Vienna Conference on Instrumentation – February 27, D. Attié, A. Bellerive, K. Boudjemline, P. Colas, M. Dixit, A. Giganon, I. Giomataris, V. Lepeltier, S. Liu, J.-P. Martin, K. Sachs, Y. Shin and S. Turnbull COSMo (Carleton, Orsay, Saclay, Montreal) Collaboration Micromegas TPC studies at high magnetic fields using the charge dispersion signal

Vienna Conference on Instrumentation – February 27, The Time Projection Chamber (TPC) for the International Linear Collider (ILC) will need to measure about 200 track points with a transverse resolution close to 100 µm. Pad width of 2 mm provides good two-track separation. With the choice of 2 mm x 6 mm pads, the ILC-TPC will have ~ 1.2 × 10 6 channels. But is still too wide to give the target resolution (  0 ~ pitch/√12). Not enough charge sharing, even for 1 mm pitch pads (in the case of Micromegas:  avalanche ~ 12 µm) Pads narrower than 1 mm are needed. This has consequences on: –Electronic cost –Material budget –Cooling aspects Expectations for ILC-TPC

Vienna Conference on Instrumentation – February 27, Disperse the signal sharing the charge between several neighbouring pads after amplification, using a resistive coating on an insulator The charge arrives on the central pad and is spread over the others pads which see a fraction of the charge after a delay Then we fit the signal over the pads which gives us the track position M.S.Dixit and A. Rankin NIM A566 (2006) 281 Solution: use a charge dispersion signal 2 x 6 mm 2 pads Simulation Data

Vienna Conference on Instrumentation – February 27, Micromegas is a parallel plate gas avalanche detector with a small gap Micromesh held by pillars 50  m above the anode plane Micromegas: gas amplification system E drift = 100-1kV/cm E amp = 50k-100 kV/cm Micromesh Drift electrode Amplification gap ~ 50  m Conversion gap > 3 mm ― Pillars ― Charge particle Pads

Vienna Conference on Instrumentation – February 27, Charge dispersion in a MPGD with a resistive anode  (r,t) integral over pads  (r) r (mm) M.S.Dixit et.al., NIM A518 (2004) 721 Modified MPGD anode with a high resistivity film bonded to a readout plane with an insulating spacer Point charge at r = 0 & t = 0 disperses with time The charge density  (r,t) at (r,t) is a solution of the 2D Telegraph equation The anode charge density is time dependent and sampled by readout pads 2D Telegraph equation: Q(t) resistive foil glue pads PCB mesh t (ns)

Vienna Conference on Instrumentation – February 27, Drift Gap MESH Amplification Gap Al-Si Cermet on mylar Charge dispersion in a MPGD with a resistive anode 25 µm mylar with Cermet (Al-Si) of 1 M  /□ glued onto the pads with 50 µm thick dry adhesive The Cermet is a composite material composed of ceramic and metallic materials Micromegas detector + resistive anode

Vienna Conference on Instrumentation – February 27, The resistive foil prevents charge accumulation, thus prevents sparks Gains higher than obtained with standard anodes can be reached Micromegas gain ― Without resistive foil ― With resistive foil (current) ― Without resistive foil ― With resistive foil (current) ArIso5%

Vienna Conference on Instrumentation – February 27, The 5 T cosmic-ray test at DESY Carleton TPC + 10 x 10 cm 2 Micromegas (50  m gap) + resistive anode 128 pads: tracking pads (2 x 6 mm 2 ) in 7 rows - 2 trigger pads (36 x 6 mm 2 ) in 2 rows Drift length: 15.7 cm Aleph charge preamplifiers MHz FADCs digitizer

Vienna Conference on Instrumentation – February 27, weeks of data using the 5 T magnet (thanks to DESY and T. Behnke et al.) 2 gas mixtures used: - Ar+5% isobutane: easy gas, for reference - Ar+3% CF 4 +2% isobutane: · so-called T2K gas, · good trade-off for safety, · high velocity (7.2 cm/  s at 200 V/cm), · low longitudinal diffusion, · large w  20 at 5 T ie D Tr = 19  m/√cm. Most data taken - at 5 T (to limit the diffusion) and - at 0.5 T (low enough field to check the effect of diffusion) 55 Fe source used for gain measurements placed inside the chamber The 5 T cosmic-ray test at DESY

Vienna Conference on Instrumentation – February 27, Gain dependence on magnetic field  Micromegas gain constant to within ~ 0.5 % up to 5 Tesla ! Micromegas gain vs. magnetic field with a 55 Fe source for Ar+5%C 4 H 10 Magnetic field (T) Gain relative to B = 1T

Vienna Conference on Instrumentation – February 27, Cosmic-ray data taken at DESY Sample cosmic ray tracks B = 0.5 T 6 mm 2 mm

Vienna Conference on Instrumentation – February 27, The Pad Response Function (PRF) is a measure of signal size as a function of track position relative to the pad. The pulse shape is variable and non-standard because of both the rise time & pulse amplitude depend on track position. The PRF amplitude for longer drift distances is lower due to Z dependent normalization. Charge dispersion pulses & pad response function

Vienna Conference on Instrumentation – February 27, The PRFs are not Gaussian. The PRF depends on track position relative to the pad: PRF = PRF(x,z). PRFs determined from the data and parameterized by a ratio of two symmetric 4th order polynomials: a 2, a 4, b 2 & b 4 can be written down in terms of: - FWHM =  (z) - base width  (z) of the PRF. Parameterization of the PRFs The parameters depend on TPC gas and operational details   x/mm Amplitude

Vienna Conference on Instrumentation – February 27, Track fit using the PRF For a given track: x track = x 0 + tan(  ) y row Y row is the y position of the row and x 0 &  the track fitting parameters Determination of x 0 &  by fitting the PRF to the pad amplitude by minimizing  2 for the entire event Definitions of the different stages: - residual: x row -x track - bias: mean of residual x row -x track = f(x track ) - resolution: geometric mean of the standard deviations of track residuals 2 mm 6 mm x y x0x0 

Vienna Conference on Instrumentation – February 27, Pad Response Function (PRF) 0 < z < 1 cm1 < z < 2 cm2 < z < 3 cm 3 < z < 4 cm 4 < z < 5 cm 5 < z < 6 cm6 < z < 7 cm7 < z < 8 cm 8 < z < 9 cm 9 < z < 10 cm 10 < z < 11 cm11 < z < 12 cm12 < z < 13 cm 13 < z < 14 cm 14 < z < 15 cm normalized amplitude xtrack – xpad (mm) 4 pads / ±4 mm T2K gas B = 5 T 15 z regions / 1 cm step

Vienna Conference on Instrumentation – February 27, Residuals : x row -x track 0 < z < 1 cm1 < z < 2 cm 2 < z < 3 cm 12 < z < 13 cm 13 < z < 14 cm 14 < z < 15 cm

Vienna Conference on Instrumentation – February 27, Average residual vs x position bias before bias after correction row 1 ± 20  m row 2 row 3 A bias of up 100  m is observed attributed to the charge spread non-uniformity due to: - inhomogeneities in the gap size - non-uniformity of the foil resistivity (± 0.15 mm) (± 14 mm)

Vienna Conference on Instrumentation – February 27, Resolution at 0.5T vs. gain B = 0.5 T, resolution fit bywhere Resolution  0 (  at z = 0) ~ 50 µm still good at low gain (will minimize ion feedback) Mean of N eff = 27 (value measured before ~ 22) Gain = 4700 Gain = 2500 N eff =25.2±2.1 N eff =28.8±2.2   0 = 1/40 of pad pitch

Vienna Conference on Instrumentation – February 27, Analysis:- Curved track fit - E P < 2 GeV - |  | < 0.05 (~3°) Resolution at 5T vs. gas mixtures Ar Iso (95:5) B = 5T Ar Iso (95:5) B = 5T 50  m   ~ 50 µm independent of the drift distance Extrapolate to B = 4T with T2K gas for 2x6 mm2 pads: D Tr = 23.3 µm/  cm, N eff ~ 27, 2 m drift distance,  Resolution of  Tr  80  m will be possible !!!

Vienna Conference on Instrumentation – February 27, Conclusions Micromegas with a resistive anode has been successfully operated in a 5 T magnetic field.  ~ 50  m over 15 cm (transverse diffusion negligible)  extrapolates to 80  m at 2 meters. The Ar+3% CF 4 +2% Isobutane gas mixture is promising. Can be also used with bulk technology for T2K experiment (See A. Sarrat’s poster A22).