1. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC1 Status report of Berkeley- Orsay-Saclay Since Montpellier, we took data with magnetic field, with Ar-Isobutane and Ar CF 4Since Montpellier, we took data with magnetic field, with Ar-Isobutane and Ar CF 4 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. 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

2. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC2 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 Acquisition

3. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC3 P10, Vmesh = 356 V

4. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC4 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 )

5. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC5 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)

6. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC6 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

7. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC7 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)

8. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC8 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)

9. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC9 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 55 Fe source (June 2002). I=I 0 exp(-ad) -> Magboltz reliable

10. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC10 Ar CF 4 : diffusion At 200 V/cm, for Ar+3%CF 4, Monte Carlo predicts D z = 240  m/sqrt(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/sqrt(cm) D t (B) = Dt(B=0)/  (1+  2  2 ),  =eB/m e  = 4.5 at 1T => expect 75  m/sqrt(cm) for B=1T

11. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC11 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 = 63+-13  m/  cm Consistent with expectation of 75  m.  x 2 (mm 2 ) Drift time (50 ns ticks)

12. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC12 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

13. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC13 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

14. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC14 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.)

15. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC15 Micromegas gain with a resistive anode Argon/Isobutane 90/10 Resistive anode suppresses sparking, stabilizing Micromegas

16. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC16 Starting tests of a Micromegas+pixel detector With H. Van der Graaf, A. Fornaini, et al. (NIKHEF, Amsterdam) AsGa 55x55  m 2 pixels read out by a CMOS integrated electronics 3-GEM device already saw particles The smallest Micromegas ever built

17. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC17 Natural limitation of the ion back-flow See V. Lepeltier’s poster, session C 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

18. LCTPC meeting, Feb. 11, 2004P. Colas - Micromegas TPC18 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)