Resistive anode to improve the resolution of MPGD TPCs

Madhu Dixit

Carleton University & TRIUMF

Paris 12 December 2006

3rd Symposium on Large TPCs for Low Energy Rare Event Detection

12/12/2006 TPC Rare Event Symposium - Paris 2

•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 R&D 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

The large TPC challenge:How to reduce complexity and achieve good resolution at an affordable costs

12/12/2006 TPC Rare Event Symposium - Paris 3

•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 diffusionThe 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.

Limits to the TPC position resolution

€

x2 ≈

DTr2 ⋅z

Neff

€

x2 ⇒

w2

12as z ⇒ 0

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For small diffusion, less precise centroid for wide pads

Micro PatternGas Detector

Proportionalwire

Anode pads Cathode pads

Induced cathode signal determined by geometry

Direct signal on the MPGD anode pad

width w width w

Accurate centroid determination possible with wide pads

Pad width limits the MPGD-TPC resolutionExB angle effects limit the wire/pad TPC resolution

eff

Trx N

zD ⋅+≈

22

02 [ ]12

1 2220

2 wzDN Tr

effx ++≈ σσ

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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.

12/12/2006 TPC Rare Event Symposium - Paris 6

Position sensing from charge dispersion in a MPGD with a resistive anode

Position sensing on a resistive anode proportional wire from charge division

Solution for charge density (L ~ 0)

Telegraph equation (1-D):2

2

2

2 1xQ

RCtQ

tQ

RL

∂∂

=∂∂

+∂∂

€

Q(x, t) =RC

4π t

−x 2RC

4 te

€

∂Q

∂t=

1

RC

∂2Q

∂r2+

1

r

∂Q

∂r

⎡

⎣ ⎢

⎤

⎦ ⎥

€

Q(r, t) =RC

2t

−r 2RC

4 te

Deposit point charge at t=0

Equivalent to Telegraph equation in 2-D

Generalize 1 D proportional wire charge division to a 2 D RC network

Solution for charge density in 2-D

Position sensing from charge dispersion in MPGDs with a resistive anode

12/12/2006 TPC Rare Event Symposium - Paris 7

•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.

€

∂ρ∂t

=1

RC

∂2ρ

∂r2+

1

r

∂ρ

∂r

⎡

⎣ ⎢

⎤

⎦ ⎥

€

⇒ ρ(r, t) =RC

2t

−r2RC

4 te

ρ(r,t) integral over pads

ρ(r) Q

mm ns

Charge dispersion in a MPGD with a resistive anode

Equation for surface charge density function on the 2-dim. continuous RC network:

M.S.Dixit et.al., Nucl. Instrum. Methods A518 (2004) 721.

12/12/2006 TPC Rare Event Symposium - Paris 8

50 m pillars

Drift Gap

MESHAmplification Gap

Al-Si Cermet on mylar

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

12/12/2006 TPC Rare Event Symposium - Paris 10

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

Point resolution for GEM Charge dispersion readout (Ar+10%CO2) Collimated ~ 4.5 keV x rays, Spot size ~ 50 m

2x6 mm2 pads

12/12/2006 TPC Rare Event Symposium - Paris 11

•15 cm drift length with GEM or Micromegas readout •B=0•Ar+10% 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.

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

12/12/2006 TPC Rare Event Symposium - Paris 12

GEM & Micromegas track Pad Response Functions Ar+10%CO2 2x6 mm2 pads

GEM PRFs Micromegas PRFs

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

The pad response function (PRF) amplitude for longer drift distances is lower due to Z dependent normalization.

12/12/2006 TPC Rare Event Symposium - Paris 13

Transverse resolution (B=0) for cosmic rays Ar+10%CO2

R.K.Carnegie et.al., NIM A538 (2005) 372

K. Boudjemline et.al., submitted to NIM

€

02 +

CD2

Nez

To be published

Compared to conventional readout, charge dispersion gives better resolution for the GEM and the Micromegas.

12/12/2006 TPC Rare Event Symposium - Paris 14

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

First tests in a magnetic field (Oct, 2005)

Micromegas TPC - charge dispersion readout

Canada, France, Germany, Japan (Carleton, Montreal, Saclay, Orsay, MPI (Munich), KEK, Kinnki, Kogakuin, Saga, Tsukuba and TUAT)

12/12/2006 TPC Rare Event Symposium - Paris 15

Extrapolate to B = 4T Use DTr = 25 µm/cm

Resolution (2x6 mm2 pads) Tr 100 m (2.5 m drift)

Transverse spatial resolution Ar+5%iC4H10 E=70V/cm DTr = 125 µm/cm (Magboltz) @ B= 1T

€

x = σ 02 +

Cd2 ⋅z

Neff

4 GeV/c + beam ~ 0°, ~ 0°

0= (52±1) m Neff = 220 (stat.)

Micromegas TPC 2 x 6 mm2 pads

•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)

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(Carleton-Orsay-Saclay-Montreal) COSMo TPC track display

Tests in the 5 T magnet test facility at DESY(Nov-Dec, 2006)

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Micromegas resistive readout2 mm x 6 mm pads

Preliminary

Preliminary

COSMo TPC - Transverse Resolution B= 5 T DT= 19 m/cm

~ 50 m average

Cosmic ray tracks

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Micromegas resistive readout

2 mm x 6 mm pads

Preliminary

Preliminary

~ 50 m average

Cosmic ray tracks

COSMo TPC - Transverse Resolution B= 5 T Ar/C4H10 95/5 DT= 27 m/cm

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Simulating the charge dispersion phenomenon

•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.

M.S.Dixit and A. Rankin, Nucl. Instrum. Methods A566 (2006) 281.

12/12/2006 TPC Rare Event Symposium - Paris 20

€

ρδ(x, y, t) =τ

4πtexp −τ x2 + y2

( ) 4t[ ] where τ = RC

€

Qpad =Nqe

4erf (

xhigh

2σ xy

) − erf (xlow

2σ xy

) ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥erf (

yhigh

2σ xy

) − erf (ylow

2σ xy

) ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

xhigh, xlow, yhigh, ylow define the pad boundaries &

€

xy = 2t /τ + w2

The charge density function for a point charge in Cartesian coordinates:

Simulation for a single charge cluster(tracks can be simulated by superposition)

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)

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I(t) incorporates intrinsic rise time, longitudinal diffusion & electronics shaping times as time dependent effects.

I(t) =1

2Trise

exp 2a2 / 2 −at( ) erft−Trise −

2a 2

⎛

⎝⎜⎞

⎠⎟+1

⎡

⎣⎢

⎤

⎦⎥−

exp 2b2 / 2 −bt( ) erft−Trise −

2b 2

⎛

⎝⎜⎞

⎠⎟+1

⎡

⎣⎢

⎤

⎦⎥−

exp 2a2 / 2 −a t−Trise( )( ) erft−Trise −

2a 2

⎛

⎝⎜⎞

⎠⎟+1

⎡

⎣⎢

⎤

⎦⎥+

exp 2b2 / 2 −b t−Trise( )( ) erft−Trise −

2b 2

⎛

⎝⎜⎞

⎠⎟+1

⎡

⎣⎢

⎤

⎦⎥+

exp 2a2 / 2 −at( ) erft− 2a 2

⎛

⎝⎜⎞

⎠⎟+ erf

t−Trise −2a

2⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢

⎤

⎦⎥−

exp 2b2 / 2 −bt( ) erft− 2b 2

⎛

⎝⎜⎞

⎠⎟+ erf

t−Trise −2b

2⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢

⎤

⎦⎥

⎡

⎣

⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥

€

a = 1 tf ; b = 1 tf +1 tr

(2)

(1) and (2) are convoluted numerically for the model simulation.

12/12/2006 TPC Rare Event Symposium - Paris 22

Charge dispersion spot x-ray signal for GEM Simulation versus measurement (Ar+10%CO2)

(2 x 6 mm2 pads) Collimated ~ 50 m 4.5 keV x-ray spot on pad centre.

Simulated primary pulse is normalized to the data.

Difference = induced signal (not included in simulation) studied previously: MPGD '99 (Orsay), LCWS 2004 Paris

Primary pulse normalization used for the simulated secondary pulse

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GEM TPC charge dispersion simulation (B=0) Cosmic ray track, Z = 67 mm Ar+10%CO2

Centre pulse used for simulation normalization - no other free parameters.

2x6 mm2 pads

Simulation

Data

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(from a talk by F.Sánchez (Universitat Autònoma de Barcelona)

Application to T2K TPC

But better momentum resolution would be useful:

Better background rejection = More channels => $$?

Can one do it with the presently chosen pad dimensions?

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

12/12/2006 TPC Rare Event Symposium - Paris 25

T2K simulation for 8 x 8 mm2 padsTrack crosses no pad row or column boundaries

Ar+10% CO2 , vDrift = 28 m/ns (E = 300 V/cm) Aleph preamp tRise = 40 ns, tFall = 2 s

(ns) (ns)

Track at z = 175 mm, x = 0, = 0 (uniform ionization)

Anode surface resistivity 150 K/, dielectric gap = 75 m, K = 2

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(mm)10-10-20 200

Rel

ati v

e am

p lit

u de

Pad response function

Micromegas TPC with resistive readout - Simulated PRF 8 x 8 mm2 pads, Ar+10% CO2@ 300 V/cm, 175 mm drift distance

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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.