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