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Higher Gain Can Be Obtained In Argon Isobutane Gas

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    August 2018
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Performance Studies of BULK Micromegas with Different Amplification Gaps Purba Bhattacharya1, Sudeb Bhattacharya1, Nayana Majumdar1, Supratik Mukhopadhyay1, Sandip Sarkar1, Paul Colas2, David Attie2 1 Applied Nuclear Physics Division, Saha Institute of Nuclear Physics, Kolkata, India 2 DSM/IRFU, CEA/Saclay, Gif-sur-Yvette CEDEX, France Motivation  Micromegas – promising candidate for TPCs including ILD main tracker  Bulk Micromegas – built using printed circuit board fabrication techniques     Important parameters that determine choice of a particular bulk over another are detector gain, gain uniformity, energy and space point resolution, comfortable operating regime (in terms of voltage settings, signal strength etc), stability and ageing characteristics (ion back-flow), capability to efficiently pave large readout surfaces with minimized dead zone (due to spacers) … These parameters are known to depend on geometry of the detector (amplification gap, mesh hole pitch, wire radius etc), electrostatic configuration within the detector, gas composition, pressure … Systematic comparison of different bulk Micromegas has been carried out to weigh out various possibilities and options and guide our choice for specific applications Comparison with numerical simulations using Garfield has been performed to verify the mathematical models and confirm our understanding of the device physics BULK MICROMEGAS Details of BULK Micromegas:  10x10 cm2 active area  Amplification gap: 64 m, 128 m, 192 m and 220 m  Stainless steel mesh, wire diameter 18 m, pitch 63 m/ 78 m  Dielectric Spacer, diameter 400 m, pitch 2 mm Microscopic view of Bulk Micromegas Mesh Hole ~ 45m Pitch ~ 63m Spacer Diameter ~ 400m Spacing between two spacer ~ 2 mm Experimental Set Up Gas Cylinder Purification System Residual Gas Analyzer RGA Spectrum for fixed Argon – Isobutane Gas Mixture Pressure Gauge Gas Flow Out Gas Mixing System Gas Flow In Gas Chamber & Detector Pre-Amp (Model No. – 142IH) Oscilloscope Typical MCA Spectrum of 55Fe Amplifier (ORTEC 672) Power Supply (High Voltage) (N471A) Filter MultiChannel Analyzer Numerical Simulation Simulation tools Radiation Source Garfield framework: to model and Ionization Drift Volume simulate two and three dimensional drift chambers Drift and Diffusion of Electrons Ionization: HEED Drift and Diffusion: MAGBOLTZ Amplification: MAGBOLTZ Amplification Gap Transfer Gap Amplification and further Diffusion Potential, Field: neBEM (nearly exact Boundary Element Method) Readout Pads Pad Response Signal Garfield + neBEM + Magboltz + Heed Variation of Electric Field (a) With Mesh Hole Pitch (Wire Diameter: 18 m) (b) With Amplification Gap (Please note, Y-Axis is in log scale) In each case detector characteristics (gain, resolution…) changes accordingly Gain : G = Nt / Np = kP/ Np , where Nt Total number of electrons Np  Primary electrons k  Constant, depends on Preamplifier, Amplifier, MCA specification P  Peak Position (Maximum allowable voltage: Sparking limit) Variation of gain with amplification field in different argon-based gas mixture (drift field 200 V/cm) Higher gain can be obtained in Argon Isobutane Gas Mixture Variation of gain with amplification field for three different amplification gap – higher gain can be obtained with larger amplification gap leading to a comfortable operating regime (Maximum allowable voltage: Sparking limit) Variation of gain with amplification field for two different pitch – for larger pitch, sparks start at higher field and so a higher value of gain can be obtained Comparison with Simulation Results Trend similar in case of both detectors→ Simulated results considerably lower without Penning Roles of different parameters : Penning Transfer Mechanism → Increase of gain, Needs further investigation on transfer rates The simulated gain in other gas and other gap also agrees quite well with experimental data Energy Resolution : R = P/P, where p  r.m.s. of the pulse height distribution P  peak position Variation of energy resolution at 5.9 keV with gain in different argon-based gas mixture At this drift field, at higher gain, Argon Isobutane gas shows better energy resolution Variation of energy resolution with gain for three different amplification gap – 128 m shows better resolution Variation of energy resolution with gain for two different pitch – 63 m shows better resolution Comparison with Simulation Numerical estimates follow trend of measured data Gain variation and electron transparency needs further investigation Similar trend observed in other cases also Estimation of Electron Transparency  Fraction of electrons arriving in amplification region  Depends on field ratio, drift voltage  Depends on hole-pitch Every electron collision is connected with red lines,  inelastic collisions  excitations  ionizations. Experiment : Ratio of signal amplitude at a given Edrift over signal amplitude at Edrift where gain is maximum Simulation:  Microscopic tracking of electrons from randomly distributed points (100 m above mesh)  Two different models for mesh modelling: one dimensional thin wire segments for Edrift < 100 V/cm and three-dimensional polygonal approximation of cylinders for Edrift > 100 V/cm Experiment: Variation with electron transparency with field ratio for three different amplification gap At this pitch value, the electron transparency reaches maximum value at much higher drift field The larger gap detector reaches maximum value at lower drift field in comparison with smaller gap Comparison of Experimental Data with Simulation Results (Amplification Gap: 64 m and 192 m; Pitch: 63 m) Simulation Results agree quite well with Experimental Data Calculation with higher pitch (78 m) is in progress Ion Backflow  Secondary ions from amplification region drift to drift region  Distortion of electric field; degrades stable operation of detector  Micromegas micromesh stops a large fraction of these ions  Backflow fraction : Nb/Nt  (1/FR)(p/t)2 where Nb  average number of backflowing ions Nt  average total number of ions FR  field ratio, p  pitch of the mesh, t  diffusion Avalanche of Electrons (2D picture) Drift of Secondary Ions (2D picture) Simulation of IBF: Variation with IBF with Field Ratio a) for different argon based gas mixture (Amplification gap: 128 m) b) for three different amplification gap (Ar:Isobutane 90:10)  Preliminary simulation results show expected trends  Need further investigation and experimental verification Experimental Set Up: Preliminary data was taken at CEA, Saclay We are trying to build up a similar set up at SINP Value of IBF follows the theoretical prediction Besides the contribution of ions from avalanche, additional contribution from ions between drift plane and test box window affect the data – implementation of 2nd drift mesh – improvement of results Effect of Spacer (Diameter 400 m, Pitch 2 mm, Amplification Gap 128 m) Electric field in axial direction through different holes Without Spacer With Spacer  Spacers cause significant perturbation resulting in Drift lines increased field values, and particularly in the regions where Avalanche cylinders touch the mesh  Electron drift lines get distorted near the dielectric spacer Electron Transparency and Gain (Without and With Spacer) Without Spacer With Spacer Position of track above mesh 25 m 50 m 100 m 25 m 50 m 100 m Electrons crossing mesh 97.794 97.304 97.549 97.549 95.343 95.833 Electrons reaching middle of amplification area 97.794 97.304 97.549 54.902 92.892 95.343 Gain 600 594 596 338 570 584  Electrons are lost on the spacer resulting in reduced gain  Signal strength reduces and it has a longer tail Signal Summary Experiments and numerical simulations carried out using different bulk Micromegas (amplification gaps 64 m, 128 m, 192 m, 220 m; Pitch 63 m, 78 m) in several argon based gas mixtures Important detector parameters such as gain, energy resolution, transparency estimated Observed conflicting advantages of different parameters, e.g., configuration that leads to higher gain and more stable operation (amplification gap 220 m) provides less attractive energy resolution Smaller pitch (63 m ) found to be generally more useful Preliminary calculation of ion back flow compare favorably with measurements Effects of spacers on gain and signal indicated significant changes occurring around the spacer Successful comparisons with simulation indicate that the device physics is quite well understood and suitably modeled mathematically Acknowledgement 1. We acknowledge CEFIPRA for partial financial support 2. We thank our collaborators from ILC-TPC collaboration for their help and suggestions 3. We acknowledge Rui de Oliveira and the CERN MPGD workshop for technical support 4. We happily acknowledge the help and suggestions of the members of the RD51 collaboration 5. We are thankful to Abhik Jash, Deb Sankar Bhattacharya, Wenxin Wang for their help in some measurement and Pradipta Kumar Das, Amal Ghoshal for their techinal help 6. We thank our respective Institutions for providing us with necessary facilities