ISSN 0020-4412, Instruments and Experimental Techniques, 2015, Vol. 58, No. 2, pp. 197–205. © Pleiades Publishing, Ltd., 2015. Original Russian Text © E.V. Atkin, S.S. Volkov, A.G. Voronin, V.V. Ivanov, B.G. Komkov, L.G. Kudin, E.Z. Malankin, V.N. Nikulin, E.V. Roshchin, G.V. Rybakov, V.M. Samsonov, O.P. Tarasenkova, V.V. Shumikhin, A.V. Khanzadeev, E.A. Chernysheva, 2015, published in Pribory i Tekhnika Eksperimenta, 2015, No. 2, pp. 32–40.
NUCLEAR EXPERIMENTAL TECHNIQUES
A Study of the Coordinate Gas-Filled Detectors Based on the GEM and TGEM Technologies for the Muon Tracking System of the CBM Experiment E. V. Atkina, S. S. Volkova,b, A. G. Voronina,c, V. V. Ivanova,b, B. G. Komkovb, L. G. Kudinb, E. Z. Malankina, V. N. Nikulinb, E. V. Roshchina,b, G. V. Rybakovb, V. M. Samsonova,b, O. P. Tarasenkovab, V. V. Shumikhina, A. V. Khanzadeeva,b, and E. A. Chernyshevab a
National Research Nuclear University, Moscow Engineering Physics Institute, Kashirskoe sh. 31, Moscow, 115409 Russia b Petersburg Nuclear Physics Institute, National Research Centre Kurchatov Institute, Orlova roshcha, Gatchina, Leningrad oblast, 188300 Russia cSkobel’tsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119234 Russia *e-mail:
[email protected] e-mail:
[email protected] e-mail:
[email protected] Received May 20, 2014
Abstract—Prototypes of the gas-filled detectors based on the GEM and TGEM technologies have been studied in the multistage gas amplification circuit with the aim of selecting variants for tracking detectors of the muon system capable of operating at high counting rates in the CBM experiment. Two- and three-component mixtures based on Ar and He are used as the working gas. Their key characteristics are presented. The gas amplification factors are measured for various high-voltage modes. The charge collection times determining the response speed of the tested detectors are estimated. DOI: 10.1134/S0020441215010248
1. INTRODUCTION In this paper, we present the results of the investigations of position-sensitive detectors based on the GEM and TGEM technologies, which could be used as the basic elements in the tracking system of the MUCH muon detector included as a component in the Compressed Baryonic Matter (CBM) experimental setup (Fig. 1). The CBM experiment, one of the basic experiments at the FAIR accelerator facility that is under construction at GSI (Darmstadt, Germany), has been designed for studying superdense nuclear matter produced by central collisions in interactions between the extracted beam of heavy nuclei (up to uranium) with energies of 8–35 AGeV (A is the mass number of bombarding nuclei) and nuclei of the target. The baryonic density of the nuclear matter produced under the extreme conditions such as these is comparable to the density at the center of neutron stars and exceeds by ten times the conventional nuclear density. The initial version of the CBM setup was intended for detecting е+е––pairs from decays of short-lived vector mesons and mesons containing charmed quarks. Thereafter, the start variant of the experiment
implying a study of the muon decay modes of these mesons using the Muon Chambers (MUCH) muon detector, was proposed and adopted. The setup has been designed so that the MUCH detector operating in the first phase of the experiment will be subsequently replaced with the Ring Imaging Cherenkov (RICH) detector without changing the geometry of the other detecting subsystems. Results of the model calculations by the Monte Carlo method have demonstrated the possibility of performing, as a part of the CBM experiment, a full range of measurements of the muon channel in decay of light vector mesons and charmonium at various initial energies. Based on these calculations, an original concept of the muon system has been developed. The MUCH detector, the overall view of which is shown on the leader in Fig. 1, contains six absorber layers admittedly made of iron and tracking stations of position-sensitive gas-filled detectors placed in the gaps between separate absorber layers. Each tracking station contains three detecting layers measuring X and Y coordinates of a charged particle. As many as 107 events/s are expected in the CBM experiment, and each of these events contains up to
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Fig. 1. General view of the CBM experimental setup containing the dipole magnet, tracking system (STS), Cherenkov detector (RICH), muon detector (MUCH), transient radiation detector (TRD), time-of-flight detector (TOF), and electromagnetic calorimeter (ECAL).
1000 tracks of charged particles. Heavy (from the standpoint of the counting ability) experimental conditions rule out the possibility of using traditional proportional chambers. One of the promising variants consists in using position-sensitive detectors based on the GEM and TGEM technologies in the tracking stations. A test bench has been designed for this purpose. It comprises a facility for preparing a multicomponent (up to four components) gas mixture, detector and logic electronics, and software for data mapping and analysis.
Among the most promising modern gas-filled detectors are chambers with signal amplification based on a gas electron multiplier (GEM) film [1, 2]. The structure of this film is apparent on images obtained with an electron microscope (see Fig. 2a). It is a 50-μm-thick plastic film with a metal coating on both surfaces and holes ~50 μm in diameter. The film is placed in the gas volume, and a potential difference of 300–500 V is applied to it. Figure 2b shows the distribution of the electric field and illustrates the operation principle of a GEM: when a voltage is applied to the film coatings, a high electric field strength (concentration of field lines) is produced inside the holes. This electric field strength is sufficient for the appearance of the gas amplification conditions inside the holes, i.e., the number of electrons initially produced by a charged particle in the drift space of the detector substantially uncreases, as result of which a particle can be detected with a high efficiency. At a certain selection of applied voltages, almost all primary electrons, while drifting along field lines, are focused inside the GEM holes (the funnel effect), suffer gas amplification, and are collected at the anode; i.e., the is formed at the signal readout electrode through the collection of electrodes. Since the film is very thin, the major portion of ions produced by gas amplification is collected at the upper GEM plate. These factors provide a high response speed of the detector and its high counting ability. The voltage value is selected so that the probability of the discharges in the gas is minimized. These discharges may damage the film through the short-circuit
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failure between the plates. This limit on the voltage causes the gas amplification factor to decrease. Therefore, for the signal amplification required for signal detection to be obtained, several layers of the GEM films are used. The diagram of this detector is shown in Fig. 3: primary electrons are produced in the drift gap cathode–GEM1, and they increase in number upon their sequential passing through the holes in the GEM1–GEM3 gap. Afterwards, the electrons penetrating into the GEM3-anode gap induce charge at the readout electrodes. As a result, the position of the first ionization source (a track) can be determined with a high accuracy (depending on the width of the readout electrodes). The spatial resolution of the GEM detector is determined by the width of the readout electrodes (strips). At a sufficiently small width of the electrodes (0.2–0.3 mm), the charge is immediately induced on a few adjacent strips. The track position can be determined by the distribution of the charge on the strips with an accuracy of 0.020–0.050 mm. We note a high response speed of such a detector, which allows operation at flux densities of ~106 particles/(cm2 s). The film homogeneity over the thickness determines a high energy resolution (σ ~ 10–15%). The high cost (today, it is ~200 euro per square decimeter) is an essential drawback of the GEM films. High-quality GEM films are currently produced only by the detector laboratory at CERN using the precision chemical etching method. The ways for simplifying the film production technology are searched for. ThickGEM (TGEM) is a substantially cheaper analog of the GEM detector [3, 4]. The TGEM detector is produced on the basis of the standard technology accepted in the printed circuit board production. It is a flat glass-cloth laminate (or other dielectric) plate coated on both sides with a metal layer 0.3–1.0 mm thick. Holes 0.3–0.6 mm in diameter are drilled in it with a pitch of ~1 mm. As is the case of a conventional GEM, the gas amplification conditions are produced INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
in the working gas mixture inside holes when a small potential difference is applied to the plates. TGEM-based detectors are inferior to GEM detectors in the response speed and can be used to good effect at flux densities of ~105 particles/(cm2 s) or lower. In the CBM experiment, the reaction products have a sharp forward orientation; therefore, the highest counting rates are expected to be at the centers of the tracking stations. Nevertheless, TGEM detectors may be treated as candidates for the periphery of the tracking stations in the muon system of the CBM detector (the total area of this periphery is ~100 m2). 3. RESULTS OF TESTS OF THE GEM AND TGEM PROTOTYPES WITH A SINGLE GAS AMPLIFICATION STAGE 3.1 A GEM Detector Prototype The geometry of the GEM detector prototype is shown in Fig. 4. The GEM film was produced at CERN. A test bench has been developed for tests of different configurations. This test bench includes a facility for preparing a multicomponent (as many as four components) gas mixture, the detector and logic electronics, and the software for data display and analysis. The detector parameters were tested using the 55Fe source, which emits X rays with an energy of 5.9 keV. Initially, a gas mixture of 90% Ar + 10% CO2 was used. The dependence of the gas amplification factor (GAF) on the potential difference applied to the GEM was measured with a 55Fe source. This dependence is shown in Fig. 5a, and the example of a 55Fe spectrum is presented in Fig. 5b. The energy resolution of this detector is σ ~ 12%. The maximum GAF of ~2.5 × 103, at which discharges had not yet been observed, was attained when the voltage between the GEM plates was 520 V. The basic result obtained in this detector test is in the illustration of the fact that the observed parameters are close to those available from the literature, e.g., in the COMPASS experiment [5]. Vol. 58
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3.2. A TGEM detector prototype As noted above, the TGEM operates similarly to a conventional GEM detector. The TGEM is a flat glass-cloth laminate (or made of other dielectric) plate metalized on both sides, with a thickness of 0.3– 1.0 mm, in which holes 0.3–0.8 mm in diameter, are drilled with a pitch of ~1.0–1.5 mm. A dielectric rim 50–100 μm wide is produced around each hole by chemical etching of the metal coating. The TGEM quality is basically dependent on the coincidence of axes of these rims and the drilled holes. This is the main technological difficulty in TGEM production. A technology has been developed by the Petersburg Nuclear Physics Institute, which helps to attain the rim eccentricity of a few micrometers or better. In the course of TGEM production, different variants were tested, in which the following parameters were varied: the thickness of the dielectric (from 0.3 to 1.0 mm), the copper layer (from 5 to 35 μm), the hole diameter (from 0.3 to 0.9 μm), the separation between holes (from 0.8 to 2.0 mm), and the rim width (from 50 to 100 μm). By selecting the operating voltage, it was possible to attain an acceptable GAF (~1.5 × 10 4) and the energy resolution σ ~ 15–20%. As a matter of fact, all manufactured samples could be used in detector production.
The geometry of the TGEM detector prototype is shown in Fig. 6, as well as a blown up TGEM fragment in which the rims of the bare dielectric can be discerned. As an example, Fig. 7 presents the dependence of the GAF on the potential difference applied to the TGEM. The TGEM parameters are as follows: thickness, 1 mm; hole diameter, 0.6 mm; hole pitch, 1 mm; and the rim width, 70 μm. The gas mixture is 90% Ar + 10% CO2. The attainable GAF value such that discharges are not observed is ~1.5 × 10 4. Based on these investigations, the following TGEM variant has been selected: 0.5-mm-thick glasscloth laminate (FR4) with a copper foil coating 18 μm thick, a hole diameter of 0.6 mm, a hole pitch of 1 mm, and a rim diameter of 70 μm. 4. TEST RESULTS FOR THE GEM AND TGEM PROTOTYPES WITH MULTISTAGE GAS AMPLIFICATION 4.1. A Prototype of the Triple GEM Detector We tested a three-stage detector prototype assembled of GEM films 5 × 5 cm, which had been produced at CERN. The configuration of the prototype is schematically shown in Fig. 8.
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The field strength in each gap was kept constant at a level of 2 kV/cm. The energy resolution estimated by the amplitude spectrum of 55Fe was σ ~ 13%.
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Figure 9 presents the GAF dependence on the total potential difference at all GEMs (ΔVG1 + G2 + G3) in the gas mixture of Ar/CO2/iC4H10 (90/8/2). INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
Operation of the prototype in the gas mixtures of He/CF4/iC4H10 (73/25/2), He/ CF4/iC4H10 (90/8/2), and He/CO2/iC4H10 (90/8/2) was also tested. In all these cases, the prototype has demonstrated reliable performance and predictable behavior close to the above results. Figures 10a and 10b show the examples of the 55Fe signals read out of the anode with a constant field value (2 kV/cm) in the GEM–anode gap for the mixtures of Ar/CO2/iC4H10 (90/8/2) and He/CO2/iC4H10 (90/8/2), respectively. The signal width is determined by the electron collection time in a 1-mm gap between GEM1 and the anode, plus the contributions of the electronics. Ions produced at the last stage of amplification (GEM1), are collected at the upper GEM1 plate and do not escape in the anode–GEM1 gap. The time of electron collection at the anode, obtained by subtracting the contribution of electronics for the mixture of He/CO2/iC4H10 (90/8/2), is 26 ns, which is in good agreement with the calculated value of ~23 ns (Fig. 11a). For the mixture of He/CO2/iC4H10 (90/8/2), the electron collection time yields ~56 ns, which is approximately a factor of 1.5 longer than the calculated value of ~34 ns (Fig. 11b). The accuracy of the obtained values of the electron collection time is estimated to be ~30–40%. This value is such that it meets the requirements of a good agreement with the calculation in the case of the argon mixture and explains the observed difference in the case of the helium mixture. It is important that the collection time of the ion component at the final gas amplification stage (GEM1) be estimated, since the insufficiently fast ion outflow will result in accumulating the spatial charge and, as a result, in lowering of the detection efficiency at high counting rates. With this aim in mind, the anode and the lower GEM1 plate were grounded, and the signal was read out from the upper GEM1 plate via the blocking capacitor. The GEM1 thickness is 50 μm; i.e., electrons produced during amplification are instantly collected at the lower GEM1 plate, and the signal formation is mainly determined by the ion collection at the upper plate. Figures 12a and 12b present examples of the 55Fe -induced signals from the upper GEM1 plate at a constant voltage at the upper plate for the mixtures of Ar/CO2/iC4H10 (90/8/2) and He/CO2/iC4H10 (90/8/2), respectively. The times of ion collection from a 50-μm gap, obtained after subtracting the contribution of the electronics, are ~164 ns for the Ar/CO2/iC4H10 (90/8/2) mixture and ~30 ns for He/CO2/iC4H10 (90/8/2); i.e., ions are collected in the argon mixture slower by a factor of ~5.5 relative to the helium mixture. The results of testing of the GEM detector prototype with the three-stage gas amplification, performed Vol. 58
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Fig. 10. Example of the 55Fe signals read out of the GEM detector anode for the mixtures (a) Ar/CO2/iC4H10 (90/8/2); the vertical and horizontal scale divisions are 10 mV and 50 ns, respectively; and (b) He/CO2/iC4H10 (90/8/2), 20 mV, and 50 ns.
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Fig. 12. Example of the signals from 55Fe measured at the upper GEM1 plate (the lower plate being grounded) for the mixtures (a) Ar/CO2/iC4H10 (90/8/2); the vertical and horizontal scale divisions are 200 mV and 50 ns, respectively; and (b) He/CO2/iC4H10 (90/8/2), 20 mV, 50 ns. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
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Fig. 13. Diagram of the double TGEM detector prototype.
with a 55Fe radioactive source, have demonstrated that this detector type is the best suitable one for the detector base in the central regions of the first three tracking stations of the muon system in the CBM experiment. 4.2. A Prototype of the Double TGEM Detector One more implemented variant of the detector with the two-stage gas amplification–a double TGEM (a DTG)–is schematically shown in Fig. 13. The parameters of each TGEM are as follows: 0.5-mm-
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thick glass-cloth laminate (FR4); hole diameter, 0.6 mm; hole pitch, 1 mm; and rim width, 70 μm. The holes were simultaneously drilled in three stacked plates. Figure 14 shows the GAF values obtained for the DTG detector in the gas mixture of Ar/CO2/iC4H10 (90/8/2) with the 55Fe source versus the voltages applied in different gaps. The GAF value of ~30 × 103 in the mixture of Ar/CO2/iC4H10 (90/8/2) for the DTG detector (see Fig. 14d) is attained without apparent problems. The energy resolution is σ ~ 13%. The dependence of the GAF on a simultaneous change of voltages applied to TGEM1 and TGEM2 in the gas mixture of He/CF4/iC4H10 (75/23/2) is presented in Fig. 15. We also investigated mixtures based on He in a mixture with CO2, with other values of the CF4 percentage, and without isobutane added. In all cases, the DTG detector has demonstrated stable performance at GAF values of ~(5–7) × 10 4. Examples of the anode signals from 55Fe at a fixed field strength of 2 kV/cm in the TGEM1–anode gap are presented in Figs. 16a and 16b for the mixtures Ar/CO2/iC4H10 (90/8/2) and He/CO2/iC4H10 (90/8/2), respectively. As was the case of the GEM 8.0
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detector prototype (see paragraph 4.1), the signal width is determined by the collection time of electrons in the 1-mm gap between the TGEM1 and anode plus the contribution of the electronics. Since the gaps and
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the field strengths in the gaps for the GEM and TGEM detectors are equal, the electron collection times are expected to be equal. For the mixture of Ar/CO2/iC4H10 (90/8/2), the collection time of electrons at the anode (~26 ns), obtained by subtracting the contribution of the electronics, agreed with the time measured for the GEM prototype. For the mixture of He/CO2/iC4H10 (90/8/2), the electron collection time is estimated to be ~44 ns (for the GEM detector, this value is 56 ns). We note that the electron collection times estimated for two detector variants provide self-consistent results. The other conditions being equal, collection of electrons in the Ar/CO2/iC4H10 (90/8/2) mixture proceeds a factor of ~1.5 faster than in the He/CO2/iC4H10 (90/8/2) mixture. As was the case with the GEM detector, the collection time of the ion component inside TGEM1 was estimated (the final stage of amplification). The anode and the lower plate of TGEM1 were grounded, and the signal was read out from the upper TGEM1 plate. The TGEM1 thickness is 500 μm; i.e., in comparison with a 50-μm-thick GEM, the ion collection time is expected to be longer by an order of magnitude. The examples of the 55Fe signals from the upper plate of
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TGEM1 at a constant applied voltage are presented in Figs. 17a and 17b for the Ar/CO2/iC4H10 (90/8/2) and He/CO2/iC4H10 (90/8/2) mixtures, respectively. The separation of the electron and ion components (the “tails” toward longer times) are apparent. The obtained ion collection times (with the contribution of the electronics included) inside the TGEM are ~2.9 μs for the mixture of Ar/CO2/iC4H10 (90/8/2) and ~500 ns for the mixture of He/CO2/iC4H10 (90/8/2); i.e., in the argon mixture, ions are collected slower by a factor of ~6 than in the helium mixture. The estimated ion collection times for GEM and TGEM in gas mixtures based on argon and helium provide self-consistent results within the limits of our accuracy. The variant of the double TGEM detector is a promising candidate for use as a basic one for the peripheral region of the muon system in the CBM experiment. 5. CONCLUSIONS The coordinate gas-filled detector prototypes based on the GEM and TGEM technologies, as well as their combinations in the multistage gas amplification scheme have been tested in the laboratory in order to select variants of tracking detectors of the muon system for the CBM experiment. These tests have made it possible for us to gain experience and become familiar with the technology of using them as the basis in developing modern micro pattern detectors. At this phase of our study, we have drawn a conclusion that, basically, all detectors under investigation and their combinations could find application in the design of the muon system for the CBM experiment. Nevertheless, for the final selection, testing must be performed
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on the accelerator beam, where the conditions close to the requirements of the CBM experiment can be created. ACKNOWLEDGMENTS This work was supported by the Ministry of Education and Science of the Russian Federation (grant no. 14.A12.31.0002 of June 24, 2013) in accordance with the Russian Federation Government Regulation no. 220. REFERENCES 1. Sauli, F., Nucl. Instrum. Methods Phys. Res., A, 1997, vol. 386, p. 531. 2. Bachmann, S., Bressan, A., Capeans, M., Deutel, M., Kappler, S., Ketzer, B., Polouektov, A., Ropelewski, I., Sauli, F., Schulte, E., Shekhtman, l., and Sokolov, A, Nucl. Instrum. Methods Phys. Res., A, 2002, vol. 479, p. 294. 3. Breskin, A., Alon, R., Cortesi, M., Chechik, R., Miyamoto, J., Dangendorf, V., Maia, J.M., and Dos Santos, J.M.F., Nucl. Instrum. Methods Phys. Res., A, 2009, vol. 598, p. 107. 4. Chechik, R., Breskin, A., Shalem, C., and Mörmann, D., Nucl. Instrum. Methods Phys. Res., A, 2004, vol. 535, p. 303. 5. Altunbas, C., Capéans, M., Dehmelt, K., Ehlers, J., Friedrich, J., Konorov, I., Gandi, A., Kappler, S., Ketzer, B., De Oliveira, R., Paul, S., Placci, A., Ropelewski, L., Sauli, F., Simon, F., and van Stenis, M., Nucl. Instrum. Methods Phys. Res., A, 2002, vol. 490, p. 177.
Translated by N. Goryacheva
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