Resistive anode to improve the resolution of MPGD TPCs

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Resistive anode to improve the resolution of MPGD
TPCs

• Madhu Dixit
• Carleton University TRIUMF

3rd Symposium on Large TPCs for Low Energy Rare
Event Detection
Paris 12 December 2006
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The large TPC challengeHow to reduce complexity
and achieve good resolution at an affordable costs

• Several large MPGD-TPCs proposed for HEP and for
  rare event detection e.g.
• ILC TPC 1,500,000 channels (2 mm x 6 mm pads)
• T2K TPC 100,000 channels (7 mm x 9 mm pads)
• ILC challenge ?Tr 100 ?m for all tracks (2 m
  drft)
• ILC MPGD-TPC prototype RD indicates 2 mm wide
  pads too wide, need 1 mm or narrower pads
• New MPGD readout concept of charge dispersion
  developed to achieve good resolution with wide
  pads.
• The proof of concept and ILC-TPC prototype test
  results
• Possible application to T2K TPC - improving
  resolution without resorting to narrower pads

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Limits to the TPC position resolution

• The physics limit of TPC resolution comes from
  transverse diffusion
• Neff
  effective no. of electrons contributing to
  position determination.
• For best resolution, choose a gas with smallest
  diffusion
• The rule applies to the wire TPCs. They use
  induced cathode pad signals for position
  determination. But ExB track angle systematic
  effects degrade wire TPC resolution.
• ExB effect does not limit the MPGD-TPC. But there
  are no comparable induced cathode pad signals.
• The MPGD-TPC resolution is limited by pad width
  w. The resolution gets worse for wide pads in
  absence of diffusion.

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Pad width limits the MPGD-TPC resolutionExB
angle effects limit the wire/pad TPC resolution
For small diffusion, less precise centroid for
wide pads
Accurate centroid determination possible with
wide pads
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How to get good MPGD resolution with wide pads?

• Find a mechanism similar to proportional wire
  induced cathode pad signals
• Charge dispersion - a new geometrical pad signal
  induction mechanism for the MPGD readout that
  makes position determination insensitive to pad
  width.

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Position sensing from charge dispersion in a MPGD
with a resistive anode
Position sensing on a resistive anode
proportional wire from charge division
Telegraph equation (1-D)
Deposit point charge at t0
Solution for charge density (L 0)
Generalize 1 D proportional wire charge division
to a 2 D RC network
Position sensing from charge dispersion in MPGDs
with a resistive anode
Equivalent to Telegraph equation in 2-D
Solution for charge density in 2-D
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Charge dispersion in a MPGD with a resistive anode

• Modified MPGD 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
M.S.Dixit et.al., Nucl. Instrum. Methods A518
(2004) 721.
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Resistive anode Micromegas
530 k?/? Carbon loaded Kapton resistive anode was
used with GEM. This was replaced with higher
resistivity 1 M?/? Cermet for tests with
Micromegas.
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The proof of concept - GEM charge dispersion x
ray tests
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Point resolution for GEM Charge dispersion
readout (Ar10CO2) Collimated 4.5 keV x rays,
Spot size 50 ?m
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

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Learning to track with charge dispersion Cosmic
ray tests no magnetic field

• 15 cm drift length with GEM or Micromegas
  readout
• B0
• Ar10 CO2 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.
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GEM Micromegas track Pad Response Functions
Ar10CO2 2x6 mm2 pads
The pad response function (PRF) amplitude 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 than
GEM after avalanche gain
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Transverse resolution (B0) for cosmic rays
Ar10CO2
R.K.Carnegie et.al., NIM A538 (2005) 372
K. Boudjemline et.al., submitted to NIM
To be published
Compared to conventional readout, charge
dispersion gives better resolution for the GEM
and the Micromegas.
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First tests in a magnetic field (Oct,
2005)Micromegas TPC - charge dispersion readout

• 4 GeV/c KEK PS ?2 hadron test beam
• Super conducting 1.2 T magnet
• Inner diameter 850 mm
• Effective length 1 m

Canada, France, Germany, Japan (Carleton,
Montreal, Saclay, Orsay, MPI (Munich), KEK,
Kinnki, Kogakuin, Saga, Tsukuba and TUAT)
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Transverse spatial resolution Ar5iC4H10
E70V/cm DTr 125 µm/?cm (Magboltz) _at_ B 1T
Micromegas TPC 2 x 6 mm2 pads
4 GeV/c ? beam? 0, ? 0

• Strong suppression of transverse diffusion at 4
  T.
• Examples
• DTr 25 ?m/?cm (Ar/CH4 91/9)
  Aleph TPC gas
• 20 ?m/?cm (Ar/CF4 97/3)

Extrapolate to B 4T Use DTr 25 µm/?cm
Resolution (2x6 mm2 pads) ?Tr ? 100 ?m (2.5 m
drift)
s0 (521) mm Neff 22?0 (stat.)
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Tests in the 5 T magnet test facility at DESY
(Nov-Dec, 2006)
(Carleton-Orsay-Saclay-Montreal) COSMo TPC track
display
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COSMo TPC - Transverse Resolution B 5 T DT 19
?m/?cm
Micromegas resistive readout
2 mm x 6 mm pads
Cosmic ray tracks
Preliminary
50 ?m average
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COSMo TPC - Transverse Resolution B 5 T
Ar/C4H10 95/5 DT 27 ?m/?cm
Micromegas resistive readout
2 mm x 6 mm pads
Cosmic ray tracks
Preliminary
50 ?m average
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Simulating the charge dispersion phenomenon
M.S.Dixit and A. Rankin, Nucl. Instrum. Methods
A566 (2006) 281.

• The charge dispersion equation describe the time
  evolution of a point like charge deposited on the
  MPGD resistive anode at t 0.
• To compare to experiment, one needs to include
  the effects of
• Longitudinal transverse diffusion in the gas.
• Intrinsic rise time Trise of the detector charge
  pulse.
• The effect of preamplifier rise and fall times tr
  tf.
• And for particle tracks, the effects of primary
  ionization clustering.

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Simulation for a single charge cluster(tracks
can be simulated by superposition)
The charge density function for a point charge in
Cartesian coordinates
Physics effects included in simulation in two
parts 1) as effects which depend on spatial
coordinates x y, or 2) as effects which depend
on time. 1) The spatial effects function includes
charge dispersion phenomena transverse size w
of the charge cluster due to transverse diffusion.

Qpad(t) is the pad signal from charge dispersion
when a charge Nqe of size w is deposited on the
anode at t 0
(1)
xhigh, xlow, yhigh, ylow define the pad
boundaries
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(2)
I(t) incorporates intrinsic rise time,
longitudinal diffusion electronics shaping
times as time dependent effects.
(1) and (2) are convoluted numerically for the
model simulation.
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Charge dispersion spot x-ray signal for GEM
Simulation versus measurement (Ar10CO2)(2 x 6
mm2 pads) Collimated 50 ?m 4.5 keV x-ray spot
on pad centre.
Difference induced signal (not included in
simulation) studied previously
MPGD '99 (Orsay), LCWS 2004 Paris
Primary pulse normalization used for the
simulated secondary pulse
Simulated primary pulse is normalized to the
data.
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GEM TPC charge dispersion simulation (B0)
Cosmic ray track, Z 67 mm Ar10CO2
2x6 mm2 pads
Simulation Data
Centre pulse used for simulation normalization -
no other free parameters.
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Application to T2K TPC

• 7x9 mm2 pads
• 10 ?p/p (1 GeV/c)
• Good enough
• Requirement limited by Fermi motion

(from a talk by F.Sánchez (Universitat Autònoma
de Barcelona)
But better momentum resolution would be
useful Better background rejection More
channels gt ? Can one do it with the presently
chosen pad dimensions?
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T2K simulation for 8 x 8 mm2 padsTrack crosses
no pad row or column boundariesAr10 CO2 ,
vDrift 28 ?m/ns (E 300 V/cm) Aleph preamp
tRise 40 ns, tFall 2 ?s
Anode surface resistivity 150 K?/?, dielectric
gap 75 ?m, K 2
Track at z 175 mm, x 0, ? 0 (uniform
ionization)
(ns)
(ns)
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Micromegas TPC with resistive readout - Simulated
PRF 8 x 8 mm2 pads, Ar10 CO2_at_ 300 V/cm, 175 mm
drift distance
(mm)
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Summary

• Traditional MPGD-TPC has difficulty achieving
  good resolution with wide pads
• With charge dispersion, the charge can be
  dispersed in a controlled way such that wide pads
  can be used without sacrificing resolution. We
  have achieved excellent resolution with wide pads
  both for the GEM and the Micromegas.
• At 5 T, an average 50 ?m resolution has been
  demonstrated with 2 x 6 mm2 readout pads for
  drift distances up to 15 cm.
• The ILC-TPC resolution goal, 100 ?m for all
  tracks, appears feasible. Good control of
  systematics will be needed.
• For T2K, it appears possible to achieve better
  resolution than design goal if charge dispersion
  readout is used.
• Good understanding of charge dispersion. The
  simulation can be used to optimize charge
  dispersion TPC readout.