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Status report of Berkeley-Orsay-Saclay

Status report of Berkeley-Orsay-Saclay. 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 ,

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Status report of Berkeley-Orsay-Saclay

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  1. Status report of Berkeley-Orsay-Saclay P. Colas1, I. Giomataris1, V. Lepeltier2, M. Ronan3 1) DAPNIA Saclay, 2) LAL Orsay, 3) LBNL Berkeley With help from F. Bieser3, R. Cizeron2, C. Coquelet1, E. Delagnes1, A. Giganon1, G. Guilhem2, V. Puill2, Ph. Rebourgeard1, J.-P Robert1 • Since Montpellier, we took data with magnetic field, with Ar-Isobutane and Ar CF4 • Analysis in progress (mainly Mike Ronan) • We re-analysed old data on feedback and attachment • Vienna Conference in Preparation (PC talk, VL Poster on feedback) • New student Max Chefdeville P. Colas - Micromegas TPC

  2. Electronics and Data Acquisition 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) P. Colas - Micromegas TPC

  3. P10, Vmesh = 356 V P. Colas - Micromegas TPC

  4. Data have been taken with the following gas mixtures: Ar-CH4 10% (v = 5cm/ms @ 120 V/cm) Ar-Isobutane 5% (v = 4.15 cm/ms @ 200 V/cm) Ar-CF4 3% (v = 8.6 cm/ms @ 200 V/cm) Hit total amplitude (ADC counts) Curvature 1/R (mm-1) P. Colas - Micromegas TPC

  5. Ar CF4: a ‘magic gas’ for the Micromegas TPC 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) P. Colas - Micromegas TPC

  6. Ar CF4: drift velocity tmax = (103+-3) x 50ns First 15 buckets used for ped. calculation 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/ms consistent with Magboltz (by S. Biagi) 8.6 cm/ms. Cross-check: for Ar:isobutane (95:5) we find v = 4.2+-0.1cm/ms Magboltz: 4.15 cm/ms P. Colas - Micromegas TPC

  7. Ar CF4: gain Measurements using a simple device with a 25 MBq 55Fe 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) P. Colas - Micromegas TPC

  8. Ar CF4: 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) P. Colas - Micromegas TPC

  9. Ar CF4: attachment Measurement with a d=1.29 cm drift setup. Measure signal amplitude from a 55Fe source (June 2002) . I=I0 exp(-ad) -> Magboltz reliable 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 P. Colas - Micromegas TPC

  10. Ar CF4: diffusion At 200 V/cm, for Ar+3%CF4, Monte Carlo predicts Dz = 240 mm/sqrt(cm) => 2.5 mm 2-track resolution in z at 1 meter => 250 mm z hit resolution at 1 meter Dt(B=0) = 350 mm/sqrt(cm) Dt(B) = Dt(B=0)/(1+w2t2), w=eB/me wt = 4.5 at 1T => expect 75 mm/sqrt(cm) for B=1T P. Colas - Micromegas TPC

  11. Ar CF4: transverse diffusion sx2 (mm2) Measurement in the cosmic setup : Compute the rms width of hits for each track. Plot sx2as a function of the drift time fit to a straight line Obtain the transverse diffusion constant at B=1T: Dt = 63+-13 mm/cm Consistent with expectation of 75 mm. Drift time (50 ns ticks) P. Colas - Micromegas TPC

  12. Potential point resolution Extrapolate to B=4T (wt=18) Dt = 15 mm/cm => track width = 150 mm at 1 m For 1cm long pads (100 electrons) : potential resolution of 15 mm at 1m ! But this would require O(200mm)-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 wt ~ 4.5 P. Colas - Micromegas TPC

  13. microMegas orMicromegas GEM & Micromegas Tests at Carleton • 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 Carbon loaded Kapton ~ 0.5 MW/o Thickness ~ 30 m Amplification withGEM P. Colas - Micromegas TPC

  14. Micromegas TPC Resolution with 2mm pads Collaboration with Carleton Univ., Ottawa (M. Dixit, K. Sachs, et al.) Pad response function width from charge dispersion on resistive anode X-ray spot position 15.35 mm Pad edges @ 15 mm and 17 mm Centre @ 16 mm Measured spatial resolution :68 mm P. Colas - Micromegas TPC

  15. Micromegas gain with a resistive anode Argon/Isobutane 90/10 Resistive anode suppresses sparking, stabilizing Micromegas P. Colas - Micromegas TPC

  16. Starting tests of a Micromegas+pixel detector With H. Van der Graaf, A. Fornaini, et al. (NIKHEF, Amsterdam) AsGa 55x55 mm2 pixels read out by a CMOS integrated electronics The smallest Micromegas ever built 3-GEM device already saw particles P. Colas - Micromegas TPC

  17. Natural limitation of the ion back-flow See V. Lepeltier’s poster, session C S1/S2 ~ Eamplif / Edrift Electrons suffer diffusion (s=12mm in the 50mm gap) -> spread over distances comparable to the grid pitch l=25-50mm Ions follow field lines, most of them return to the grid, as the ratio amplification field / drift field is large If s/l >0.5, the fraction of back-flowing ions is 1/(field ratio) S1 S2 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. P. Colas - Micromegas TPC

  18. Conclusions • A large TPC read out by Micromegas is now in operation in a 2T magnet. • Experimental tests 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) P. Colas - Micromegas TPC

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