status report of berkeley-orsay-saclay

LCTPC meeting, Feb. 11, 2004 P. Colas - Micromegas TPC 1

Status report of Status report of Berkeley-Orsay-SaclayBerkeley-Orsay-Saclay

Status report of Status report of Berkeley-Orsay-SaclayBerkeley-Orsay-Saclay

• Since Montpellier, we took data with magnetic field, with Ar-Since Montpellier, we took data with magnetic field, with Ar-

Isobutane and Ar CFIsobutane and Ar CF44

• Analysis in progress (mainly Mike Ronan)Analysis in progress (mainly Mike Ronan)

• We re-analysed old data on feedback and attachmentWe re-analysed old data on feedback and attachment

• Vienna Conference in Preparation (PC talk, VL Poster on feedback)Vienna Conference in Preparation (PC talk, VL Poster on feedback)

• New student Max ChefdevilleNew student Max Chefdeville

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


From the STAR TPC (from Berkeley)

1024 readout channels with preamplification, 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)


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: 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 55Fe source (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/sqrt(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/sqrt(cm)

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

=eB/me

= 4.5 at 1T

=> expect 75 m/sqrt(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 = 63+-13 m/cm

Consistent with expectation of 75 m.

x2 (mm2)

Drift time (50 ns ticks)


Potential point resolutionPotential point resolutionPotential point resolutionPotential point resolution

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


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


Starting tests of a Micromegas+pixel detector

Starting tests of a Micromegas+pixel detector

With H. Van der Graaf, A. Fornaini, With H. Van der Graaf, A. Fornaini, et alet al. (NIKHEF, . (NIKHEF, Amsterdam)Amsterdam)

AsGa 55x55 AsGa 55x55 mm22 pixels read out pixels read out by a CMOS integrated electronicsby a CMOS integrated electronics

3-GEM 3-GEM device device already already saw saw particlesparticles

The smallest The smallest Micromegas ever builtMicromegas ever built


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, session poster, session CC

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


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)

status report of berkeley-orsay-saclay

Documents

colas micromegas tpcar

colas micromegas tpcfrom

colas micromegas tpcp10

colas micromegas tpcdata

micromegas tpcintroduction

cmms magboltz

cosmic data

ar cf4analysis