Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 1 Micromegas TPC R&D (and wire...

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Prague , Novembe r 15-18th, 2002 Vincent Lepeltier Micro megas TPC R&D 1 Micromegas TPC R&D Micromegas TPC R&D (and wire chamber) (and wire chamber) First measurements with a 2T First measurements with a 2T magnetic field magnetic field ion feed-back ion feed-back New studies ( New studies ( mainly mainly simulations) simulations) on: on: some gas properties some gas properties ion feedback ion feedback Cosmic set-up under construction Cosmic set-up under construction How to improve the r-φ How to improve the r-φ resolution? resolution? Conclusion Conclusion F. Bieser 1 , R. Cizeron 2 , P. Colas 3 , C. Coquelet 3 ,E. Delagnes 3 , B. Genolini 4 , A. Giganon 3 , Y. Giomataris 3 , G. Guilhem 2 , S. Herlant 3 , J. Jeanjean 2 , V. Lepeltier 2 , J. Martin 3 , A. Olivier 3 , J. Peyré 4 , J. Pouthas 4 , Ph. Rebourgeard 3 , M. Ronan 1 (and many others, not mentioned) 1) LBL, 2) LAL Orsay, 3) DAPNIA Saclay, 4) IPN Orsay ECFA-DESY workshop workshop

Transcript of Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 1 Micromegas TPC R&D (and wire...

Page 1: Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 1 Micromegas TPC R&D (and wire chamber) First measurements with a 2T magnetic fieldFirst.

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Micromegas TPC R&D Micromegas TPC R&D (and wire chamber)(and wire chamber) Micromegas TPC R&D Micromegas TPC R&D (and wire chamber)(and wire chamber)

• First measurements with a 2T First measurements with a 2T

magnetic fieldmagnetic field

ion feed-backion feed-back

• New studies (New studies (mainlymainly simulations) on: simulations) on: – some gas properties some gas properties

– ion feedbackion feedback

• Cosmic set-up under constructionCosmic set-up under construction

• How to improve the r-φ resolution?How to improve the r-φ resolution?

• ConclusionConclusion

F. Bieser1, R. Cizeron2, P. Colas3, C. Coquelet3,E. Delagnes3, B. Genolini4, A. Giganon3,Y. Giomataris3, G. Guilhem2, S. Herlant3, J. Jeanjean2, V. Lepeltier2, J. Martin3, A. Olivier3, J. Peyré4, J. Pouthas4, Ph. Rebourgeard3, M. Ronan1 (and many others, not mentioned)

1) LBL, 2) LAL Orsay, 3) DAPNIA Saclay, 4) IPN Orsay

ECFA-DESY workshopworkshop

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First measurements in a magnetic fieldFirst measurements in a magnetic fieldFirst measurements in a magnetic fieldFirst measurements in a magnetic field By P. Colas1, Y. Giomataris1, J. Jeanjean2, V. Lepeltier2, J. Martin1, A. Olivier1

1) DAPNIA Saclay 2) LAL Orsay

We took data end of June with a small-gap wire-chamber TPC and a micromegas TPC (both 1cm drift) in a 1-2 Tesla magnetic field

The primary ionization was provided by

•an 55Fe X-ray source (25 MBq, gain typically 100,000) for the wire

chamber

•a 90Sr -ray source (1 GBq, gain a few 100) for the micromegas

The currents from the supplies were monitored, allowing ion feedback

measurements

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55Fe

0 +2kV 0 - 300 V

Cathodegridwires

90Sr

0 -340 V - 640 V

Cathodemeshanode

2mm 2mm 1cm 50 m 1cm

Small-gap wire TPC Micromegas TPC

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ION FEEDBACK MEASUREMENTS

Vmesh

Vdrift

I2 (mesh)

I1 (drift)

X-ray gun or -source

primary ions + feedback

I1+I2 ~ G x primary current

Obtain primary from G=1 ( at small Vmesh)

Eliminate G between the 2 equations to obtain the feedback fraction

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2T SaclayNMR magnet

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B//E

The total current (primary ionisation x gain) is reasonably constant with B in the wire chamber case.

The current increase seen with micromegas is very likely due to in increase in primary ionisation

( electrons are spiraling

better electron collection)

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The ion feedback does not depend significantly on the magnetic field

It is in quite good agreement with prediction for Ar-CH4 (90:10) and for a 500 lpi grid.

ED/EA

~4xED/EA

90Sr -source

Ar-CH4 10%

expected value

measurement

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The ion feedback does not depend on B for the wire chamber (however the MC predicts a 50% increase from 0 to 2 Tesla)

An effective field ratio can be defined, and the ion feedback is equal to the field ratio .

The feedback is a bit worse for Ar-CH4, as expected from a lower diffusion

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new gas studies (simulations)

amplification properties

the quencher has:

•a very small influence on the optimal gap, which is determined mainly by the dominant atomic mass (argon)

•a large influence on the gain

max. gain at ~25 μm

with Argon

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Stationnarity of the gainStationnarity of the gainStationnarity of the gainStationnarity of the gain

• VeryVery The gain is stationnary (maximal) as a function of the gap around a few 10m.

decrease of the gas atomic mass the optimal gap and the maximum gain

good dE/dx potential

for a micromegas TPC

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Gas studies Constraints on the gas mixture:

Drift properties: to obtain a high drift velocity plateau at low E-field, an Ar-dominated carrier is required (good also for dE/dx)

Hydrogen should be avoided because of neutron background: no CH4

Use of CF4 as a quencher improves T

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Gas studies

Ar-CF4 (1 to 4%) mixtures ~vxB/E

1% ~24

2% ~ 19

4% ~14

3% ~16

Drift properties:

A plateau drift velocity of ~8 cm/s with E<200 V/cm can be obtained with Ar-CF4 mixtures (confirmed by measurements)

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Gas studies

attachment is negligible below 500 V/cm (supported by our measurements) and above 15 kV/cm (dominated by Townsend)

with Micromegas, the transition between drift and multiplication spaces is very short (a few m) so the loss of electrons is expected to be negligible (a few %)

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Ar CF4 (98:2) has many nice properties:

drift velocity plateau at 180 V/cm

with 7 cm/s

good gain

electron attachment is negligible

below ~500V/cm

(need absorption less than 1/(10m)

to be checked with the prototype

was suspected of aging.

--> aging test (presented by Paul C. at St- Malo and Jeju):

NO aging

100

1000

104

105

360 380 400 420 440 460

Gain Ar+2% CF4Edrift=100 V/cm

Ga

in

HV2 (V)

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Ion feed-back modelisation: Ion feed-back modelisation: funnel effectfunnel effect

Ion feed-back modelisation: Ion feed-back modelisation: funnel effectfunnel effect

• VeryVeryS1/S2 ~ Eamplif / Edrift (Gauss theorem)

due to diffusion of the electrons in the avalanche, the ions are unlikely to follow back the field lines to the drift space.

the dominant parameter

is the ratio diff/pitch

typically for 100m and 40kV/cm at 1 atm.:

~12-15 m

ion feed-back and

related space charge

effects are suppressed

S1

S2

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Hypotheses on the avalanche

Gaussian diffusionPeriodical structure

l

2

Avalanche Dispersion

Ion feedback theoryIon feedback theoryIon feedback theoryIon feedback theory

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ion feedback

0,2 0,3 0,4 0,5 0,6 0,7 0,8

sigma/l

ion

fe

ed

ba

ck

/ f

ield

ra

tio

Field ratio

Feedback 2D

Feedback 3D

conclusion: the ion feedback is close to its optimum (equal to the field ratio) if /l > 0.5

1000 l.p.i. meshes (for usual gases and 50-100 μm gap)

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ion feedback measurements

as expected scales as 1 / field ratio reasonably well (/l ~0.7)

Ar + 10% isobutane

1500 lpi mesh

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Building a TPC for a cosmic test in a magnetic Building a TPC for a cosmic test in a magnetic field field

Building a TPC for a cosmic test in a magnetic Building a TPC for a cosmic test in a magnetic field field

• 2 tesla magnet brought to operation end of March 20022 tesla magnet brought to operation end of March 2002

• STAR front-end electronics. STAR front-end electronics. Full wave sampling on 1028 Full wave sampling on 1028

channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits

ADC, 512 time buckets deep, low noiseADC, 512 time buckets deep, low noise

• removable detector endplate removable detector endplate (plan to test micromegas, (plan to test micromegas,

wires, +options for e-cloud spreading)wires, +options for e-cloud spreading)

• Supplies recuperated from LEPSupplies recuperated from LEP

• Copper grid (Nickel was shown to deform strongly in a Copper grid (Nickel was shown to deform strongly in a

magnetic field)magnetic field)

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TPC for the cosmic test

Field cage

Detector

Front end electronics

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2x10 mm2 pads

1024 pads

1x10 mm2 pads

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STAR READOUT ELECTRONICS

TEST BENCH

Front end cards

Pulse generator

Mother board

Optical link

VME processor

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HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?

1. problem:

the intrinsic optimal r-φ resolution is roughly

σrφ = σtr.diff./Ne , with σtr.diff.(цm) =500xLdrift/(1+22)

2. in our case: Ne ~ 60 for a 6mm-long pad

and ~ 17 with Ar-2%CF4 at 1 atm. , 180V/cm and 4T

for Ldrift ~250cm: tr.diff. =500 m and r = 62 m

“ 100cm: tr.diff. =300 m and r = 40 m

“ 25cm: tr.diff. =150 m and r = 20 m

micr. ~ 20 m

3. (rectangular) optimal pad width l < 3-4 tr.diff in order to spread over 2-3

pads

l ~ 500m ... and 6 millions channels !!!

if l =2mm need to diffuse charge over ~500-600 m

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HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?

1. Chevron pads

2. Diffusion of electrons AFTER multiplication

3. Resistive sheet

4. Pad segmentation

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DIFFUSION OF ELECTRONS AFTER MULTIPLICATION

------------------------

------------------------

micromegas meshmesh

drift

E~40 kV/cm multiplication

E~ 160V/cm

E~ 8-10 kV/cm diffusion ~1cm ~500m

pad plane

But: 1/ bad electron transparency from multiplication to diffusion region (20%?)

2/ attachment problem with CF4

will be tested soon with Subatech lab (Nantes) at CERN or PSI

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for example: diffuse over 1cm at ~8-10kV/cm ~ 500m

but: attachment+Townsend many + and - ions producing a very long

( >100s !!!) and huge signal Nions~ ~ 2xe2xe1010 ~2000020000

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USE OF A RESISTIVE SHEET

------------------------

micromegas mesh

resistive sheet~1M/

drift

E~40 kV/cm multiplication

E~ 160V/cm

50 m mylar pad plane

idea: the resistive sheet spreads the signal over a larger surface cf Kisten Sachs’s talk at Jeju and Madhu Dixit’s at Prague

will be tested soon at Saclay with a plotted micro-mesh

simple calculations made by Eric Delagnes at Saclay

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CONCLUSIONCONCLUSIONCONCLUSIONCONCLUSION• A strong collaboration within the PRC group is building a cosmic

test for a micromegas (and an asymmetric wire chamber) TPCs in a 2T magnetic field.

• In parallel, tests of various aspects of the micromegas behavior are conducted. They allow to assess the potentialities of this technology. Operation of a micromegas device in a magnetic field has been successfully tried for the first time.

• Ion feedback, aging, gain, drift velocities have been studied for

several gases. Ar+2% CF4 seems to be a very promising

mixture for the LC TPC in all these respects.

• Within a few months, 2 new high energy exp’ts using micromegas have successfully started (COMPASS and NA48-KABES at CERN)