1. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC1 First test of a Micromegas TPC in a magnetic field Advantages of a Micromegas TPC for the Linear ColliderAdvantages of a Micromegas TPC for the Linear Collider The Berkeley-Orsay-Saclay cosmic setupThe Berkeley-Orsay-Saclay cosmic setup A magic gas mixture: ArCF 4A magic gas mixture: ArCF 4 –gain, drift velocity, diffusion, attachment, aging Ion back-flow suppression in MicromegasIon back-flow suppression in Micromegas Conclusion and outlook:Conclusion and outlook: resistive anode and pixels resistive anode and pixels P. Colas 1, I. Giomataris 1, V. Lepeltier 2, M. Ronan 3 1) DAPNIA Saclay, 2) LAL Orsay, 3) LBNL Berkeley With help from F. Bieser 3, R. Cizeron 2, C. Coquelet 1, E. Delagnes 1, A. Giganon 1, G. Guilhem 2, V. Puill 2, Ph. Rebourgeard 1, J.-P Robert 1 Conference on instrumentation

2. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC2 A LARGE TPC FOR THE LINEAR COLLIDER The 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.

3. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC3 2x10 mm 2 pads 1x10 mm2 pads THE BERKELEY-ORSAY-SACLAY COSMIC-RAY SETUP Field cage Detector Front end electronics 50 cm drift length, 50 cm diameter 1024 channels In operation since July 2003 Data taking with field in Nov. 2003 The largest Micropattern TPC ever built

4. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC4 Field cage assembly in Orsay 2T magnet in Saclay

5. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC5 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 Acquisition

6. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC6 P10, Vmesh = 356 V

7. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC7 Data have been taken with the following gas mixtures: Ar-CH 4 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 muons: minimum-ionizing and low multiple scattering

8. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC8 Ar CF 4 : a ‘magic gas’ for the Micromegas TPC Introduction: Introduction: choosing a gas mixture for the TPC R1) Have enough primary electrons R2) Have a velocity plateau at low enough voltage ( 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) -> CO 2 too slow, needs too high a field. ->ArCF 4 OK, but attachment? Aging? Reactivity? => USE MEASUREMENTS (June 2002, Nov. 2003) AND SIMULATIONS (Magboltz by S. Biagi)

9. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC9 Ar CF 4 : 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 t max = (103+-3) x 50ns First 15 buckets used for ped. calculation

10. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC10 Ar CF 4 : gain Measurements using a simple device with a 25 MBq 55 Fe source (April 2002). Note hyperexponential, but stable, behaviour at high voltage In the cosmic data taking Micromegas was operated at a gain of about 2000. (50 micron gap with 350 V) Ar CF 4 : aging CF 4 known to be aggressive to some materials. However: - no sign of aging after months of operation with various detectors/mixtures -X-ray aging test performed (20 l/h Ar+5%CF 4 ): 2 mC/mm 2 = 13000 years of LC

11. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC11 Ar CF 4 : attachment Fear: there exists an attachment resonance in CF 4 (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)

12. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC12 Ar CF 4 : 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=I 0 exp(-ad) -> Magboltz reliable

13. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC13 Ar CF 4 : diffusion At 200 V/cm, for Ar+3%CF 4, Monte Carlo predicts D z = 240  m/  cm => 2.5 mm 2-track resolution in z at 1 meter => 250  m z hit resolution at 1 meter D t (B=0) = 350  m/  cm D t (B) = D t (B=0)/  (1+  2  2 ),  =eB/m e  = 4.5 at 1T => expect 75  m/  cm) for B=1T

14. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC14 Ar CF 4 : transverse diffusion Measurement in the cosmic setup : Compute the rms width of hits for each track Plot  x 2 as a function of the drift time fit to a straight line Obtain the transverse diffusion constant at B=1T: D t = 64+-13  m/  cm Consistent with expectation of 75  m.  x 2 (mm 2 ) Drift distance (cm)

15. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC15 Potential point resolution Extrapolate to B=4T (  =18) D t = 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(200  m)-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%, V mesh = 340 V B = 1 Tesla  ~ 4.5

16. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC16 Position sensing from charge dispersion in micropattern gas detectors with a resistive anode (Dixit et al.) or Micromegas

17. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC17 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 with GEM microMegas or Micromegas Thickness ~ 30  m

18. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC18 Micromegas 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.)

19. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC19 Micromegas gain with a resistive anode Argon/Isobutane 90/10 Resistive anode suppresses sparking, stabilizing Micromegas

20. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC20 First data from a Micromegas+pixel detector Medipix2 CMOS chip in 0.25  m technology with 65000 55x55  m 2 Si pixels, noise about 200e-/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 a Micromegas (Ar+5% isobutane) Smallest Micromegas ever built see also poster P.B22

21. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC21 First data from a Micromegas+pixel detector 3-GEM device already saw particles (March ’03, Feb. ’04) Larger spread (O(1mm)) due to mm-thick gaps with kV/cm fields 100 mm drift GAIN = 40 000 No source, 1s 55 Fe, 1s 55 Fe, 10s 55 Fe, 2s Clear signal from an iron 55 source (220 e- per photon) 300  x500  clouds as expected 15 mm drift GAIN=700 Feb. 13, ‘04 MICROMEGAS + PIXELS :very promising device + powerful tool for studying Micromegas GEMGEMGEMGEM MICROMEGASMICROMEGASMICROMEGASMICROMEGAS

22. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC22 6 and 3 keV photons as seen by a Micromegas+Medipix2 detector 500  400 

23. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC23 Stability of operation of Micromegas in a magnetic field B=1 tesla 15 cm TPC Stability of the position and width of the 55 Fe peak as a function of the field

24. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC24 Natural limitation of the ion back-flow See V. Lepeltier’s poster, P.C13 Electrons suffer diffusion (  =12  m in the 50  m gap) -> spread over distances comparable to the grid pitch l=25-50  m 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 back flow is suppressed by O(10 -3 ) For gains of 500-1000, this is not more than the primary ionisation In the test, the gain was 310. Data from dec 2001, reanalysed. S1/S2 ~ E amplif / E drift S1 S2

25. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC25 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)

26. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC26 Conclusions A large TPC read out by Micromegas is now in operation in a 2T magnet. andExperimental tests and Monte Carlo studies allowed us to limit the choice of gas mixtures for Micromegas. Ar-CF 4 is one of the favorite with possibly an isobutane or CO 2 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)

27. Vienna, Feb. 17, 2004P. Colas - Micromegas TPC27 Outlook 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