January 10, 2008 MiPGD TPC resolution and gas 1

Micromegas TPCaddendum on measurements

P. Colas, SaclayLectures at the TPC school, Tsinghua University, Beijing,

January 7-11, 2008

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METHOD FOR MEASURING THE DRIFT VELOCITIES

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Setup for drift velocity measurements

LASER

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Time (s)Time (ns)

t t

Mesh signal Drift electrode signal

Electron and ion drift velocities are obtained from the time they take to cross the 3mm drift gap

MiPGD TPC resolution and gasMiPGD TPC resolution and gas 55January 10, 2008January 10, 2008

0

0.005

0.01

0.015

0.02

0 1 2 3 4 5

Vitesse ionique He+10 % Isobutane

vite

sse

ion

iqu

e (

cm/µ

s)

Champ (kV/cm)

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X-ray source

1500 lpi micromegas 500 lpiMesh current measuremt

on the HV supply

SETUP USED FOR ION BACKFLOWAND AGING STUDIES

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Data taken in the 4 GeV beam at KEK in June 2005

Gas: Ar+5% isobutane

‘Multi-Prototype TPC’ (Ron Settles) to test various technologies

Saclay-Orsay Micromegas endplate

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beam

55Fe

384 pads, 2.3x6.3 mm2

Read out by ALEPH electronics

Max drift 26 cm

permanent monitoring by reading the mesh signal of a 55Fe source

JTPC online event display (D. Karlen)

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Drift velocity Drift velocity measurementmeasurement• Cross-check of gas purity and MC simulationCross-check of gas purity and MC simulation

Using a beam at 45 deg. Look at time distribution on one pad. Max time gives drift time over 26.08+-0.02 cm (add trig. delay)

1 cm scint.

cathode

Vdrift (Ar+5%iso, E=220V/cm) = 4.181 +- 0.034 cm/s

In agreement with Magboltz : 4.173 +- 0.016

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Measurement of the diffusion Measurement of the diffusion coefficient at B=0, 0.5 and 1Tcoefficient at B=0, 0.5 and 1T

Magnetic Magnetic fieldfield

0 T0 T 0.5 T0.5 T 1 T1 T

Global Global likelihoolikelihoodd

488 488 ± ± 1111

314 314 ± ± 1515

209 209 ± 7± 7

PRF PRF widthwidth

(stat.onl(stat.only)y)

475 475 ± ± 33 293 293 ± ± 44 194 194 ± ± 1818

MagboltzMagboltz 469.3469.3 284.1284.1 192.6192.6

in /√cm

2 methods: global likelihood fit of the track width to all pad charges (shown here), or width of the PRF (slope at large distance is unbiased)

Good agreement between the two methods and good agreement with Magboltz.

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Study of the resolution Study of the resolution (theory)(theory)At large drift distance, transverse

diffusion dominates: resol ~ CD√z/√Neff

Neff different from Ntot because of

-Ionisation fluctuations 1/<1/N>

-Gain fluctuations: x <G2>/<G>2

At small distance, hodoscope effect: not enough charge spreading by diffusion to encompass more than 1 pad

Ex: for 60 e- total, Neff=21.2±2.7

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Resolution measurement Resolution measurement B=0B=0

r.m.s. of the residuals (√withwo)

2 methods for the track: global likelihood fit or 2 fit

Note: bias at small z - the track is reconstructed close to the middle of the central pad (hollow points)

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Resolution measurement B=0.5 Resolution measurement B=0.5 and 1Tand 1T

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ScalingScaling(use dimensionless quantities scaled by the pad width w)

The resolution dependance on z has two regimes:

At large z the asymptotic behaviour follows the diffusion limit

At low z the effect of finite pad size dominates.

For typical values of Neff, the optimal resolution is about 10% of the pad size.

1/√12

1/√(12.Neff)

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Extrapolation to ILC-Extrapolation to ILC-TPCTPC

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CONCLUSIONSCONCLUSIONS

• A clear understanding of the basic limitations on A clear understanding of the basic limitations on the resolution of a Micromegas TPC with standard the resolution of a Micromegas TPC with standard pad readout has been obtainedpad readout has been obtained

• The role of ionisation statistics, gas gain The role of ionisation statistics, gas gain fluctuations and finite pad size have been clearly fluctuations and finite pad size have been clearly assessed, opening the way to optimizationassessed, opening the way to optimization

• A good agreement is found with beam test resultsA good agreement is found with beam test results• For Linear Collider applications, For Linear Collider applications, standardstandard 2x6mm 2x6mm22

pads will not give the target resolution. Ongoing pads will not give the target resolution. Ongoing developments will be necessary: charge developments will be necessary: charge spreading or pixel readoutspreading or pixel readout