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Development of high resolution Micro-Pattern GasDetectors with wide readout padsTo cite this article M Dixit 2010 JINST 5 P03008

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2010 JINST 5 P03008

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PUBLISHED March 22 2010

1st INTERNATIONAL CONFERENCE ON MICRO PATTERN GASEOUS DETECTORSJUNE 12ndash15 2009KOLYMPARI CRETE GREECE

Development of high resolution Micro-Pattern GasDetectors with wide readout pads

M Dixit

TRIUMF and Carleton UniversityOttawa Canada

E-mail msdphysicscarletonca

ABSTRACT A Micro Pattern Gas Detector (MPGD) requires 200 microm wide anode readout pads toachieve sim 40 microm resolution With the development of the new charge dispersion readout conceptfor MPGDs with a resistive anode comparable resolution can be achieved with order of magnitudewider pads We present here an overview and present status of the charge dispersion MPGD readouttechnology The development of MPGDs with a resistive anode may permit experiments withlarge area high resolution tracking requirements to consider using MPGDs which would have beenotherwise prohibitive due to excessive readout channel count

KEYWORDS Time projection Chambers (TPC) Detector modelling and simulations II (electricfields charge transport multiplication and induction pulse formation electron emission etc) Mi-cropattern gaseous detectors (MSGC GEM THGEM RETHGEM MICROMEGAS InGrid etc)Gaseous imaging and tracking detectors

ccopy 2010 IOP Publishing Ltd and SISSA doi1010881748-0221503P03008

2010 JINST 5 P03008

Contents

1 Introduction 1

2 Position sensing from charge dispersion in MPGDs Theory and simulation 3

3 Charge dispersion signal characteristics and data analysis 5

4 Recent developments 8

5 Conclusions and outlook 9

1 Introduction

The Micro Pattern Gas Detectors (MPGD) [1] such as GEMs [2] and Micromegas [3] are nowwidely used in experimental physics A MPGD tracker can achievesim 40 microm spatial resolution with200 microm pitch anodes although it has many more readout channels than the previous generation ofwire chamber based detectors The larger channel count has not been a issue since the areas coveredby MPGD trackers have been relatively modest until recently There are several new projects onthe horizon however where MPGDs could be used with advantage but have to cover significantlylarger areas than in the past

An example where the MPGD would make a big difference is to use it for the Time Pro-jection Chamber (TPC) [4] readout The use of MPGDs for the TPC readout would eliminate amajor source of systematic error from the ExB effect in r-φ measurement for the conventionalproportional-wirecathode-pad TPC [5 6] A large volume MPGD readout TPC is presently beingdeveloped for main charged particle tracking detector for ILD experiment [7] at International Lin-ear Collider (ILC) [8] The resolution requirements for the ILD-TPCsim 100 microm for all tracks up to2 meter drift are close to the fundamental limit from diffusion and have never been achieved beforeThe conventional MPGD readout technology in principle achieve the ILD-TPC resolution goal forthe GEM readout option with sim 1 mm wide pads but will require over 3 million readout channels

Micromegas are a candidate technology for muon tracking chambers for the Super LHC AT-LAS upgrade [9 10] The areas to be covered could be in excess of sim 500 m2 depending on RampDprogress To minimize detector cost and complexity it would be desirable to use wide readout padswithout sacrificing resolution

The first set of large TPCs to be constructed with MPGD readout have recently been commi-sioned for the near detector for T2K experiment [11] at JPARK The T2K TPCs use Micromegas tocover an area of sim 9 m2 Since the momentum resolution requirements are relatively modest large7 mm times 10 mm pads were chosen to keep the channel count down to sim 80000

The ExB systematics have so far limited the transverse resolution performance of conventionalproportional wire TPCs in a magnetic field However studies carried out to understand the perfor-mance of ALEPH TPC indicate that the resolution for tracks which crossed the wire at an angle

ndash 1 ndash

2010 JINST 5 P03008

Figure 1 The concept of charge dispersion illustrated for a double GEM detector The avalanche is initiallyconfined within 10 to 15 microm but the charge disperses quickly to cover the resistive anode surface andinduces signals on multiple readout pads making possible an accurate charge position determination fromcentre of gravity

which canceled the Lorentz angle could be quite good sim 100 microm even at 2 meter drift [6] But forthe ExB effect ALEPH TPC could have achieved excellent resolution from the centre of gravityof induced signals on 7 mm wide cathode readout pads This is not surprising since the techniquebasically depends on wire-pad readout geometry and electrostatics In contrast MPGDs have sofar relied largely on the use of narrow sub-millimeter anode strips or pads as a means to achievinggood resolution

A technique to read out the MPGD charge signal has been developed which can use order ofmagnitude wider pads with no loss of resolution but requires certain modifications to the anodereadout structure [12] The conventional MPGD anode plane is replaced by a composite struc-ture consisting of a high surface resistivity thin film laminated to the readout pad plane with anintermediate insulating spacer The structure forms a distributed 2-dimensional RC network andan avalanche charge arriving at the surface of the anode will disperse with the time constant of thesystem Figure 1 illustrates the charge dispersion readout concept for a double GEM detector

The dispersion of charge on the resistive anode surface induces signals on multiple readoutpads which can then be used to compute the position centroid The process is completely describedby the detector material properties and geometry and in contrast to diffusion which is statistical innature there is no loss of accuracy in determining the centroid of a wider distribution Excellentresolution can therefore be achieved by using wide pads comparable in width to those used for theproportional wire-cathode pad TPCs

The new readout concept has been extensively tested and proven to work both with GEMs andMicromegas For the Micromegas there is the added benefit of reduced sparking and the potentialfor stable high gain operation Figure 2 illustrates the implementation of charge dispersion readoutconcept for our early studies with Micromegas

We had previously reported an unprecedentedsim 50 microm resolution with 2 mmtimes 6 mm pads incosmic ray tests of a small prototype Micromegas TPC at 5 Tesla [13] This data has recently beenreanalyzed with improved analysis techniques and the resolution achieved is better than 40 microm

A theoretical model of charge dispersion has been developed [14] Simulations based on

ndash 2 ndash

2010 JINST 5 P03008

Figure 2 Charge dispersion MPGD readout technique is equally applicable to the GEM and the Mi-cromegas The figure shows the details of the resistive anode structure as implemented for our initial studieswith Micromegas

the model can explain in detail all features of experimental data The following sections givean overview charge dispersion phenomena in MPGDs with a resistive anode model simulationstechniques of data analysis and recent developments and the outlook

2 Position sensing from charge dispersion in MPGDs Theory and simulation

The resistive anode and the readout plane together can be looked upon to form a distributed 2-dimensional RC network in the finite element approximation We start with the charge divisionmethod [15] of measuring the position on a proportional wire described by one-dimensional Tele-graph equation For an avalanche charge arriving at t = 0 the space-time evolution of the chargedensity per unit length ρ is given by

partρ

part t=

1τ

part 2ρ

partx2 where τ = RC

Here R is resistance per unit length and C the capacitance per unit length for the wireIn analogy with the one dimensional Telegraph equation we write the 2-dimensional equation

for charge density on the resistive surface

partρ

part t=

1τ

[part 2ρ

part r2 +1r

partρ

part r

]where in this case τ = RC is the system time constant per unit area with R the anode surfaceresistivity and C the capacitance density per unit area for the readout structure

A localized charge cluster arrives at the resistive anode surface at t = 0 The solution for ρ fora resistive anode of finite size is an infinite Fourier series An approximation that works quite wellis to assume that the anode boundaries are at infinity in which case a closed form solution can bewritten down

ρ(r t) =τ

2texp(minusr2

τ4t)

ndash 3 ndash

2010 JINST 5 P03008

Figure 3 (a) Dependence of charge density function on time in a MPGD with a resistive anode (b) Thepads integrate the time dependent charge density and both the rise time and pulse height of signal depend onthe pad position with respect to the avalanche charge

The charge density function ρ varies with time and is capacitively sampled by the readout padsFigure 3 (a) shows the time evolution of the charge density for an initially localized charge cluster

The charge signal on a pad can be computed [14] by integrating the time dependent chargedensity function over the pad area The shape and the pulse height of the charge pulse on a paddepends on the pad geometry the location of the pad with respect to the initial charge and the RCtime constant of the system mdash figure 3 (b)

The charge dispersion MPGD TPC readout has been extensively tested with and without amagnetic field with cosmic rays [13 16 17] Beam tests have been carried out with hadron andelectron beams in a 1 Tesla magnet [18 19] Good resolution has been achieved with wide padsboth for the GEM and the Micromegas readout Pad width is no longer a resolution limiting factorand the dependence of resolution on the drift distance is close to the expectation from transversediffusion and electron statistics

Detailed simulations based on the theoretical model have been done to understand the charac-teristics of charge dispersion signals Initial ionization clustering electron drift diffusion effectsthe MPGD gain the intrinsic detector pulse-shape and electronics effects have been included [14]All aspects of charge dispersion phenomena can be simulated from first principle including pulseshapes mdash see figure 4 (c) and the pad response function (PRF ) Signals for both rectangular and

ndash 4 ndash

2010 JINST 5 P03008

Figure 4 Simulation of charge dispersion signals from first principle for a GEM TPC Cosmic ray track atz = 67 mm B = 0 Ar + 10 CO2 gas (a) TPC pad layout The 2 times 6 mm2 pads in the five central rowswere used for tracking and the two long pads in the outer rows for triggering The figure shows the firstand the last pad number in each row (b) Charge preamplifier signals on the five central pads in each of thetracking rows (c) Measured and simulated signals on pads 28 to 32 Only the centre pad signal was usedfor normalization with no other free parameters

keystone shaped pads can be simulated The results are in excellent agreement with measurementsand the simulation can be used to optimize MPGD charge dispersion readout designs for experi-ments

3 Charge dispersion signal characteristics and data analysis

In learning how to use the complicated MPGD charge dispersion signal with variable pulse shapesto measure track coordinates we have dispensed with the traditional method of using the mainshaper-amplifier output signal and use instead the front-end charge preamplifier signal so the detailsof the pulse characteristics can be studied

For normal MPGDs with conventional anode readout there is a charge signal only if the chargeis finally collected by the pad Depending on the transverse diffusion in the gas one or more pads ina row may collect part of the track charge For the conventional readout all pad preamplifier pulseshave the same shape eg rise times are the same and the maximum pulse height is proportionalto the charge collected by the pad The dependence of computed pad signal amplitude on the track

ndash 5 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 1 The concept of charge dispersion illustrated for a double GEM detector The avalanche is initiallyconfined within 10 to 15 microm but the charge disperses quickly to cover the resistive anode surface andinduces signals on multiple readout pads making possible an accurate charge position determination fromcentre of gravity

which canceled the Lorentz angle could be quite good sim 100 microm even at 2 meter drift [6] But forthe ExB effect ALEPH TPC could have achieved excellent resolution from the centre of gravityof induced signals on 7 mm wide cathode readout pads This is not surprising since the techniquebasically depends on wire-pad readout geometry and electrostatics In contrast MPGDs have sofar relied largely on the use of narrow sub-millimeter anode strips or pads as a means to achievinggood resolution

A technique to read out the MPGD charge signal has been developed which can use order ofmagnitude wider pads with no loss of resolution but requires certain modifications to the anodereadout structure [12] The conventional MPGD anode plane is replaced by a composite struc-ture consisting of a high surface resistivity thin film laminated to the readout pad plane with anintermediate insulating spacer The structure forms a distributed 2-dimensional RC network andan avalanche charge arriving at the surface of the anode will disperse with the time constant of thesystem Figure 1 illustrates the charge dispersion readout concept for a double GEM detector

The dispersion of charge on the resistive anode surface induces signals on multiple readoutpads which can then be used to compute the position centroid The process is completely describedby the detector material properties and geometry and in contrast to diffusion which is statistical innature there is no loss of accuracy in determining the centroid of a wider distribution Excellentresolution can therefore be achieved by using wide pads comparable in width to those used for theproportional wire-cathode pad TPCs

The new readout concept has been extensively tested and proven to work both with GEMs andMicromegas For the Micromegas there is the added benefit of reduced sparking and the potentialfor stable high gain operation Figure 2 illustrates the implementation of charge dispersion readoutconcept for our early studies with Micromegas

We had previously reported an unprecedentedsim 50 microm resolution with 2 mmtimes 6 mm pads incosmic ray tests of a small prototype Micromegas TPC at 5 Tesla [13] This data has recently beenreanalyzed with improved analysis techniques and the resolution achieved is better than 40 microm

A theoretical model of charge dispersion has been developed [14] Simulations based on

ndash 2 ndash

2010 JINST 5 P03008

Figure 2 Charge dispersion MPGD readout technique is equally applicable to the GEM and the Mi-cromegas The figure shows the details of the resistive anode structure as implemented for our initial studieswith Micromegas

the model can explain in detail all features of experimental data The following sections givean overview charge dispersion phenomena in MPGDs with a resistive anode model simulationstechniques of data analysis and recent developments and the outlook

2 Position sensing from charge dispersion in MPGDs Theory and simulation

The resistive anode and the readout plane together can be looked upon to form a distributed 2-dimensional RC network in the finite element approximation We start with the charge divisionmethod [15] of measuring the position on a proportional wire described by one-dimensional Tele-graph equation For an avalanche charge arriving at t = 0 the space-time evolution of the chargedensity per unit length ρ is given by

partρ

part t=

1τ

part 2ρ

partx2 where τ = RC

Here R is resistance per unit length and C the capacitance per unit length for the wireIn analogy with the one dimensional Telegraph equation we write the 2-dimensional equation

for charge density on the resistive surface

partρ

part t=

1τ

[part 2ρ

part r2 +1r

partρ

part r

]where in this case τ = RC is the system time constant per unit area with R the anode surfaceresistivity and C the capacitance density per unit area for the readout structure

A localized charge cluster arrives at the resistive anode surface at t = 0 The solution for ρ fora resistive anode of finite size is an infinite Fourier series An approximation that works quite wellis to assume that the anode boundaries are at infinity in which case a closed form solution can bewritten down

ρ(r t) =τ

2texp(minusr2

τ4t)

ndash 3 ndash

2010 JINST 5 P03008

Figure 3 (a) Dependence of charge density function on time in a MPGD with a resistive anode (b) Thepads integrate the time dependent charge density and both the rise time and pulse height of signal depend onthe pad position with respect to the avalanche charge

The charge density function ρ varies with time and is capacitively sampled by the readout padsFigure 3 (a) shows the time evolution of the charge density for an initially localized charge cluster

The charge signal on a pad can be computed [14] by integrating the time dependent chargedensity function over the pad area The shape and the pulse height of the charge pulse on a paddepends on the pad geometry the location of the pad with respect to the initial charge and the RCtime constant of the system mdash figure 3 (b)

The charge dispersion MPGD TPC readout has been extensively tested with and without amagnetic field with cosmic rays [13 16 17] Beam tests have been carried out with hadron andelectron beams in a 1 Tesla magnet [18 19] Good resolution has been achieved with wide padsboth for the GEM and the Micromegas readout Pad width is no longer a resolution limiting factorand the dependence of resolution on the drift distance is close to the expectation from transversediffusion and electron statistics

Detailed simulations based on the theoretical model have been done to understand the charac-teristics of charge dispersion signals Initial ionization clustering electron drift diffusion effectsthe MPGD gain the intrinsic detector pulse-shape and electronics effects have been included [14]All aspects of charge dispersion phenomena can be simulated from first principle including pulseshapes mdash see figure 4 (c) and the pad response function (PRF ) Signals for both rectangular and

ndash 4 ndash

2010 JINST 5 P03008

Figure 4 Simulation of charge dispersion signals from first principle for a GEM TPC Cosmic ray track atz = 67 mm B = 0 Ar + 10 CO2 gas (a) TPC pad layout The 2 times 6 mm2 pads in the five central rowswere used for tracking and the two long pads in the outer rows for triggering The figure shows the firstand the last pad number in each row (b) Charge preamplifier signals on the five central pads in each of thetracking rows (c) Measured and simulated signals on pads 28 to 32 Only the centre pad signal was usedfor normalization with no other free parameters

keystone shaped pads can be simulated The results are in excellent agreement with measurementsand the simulation can be used to optimize MPGD charge dispersion readout designs for experi-ments

3 Charge dispersion signal characteristics and data analysis

In learning how to use the complicated MPGD charge dispersion signal with variable pulse shapesto measure track coordinates we have dispensed with the traditional method of using the mainshaper-amplifier output signal and use instead the front-end charge preamplifier signal so the detailsof the pulse characteristics can be studied

For normal MPGDs with conventional anode readout there is a charge signal only if the chargeis finally collected by the pad Depending on the transverse diffusion in the gas one or more pads ina row may collect part of the track charge For the conventional readout all pad preamplifier pulseshave the same shape eg rise times are the same and the maximum pulse height is proportionalto the charge collected by the pad The dependence of computed pad signal amplitude on the track

ndash 5 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 2 Charge dispersion MPGD readout technique is equally applicable to the GEM and the Mi-cromegas The figure shows the details of the resistive anode structure as implemented for our initial studieswith Micromegas

the model can explain in detail all features of experimental data The following sections givean overview charge dispersion phenomena in MPGDs with a resistive anode model simulationstechniques of data analysis and recent developments and the outlook

2 Position sensing from charge dispersion in MPGDs Theory and simulation

The resistive anode and the readout plane together can be looked upon to form a distributed 2-dimensional RC network in the finite element approximation We start with the charge divisionmethod [15] of measuring the position on a proportional wire described by one-dimensional Tele-graph equation For an avalanche charge arriving at t = 0 the space-time evolution of the chargedensity per unit length ρ is given by

partρ

part t=

1τ

part 2ρ

partx2 where τ = RC

Here R is resistance per unit length and C the capacitance per unit length for the wireIn analogy with the one dimensional Telegraph equation we write the 2-dimensional equation

for charge density on the resistive surface

partρ

part t=

1τ

[part 2ρ

part r2 +1r

partρ

part r

]where in this case τ = RC is the system time constant per unit area with R the anode surfaceresistivity and C the capacitance density per unit area for the readout structure

A localized charge cluster arrives at the resistive anode surface at t = 0 The solution for ρ fora resistive anode of finite size is an infinite Fourier series An approximation that works quite wellis to assume that the anode boundaries are at infinity in which case a closed form solution can bewritten down

ρ(r t) =τ

2texp(minusr2

τ4t)

ndash 3 ndash

2010 JINST 5 P03008

Figure 3 (a) Dependence of charge density function on time in a MPGD with a resistive anode (b) Thepads integrate the time dependent charge density and both the rise time and pulse height of signal depend onthe pad position with respect to the avalanche charge

The charge density function ρ varies with time and is capacitively sampled by the readout padsFigure 3 (a) shows the time evolution of the charge density for an initially localized charge cluster

The charge signal on a pad can be computed [14] by integrating the time dependent chargedensity function over the pad area The shape and the pulse height of the charge pulse on a paddepends on the pad geometry the location of the pad with respect to the initial charge and the RCtime constant of the system mdash figure 3 (b)

The charge dispersion MPGD TPC readout has been extensively tested with and without amagnetic field with cosmic rays [13 16 17] Beam tests have been carried out with hadron andelectron beams in a 1 Tesla magnet [18 19] Good resolution has been achieved with wide padsboth for the GEM and the Micromegas readout Pad width is no longer a resolution limiting factorand the dependence of resolution on the drift distance is close to the expectation from transversediffusion and electron statistics

Detailed simulations based on the theoretical model have been done to understand the charac-teristics of charge dispersion signals Initial ionization clustering electron drift diffusion effectsthe MPGD gain the intrinsic detector pulse-shape and electronics effects have been included [14]All aspects of charge dispersion phenomena can be simulated from first principle including pulseshapes mdash see figure 4 (c) and the pad response function (PRF ) Signals for both rectangular and

ndash 4 ndash

2010 JINST 5 P03008

Figure 4 Simulation of charge dispersion signals from first principle for a GEM TPC Cosmic ray track atz = 67 mm B = 0 Ar + 10 CO2 gas (a) TPC pad layout The 2 times 6 mm2 pads in the five central rowswere used for tracking and the two long pads in the outer rows for triggering The figure shows the firstand the last pad number in each row (b) Charge preamplifier signals on the five central pads in each of thetracking rows (c) Measured and simulated signals on pads 28 to 32 Only the centre pad signal was usedfor normalization with no other free parameters

keystone shaped pads can be simulated The results are in excellent agreement with measurementsand the simulation can be used to optimize MPGD charge dispersion readout designs for experi-ments

3 Charge dispersion signal characteristics and data analysis

In learning how to use the complicated MPGD charge dispersion signal with variable pulse shapesto measure track coordinates we have dispensed with the traditional method of using the mainshaper-amplifier output signal and use instead the front-end charge preamplifier signal so the detailsof the pulse characteristics can be studied

For normal MPGDs with conventional anode readout there is a charge signal only if the chargeis finally collected by the pad Depending on the transverse diffusion in the gas one or more pads ina row may collect part of the track charge For the conventional readout all pad preamplifier pulseshave the same shape eg rise times are the same and the maximum pulse height is proportionalto the charge collected by the pad The dependence of computed pad signal amplitude on the track

ndash 5 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 3 (a) Dependence of charge density function on time in a MPGD with a resistive anode (b) Thepads integrate the time dependent charge density and both the rise time and pulse height of signal depend onthe pad position with respect to the avalanche charge

The charge density function ρ varies with time and is capacitively sampled by the readout padsFigure 3 (a) shows the time evolution of the charge density for an initially localized charge cluster

The charge signal on a pad can be computed [14] by integrating the time dependent chargedensity function over the pad area The shape and the pulse height of the charge pulse on a paddepends on the pad geometry the location of the pad with respect to the initial charge and the RCtime constant of the system mdash figure 3 (b)

The charge dispersion MPGD TPC readout has been extensively tested with and without amagnetic field with cosmic rays [13 16 17] Beam tests have been carried out with hadron andelectron beams in a 1 Tesla magnet [18 19] Good resolution has been achieved with wide padsboth for the GEM and the Micromegas readout Pad width is no longer a resolution limiting factorand the dependence of resolution on the drift distance is close to the expectation from transversediffusion and electron statistics

Detailed simulations based on the theoretical model have been done to understand the charac-teristics of charge dispersion signals Initial ionization clustering electron drift diffusion effectsthe MPGD gain the intrinsic detector pulse-shape and electronics effects have been included [14]All aspects of charge dispersion phenomena can be simulated from first principle including pulseshapes mdash see figure 4 (c) and the pad response function (PRF ) Signals for both rectangular and

ndash 4 ndash

2010 JINST 5 P03008

Figure 4 Simulation of charge dispersion signals from first principle for a GEM TPC Cosmic ray track atz = 67 mm B = 0 Ar + 10 CO2 gas (a) TPC pad layout The 2 times 6 mm2 pads in the five central rowswere used for tracking and the two long pads in the outer rows for triggering The figure shows the firstand the last pad number in each row (b) Charge preamplifier signals on the five central pads in each of thetracking rows (c) Measured and simulated signals on pads 28 to 32 Only the centre pad signal was usedfor normalization with no other free parameters

keystone shaped pads can be simulated The results are in excellent agreement with measurementsand the simulation can be used to optimize MPGD charge dispersion readout designs for experi-ments

3 Charge dispersion signal characteristics and data analysis

In learning how to use the complicated MPGD charge dispersion signal with variable pulse shapesto measure track coordinates we have dispensed with the traditional method of using the mainshaper-amplifier output signal and use instead the front-end charge preamplifier signal so the detailsof the pulse characteristics can be studied

For normal MPGDs with conventional anode readout there is a charge signal only if the chargeis finally collected by the pad Depending on the transverse diffusion in the gas one or more pads ina row may collect part of the track charge For the conventional readout all pad preamplifier pulseshave the same shape eg rise times are the same and the maximum pulse height is proportionalto the charge collected by the pad The dependence of computed pad signal amplitude on the track

ndash 5 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 4 Simulation of charge dispersion signals from first principle for a GEM TPC Cosmic ray track atz = 67 mm B = 0 Ar + 10 CO2 gas (a) TPC pad layout The 2 times 6 mm2 pads in the five central rowswere used for tracking and the two long pads in the outer rows for triggering The figure shows the firstand the last pad number in each row (b) Charge preamplifier signals on the five central pads in each of thetracking rows (c) Measured and simulated signals on pads 28 to 32 Only the centre pad signal was usedfor normalization with no other free parameters

keystone shaped pads can be simulated The results are in excellent agreement with measurementsand the simulation can be used to optimize MPGD charge dispersion readout designs for experi-ments

3 Charge dispersion signal characteristics and data analysis

In learning how to use the complicated MPGD charge dispersion signal with variable pulse shapesto measure track coordinates we have dispensed with the traditional method of using the mainshaper-amplifier output signal and use instead the front-end charge preamplifier signal so the detailsof the pulse characteristics can be studied

For normal MPGDs with conventional anode readout there is a charge signal only if the chargeis finally collected by the pad Depending on the transverse diffusion in the gas one or more pads ina row may collect part of the track charge For the conventional readout all pad preamplifier pulseshave the same shape eg rise times are the same and the maximum pulse height is proportionalto the charge collected by the pad The dependence of computed pad signal amplitude on the track

ndash 5 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 5 Cosmic ray tests in a 5 Tesla magnet at DESY Figure (a) shows the readout pad layout The outerrows with single long pads were used for triggering and the seven central rows with 2 mm x 6 mm pads usedfor tracking For a conventional MPGD in a 5 Tesla magnetic field the track charge signal would have beenconfined to a single pad per row Due to charge dispersion signals with good SN are observed on multiplepads per row

position is characterized by pad response function (PRF) The PRF amplitude for a conventionalMPGD readout required for resolution and tracking studies can be evaluated from the knowndiffusion properties of the gas and readout geometry

Every pulse is different for a MPGD with charge dispersion readout The pulse height therise time and the fall time are all determined by track position relative to the pad Signals ondirect charge collecting pads have a larger pulse height and both the rise-time and the fall time aresignificantly faster than for nearby pads which only see the dispersed charge signal Pulses on padsfarther away have smaller pulse heights and also slower rise and decay times mdash see figures 4 (c)

With variable pulse shape and both the pulse rise time and pulse height carrying positioninformation there is no obvious unique recipe to define the PRF amplitude The width and thedetailed shape of the PRF will depend on how the charge pulse measurement is used by a PRFalgorithm to compute the pad signal amplitude

The PRF width should not be too large so as not to lose measurement accuracy due to padsfarther away with small noisy pulses In principle once an algorithm has been chosen the PRFcan be computed from theory as demonstrated in Reference [14] However local RC inhomo-geneities in the readout structure make the experimental PRF deviate from theory and introduceposition dependent systematic bias in measurements We had previously measured the PRF andbias experimentally for a collimated soft x-ray source [12] One could similarly experimentally de-

ndash 6 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 6 (a) The pad response function (PRF) for 2 mm x 6 mm pads at z = 1 cm for Micromegas TPCFor T2K gas (ArCF4iC4H109532) at 5 T the transverse diffusion at 200 Vcm was sim 19 microm

radiccm (b)

The PRF FWHM2 as a function of drift distance

termine the PRF and bias for charged particle tracking using an external high precision referencedetector to measure the track coordinate With no such option available until now we have usedthe MPGD-TPC track data itself to determine the PRF

An algorithm to compute the PRF amplitude was developed [16] which integrated pad pream-plifier charge pulses within a time window with width determined by the details of the pulse shapeThe PRF and systematic effects were both determined empirically from the internal consistency ofa subset of data used only for calibration The PRF and systematic effect corrections are appliedto the remaining data set for resolution studies The data analysis techniques were developed usingcharge dispersion GEM-TPC cosmic ray data The details are described in Reference [16]

The 5 Tesla cosmic ray test of Carleton Orsay Saclay Montreal (COSMo) Micromegas TPC atDESY [13] best exemplifies the concepts and analysis techniques referred to above The COSMoTPC was tested with the so called T2K gas ArCF4iC4H109532 a candidate gas for the ILDTPC The T2K gas at 5 T has a large ωτ sim 20 which reduces transverse diffusion to DTr 19 microm

radiccm The track charge width at the end of 16 cm maximum drift TPC is completely

negligible compared to 2 mm width of 6 mm long readout pads Nonetheless the dispersion oftrack ionization charge is clearly visible in the TPC event display for cosmic ray events as shownin figure 5 (a) and (b)

The PRF at 1 cm drift distance as determined from the calibration data subset is shown infigure 6 (a) Figure 6 (b) shows the variation of PRF FWHM2 with drift distance Althoughtransverse diffusion was negligible there is a slight increase in PRF width with distance due tolarger longitudinal diffusion which contributes to increased charge dispersion

A bias in position determination of up to 50 microm before correction was observed The bias

ndash 7 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 7 Inhomogeneity in resistive anode RC structure can lead to track position dependent systematiceffects The resulting bias in position determination is intrinsic to the detector and is therefore easily removedby calibration The figure shows the bias as a function of track position for row 4 mdash see figure 5 (a) for thepad layout (a) the initial bias and (b) the remaining bias after calibration is less than 20 microm

for row 4 before and after correction is shown in figure 7 As stated earlier the bias is due toa non-uniform RC due to inhomogeneities in the dielectric gap size and the resistivity of the foilHowever since the bias is due to material properties and detector geometry it does not change withtime and can be removed by calibration The figure 7 (b) shows the bias remaining after correctionwhich is negligible

4 Recent developments

As mentioned earlier in learning how to extract position information from complex pulse shapesfor the charge dispersion readout a variable width integration time window algorithm was devel-oped [16] to compute the PRF from data This was our first attempt to do so and the methodworked quite well in that we achieved a flat sim 50 microm resolution with 2 mm x 6 mm pads at 5 Tover the full 16 cm TPC drift length [13] However the PRF algorithm was sensitive to TPC gaselectron transport parameters and maximum drift length and needed to be fine-tuned for each newconfiguration

A new more robust algorithm to compute PRF has been developed recently which does notneed fine tuning The variable width integration time window technique did not treat all pulses thesame way For the new algorithm all pad pulses are treated the same way and the PRF amplitude issimply calculated by integrating within a fixed time window Data collected in our 5 T cosmic raytests at DESY were reanalyzed with the new PRF algorithms and the results shown in figure 8 aresignificantly better mdash a flat sim 35 microm resolution [21] over the full 16 cm TPC drift length Further-more the new PRF algorithm improves the tracking efficiency as there were fewer track fit failuresWork is also presently in progress to measure the time resolution from the reanalysis of our previousdata Studies to date indicate that a time resolution of 7-8 ns can be achieved for the charge disper-sion readout with data digitized at 25 MHz The details and results will be published later [22]

A Large Prototype of TPC for the ILD detector at ILC has been built by the LCTPC col-laboration and is being tested at DESY in a 1 T magnet with a 5 GeV electron beam The Bulk

ndash 8 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

Figure 8 DESY 5 T cosmic ray data reanalyzed with a new improved PRF amplitude algorithm Withtransverse diffusion negligible a resolution of sim 35 microm was measured over the full 16 cm TPC drift lengthThe new fixed window integration algorithm gives better resolution than achieved previously with improvedtracking efficiency and requires no tuning when TPC operational parameters change

Micromegas module has a resistive anode with the readout using AFTER electronics designed forT2K TPCs [11] As our understanding of charge dispersion phenomena has improved we wereable to use the shaped signal from the main amplifier for the PRF determination and resolutionanalysis A resolution of sim 60 microm was achieved with 3 mm x 7 mm pads at zero drift distanceThe results are described elsewhere [19]

5 Conclusions and outlook

The conventional MPGD achievessim 40 microm resolution using 200 microm wide anode readout pads Anorder of magnitude wider pads can be used to achieve comparable resolution using the charge dis-persion readout method for a MPGD with a resistive anode With existing fabrication techniquesthere is a bias in position measurements which can be corrected by calibration However it is de-sirable to reduce bias as larger area detectors are built Bias can be minimized by improving theRC uniformity of the resistive anode structure Resistive films with more uniform distribution ofsurface resistivity are needed and at present there is no reliable source One also needs to improvethe resistive anode filmreadout PCB lamination techniques to minimize the point-to-point varia-tion of local capacitance density Some progress has been made in that direction recently at the

ndash 9 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

CERN TS-DEM workshop Lastly RampD is required to ensure the radiation hardness of resistiveanode MPGD readout structure The development of MPGDs with resistive anodes may enableexperiments with large area tracking requirements to use MPGDs which may have not have beenpossible until now

Acknowledgments

I have benefitted from my interactions with and been helped by a large number of individualsduring the course of this work David Nygren first pointed out the possibility of dispersing theMPGD avalanche charge on a resistive anode as a means to achieving good resolution with widepads To my colleagues and coworkers in Canada J Dubeau J-P Martin Kirsten Sachs A Bel-lerive K Boudjemline Hans Mes and R Carnegie my sincere thanks for their help and supportthroughout E Neuheimer and S Liu helped in designing building testing and repairing muchof the specialized electronics used for these measurements Y Shin our software support special-ist made important contributions to developing the DAQ and analysis software Morley OrsquoNeillVance Strickland and Matt Bowcock at Carleton helped with mechanical design and fabricationand Philippe Gravelle in solving technical problems

Among the many students who worked with me on this project over the years Alasdair Rankinand Stephen Turnbull stand out for their singular contributions to the hardware development andto the data acquisition and analysis and simulation software The charge preamplifiers used forthese measurements came from the Aleph TPC at CERN and I thank Ron Settles for making theseavailable to us Finally I should mention my research colleagues from Saclay Ioannis GiomatarisPaul Colas David Attie and Vincent Lepeltier1 from Orsay The work described here could nothave been done without their help and I am especially thankful to them

This research was supported by a project grant from the Natural Sciences and Engineering Re-search Council of Canada TRIUMF receives federal funding via a contribution agreement throughthe National Research Council of Canada

References

[1] F Sauli and A Sharma Micropattern gaseous detectors Ann Rev Nucl Part Sci 49 (1999) 341

[2] F Sauli GEM a new concept for electron amplification in gas detectors Nucl Instrum Meth A 386(1997) 531

[3] Y Giomataris P Rebourgeard JP Robert and G Charpak Micromegas a high-granularityposition-sensitive gaseous detector for high particle-flux environments Nucl Instrum Meth A 376(1996) 29

[4] DR Nygren A Time Projection Chamber - 1975 presented at 1975 PEP Summer Study PEP 198(1975) and included in proceedings

[5] CK Hargrove et al The spatial resolution of the Time Projection Chamber at TRIUMF NuclInstrum Meth 219 (1984) 461

[6] SR Amendolia et al The spatial resolution of the ALEPH TPC Nucl Instrum Meth A 283 (1989)573

1Deceased

ndash 10 ndash

2010 JINST 5 P03008

[7] httpwwwilcildorgdocumentsild-letter-of-intentLOIpdf

[8] httpwwwlinearcolliderorgcmspid=1000025

[9] httpstwikicernchtwikibinviewfileAtlasMuonMicromegasrev=2filename=ATLAS-RD-MuMegas-v04pdf

[10] S Tapprogge ed Outline of RampD activities for ATLAS at an upgraded LHCCERN-ATL-COM-GEN-2005-002 23 Jan 2005

[11] J Bouchez et al Bulk Micromegas detectors for large TPC applications Nucl Instrum Meth A 574(2007) 425

[12] MS Dixit J Dubeau JP Martin and K Sachs Position sensing from charge dispersion inmicro-pattern gas detectors with a resistive anode Nucl Instrum Meth A 518 (2004) 721[physics0307152]

[13] M Dixit et al Micromegas TPC studies at high magnetic fields using the charge dispersion signalNucl Instrum Meth A 581 (2007) 254 [physics0703243]

[14] MS Dixit and A Rankin Simulating the charge dispersion phenomena in Micro Pattern GasDetectors with a resistive anode Nucl Instrum Meth A 566 (2006) 281

[15] V Radeka and P Rehak Charge dividing mechanism on resistive electrode in position-sensitivedetectors IEEE Trans Nucl Sci 26 (1979) 225

[16] K Boudjemline MS Dixit JP Martin and K Sachs SPatial resolution of a GEM readout TPCusing the charge dispersion signal Nucl Instrum Meth A 574 (2007) 22

[17] A Bellerive et al Spatial resolution of a Micromegas-TPC using the charge dispersion signal inProceedings of International Linear Collider Workshop LCWS2005 Stanford USA[physics0510085]

[18] K Boudjemline et al Resolution studies in Micromegas-TPC using charge dispersion in magneticfield presented at IEEE NSSMIC Conf SanDiego California Oct 2006

[19] D Attie et al Beam test of the Micromegas ILC-TPC Large Prototype paper presented atMPGD2009 Conference Crete June 2009

[20] RJ McKee et al Precise calibration of a Ge(Li)-spectrometer using a digital to analog converterNucl Instrum Meth 92 (1971) 421

[21] S Turnbull et al Resolution of charge dispersion readout MPGD-TPC - a reanalysis based on animproved algorithm to be submitted to Nucl Instrum Meth A

[22] R Woods et al Measurement of time resolution for a Micromegas TPC with charge dispersionreadout to be submitted to Nucl Instrum Meth A

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