View
216
Download
0
Category
Tags:
Preview:
Citation preview
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
Spacing between two spacer ~ 2 mmSpacer Diameter ~ 400m
Mesh Hole ~ 45m
Pitch ~ 63m
Gas Mixing System
Gas Chamber & Detector
Amplifier(ORTEC 672)
OscilloscopeMulti-
Channel Analyzer
Power Supply(High Voltage)
(N471A)
Pre-Amp(Model No. –
142IH)
Residual Gas
Analyzer
Filter
Gas Flow In
Gas Cylinder
Purification System
Pressure Gauge
Gas Flow Out
Experimental Set Up
Typical MCA Spectrum of 55Fe
RGA Spectrum for fixed Argon – Isobutane Gas Mixture
Pad Response
Readout Pads
Amplification GapAmplification and further Diffusion
Radiation Source
Ionization
Drift and Diffusion of Electrons
Drift Volume
Transfer Gap
Signal
Numerical SimulationSimulation tools
Garfield framework: to model and simulate two and three dimensional drift chambers
Ionization: HEED
Drift and Diffusion: MAGBOLTZ
Amplification: MAGBOLTZ
Potential, Field: neBEM (nearly exact Boundary Element Method)
Garfield + neBEM + Magboltz + Heed
With Amplification Gap
In each case detector characteristics (gain, resolution…) changes
accordingly
Variation of Electric Field
With Mesh Hole Pitch (Wire Diameter: 18 m)
(b)
(a)
(Please note, Y-Axis is in log scale)
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
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
(Maximum allowable voltage: Sparking limit)
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
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
(Maximum allowable voltage: Sparking limit)
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
Every electron collision is connected with red lines, inelastic collisions excitations ionizations.
Fraction of electrons arriving in amplification region
Depends on field ratio, drift voltage
Depends on hole-pitch
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
Avalanche of Electrons (2D picture) Drift of Secondary Ions (2D picture)
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
Simulation of IBF:
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
Variation with IBF with Field Ratio
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
Drift lines and
Avalanche
Spacers cause significant perturbation resulting in increased field values, particularly in the regions where cylinders touch the mesh
Electron drift lines get distorted near the dielectric 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
Electron Transparency and Gain (Without and With Spacer)
Signal
Electrons are lost on the spacer resulting in reduced gain
Signal strength reduces and it has a longer tail
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
Recommended