First test of a Micromegas TPC in a magnetic field

Vienna, Feb. 17, 2004 P. Colas - Micromegas TPC 1

First test of a Micromegas First test of a Micromegas TPC in a magnetic fieldTPC in a magnetic field

First test of a Micromegas First test of a Micromegas TPC in a magnetic fieldTPC in a magnetic field

• Advantages of a Micromegas Advantages of a Micromegas

TPC for the Linear ColliderTPC for the Linear Collider

• The Berkeley-Orsay-Saclay The Berkeley-Orsay-Saclay

cosmic setupcosmic setup

• A magic gas mixture: ArCFA magic gas mixture: ArCF44

– gain, drift velocity, diffusion, gain, drift velocity, diffusion,

attachment, agingattachment, aging

• Ion back-flow suppression in Ion back-flow suppression in

MicromegasMicromegas

• Conclusion and outlook: Conclusion and outlook:

resistive anode and pixelsresistive anode and pixels

P. Colas1, I. Giomataris1, V. Lepeltier2, M. Ronan3

1) DAPNIA Saclay, 2) LAL Orsay, 3) LBNL BerkeleyWith help from

F. Bieser3, R. Cizeron2, C. Coquelet1, E. Delagnes1, A. Giganon1,

G. Guilhem2, V. Puill2, Ph. Rebourgeard1, J.-P Robert1

Conference on instrumentation


A LARGE TPC FOR THE LINEAR A LARGE TPC FOR THE LINEAR COLLIDERCOLLIDERThe tracker of the LC detector should be a TPC with

- good 2-track separation (<8mm)

- 10 times better momentum resolution than at LEP

- low ion back-flow

- possibility to operate with a H-less gas (200 n/BX)

- High B field (4T) to wipe out the (large) background

We proposed a Micromegas readout to fulfill these requirements

(see also J. Kaminski et al.’s poster P.A9 for the GEM TPC)

Using recent measurements, simulations and recently re-analysed data, we show that all the requirements can be met.


2x10 mm2 pads

1x10 mm2 pads

THE BERKELEY-ORSAY-SACLAY COSMIC-RAY THE BERKELEY-ORSAY-SACLAY COSMIC-RAY SETUP SETUP

Field cage

Detector

Front end electronics

50 cm drift length,

50 cm diameter

1024 channels

In operation since July 2003Data taking with field in Nov. 2003

The largest The largest Micropattern Micropattern TPC ever builtTPC ever built


Field cage assembly in Orsay

2T magnet in Saclay


From the STAR TPC (from Berkeley)

1024 readout channels with pre-amplification, shaping, 20 MHz sampling over 10 bit ADC.

VME-based DAQ.

Very steady data taking conditions, with mesh currents below 0.3 nA and essentially no sparking.Trigger rate 2.5 Hz. DAQ rate 0.1 Hz.

Display and reconstruction using Java code from Dean Karlen, adapted by Mike Ronan. Java-based analysis (JAS3 and AIDA)

Electronics and Data Electronics and Data

AcquisitionAcquisition Electronics and Data Electronics and Data

AcquisitionAcquisition


P10, Vmesh = 356 V


Data have been taken with the following gas mixtures:

Ar-CH4 10% (v = 5cm/s @ 120 V/cm)

Ar-Isobutane 5% (v = 4.15 cm/s @

200 V/cm)

Ar-CF4 3% (v = 8.6 cm/s @ 200 V/cm)

Hit total amplitude (ADC counts)

Curvature 1/R (mm-1)

Mainly few-GeV Mainly few-GeV muons: minimum-muons: minimum-ionizing and low ionizing and low multiple scatteringmultiple scattering


Ar CFAr CF44: a ‘magic gas’ for the Micromegas : a ‘magic gas’ for the Micromegas TPCTPC

Ar CFAr CF44: a ‘magic gas’ for the Micromegas : a ‘magic gas’ for the Micromegas TPCTPC

Introduction:Introduction: choosing a gas mixture for the TPC

R1) Have enough primary electrons

R2) Have a velocity plateau at low enough voltage (<200 V/cm -> 50 kV for 2.5m)

R3) Keep cost reasonable (large volume)

-> Requirements 1 to 3 point to Argon as carrier gas

R4) Avoid Hydrogen (200 n/bx at TESLA)

-> CO2 too slow, needs too high a field.

->ArCF4 OK, but attachment? Aging? Reactivity?

=> USE MEASUREMENTS (June 2002, Nov. 2003)

AND SIMULATIONS (Magboltz by S. Biagi)


Ar CFAr CF44: drift velocity: drift velocityAr CFAr CF44: drift velocity: drift velocity

Measurement of the drift velocity in Ar:CF4(97:3) at E=200 V/cm

5150 ns + 200 ns (trigger delay)

for 47.9 cm drift:

V = 9.0 +- 0.3 cm/s consistent with Magboltz (by S. Biagi) 8.6 cm/s.

Cross-check: for Ar:isobutane (95:5) we find v = 4.2+-0.1 cm/s

Magboltz: 4.15 cm/s

tmax = (103+-3) x 50ns

First 15 buckets used for ped. calculation


Ar CFAr CF44: gain: gainAr CFAr CF44: gain: gain Measurements using a simple device with a 25 MBq 55Fe source (April 2002).

Note hyperexponential, but stable, behaviour at high voltageIn the cosmic data taking

Micromegas was operated at a gain of about 2000.

(50 micron gap with 350 V)

Ar CFAr CF44: aging: agingAr CFAr CF44: aging: aging

CFCF44 known to be known to be aggressive to some aggressive to some materials. However:materials. However:

- no sign of aging after - no sign of aging after months of operation with months of operation with various detectors/mixturesvarious detectors/mixtures-X-ray aging test performed -X-ray aging test performed (20 l/h Ar+5%CF(20 l/h Ar+5%CF44):): 2 mC/mm 2 mC/mm22 = 13000 years = 13000 years of LCof LC


Ar CFAr CF44: attachment: attachmentAr CFAr CF44: attachment: attachment

Fear: there exists an attachment resonance in CF4 (narrow but present)

Attachment coefficient must be << 0.01 cm-1 for a 2.5 m TPC

MC predicts :

-No attachmt in the drift region (E<400 V/cm)

-attachmt overwhelmed by Townsend in the amplification region (E>10kV/cm)


Ar CFAr CF44: attachment: attachmentAr CFAr CF44: attachment: attachment

Measurement with the cosmic setup:

Exponential fit to the mean signal vs distance gives the limit:

Attachment < 4.1 10-3 cm-1 @ 90% C.L.

-> OK for a large TPC

Truncated mean signal vs drift time

Measurement with a d=1.29 cm drift setup. Measure signal amplitude from a focused laser (June 2002) .

I=I0 exp(-ad)

-> Magboltz reliable


Ar CFAr CF44: diffusion: diffusionAr CFAr CF44: diffusion: diffusion

At 200 V/cm, for Ar+3%CF4,

Monte Carlo predicts

Dz = 240 m/cm

=> 2.5 mm 2-track resolution in z at 1 meter

=> 250 m z hit resolution at 1 meter

Dt(B=0) = 350 m/cm

Dt(B) = Dt(B=0)/(1+22),

=eB/me

= 4.5 at 1T

=> expect 75 m/cm) for B=1T


Ar CFAr CF44: transverse diffusion: transverse diffusionAr CFAr CF44: transverse diffusion: transverse diffusion

Measurement in the cosmic setup :

Compute the rms width of hits for each track

Plot x2 as a function of

the drift time

fit to a straight line

Obtain the transverse diffusion constant at B=1T:

Dt = 64+-13 m/cm

Consistent with expectation of 75 m.

x2 (mm2)

Drift distance (cm)


Potential point Potential point resolutionresolution

Potential point Potential point resolutionresolution

Extrapolate to B=4T (=18)

Dt = 15 m/cm

=> track width = 150 m at 1 m

For 1cm long pads (100 electrons) : potential resolution of 15 m at 1m !

But this would require O(200m)-wide pads: too many channels (but good for microTPC).

Two ways out:

=> spread the charge after amplification (resistive anode for instance)

=> go to digital pixels (300x300 microns have been shown to be optimal by M. Hauschild)

Ar-CF4 3%, Vmesh = 340 VB = 1 Tesla ~ 4.5


Position sensing from charge dispersion in micropattern gas detectors with a resistive anode (Dixit et al.)

Position sensing from charge dispersion in micropattern gas detectors with a resistive anode (Dixit et al.)

or Micromegas


GEM & Micromegas Tests at CarletonGEM & Micromegas Tests at CarletonGEM & Micromegas Tests at CarletonGEM & Micromegas Tests at Carleton

Carbon loaded Kapton ~ 0.5 M/

•Resistive anode spreads the avalanche cluster charge•Position obtained from centroid of dispersed charge sampled by several pads •In contrast to the GEM, in Micromegas there is little transverse diffusion after gain which makes centroid determination difficult•resistive foil C-loaded Kapton

Amplification withGEM

microMegas

orMicromegas

Thickness ~ 30 m


Micromegas TPC Resolution with 2mm padsMicromegas TPC Resolution with 2mm pads

X-ray spot position 15.35 mm

Pad edges @ 15 mm and 17 mm Centre @ 16 mm

Measured spatial resolution :68 m

Pad response function width from charge dispersion on resistive anode

Collaboration with Carleton Univ., Ottawa (M. Dixit, K. Sachs, et al.)


Micromegas gain with a resistive anode

Micromegas gain with a resistive anode

Argon/Isobutane 90/10

Resistive anode suppresses sparking, stabilizing Micromegas


First data from a Micromegas+pixel detector


Medipix2 CMOS chip in 0.25Medipix2 CMOS chip in 0.25m m technology with 65000 55x55 technology with 65000 55x55 mm22 Si pixels, noise about Si pixels, noise about 200e-/channel200e-/channel

Harry van der Graaf, Alessandro Fornaini, Jan Timmermans, Jan Visschers (NIKHEF Amsterdam)

Paul Colas, Y. Giomataris (DAPNIA Saclay)Erik Heijne (CERN/MEDIPIX2)

Jurriaan Schmitz (Univ. Twente/MESA)

After an amplification stage by After an amplification stage by a Micromegas (Ar+5% a Micromegas (Ar+5% isobutane)isobutane)

Smallest Smallest Micromegas ever Micromegas ever builtbuilt

see also see also poster poster P.B22P.B22




3-GEM device already 3-GEM device already saw particlessaw particles

(March ’03, Feb. ’04)(March ’03, Feb. ’04)

Larger spread Larger spread (O(1mm)) due to mm-(O(1mm)) due to mm-thick gaps with kV/cm thick gaps with kV/cm fieldsfields

100 mm drift100 mm drift

GAIN = 40 000GAIN = 40 000

No source, 1sNo source, 1s 5555Fe, 1sFe, 1s 5555Fe, 10sFe, 10s

5555Fe, 2sFe, 2s

Clear signal Clear signal from an from an iron 55 iron 55 source (220 source (220 e- per e- per photon)photon)

300300x500x500 clouds as clouds as expectedexpected

15 mm drift15 mm drift

GAIN=700GAIN=700

Feb. 13, Feb. 13, ‘04‘04

MICROMEGAS + MICROMEGAS + PIXELS :very PIXELS :very promising device promising device + powerful tool + powerful tool for studying for studying MicromegasMicromegas

GGEEMM

MMICICRROOMMEEGGAASS


6 and 3 keV photons as seen by a Micromegas+Medipix2 detector

500

400


Stability of operation of Stability of operation of Micromegas in a magnetic fieldMicromegas in a magnetic field

Stability of operation of Stability of operation of Micromegas in a magnetic fieldMicromegas in a magnetic field B=1

tesla

15 cm TPCStability of the position and width of the 55Fe peak as a function of the field


Natural limitation of the ion back-flowNatural limitation of the ion back-flowNatural limitation of the ion back-flowNatural limitation of the ion back-flow See V. See V. Lepeltier’s Lepeltier’s poster, P.C13poster, P.C13

Electrons suffer diffusion (=12m in the 50m gap) -> spread over distances comparable to the grid pitch l=25-50m

Ions follow field lines, most of them return to the grid, as the ratio amplification field / drift field is large

If /l >0.5, the fraction of back-flowing ions is 1/(field ratio)

In Micromegas, the ion In Micromegas, the ion back flow is suppressed by back flow is suppressed by O(10O(10-3-3))

For gains of 500-1000, this For gains of 500-1000, this is not more than the is not more than the primary ionisationprimary ionisation

In the test, the gain was 310. In the test, the gain was 310.

Data from dec 2001, reanalysed.Data from dec 2001, reanalysed.

S1/S2 ~ Eamplif / EdriftS1

S2S2


Good understanding and stability of the ion feedback : need for manufacturing large grids at the 25 micron pitch.

Optimal case

(reached for a 1000 lpi grid)


ConclusionsConclusionsConclusionsConclusions•A large TPC read out by Micromegas is now in operation in a 2T magnet.

•Experimental tests and and Monte Carlo studies allowed us to limit the choice of gas mixtures for Micromegas. Ar-CF4 is one of the favorite with possibly an isobutane or CO2 admixture.

•Theoretical and experimental studies have demonstrated that the ion feedback can be suppressed down to the 3 permil level.

•Tests with simple dedicated setups have proven the principle of operation and shown that the performances of Micromegas hold in a magnetic field (March 2002, June 2002 and January 2003 data taking)


OutlookOutlookOutlookOutlook•The position resolution issue at an affordable cost has to be addressed. There exists at least two possibilities:

•Pixel readout

•Spread the charge after gain (resistive anode coating for instance)

For both, the «proof of principle» has been done

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First test of a Micromegas TPC in a magnetic field