Discharge reduction technologies for Micromegas detectors in high hadron flux environments

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1 Journal of Instrumentation OPEN ACCESS Discharge reduction technologies for Micromegas detectors in high hadron flux environments To cite this article: M Vandenbroucke View the article online for updates and enhancements. Related content - New pixelized Micromegas detector with low discharge rate for the COMPASS experiment D Neyret, P Abbon, M Anfreville et al. - Performance of large pixelised Micromegas detectors in the COMPASS environment F Thibaud, P Abbon, V Andrieux et al. - Micromegas study for the slhc environment T Alexopoulos, D Attié, M Boyer et al. Recent citations - MICRO-PATTERN GASEOUS DETECTOR TECHNOLOGIES AND RD51 COLLABORATION MAXIM TITOV and LESZEK ROPELEWSKI This content was downloaded from IP address on 31/10/2018 at 20:14

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: November 14, 2011 ACCEPTED: April 19, 2012 PUBLISHED: May 28, nd INTERNATIONAL CONFERENCE ON MICRO PATTERN GASEOUS DETECTORS, 29 AUGUST 1 SEPTEMBER 2011, KOBE, JAPAN Discharge reduction technologies for Micromegas detectors in high hadron flux environments M. Vandenbroucke 1,2 a CEA Saclay/DSM/IRFU/SPhN, Saclay, France Maxence.Vandenbroucke@cea.fr ABSTRACT: R&D program on Micromegas detectors for high flux hadron environment is ongoing at CEA Saclay, lead by the COMPASS and CLAS12 groups. The goal is to reduce the discharge probability in presence of intense highly ionizing beam. In order to achieve full efficiency at the high flux of hadrons/s foreseen at the COMPASSII experiment, a reduction of the spark rate by a factor 10 to 100 compared to the present COMPASS Micromegas detectors is needed. Two approaches have been followed. The first one is based on a pre-amplification stage with a GEM foil and the second one using resistive layers. To study and evaluate these solutions, two prototypes featuring an additional GEM foil were extensively studied, while six resistive detectors, including two of them featuring buried resistance scheme proposed by R. de Olivera et al., were built and tested. These detectors have been characterized at CERN during two periods of beam tests: one at the Proton-Synchrotron (PS) using of 0.3 to 3 GeV/c pion beam, and the other one during the RD51 test beam period at the Super-Proton-Synchrotron (SPS) using 200 GeV/c muons. The GEM pre-amplification solution matches the COMPASSII requirements and simulations successfully reproduce the results. The good performances of the resistive design open another promising alternative. KEYWORDS: Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MI- CROPIC, MICROMEGAS, InGrid, etc); Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc) 1 Corresponding author 2 On behalf of the COMPASS and CLAS12 CEA Saclay groups c 2012 IOP Publishing Ltd and Sissa Medialab srl doi: / /7/05/c05014

3 Contents 1 Introduction Motivation and technologies Discharge reduction technologies Principle of discharge rate reduction with a GEM pre-amplification Discharge reduction with embedded resistive structures 2 2 Experimental tests and results Detectors and setup Discharge rate measurement with 0.2 to 3 GeV/c hadron beam Performances measurement with 170 GeV/c muon beam 4 3 Simulation of the Micromegas + GEM detector Model and simulation principle Simulation results 5 4 Conclusion 7 1 Introduction A R&D has been triggered at CEA Saclay by the COMPASS and the CLAS12 groups to reduce the discharge rate and impact in Micromegas detectors. The motivation of this R&D and the discharges reduction technologies are introduced in Sec Results of two test beam campaigns at CERN will reported in section 2 to expose the performances of the different technologies. Simulations of the Micromegas + GEM solutions are presented in section Motivation and technologies COMPASS [1] is the first experiment that used the Micromegas detectors in a large scale experiment. They have shown great performances [2] with 90 µm spatial resolution, 10 ns time resolution and an efficiency from 98% to 96% near the beam area where the electronic occupancy is high. These detectors cannot sustain the rate in the beam area, and are blinded by a 5 cm disc filling the amplification gap in the center. COMPASS is situated at the CERN SPS which delivers a muon beam of µ/s or a hadron beam of 10 7 h/s. With these particle rates the discharge probability is 0.02 discharge/s in the muon beam and 0.1 discharge/s with hadrons. The next generation of Micromegas detectors, for the COMPASSII [3] experiment, will have to be instrumented in the beam area and be able to stand a 5 times higher hadron flux. With the actual Micromegas detectors, this would lead to several discharges per spill, causing gain stability and aging problem. This discharge probability has to be decrease by a factor 10 to 100, this paper reports on this effort. Other foreseen upgrades are detailed in [4]. 1

4 Figure 1. Left: Schematic side view of a GEM + Micromegas detector; the GEM foil (red/green dashed line) is placed over the micro-mesh (thin black line). Blue triangles represent electrons. Right: the 3 resistive designs tested here: (A): A carbon loaded kapton layer (black) is disposed on an insulator layer (green) that covers the copper strips (red). (B) a resistive paste (black) is laid out on the copper strips (red). (C) a resistive pad (gray) is connected to the copper strip (red) via a buried resistor (black). The gray and dotted line represents the micro-mesh embedded in the vertical coverlay pillars of the bulk technology. (not to scale) 1.2 Discharge reduction technologies Two types of discharge rate reduction solutions have been investigated, GEM pre-amplification and resistive structures Principle of discharge rate reduction with a GEM pre-amplification A single GEM foil is placed at 1 or 2 mm over the micro-mesh and is used at a gain below 20. The GEM pre-amplification lowers the discharge probability by receding from the Raether limit via two effects. First the gain is shared between the Micromegas and the GEM. This allows operating the Micromegas at low voltage, lowering the number of charges created in the amplification gap. The second effect is based on the diffusion of the primary electron cloud at the exit of a GEM hole. Moreover charges diffuse in between the GEM and the Micromegas, in the transfer gap, heavily lowering the electron density at the readout electrode Discharge reduction with embedded resistive structures A protective resistive structure is laid on the readout electrodes to prevent the development of a high amplitude discharge between the micro-mesh and readout strips. Several designs have been considered whereas the discharge quenching mechanism principle remains the same. Resistive technologies use the charging property of a resistive material. When a discharge is igniting, the potential on the resistive surface increases toward the mesh potential, canceling the electric field and quenching the discharge. Physics signals are read by capacitive effect in hundreds of nanoseconds before charges flow out of the layer in microseconds, depending on the resistive architecture chosen. Figure 1 describes the 3 different designs used in these tests. The challenge of resistive technologies is to obtain a robust design that allows a fast evacuation of the charges in order to avoid local charge space. Such a local charge density would momentarily lower the gain, impacting the efficiency, time, space and energy resolutions of such a detector. 2

5 2 2.1 Experimental tests and results Detectors and setup 17 detectors have been build for these tests on a standard basis consisting of a 0.8 mm thick Printed Circuit Board (PCB) with an active area made of 144 copper strips of 10 cm by 270 µm at a pitch of 400 µm, see figure 2. The woven stainless steel micro-mesh of 18 µm thick wires, spaced by 45 µm, was integrated at 128 µm over the readout PCB using the bulk process [5]. The detectors were operated with Ar+5%iC4 H10. The test bench consists of 12 detectors read by six front-end electronics cards based on the AFTER chip [6] triggered by the coincidence of three cm2 plastic scintillators placed at both ends of the bench, see figure 2. Two Micromegas plus GEM detectors and 6 resistive detectors constituted the main prototype group to be tested. The two first and last detectors have been used as fixed tracking detectors. This method provides a fixed track error only depending on the assumed resolution of a standard detector (well known [7]) and the relative position of the detector to be characterized. A more complex track reconstruction algorithm, using all detectors to fit a straight line, has been used to crosscheck these results with success. 2.2 Discharge rate measurement with 0.2 to 3 GeV/c hadron beam Low momentum hadrons of 0.2 to 3 GeV/c easily interact with matter, they create enough primary charges to measure precisely the discharge probability. The rate of h/s allows accessing probabilities down to Discharges are counted using a voltage discriminator on the micromesh signal. Detailed analysis of the discharge rate can be found in [8]. On the Micromegas + GEM detectors the discharge reduction factor was measured to be 10 to 500 depending on the field configuration. At high GEM gain, the detector with a 2 mm transfer gap has shown a better discharge reduction up to an order of magnitude less than the 1 mm one. This is explained by the larger diffusion that happens in the transfer gap. No discharges were recorded for resistive detectors. However, the discharge acquisition system was calibrated to recorded standard Micromegas discharges of hundred of volts and not the resistive 3 Figure 2. Left: the different parts of a standard prototype with a GEM foil. A: the PCB basis mounted on an aluminum frame. B: the 25 µm aluminized Mylar drift electrode. C: gas vessel aluminum frame and Mylar window. D: cm2 GEM foil. Right: the test bench with the 12 detectors on the PS beam line.

6 Efficiency MM+GEM_2mm_ 0.3GeV/C MM+GEM_1mm_ 0.3GeV/C MM+GEM_2mm_ 0.5GeV/C MM+GEM_1mm_ 0.5GeV/C MM+GEM_2mm_ 1GeV/C MM+GEM_1mm_ 1GeV/C MM+GEM_2mm_ 2GeV/C MM+GEM_1mm_ 2GeV/C MM+GEM_2mm_ 3GeV/C MM+GEM_1mm_ 3GeV/C MM+GEM_2mm_0.3GeV/C MM+GEM_1mm_0.3GeV/C MM+GEM_2mm_0.5GeV/C MM+GEM_1mm_0.5GeV/C MM+GEM_2mm_1GeV/C MM+GEM_1mm_1GeV/C MM+GEM_2mm_2GeV/C MM+GEM_1mm_2GeV/C MM+GEM_2mm_3GeV/C MM+GEM_1mm_3GeV/C Figure 3. Efficiency versus gain for various momentum for Micromegas + GEM with a 0.2 to 3 GeV/c hadron beam. Blue lines represent positive hadrons and red is for negative. detector discharges of amplitude in the range of 1 V [9]. We consider the impact of these small discharges is negligible. This statement is supported by the observation of efficiency loss in respect of hadron momentum. Figure 3 shows that Micromegas + GEM detectors efficiency decrease at high-gain faster with low energy particles. The higher energy loss of these particles increases the spark probability from 1 to for a gain of [8]. The dead-time resulting from discharges causes the loss of efficiency. Since this is not observed with resistive detectors, the low amplitude discharges must not impact detector performances. 2.3 Performances measurement with 170 GeV/c muon beam The test bench was placed on the H4 line of the SPS accelerator at CERN during the RD51 collaboration [10] dedicated beam time. The 170 GeV/c muon beam was contaminated with 40% of hadrons. The measured performances have not been impacted thanks to a quality cut on the reconstructed tracks: one and only one cluster is requested in the 4 tracking detectors per event. Efficiencies are corrected from the background, and resolutions are corrected from track error. Micromegas + GEM detectors have shown efficiency up to 98%, as seen in figure 4, and a spatial resolution better than 70 µm, see figure 5. The detector with a 2 mm gap between the GEM foil and the Micromegas shows, on average, a 10 µm better spatial resolution than the one with a 1 mm gap thanks to larger diffusion. Resistive detectors have shown efficiency under 20%, except the one with buried resistors. The low efficiency of the other detectors has been identified as a production issue. These detectors have also shown 4 times lower gain than standard detectors. The 99% efficiency of the BR2 detector is better than that of the 2 Micromegas + GEM detectors. Whereas its 90 µm spatial resolution and its difference with the other detectors are not fully understood and require further investigation. Several different weighted methods for cluster position reconstruction have been tried (logarithm, exponential, η algorithm) to lower this resolution instead of the usual weighted mean method without success. 4

7 The test setup was not designed to record the phase between the trigger system and the AF- TER front-end clock, therefore the time resolution could not be measured under 25 ns that all the working detectors reached. 3 Simulation of the Micromegas + GEM detector To have a complete understanding of the GEM pre-amplification solution, a simulation of the gaseous and field effects has been performed using GARFIELD++, that solves the Boltzmann transport equation, with the MAGBOLTZ [11] package for gaseous processes. One of the main interest of this simulation is the understanding of the effect of the electric field between the GEM foil and the micro-mesh, the transfer field E T R. This defines the sensitivity of the detector to the precision of the mechanical holding of the GEM foil. Results on gain and ion feedback are presented in section Model and simulation principle The simulation is based on the MAGBOLTZ2 package, which contains an accurate description of the numerous excitation and ionization modes in common gases and gas mixtures. The geometry of the detector is defined via the electromagnetic field map of the elementary pattern of the GEM hole and the Micromegas using a finite element solver, see figure 6. These simulations take extensive computing time scaling with the number of electrons created. That is why Ar+10%CO 2, low gain and 1 mm transfer gap has to be used. Even though this gas has different properties compare to Ar+5%iC 4 H 10, the general behavior of amplification in a plane detector is supposed to be dominated by the electric field ratio. Absolute amplification numbers are not comparable. 3.2 Simulation results Simulations of the gain of the Micromegas + GEM detector in respect to E TR, see figure 7, well reproduce the shape of lab gain measurement made with 55 Fe. The simulation allows us to access the different components of the amplification process. For a given HV settings on the GEM and Micromegas, simulations show the gain resulting of the convolution of 3 effects: the electron extraction at the exit of the GEM holes, the electron transparency of the micro-mesh and electron captures in the transfer field. When E TR < 100 V/mm the gain is dominated by the GEM extraction. Then when 100 V/mm < E TR < 900 V/mm the effects compensate each others, providing a range of field values where the gain is independent of E TR. Over 900 V/mm, ionization in the transfer gap start and the gain grows exponentially with E TR. The number of ions drifting back in the drift volume, over the GEM foil, per incoming electron, i.e. ion feedback, is a key quantity for gaseous detectors. An average of 12 ions per incoming electron, at a gain of 40 is observed in this simulation for a moderate transfer field (100 V/mm < E T R < 800 V/mm). This important ion feedback makes the use of such a detector unsuitable with large drift volumes and high particle flux. For a plane detector, this value is compatible with the COMPASS II foreseen rate. 5

8 Efficiency MM+GEM_1mm_260V MM+GEM_2mm_260V MM+GEM_1mm_280V MM+GEM_2mm_280V MM+GEM_1mm_300V MM+GEM_2mm_300V MM+GEM_1mm_320V MM+GEM_2mm_320V Efficiency Buried resistor 1 Buried Resistor 2 Resist Kapton Figure 4. Efficiency plateau for Micromegas + GEM detectors (top) and resistive Micromegas (bottom) in 170 GeV/c muon beam. The legend indicates the applied voltage on the GEM foil. Buried resistor 1 and 2 use the C structure of figure1 Resist Kapton is type A. Res. (mm) Resolution (mm) MM+GEM_1mm_260V MM+GEM_2mm_260V MM+GEM_1mm_280V MM+GEM_2mm_280V MM+GEM_1mm_300V MM+GEM_2mm_300V MM+GEM_1mm_320V MM+GEM_2mm_320V Buried Resistor Buried Resistor Figure 5. Resolution versus gain for Micromegas + GEM (top) and resistive (bottom) in 170 GeV/c muon beam. Buried resistor 1 and 2 use the C structure of figure1. 6

9 Figure 6. Left: Model used for computing the electric field. Elementary pattern of the GEM (at the back) and the micro-mesh (front). Right: Event display of the simulation of 2 electrons dropped over the GEM foil. Orange lines represent the path of electron, black lines ions and the colored dots are the different ionization/excitation in the gas Etr(V/mm) Figure 7. Left: Simulated gain of the Micromegas + GEM detector and the contribution of the GEM stage, transfer gap and Micromegas. On the right, gain (blue curve) and energy resolution (red curve) measurement on the Micromegas + GEM detector used in the test beam. The stability of the gain with E T R between 100 V/mm and 800 V/mm is reproduced by simulation but not the absolute values since simulations are constrain to low gain. 4 Conclusion E E The next generation of Micromegas detectors for the COMPASS II experiment requires new technologies in order to reduce the discharge probability by a factor of 10 to 100 compared to the present detectors. Two different paths have been investigated to achieve this goal: a pre-amplifying GEM foil and an integrated resistive structure on the readout electrodes. 17 prototypes of 6 10 cm 2 have been tested on two test beam periods at CERN, one with 0.3 to 3 GeV/c hadrons for discharge measurement, and the other using 170 GeV/c muons for performance measurements. The Micromegas+GEM detectors have shown a discharge rate reduction of 10 to 500, reaching a spark probability of 10 7 at a gain of 10 4, 70 µm spatial resolution and 98% efficiency. The behavior of the amplification mechanism and the ion feedback have been successfully simulated. This solution matches the COMPASS II requirement and full-size tests [12] have confirmed the 7

10 viability in the COMPASS environment. The prototype with the buried resistor scheme has shown good performances with 99% efficiency, 100 µm spatial resolution and no discharge. This technology offers the advantages of being monolithic and is in principle compatible with industrial PCB production processes. The performances have to be proven to be reproducible with a full scale detector in the COMPASS environment. It provides a very elegant technical solution with the advantages of a single amplification stage detector. However it does not have the maturity of the GEM pre-amplification technology and both solutions are considered for COMPASSII. Acknowledgments This study was conducted within the framework of the RD51 collaboration of and was supported in part by the Excellence Cluster P2I. We would like to thank the CERN Detector Lab and the TE-MPE-EM lab of Rui De Olivera, in particular Olivier Pizzirusso. We also thank Rob Veenhof for his precious help with MAGBOLTZ simulations. References [1] P. Abbon et al., The compass experiment at cern, Nucl. Instrum. Meth. A 577 (2007) 455. [2] F. Kunne et al., Micromegas: Large-size high-rate trackers in the high energy experiment compass, IEEE Nucl. Sci. Symp. Conf. Rec. 6 (2006) [3] F. Gautheron, Compass-ii proposal, Tech. Rep. CERN-SPSC SPSC-P-340, CERN, Geneva, May, [4] D. Neyret et al., New pixelized micromegas detector for the compass experiment, 2009 JINST 4 P [5] I. Giomataris et al., Micromegas in a bulk, Nucl. Instrum. Meth. A 560 (2006) 405. [6] P. Baron et al., After, an asic for the readout of the large t2k time projection chambers, IEEE Trans. Nucl. Sci. 55 (2008) [7] S. Procureur et al., Discharge studies in micromegas detectors in a 150 gev/c pion beam, Nucl. Instrum. Meth. A 659 (2011) 91. [8] G. Charles et al., Discharge studies in micromegas detectors in low energy hadron beams, Nucl. Instrum. Meth. A 648 (2011) 174. [9] T. Alexopoulos et al., A spark-resistant bulk-micromegas chamber for high-rate applications, Nucl. Instrum. Meth. A 640 (2011) 110. [10] Rd51 proposal, final version. [11] S. Biagi, Magboltz 2, [12] D. Neyret and et al., New pixelized micromegas detector with low discharge rate for the compass experiment, in proceedings of 2 nd International Conference on Micro Pattern Gaseous Detectors, 29 August 1 September 2011, Kobe, Japan. 8