ILC TPC resolution studies with charge dispersion in MPGDs with a resistive anode

ILC TPC resolution studies with charge dispersion in MPGDs with a resistive anode

Madhu DixitCarleton University & TRIUMF

IPNS KEK 14/05/2005

M. Dixit IPNS KEK 14/10/05 2

The International Linear Collider (ILC)• 2007 - LHC ready to explore new physics and to search for Higgs,

supersymmetry, hidden new dimensions, etc.• ILC critical to understanding LHC discoveries

– Detailed study of Higgs and SUSY particles– Precision measurements

• Higgs e+ e- -> Z H -> l l X • ∆Mtop≈ 100 MeV, ∆top≈ 2%• ∆MZ & ∆MW ≈ 5 MeV (from 30 MeV)• ∆(sin2) ≈ 10-5 (from 2·10-4)

• Global Design Effort (GDE) timeline:– 2006 accelerator CDR– 2008 accelerator TDR, experimental collaborations, detector CDRs– 2009 - Detector TDRs, construction – 2015 - Physics at ILC

• ILC tracker resolution ∆(1/pT) ~ 5 x10-5 (GeV/c)-1 (10 times better than at LEP!)


ILC tracker requirements• Small cross sections 100 fb, low rates, no fast trigger.

• Higgs measurements & SUSY searches require:– High granularity continuous tracking for good pattern recognition.

– Good energy flow measurement in tight high multiplicity jets.

– Excellent primary and secondary b, c, decay vertex reconstruction.

• TPC is an ideal tracker for ILC.– Momentum resolution goal (1/pT) ~ 5.10-5 (GeV-1) achievable with vertex

+ Si inner tracker + TPC with ∆(1/pT) ~ 2 x 10-4 (GeV-1)

• ILC TPC tracker goals:– 200 track points with (r, ) = 100 m, (r, z) = 500 m

– 2 track resolution < 2mm in (r, ) and < 5 mm in (r, z)

– dE/dx resolution < 5%


ILC detector concepts

Silicon tracker (B=5T)SiW ECAL

“SiD”

TPC (B=4T)SiW ECAL (medium)

“LDC”

TPC (B=3T)W/Scint ECAL (large)

“GLD”


ILC TPC (TESLA design)

cm

E

B = 4 T


ExB cancels track angle effect

TPC wire/pad readout

100 µm

•Average Aleph resolution ~ 150 µm•About 100 µm best for all drift distances• Limit from diffusion (10 cm drift) ~ 20 µm; (2 m drift) ~ 90 µm

Conventional TPCs never achieve their potential! Example:Systematic effects in Aleph TPC at LEP


•Transverse diffusion sets the ultimate limit on TPC resolution.

•ILC TPC resolution goals close to the diffusion limit.

•Wire/pad TPC resolution inherently limited by ExB & track angle systematic effects.•A TPC read out with a MPGD endcap could meet the ILC resolution challenge if the precision of pad charge centroid determination could be improved. •What is the best achievable resolution with conventional techniques?

Micro Pattern Gas Detector (MPGD) Readout for ILC TPC


Micro Pattern Gas Detectors (MPGD)Unlike wires, MPGDs have no preferred direction - negligible ExB effect

MPGDs achieve excellent 40 µm resolution with 200 µm wide pads.Conventional wire readout TPCs use cathode pads of width ~ a few mm.Proposed ILC TPC channel count ~ 1.5x106 with 2 mm wide pads. Narrower pads would lead to increased detector cost & complexity.

The Gas Electron Multiplier (GEM) Micromegas

~ 50 m~ mm

Drift regionDrift region

AmericaCarleton U

Cornell/PurdueLBNLMIT

U MontrealU Victoria

EuropeEuropeRWTH Aachen RWTH Aachen

DESYDESYU HamburgU HamburgU KarlsruheU Karlsruhe

UMM KrakowUMM KrakowMPI-MunichMPI-Munich

NIKHEFNIKHEFBINP NovosibirskBINP Novosibirsk

LAL OrsayLAL OrsayIPN OrsayIPN OrsayU RostockU Rostock

CEA SaclayCEA SaclayPNPI St. PetersburgPNPI St. Petersburg

Asian ILC gaseous-Asian ILC gaseous-tracking groupstracking groups

Chiba UChiba UHiroshima UHiroshima U

Minadamo SU-IITMinadamo SU-IITKinki UKinki UU OsakaU OsakaSaga USaga U

Tokyo UATTokyo UATU TokyoU Tokyo

NRICP TokyoNRICP TokyoKogakuin U TokyoKogakuin U Tokyo

KEK TsukubaKEK TsukubaU TsukubaU Tsukuba

Other USAOther USA MIT (LCRD)MIT (LCRD)

Temple/Wayne Temple/Wayne State (UCLC)State (UCLC)

YaleYale

Worldwide R&D effort for ILC TPC Ron Settles main coordinator

Large task listLarge task listResolution studies.Resolution studies.Ion feedback studies.Ion feedback studies.Gas studies for better Gas studies for better resolution and low neutron resolution and low neutron background.background.Low mass field cage and Low mass field cage and endcap.endcap.High density low power High density low power electronics.electronics.Analysis and simulation Analysis and simulation software.software.

ILC TPC R&D plans1) Demonstration phase1) Demonstration phase

•Continue work for ~1 year with small prototypes on mapping out parameter space, Continue work for ~1 year with small prototypes on mapping out parameter space, understanding resolution, etc, to prove feasibility of an MPGD TPC.understanding resolution, etc, to prove feasibility of an MPGD TPC. For Si-based For Si-based ideas this will include a basic proof-of-principle.ideas this will include a basic proof-of-principle.

2) Consolidation phase2) Consolidation phase•Build and operate “large” prototype (Ø ≥ 70cm, drift ≥ 50cm) which allows any Build and operate “large” prototype (Ø ≥ 70cm, drift ≥ 50cm) which allows any MPGD technology, to test manufacturing techniques for MPGD endplates, fieldcage MPGD technology, to test manufacturing techniques for MPGD endplates, fieldcage and electronics. Design work would start in ~1/2 year, building and testing another ~ and electronics. Design work would start in ~1/2 year, building and testing another ~ 2 years.2 years.

3) Design phase3) Design phase•After phase 2, the decision as to which endplate technology to use for the LC TPC After phase 2, the decision as to which endplate technology to use for the LC TPC would be taken and final design started.would be taken and final design started.

Point resolution, MicromegasSaclay/Orsay/Berkeley

--Diffusion measurements (r,) < 100 m possible--At moment only achieved for short drift (intrinsic ) for gain~5000 (350V mesh), noise~1000 e--ongoing effort…

B = 1T1x10mm^2 pads

mm^2, B = 1T

Many groups are working on MPGD TPC R&D. Prototype results from “Some Results -Summer 2005” (Ron Settles)

Prototype results contd.

Point resolution GEM

DESY group. (r,) measured for GEMs with 2x6mm2 pads

30cm

B=4TGas:P5

Victoria group has achieved ~ 100 µm resolution for short drift distances with narrower 1.2x7mm2 pads.

Prototype Results - cont.

Point resolution, GEM

--Example of (r,) measured at Aachen GEMs with 2x6mm2 pads by comparing track position with a Si hodoscope.

--In general (also for Micromegas) the resolution is not as good as expected from diffusion.

PRELIMINARY!

•Narrower pads leading to increased complexity & a larger number of readout channels.•Disperse track charge after gas gain over a larger area to improve pad centroid with wide pads.

•For the GEM, large transverse diffusion in the high E-field field in transfer and induction gaps provides a natural mechanism to disperse the cluster charge. •Measurements with prototype GEM-TPCs have shown that this increased diffusion is insufficient in a high B field for the ILC-TPC to achieve the resolution target with ~ 2 mm pads. •Explore other concepts to disperse the charge

Ideas to improve the MPGD TPC resolution


Carleton setup for MPGD resolution studies with x rays

•Point source ~ 50 µm collimated 4.5 keV x rays. •Aleph TPC preamps. Rise= 40 ns, Fall = 2 s.•DAQ - 500 MHz Tektronix digital scope.


An idea - Measure the induced signals in a GEM

GEM Proportional wire

Cathode padsAnode pads

• Short ~ 200 ns signalWe measure x y 70 µm •But this technique requires expensive high frequency pulse shape sampling electronics


Another idea- Position sensing from charge dispersion in MPGDs with a resistive anode

Analogy:Analogy: Charge division is used to measure the avalanche position on a proportional wire.Deposit point charge at t=0

Solution for charge density on the wire(L ~ 0)

Telegraph equation (1-D): 2

2

2

2 1xQ

RCtQ

tQ

RL

∂∂

=∂∂

∂∂

€

Q(x,t) = RC4π t

−x 2RC4 te

Generalize the concept of 1-D charge division to 2-D


Charge dispersion in a GEM with a resistive anode


Equivalent circuit for currents in a GEM with an intermediate resistive anode

Resistive anode foil

Signal pickup pads

Current generators

Pad amplifier


Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width)

A photon event in the resistive anode GEM test cell


•Modified GEM anode with a high resistivity film bonded to a readout plane with an insulating spacer.•2-dimensional continuous RC network defined by material properties & geometry.•Point charge at r = 0 & t = 0 disperses with time.•Time dependent anode charge density sampled by readout pads.Equation for surface charge density function on the 2-dim. continuous RC network:

€

∂ρ∂t

= 1RC

∂2ρ∂r2 + 1

r∂ρ∂r

⎡ ⎣ ⎢

⎤ ⎦ ⎥

€

⇒ ρ(r, t) = RC

2t

−r 2RC4 te

ρ(r,t) integral over pads

ρ(r) Q

r / mmmm ns

Improving resolution with charge dispersion in a MPGD with a resistive anode M.S.Dixit et.al., Nucl. Instrum. Methods A518 (2004) 721.


Charge dispersion signal for a GEM Simulation versus measurement

(2 mm x 6 mm pads) Collimated ~ 50 m 4.5 keV x-ray spot on pad centre.

Primary signal: Fast large amplitude main pulse on charge collecting pad. Simulated primary pulse is normalized to the data.

Difference = induced signal (not included in simulation) studied previously: MPGD '99 (Orsay), LCWS '00

Secondary signal: The dispersion pulse on the neighboring pad is slower & smaller. Simulated secondary pulse normalization is the same as for the primary.

Detailed simulation includes effects of, longitudinal & transverse diffusion, gas gain, detector pulse formation, charge dispersion & preamplifier rise and fall time effects. For tracks, include effects of unequal primary clusters.


50 m pillars

Drift Gap

MESHAmplification Gap

Al-Si Cermet on mylar

Resistive anode Micromegas530 k/ Carbon loaded Kapton resistive anode was used with GEM. This was replaced with more uniform higher resistivity 1 M/ Cermet for Micromegas.


Charge dispersion signals in MicromegasSingle event (2 mm wide pads)

2 x 4 channel TektronixX-ray spot centred on pad 2

Primary signal

Two 1st neighbors

2nd neighbor (note different scale)

Ar/CO2 90/10, Gain ~ 30001st neighbor peak ~ 100 ns after the primary pulse peakSlow rising 2nd neighbor pulse ~ 25 MHz digitization could replace pulse shape sampling


GEM pad response function for collimated x rays Simulation versus measurement

Measured PRF deviates from simulation due to anode RC nonuniformities.

(Solid line)

Pad 23

Pad 24

Pad 22

Scan across width

Ionization from 50 m collimated x-rays.2x6 mm2 pads


•GEM resolution ~ 70 m. •Similar resolution measured for a Micromegas with a resistive anode readout using 2 mm x 6 mm pads

Resistive anode double-GEM spatial resolutionCollimated ~ 50 m x-ray spot

2x6 mm2 pads


Resistive anode suppresses sparking stabilizing Micromegas Extremely high gains without breakdown are possible

A fringe benefit: High Micromegas gain with a resistive anode

Argon/Isobutane 90/10


•15 cm drift length with GEM or Micromegas readout •B=0 (so far)•Ar:CO2/90:10 chosen to simulate low transverse diffusion in a magnetic field.•Aleph charge preamps. Rise= 40 ns, Fall = 2 s. •200 MHz FADCs rebinned to digitization effectively at 25 MHz.•60 tracking pads (2 x 6 mm2) + 2 trigger pads (24 x 6 mm2).The GEM-TPC resolution was first measured with conventional direct charge TPC readout.

The resolution was next measured with a charge dispersion resistive anode readout with a double-GEM & with a Micromegas endcap.

Carleton cosmic ray test MPGD-TPC


Simulation - GEM TPC cosmic event with charge dispersion

(track Z drift distance ~ 67 mm, Ar/CO2 90/10 gas)Detailed model simulation including longitudinal & transverse diffusion, gas gain, detector pulse formation, charge dispersion & preamp rise & fall time effects.

Centre pad amplitude used for normalization - no other free parameters.

2x6 mm2 pads

SimulationData


The pad response function (PRF)• The PRF is a measure of signal size as a function of track

position relative to the pad.• For charge dispersion non charge collecting pads have signals

in contrast to conventional direct charge readout.• Unusual highly variable charge dispersion pulse shape; both

the rise time & pulse amplitude depend on track position.• We use pulse shape information to optimize the PRF.• The PRF can, in principle, be determined from simulation. • However, system RC nonuniformities & geometrical effects

introduce bias in absolute position determination.• The position bias can be corrected by calibration. • PRF and bias determined empirically using a subset of data

which was used for calibration. The remaining data was used for resolution studies.


GEM & Micromegas PRFs for TPC trackAr:CO2 (90:10) 2x6 mm2 pads

GEM PRFs Micromegas PRFsMicromegas PRF is narrower due to the use of higher resistivity anode & smaller diffusion after avalanche gain

The pad response function maximum for longer drift distances is lower due to Z dependent normalization.

M. Dixit IPNS KEK 14/10/05 32€

PRF[x,Γ(z),Δ,a,b] = (1+ a2x 2 + a4 x 4 )(1+ b2x 2 + b4 x 4 )

a2 a4 b2 & b4 can be written down in terms of and & two scale parameters a & b.

PRFs with the GEM & the Micromegas readout• The PRFs are not Gaussian.• The PRF depends on track position relative to the pad.• PRF = PRF(x,z)• PRF can be characterized by its FWHM (z) & base

width (z).• PRFs determined from the data have been fitted to a

functional form consisting of a ratio of two symmetric 4th order polynomials.


Track fit using the the PRF

One parameter fit for xrow (track position for a given row) using Bias = Mean of residuals (xrow-xtrack) as a function of xtrack Resolution = of track residuals for tracks with || < 5

Determine x0 & by minimizing

for the entire event

€

(Ai − PRFi) ∂Ai =rows i=pads

2

2 mm

6 mm

Track at: xtrack= x0+ tan yrow


Bias corrections with GEM & with Micromegas

GEM Micromegas

2 mm pads 2 mm pads

Initial bias

Remaining bias after correction

Initial bias

Remaining bias after correction


What is the diffusion limit of resolution for a gaseous TPC?

€

x2 = σ 0

2 + Cd2 ⋅ z

Neff

0 includes noise & systematic effects. Cd = diffusion constant; z = drift distance

.

Resolution depends on electron statistics.Electron number N fluctuates from event to event.

Neff <N> the average number of electrons = 1/<1/N> the inverse of average of 1/NGain fluctuations also affect Neff


Simulation for the effective number of electrons for resolution2 mm x 6 mm pads - Ar/CO2 90/10

.

dE/dx in Argon

•Statistics of primary ionization & cluster size distribution.•dE/dx dependence on momentum.•Account for track angle & detector acceptance effects.•Use simulation to scale measured pulse heights to electron number.•Neff = 1/<1/N> determined from pulse height distribution.•Neff ≈ 38.9 10% (Naverage= 57)

Cosmic ray momentum spectrum

Total

Ionization

Measured pad pulse height distribution

Mostly muons at

sea level


Measured TPC transverse resolution for Ar:CO2 (90:10) R.K.Carnegie et.al., NIM A538 (2005) 372

R.K.Carnegie et.al., to be published

€

02 + CD

2

Nez [Neff = 38.9]

Unpublished

Compared to conventional readout, resistive readout gives better resolution for the GEM and the Micromegas readout. The z dependence follows the expectations from transverse diffusion & electron statistics.


Summary• TPC with MPGD readout is a very well suited technology for

the ILC.• Traditional readout has difficulty achieving TPC resolution

goal, unless narrower pads are used. • With charge dispersion, the cluster charge can be dispersed in

a controlled way such that relatively wide pads can be used without sacrificing resolution. With such a readout system, it may be feasible to achieve ILC TPC resolution goal with relatively wide pads both for the GEM and the Micromegas readout.

• With R&D, ILC TPC resolution goals appear within reach. • Beam test at KEK (next week) important step in developing

the MPGD readout for the ILC TPC.

ILC TPC resolution studies with charge dispersion in MPGDs with a resistive anode

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