure detectors
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
The ion trap: a new approach to gaseous microstructure detectors Oleg Bouianov* Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 28 July 2003; received in revised form 21 January 2004; accepted 10 February 2004
Abstract A new approach for building gas avalanche amplification structures, the ion trap, is proposed. This technique was devised to obtain a new class of gas amplification devices with a fast signal response due to rapid ion charge evacuation, low space charge build-up useful in high-rate applications, and low ion feedback. A design of the closed-geometry position-sensitive Ion Trap MicroStrip (ITMS) detector is proposed. The implementation of this novel microstructure detector design with 3D electrodes is based on new materials and microfabrication techniques recently developed in the field of Micro Electro Mechanical Systems (MEMS). Extensive simulations have been made to study the performance of the ITMS detector. The modelled microstructure is shown to deliver the ion feedback below 4% with drift fields up to 4.5 kV/cm and fast signals induced by 95% of all ions reaching the cathodes in B30 ns in Ar/CO2 (70/30) gas mixture. The use of the ITMS detector with a cascaded GEM preamplifier may prove to be a feasible solution to achieve the ion feedback suppression levels required in the Time Projection Chamber (TPC) applications and in operation with solid photon converters in the reflective mode. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40. n; 29.40.Cs; 85.60.Gz Keywords: Gaseous detectors; Microstructure detectors; Ion trap; Ion feedback; Ion trap microstrip detector
1. Introduction Gaseous micro-pattern position-sensitive detectors have found numerous applications in particle and nuclear physics, structural biology and scattering experiments in chemistry, X-ray crystallography and astrophysics [1–3]. The use of gas avalanche detectors in such applications as gas*Corresponding author. CERN, Geneva 23, CH-1211, Switzerland. E-mail address: [email protected] (O. Bouianov).
eous photomultipliers [4,5] and Time Projection Chambers (TPC) [6] sets a certain limit to the amount of ions produced in an avalanche flowing into the drift volume of a TPC or towards a photocathode. The TPC performance is affected by electric field distortions caused by the positive ion feedback [6]. In gaseous photomultipliers, ions impinging the photocathode can limit its lifetime, initiate secondary electron emission and reduce gain [7]. Finally, a rate-dependent space charge build-up caused by the low mobility ions collecting in the
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.02.021
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O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
drift volume is a serious problem at high rates and high multiplicities. Hence, for such applications, a technique to reduce the ion feedback is needed.
structures. The attempt is made to achieve the ion feedback suppression levels of the mesh parallel plate devices in structures featuring mechanical rigidity, better response uniformity over detector active area, and operation stability.
2. Ion feedback in the current generation of gas avalanche devices The ion feedback is the fraction of avalanchegenerated ions that leave the avalanche region and populate the volume of the detector where a drift of primary charges occurs. A family of planar gaseous position-sensitive devices, comprising for example the Micro-Strip Gas Chamber (MSGC) [8] and Micro-Dot Chamber [9], due to its open geometry has no or very low ion feedback suppression properties. The non-planar micro-pattern gaseous devices family, including the Gas Electron Multiplier (GEM) [10], Well [11], Micro-Hole and Strip Plate (MHSP) [12], etc., is characterised by a potentially better suppression of the ion feedback. These devices are made by patterning of metal electrodes laminated on each side of a thin dielectric sheet, and their cathodes are separated with the gap from the anode electrodes. The best results in suppression of the ion feedback for such devices are obtained with cascaded GEM structures. Non-planar parallel gap devices, the MICROMEGAS [13] and MicroCAT [14], feature a better confinement of the gas volume with high field where the avalanche occurs, between the anode plane and cathode mesh. Natural ion feedback suppression of these closed-geometry devices is explained by the funnel effect [15]. The ion feedback demonstrated by the MICROMEGAS detector is reported to be as low as 3% at the TPC operating conditions in DC mode [16]. In similar conditions a triple-GEM configuration delivers the ion feedback below 8% [17]. However, it is far from a target suppression level of 10 4 required in the TPC applications [6]. In the multiGEM cascade operation with solid photon converters in the reflective mode requiring a high field at the converter surface the ion feedback cannot be reduced below 10–20% [18]. This work introduces another closed-geometry approach for a design of gas multiplication
3. The ion trap approach An innovative technique, the ion trap, to build gaseous position-sensitive detectors with high confinement of the avalanche charges in a small multiplication volume featuring a fast signal response due to rapid ion charge evacuation, low space charge build-up useful in high-rate applications, and low ion feedback, is proposed here. According to the ion trap approach, the electrodes of a detector are implemented as nonplanar microstructures with more than one active surface, and the cathode electrodes are placed symmetrically with respect to the surfaces of the anode. Fig. 1 depicts the electrode layout of such a gaseous avalanche multiplication structure to illustrate the ion trap principle. A free electron originating from an ionisation, a photocathode or a preamplification structure, following a random movement pattern, drifts to the anode through a gap between symmetrical cathode electrodes at equal electric potential.
Fig. 1. The ion trap principle: electrode layout, geometry parameters and equipotential lines. Electrode dimensions T1– T4 are given to illustrate the symmetry considerations.
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Approaching the region with sufficiently high electric field the electron initiates the development of an avalanche towards the anode. Ions, created in the avalanche, move to the cathodes. In a high electric field existing in narrow gaps (T2) between the active surfaces of anode and cathodes most of the ions quickly reach the cathodes. Only a small fraction of all ions produced in a narrow median region between the cathodes is able to escape into the drift volume. This fraction constitutes the ion feedback of a detector. Diffusion makes the lateral size of the avalanche greater than that of the ion escape region. This feature of the ion trap enables building the gaseous amplification structures with a high suppression level of the ion feedback. Apart from the drift field to amplification gap field ratio that normally governs the ion feedback level, varying the value of a geometry parameter T4 allows the adjustment of the ion feedback suppression ratio in a wide range. However, there exists a trade-off between the ion feedback suppression and primary electron transmission to the anode structure. A narrow opening between the cathodes may result in low primary electron collection efficiency. A significant fraction of the avalanche volume extends into the regions between the anode and cathode where the electric field lines are parallel. In this region the ion tracks are quite straight and short leading to a fast signal due to ions sensed at either cathode. The length of the ion tracks increases for the ions produced at larger distances from the anode tip, but their fraction quickly drops due to the avalanche shape. It is expected that the signal response due to ions will have a short tail.
4. Novel microstructure detectors built upon the ion trap approach The ion trap technique can be used to build a new class of position-sensitive gaseous amplification structures based on a novel 3D electrode design. New materials and micro-fabrication techniques recently developed in the field of Micro-Electro Mechanical Systems (MEMS) allow for a formation of sophisticated 3D electrode
415
Fig. 2. 2 2 cell layout of the Ion Trap Microstrip detector.
structures made of photo-patternable plastic [19] with a metal sputtered on its surface. An early attempt to create gas amplification microstructures with these techniques has been described in Ref. [20]. As one example, this paper presents the design of a novel closed-geometry position-sensitive device, the Ion Trap MicroStrip (ITMS) detector. Its layout is shown in Fig. 2. The ITMS structure consists of a pattern of interleaved cathode and anode strip electrodes formed on a dielectric substrate. Anode strips are wide enough to carry an insulating support for metallic cathode strips with windows, laid down orthogonally to obtain a 2D sensitivity. The insulating supports add mechanical rigidity to the structure, and their width is chosen to minimise the dielectric charging effects. If a positive potential is applied to the anodes, the upper and lower cathodes can be held at earth potential to simplify coupling with sensitive preamplifiers. The properties of the ITMS detector have been thoroughly studied using the standard simulation flow [21,22].1 In Fig. 3 the electric field configuration and dimensions of the modelled structure are shown. The dimensions are chosen to comply with the fabrication process and approach the lower limit of cell granularity. All results presented 1
Maxwell 3-D Field Simulator, Ansoft Corporation.
ARTICLE IN PRESS 416
O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Fig. 3. Cross-section view, dimensions and electric field pattern in a single cell of the ITMS detector.
below were obtained with the anode voltage +350 V chosen to optimise the computation effort, and drift field 0.5 kV/cm in an Ar/CO2 (70/30) gas mixture, unless otherwise specified. Charge transport in the detector is illustrated in Fig. 4, which shows a small fraction of an avalanche from a single electron. The ion drift pattern is symmetrical, with ion charges evenly shared between the cathodes. Most of the ion tracks are short and only a small proportion of the ions escape into the drift volume. To study the ion drift pattern in detail, a socalled ion drift map of a detector cell was generated. The ion drift map is shown in Fig. 5. The cross-sectional area of a cell was scanned in such a way that an ion charge was created in each location of a gas volume and its drift was traced until a destination point. Depending where the ion track terminates, three areas shown in different
greyscale patterns were built, namely, the region of ion drift (a) to the cathode 1, (b) to the cathode 2, (c) into the drift volume. The computed ion drift map of the ITMS detector illustrates the main feature of the ion trap—a very narrow ion escape region in the volume where the avalanche multiplication occurs. This property determines the low ion feedback demonstrated by the ITMS structure. The calculated ion feedback can be found from Fig. 6. Here, the statistics from a number of events have been collected (total B105 e ), and the resulting distribution of the final z-coordinate of the ion drift path in the detector volume is shown. The ion charges are evenly shared between cathode 1 and 2, as seen in detail in Fig. 4, and only 1.5% constitutes the ion feedback. The short ion drift time is another feature of the ITMS detector. For the same event data set, the ion drift time distribution is shown in Fig. 7. In the
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
417
Fig. 4. Fragment of a simulated avalanche with the electron and ion tracks shown.
Fig. 5. Simulated ion drift map. Marked greyscale shapes outline the areas where ions drift towards the corresponding cathode electrodes or escape into the drift volume. Detector structure is shown as a dark pattern.
25 mm gap of the ITMS structure described here, about 95% of all ions reach the cathodes in B30 ns inducing a very fast signal.
Fig. 6. z-coordinate of the ion arrival position in the ITMS detector. Area of the histogram is normalised to unity.
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O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Fig. 7. Normalised ion drift time distribution.
Fig. 8. Simulated ion feedback and electron collection efficiency as a function of the drift (transfer) field.
Gas preamplification structures, such as cascaded GEMs, are often used in a combination with position-sensitive gas avalanche devices to provide higher overall gains and more stable operation. This mode of operation also enables obtaining a higher suppression of the ion feedback [23]. In such applications, in the gap between the last preamplification stage and a position sensitive detector, a sufficiently high transfer field is created to ensure the extraction of electrons produced in the holes. Fig. 8 depicts the ion feedback and
Fig. 9. Simulated absolute gas gain and ion feedback as a function of the anode voltage.
corresponding primary electron collection efficiency as a function of the field in the transfer gap. The calculated ion feedback of the studied ITMS microstructure is found to be below 4% and collection efficiency over 0.6 with drift fields up to 4.5 kV/cm, sufficient for the operation with a cascaded GEM preamplifier. Finally, the expected absolute gas gain and corresponding ion feedback was calculated for a range of anode voltages. The ion feedback tends to decrease with the anode voltage and reaches its minimum of 1.5% at Va E400 V for the given drift field, where the absolute gas gain is about 5000, as Fig. 9 suggests. High-ion feedback suppression levels can possibly be achieved when the ITMS detector is used in a combination with a cascaded GEM sufficient to the TPC applications and operation with solid photon converters in the reflective mode.
5. Conclusions and future work In this work, a new approach for building gas avalanche amplification structures, the ion trap, was presented. A new class of gas amplification devices with 3D electrodes built with the ion trap technique features rapid ion charge evacuation and low ion feedback.
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
The design of a novel closed-geometry device with a 2D position-sensitivity, the Ion Trap Microstrip detector, was proposed. Extensive simulations have been made to study the performance of the ITMS detector. The modelled microstructure was shown to deliver the ion feedback below 4% with drift fields up to 4.5 kV/ cm and fast signals induced by 95% of all ions reaching the cathodes in B30 ns in Ar/CO2 (70/30) gas mixture. The presented results show that the ion trap technique looks promising for the development of novel gas avalanche microstructure detectors in the applications requiring fast signals and low space charge build-up combined with response uniformity and mechanical robustness. The use of the ITMS detector with a cascaded GEM preamplifier may prove to be a feasible solution to achieve the ion feedback suppression levels required in the TPC applications and in operation with solid photon converters in the reflective mode. Small area (less than 10 10 cm2) prototypes of the ITMS detector are being fabricated using materials and manufacturing methods recently developed in the MEMS field. The cathode and anode strips are made of photo-patternable plastic with a metal layer sputtered on its surface. The performance tests of the prototypes are in preparation. Alternative technologies suitable to fabricate large-area detectors with a similar to the ITMS geometry are being studied. New detector designs based on the ion trap principle are also being investigated.
Acknowledgements The author is indebted to Dave Bailey from Bristol University for his help with simulation tools.
419
References [1] F. Sauli, Nucl. Instr. and Meth. A 477 (2002) 1. [2] R. Bellazzini, et al., Nucl. Instr. and Meth. A 478 (2002) 13. [3] A. Breskin, Nucl. Instr. and Meth. A 454 (2000) 26. [4] P. Carlson, et al., Nucl. Instr. and Meth. A 505 (2003) 207. [5] F. Piuz, Nucl. Instr. and Meth. A 502 (2003) 76. [6] M. Killenberg, et al., LC TPC R&D: A proposal to the DESY PRC, DESY-PRC-RD-01-03, October 2001. [7] A. Breskin, et al., Nucl. Instr. and Meth. A 478 (2002) 225. [8] A. Oed, Nucl. Instr. and Meth. A 263 (1988) 351. [9] S.F. Biagi, T.J. Jones, Nucl. Instr. and Meth. A 361 (1995) 72. [10] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [11] R. Bellazzini, et al., Nucl. Instr. and Meth. A 423 (1999) 125. [12] J.M. Maia, et al., IEEE Trans. Nucl. Sci. NS-49 (2002) 875. [13] I. Giomataris, et al., Nucl. Instr. and Meth. A 376 (1996) 29. [14] A. Sarvestani, et al., Nucl. Instr. and Meth. A 410 (1998) 238. [15] P. Colas, et al., Micromegas TPC R&D, DAPNIA-02-409, January 2003. [16] P.S. Barbeau, et al., A study of GEMs and MICROMEGAS in TPC mode, IEEE NSS 2003, Portland, October 22, 2003. [17] A. Bondar, et al., Nucl. Instr. and Meth. A 496 (2003) 325. [18] R. Chechik, et al., Recent investigations of cascaded GEM and MHSP detectors, IEEE NSS 2003, Portland, October 22, 2003. [19] H. Lorenz, et al., Sensor. Actuat. A 64 (1998) 33. [20] P. Rehak, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 1426. [21] R. Veenhof, Nucl. Instr. and Meth. A 419 (1998) 726. [22] S.F. Biagi, Nucl. Instr. and Meth. A 421 (1999) 234. [23] S. Roth, Recent developments in gaseous tracking detectors, ICHEP 2002, Amsterdam, Netherlands, July 2002.
Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
The ion trap: a new approach to gaseous microstructure detectors Oleg Bouianov* Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 28 July 2003; received in revised form 21 January 2004; accepted 10 February 2004
Abstract A new approach for building gas avalanche amplification structures, the ion trap, is proposed. This technique was devised to obtain a new class of gas amplification devices with a fast signal response due to rapid ion charge evacuation, low space charge build-up useful in high-rate applications, and low ion feedback. A design of the closed-geometry position-sensitive Ion Trap MicroStrip (ITMS) detector is proposed. The implementation of this novel microstructure detector design with 3D electrodes is based on new materials and microfabrication techniques recently developed in the field of Micro Electro Mechanical Systems (MEMS). Extensive simulations have been made to study the performance of the ITMS detector. The modelled microstructure is shown to deliver the ion feedback below 4% with drift fields up to 4.5 kV/cm and fast signals induced by 95% of all ions reaching the cathodes in B30 ns in Ar/CO2 (70/30) gas mixture. The use of the ITMS detector with a cascaded GEM preamplifier may prove to be a feasible solution to achieve the ion feedback suppression levels required in the Time Projection Chamber (TPC) applications and in operation with solid photon converters in the reflective mode. r 2004 Elsevier B.V. All rights reserved. PACS: 29.40. n; 29.40.Cs; 85.60.Gz Keywords: Gaseous detectors; Microstructure detectors; Ion trap; Ion feedback; Ion trap microstrip detector
1. Introduction Gaseous micro-pattern position-sensitive detectors have found numerous applications in particle and nuclear physics, structural biology and scattering experiments in chemistry, X-ray crystallography and astrophysics [1–3]. The use of gas avalanche detectors in such applications as gas*Corresponding author. CERN, Geneva 23, CH-1211, Switzerland. E-mail address: [email protected] (O. Bouianov).
eous photomultipliers [4,5] and Time Projection Chambers (TPC) [6] sets a certain limit to the amount of ions produced in an avalanche flowing into the drift volume of a TPC or towards a photocathode. The TPC performance is affected by electric field distortions caused by the positive ion feedback [6]. In gaseous photomultipliers, ions impinging the photocathode can limit its lifetime, initiate secondary electron emission and reduce gain [7]. Finally, a rate-dependent space charge build-up caused by the low mobility ions collecting in the
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.02.021
ARTICLE IN PRESS 414
O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
drift volume is a serious problem at high rates and high multiplicities. Hence, for such applications, a technique to reduce the ion feedback is needed.
structures. The attempt is made to achieve the ion feedback suppression levels of the mesh parallel plate devices in structures featuring mechanical rigidity, better response uniformity over detector active area, and operation stability.
2. Ion feedback in the current generation of gas avalanche devices The ion feedback is the fraction of avalanchegenerated ions that leave the avalanche region and populate the volume of the detector where a drift of primary charges occurs. A family of planar gaseous position-sensitive devices, comprising for example the Micro-Strip Gas Chamber (MSGC) [8] and Micro-Dot Chamber [9], due to its open geometry has no or very low ion feedback suppression properties. The non-planar micro-pattern gaseous devices family, including the Gas Electron Multiplier (GEM) [10], Well [11], Micro-Hole and Strip Plate (MHSP) [12], etc., is characterised by a potentially better suppression of the ion feedback. These devices are made by patterning of metal electrodes laminated on each side of a thin dielectric sheet, and their cathodes are separated with the gap from the anode electrodes. The best results in suppression of the ion feedback for such devices are obtained with cascaded GEM structures. Non-planar parallel gap devices, the MICROMEGAS [13] and MicroCAT [14], feature a better confinement of the gas volume with high field where the avalanche occurs, between the anode plane and cathode mesh. Natural ion feedback suppression of these closed-geometry devices is explained by the funnel effect [15]. The ion feedback demonstrated by the MICROMEGAS detector is reported to be as low as 3% at the TPC operating conditions in DC mode [16]. In similar conditions a triple-GEM configuration delivers the ion feedback below 8% [17]. However, it is far from a target suppression level of 10 4 required in the TPC applications [6]. In the multiGEM cascade operation with solid photon converters in the reflective mode requiring a high field at the converter surface the ion feedback cannot be reduced below 10–20% [18]. This work introduces another closed-geometry approach for a design of gas multiplication
3. The ion trap approach An innovative technique, the ion trap, to build gaseous position-sensitive detectors with high confinement of the avalanche charges in a small multiplication volume featuring a fast signal response due to rapid ion charge evacuation, low space charge build-up useful in high-rate applications, and low ion feedback, is proposed here. According to the ion trap approach, the electrodes of a detector are implemented as nonplanar microstructures with more than one active surface, and the cathode electrodes are placed symmetrically with respect to the surfaces of the anode. Fig. 1 depicts the electrode layout of such a gaseous avalanche multiplication structure to illustrate the ion trap principle. A free electron originating from an ionisation, a photocathode or a preamplification structure, following a random movement pattern, drifts to the anode through a gap between symmetrical cathode electrodes at equal electric potential.
Fig. 1. The ion trap principle: electrode layout, geometry parameters and equipotential lines. Electrode dimensions T1– T4 are given to illustrate the symmetry considerations.
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Approaching the region with sufficiently high electric field the electron initiates the development of an avalanche towards the anode. Ions, created in the avalanche, move to the cathodes. In a high electric field existing in narrow gaps (T2) between the active surfaces of anode and cathodes most of the ions quickly reach the cathodes. Only a small fraction of all ions produced in a narrow median region between the cathodes is able to escape into the drift volume. This fraction constitutes the ion feedback of a detector. Diffusion makes the lateral size of the avalanche greater than that of the ion escape region. This feature of the ion trap enables building the gaseous amplification structures with a high suppression level of the ion feedback. Apart from the drift field to amplification gap field ratio that normally governs the ion feedback level, varying the value of a geometry parameter T4 allows the adjustment of the ion feedback suppression ratio in a wide range. However, there exists a trade-off between the ion feedback suppression and primary electron transmission to the anode structure. A narrow opening between the cathodes may result in low primary electron collection efficiency. A significant fraction of the avalanche volume extends into the regions between the anode and cathode where the electric field lines are parallel. In this region the ion tracks are quite straight and short leading to a fast signal due to ions sensed at either cathode. The length of the ion tracks increases for the ions produced at larger distances from the anode tip, but their fraction quickly drops due to the avalanche shape. It is expected that the signal response due to ions will have a short tail.
4. Novel microstructure detectors built upon the ion trap approach The ion trap technique can be used to build a new class of position-sensitive gaseous amplification structures based on a novel 3D electrode design. New materials and micro-fabrication techniques recently developed in the field of Micro-Electro Mechanical Systems (MEMS) allow for a formation of sophisticated 3D electrode
415
Fig. 2. 2 2 cell layout of the Ion Trap Microstrip detector.
structures made of photo-patternable plastic [19] with a metal sputtered on its surface. An early attempt to create gas amplification microstructures with these techniques has been described in Ref. [20]. As one example, this paper presents the design of a novel closed-geometry position-sensitive device, the Ion Trap MicroStrip (ITMS) detector. Its layout is shown in Fig. 2. The ITMS structure consists of a pattern of interleaved cathode and anode strip electrodes formed on a dielectric substrate. Anode strips are wide enough to carry an insulating support for metallic cathode strips with windows, laid down orthogonally to obtain a 2D sensitivity. The insulating supports add mechanical rigidity to the structure, and their width is chosen to minimise the dielectric charging effects. If a positive potential is applied to the anodes, the upper and lower cathodes can be held at earth potential to simplify coupling with sensitive preamplifiers. The properties of the ITMS detector have been thoroughly studied using the standard simulation flow [21,22].1 In Fig. 3 the electric field configuration and dimensions of the modelled structure are shown. The dimensions are chosen to comply with the fabrication process and approach the lower limit of cell granularity. All results presented 1
Maxwell 3-D Field Simulator, Ansoft Corporation.
ARTICLE IN PRESS 416
O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Fig. 3. Cross-section view, dimensions and electric field pattern in a single cell of the ITMS detector.
below were obtained with the anode voltage +350 V chosen to optimise the computation effort, and drift field 0.5 kV/cm in an Ar/CO2 (70/30) gas mixture, unless otherwise specified. Charge transport in the detector is illustrated in Fig. 4, which shows a small fraction of an avalanche from a single electron. The ion drift pattern is symmetrical, with ion charges evenly shared between the cathodes. Most of the ion tracks are short and only a small proportion of the ions escape into the drift volume. To study the ion drift pattern in detail, a socalled ion drift map of a detector cell was generated. The ion drift map is shown in Fig. 5. The cross-sectional area of a cell was scanned in such a way that an ion charge was created in each location of a gas volume and its drift was traced until a destination point. Depending where the ion track terminates, three areas shown in different
greyscale patterns were built, namely, the region of ion drift (a) to the cathode 1, (b) to the cathode 2, (c) into the drift volume. The computed ion drift map of the ITMS detector illustrates the main feature of the ion trap—a very narrow ion escape region in the volume where the avalanche multiplication occurs. This property determines the low ion feedback demonstrated by the ITMS structure. The calculated ion feedback can be found from Fig. 6. Here, the statistics from a number of events have been collected (total B105 e ), and the resulting distribution of the final z-coordinate of the ion drift path in the detector volume is shown. The ion charges are evenly shared between cathode 1 and 2, as seen in detail in Fig. 4, and only 1.5% constitutes the ion feedback. The short ion drift time is another feature of the ITMS detector. For the same event data set, the ion drift time distribution is shown in Fig. 7. In the
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
417
Fig. 4. Fragment of a simulated avalanche with the electron and ion tracks shown.
Fig. 5. Simulated ion drift map. Marked greyscale shapes outline the areas where ions drift towards the corresponding cathode electrodes or escape into the drift volume. Detector structure is shown as a dark pattern.
25 mm gap of the ITMS structure described here, about 95% of all ions reach the cathodes in B30 ns inducing a very fast signal.
Fig. 6. z-coordinate of the ion arrival position in the ITMS detector. Area of the histogram is normalised to unity.
ARTICLE IN PRESS 418
O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
Fig. 7. Normalised ion drift time distribution.
Fig. 8. Simulated ion feedback and electron collection efficiency as a function of the drift (transfer) field.
Gas preamplification structures, such as cascaded GEMs, are often used in a combination with position-sensitive gas avalanche devices to provide higher overall gains and more stable operation. This mode of operation also enables obtaining a higher suppression of the ion feedback [23]. In such applications, in the gap between the last preamplification stage and a position sensitive detector, a sufficiently high transfer field is created to ensure the extraction of electrons produced in the holes. Fig. 8 depicts the ion feedback and
Fig. 9. Simulated absolute gas gain and ion feedback as a function of the anode voltage.
corresponding primary electron collection efficiency as a function of the field in the transfer gap. The calculated ion feedback of the studied ITMS microstructure is found to be below 4% and collection efficiency over 0.6 with drift fields up to 4.5 kV/cm, sufficient for the operation with a cascaded GEM preamplifier. Finally, the expected absolute gas gain and corresponding ion feedback was calculated for a range of anode voltages. The ion feedback tends to decrease with the anode voltage and reaches its minimum of 1.5% at Va E400 V for the given drift field, where the absolute gas gain is about 5000, as Fig. 9 suggests. High-ion feedback suppression levels can possibly be achieved when the ITMS detector is used in a combination with a cascaded GEM sufficient to the TPC applications and operation with solid photon converters in the reflective mode.
5. Conclusions and future work In this work, a new approach for building gas avalanche amplification structures, the ion trap, was presented. A new class of gas amplification devices with 3D electrodes built with the ion trap technique features rapid ion charge evacuation and low ion feedback.
ARTICLE IN PRESS O. Bouianov / Nuclear Instruments and Methods in Physics Research A 526 (2004) 413–419
The design of a novel closed-geometry device with a 2D position-sensitivity, the Ion Trap Microstrip detector, was proposed. Extensive simulations have been made to study the performance of the ITMS detector. The modelled microstructure was shown to deliver the ion feedback below 4% with drift fields up to 4.5 kV/ cm and fast signals induced by 95% of all ions reaching the cathodes in B30 ns in Ar/CO2 (70/30) gas mixture. The presented results show that the ion trap technique looks promising for the development of novel gas avalanche microstructure detectors in the applications requiring fast signals and low space charge build-up combined with response uniformity and mechanical robustness. The use of the ITMS detector with a cascaded GEM preamplifier may prove to be a feasible solution to achieve the ion feedback suppression levels required in the TPC applications and in operation with solid photon converters in the reflective mode. Small area (less than 10 10 cm2) prototypes of the ITMS detector are being fabricated using materials and manufacturing methods recently developed in the MEMS field. The cathode and anode strips are made of photo-patternable plastic with a metal layer sputtered on its surface. The performance tests of the prototypes are in preparation. Alternative technologies suitable to fabricate large-area detectors with a similar to the ITMS geometry are being studied. New detector designs based on the ion trap principle are also being investigated.
Acknowledgements The author is indebted to Dave Bailey from Bristol University for his help with simulation tools.
419
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