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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 Dixit Carleton University & TRIUMF. IPNS KEK 14/05/2005. The International Linear Collider (ILC).

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

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  1. ILC TPC resolution studies with charge dispersion in MPGDs with a resistive anode Madhu Dixit Carleton University & TRIUMF IPNS KEK 14/05/2005

  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!) IPNS KEK 14/10/05

  3. 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% IPNS KEK 14/10/05

  4. 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” IPNS KEK 14/10/05

  5.  E B = 4 T ILC TPC (TESLA design) cm IPNS KEK 14/10/05

  6. Conventional TPCs never achieve their potential!Example:Systematic effects in Aleph TPC at LEP TPC wire/pad readout ExB cancels track angle effect 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 IPNS KEK 14/10/05

  7. Micro Pattern Gas Detector (MPGD) Readout for ILC TPC • 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? IPNS KEK 14/10/05

  8. Micro Pattern Gas Detectors (MPGD)Unlike wires, MPGDs have no preferred direction - negligible ExB effect The Gas Electron Multiplier (GEM) Micromegas Drift region Drift region ~ mm ~ 50 m 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. IPNS KEK 14/10/05

  9. Worldwide R&D effort for ILC TPCRon Settles main coordinator Europe RWTH Aachen DESY U Hamburg U Karlsruhe UMM Krakow MPI-Munich NIKHEF BINP Novosibirsk LAL Orsay IPN Orsay U Rostock CEA Saclay PNPI St. Petersburg America Carleton U Cornell/Purdue LBNL MIT U Montreal U Victoria Other USA MIT (LCRD) Temple/Wayne State (UCLC) Yale Asian ILC gaseous-tracking groups Chiba U Hiroshima U Minadamo SU-IIT Kinki U U Osaka Saga U Tokyo UAT U Tokyo NRICP Tokyo Kogakuin U Tokyo KEK Tsukuba U Tsukuba Large task list Resolution studies. Ion feedback studies. Gas studies for better resolution and low neutron background. Low mass field cage and endcap. High density low power electronics. Analysis and simulation software.

  10. ILC TPC R&D plans • 1) Demonstration phase • Continue work for ~1 year with small prototypes on mapping out parameter space, understanding resolution, etc, to prove feasibility of an MPGD TPC. For Si-based ideas this will include a basic proof-of-principle. • 2) Consolidation phase • Build and operate “large” prototype (Ø ≥ 70cm, drift ≥ 50cm) which allows any MPGD technology, to test manufacturing techniques for MPGD endplates, fieldcage and electronics. Design work would start in ~1/2 year, building and testing another ~ 2 years. • 3) Design phase • After phase 2, the decision as to which endplate technology to use for the LC TPC would be taken and final design started.

  11. Many groups are working on MPGD TPC R&D.Prototype results from “Some Results -Summer 2005” (Ron Settles) mm^2, B = 1T Point resolution, Micromegas Saclay/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 = 1T 1x10mm^2 pads

  12. Prototype results contd.Point resolution GEM DESY group. (r,) measured for GEMs with 2x6mm2 pads B=4T Gas:P5 Victoria group has achieved ~ 100 µm resolution for short drift distances with narrower 1.2x7mm2 pads. 30cm

  13. Prototype Results - cont.Point resolution, GEM PRELIMINARY! --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.

  14. Ideas to improve the MPGD TPC resolution • 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

  15. 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. IPNS KEK 14/10/05

  16. An idea - Measure the induced signals in a GEM GEM Proportional wire Anode pads Cathode pads • Short ~ 200 ns signal We measure x  y  70 µm • But this technique requires expensive high frequency pulse shape sampling electronics IPNS KEK 14/10/05

  17. Another idea- Position sensing from charge dispersion in MPGDs with a resistive anode Analogy: Charge division is used to measure the avalanche position on a proportional wire.Deposit point charge at t=0 Telegraph equation (1-D): Solution for charge density on the wire(L ~ 0) Generalize the concept of 1-D charge division to 2-D IPNS KEK 14/10/05

  18. Charge dispersion in a GEM with a resistive anode IPNS KEK 14/10/05

  19. Current generators Resistive anode foil Signal pickup pads Pad amplifier Equivalent circuit for currents in a GEM with an intermediate resistive anode IPNS KEK 14/10/05

  20. A photon event in the resistive anode GEM test cell Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width) IPNS KEK 14/10/05

  21. M.S.Dixit et.al., Nucl. Instrum. Methods A518 (2004) 721. Improving resolution with charge dispersion in a MPGD with a resistive anode • 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: (r) Q (r,t) integral over pads mm ns r / mm IPNS KEK 14/10/05

  22. 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. 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. Difference = induced signal (not included in simulation) studied previously:MPGD '99 (Orsay), LCWS '00 Primary signal: Fast large amplitude main pulse on charge collecting pad. Simulated primary pulse is normalized to the data. Secondary signal: The dispersion pulse on the neighboring pad is slower & smaller. Simulated secondary pulse normalization is the sameas for the primary. IPNS KEK 14/10/05

  23. Al-Si Cermet on mylar Drift Gap MESH Amplification Gap 50 m pillars Resistive anode Micromegas 530 k/ Carbon loaded Kapton resistive anode was used with GEM. This was replaced with more uniform higher resistivity 1 M/ Cermet for Micromegas. IPNS KEK 14/10/05

  24. Charge dispersion signals in MicromegasSingle event(2 mm wide pads) 2nd neighbor (note different scale) 2 x 4 channel Tektronix X-ray spot centred on pad 2 Ar/CO2 90/10, Gain ~ 3000 1st neighbor peak ~ 100 ns after the primary pulse peak Slow rising 2nd neighbor pulse ~ 25 MHz digitization could replace pulse shape sampling Two 1st neighbors Primary signal IPNS KEK 14/10/05

  25. Scan across width Pad 22 Pad 23 Pad 24 GEM pad response function for collimated x raysSimulation versus measurement Ionization from 50 m collimated x-rays. 2x6 mm2 pads (Solid line) Measured PRF deviates from simulation due to anode RC nonuniformities. IPNS KEK 14/10/05

  26. Resistive anode double-GEM spatial resolution Collimated ~ 50 m x-ray spot 2x6 mm2 pads GEM resolution ~ 70 mm. Similar resolution measured for a Micromegas with a resistive anode readout using 2 mm x 6 mm pads IPNS KEK 14/10/05

  27. A fringe benefit: High Micromegas gain with a resistive anode Argon/Isobutane 90/10 Resistive anode suppresses sparking stabilizing Micromegas Extremely high gains without breakdown are possible IPNS KEK 14/10/05

  28. Carleton cosmic ray test MPGD-TPC 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 resolution was next measured with a charge dispersion resistive anode readout with a double-GEM & with a Micromegas endcap. The GEM-TPC resolution was first measured with conventional direct charge TPC readout. IPNS KEK 14/10/05

  29. 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. 2x6 mm2 pads Simulation Data Centre pad amplitude used for normalization - no other free parameters. IPNS KEK 14/10/05

  30. 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 signalsin 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. IPNS KEK 14/10/05

  31. GEM & Micromegas PRFs for TPC trackAr:CO2 (90:10)2x6 mm2 pads The pad response function maximum for longer drift distances is lower due to Z dependent normalization. GEM PRFs Micromegas PRFs Micromegas PRF is narrower due to the use of higher resistivity anode & smaller diffusion after avalanche gain IPNS KEK 14/10/05

  32. 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 itsFWHM (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. a2 a4 b2 & b4 can be written down in terms of  and  & two scale parameters a & b. IPNS KEK 14/10/05

  33. 6 mm   [ ] 2 2 = 2 mm rows i=pads Track fit using the the PRF Track at: xtrack= x0+ tan() yrow Determine x0 &  by minimizing2 for the entire event 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 IPNS KEK 14/10/05

  34. Bias corrections with GEM & with Micromegas Initial bias Initial bias Remaining bias after correction Remaining bias after correction 2 mm pads 2 mm pads Micromegas GEM IPNS KEK 14/10/05

  35. What is the diffusion limit of resolution for a gaseous TPC? Resolution depends on electron statistics. Electron number N fluctuates from event to event. 0includes noise & systematic effects. Cd = diffusion constant; z = drift distance . Neff <N> the average number of electrons = 1/<1/N> the inverse of average of 1/N Gain fluctuations also affectNeff IPNS KEK 14/10/05

  36. Simulation for the effective number of electrons for resolution2 mm x 6 mm pads - Ar/CO2 90/10 Cosmic ray momentum spectrum Measured pad pulse height distribution dE/dx in Argon Mostly muons at sea level Total Ionization . 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) IPNS KEK 14/10/05

  37. 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 Unpublished [Neff = 38.9] 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. IPNS KEK 14/10/05

  38. 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. IPNS KEK 14/10/05

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