Point resolution measurements of a Micromegas with a resistive anode in an X-ray source Dan Burke1, P. Colas2, M. Dixit1, I. Giomataris2, V. Lepeltier3, A. Rankin1, K. Sachs1 1 Carleton University Ottawa 2 CEA-DAPNIA Saclay 3 LAL Orsay Using a 3-6 kV X-ray source we test whether the expected resolution improvement from the resistive layer holds for Micromegas Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

Motivation for a resistive readout Goal for point resolution for the LC-TPC : about 100 microns. Pads cannot be too small : too many electronic channels, too little ionisation. 2mm x 6 mm rough guess optimum Track width due to diffusion at 3T: 0.65 mm with Ar+5%isobutane, 0.27 mm with Ar+3%CF4 -> too small for a barycenter, the charge is on one pad! Need to spread the charge. M. Dixit suggests a resistive-capacitive continuous network: resistive coating on the anode. Resolutions of 70 mm (consistent with X-ray beam diameter) already demonstrated (Dixit et al., NIM) for double GEMs with 1.5 mm strips. Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

P. Colas - Resistive anode Micromegas The setup Micromegas detector with a 6-mm conversion gap. Al-Si Cermet laminated with a glue foil 1 MW/square, excellent quality 3-6 KeV photons from an X-ray gun with a 40 mm pinhole collimator producing a 70 mm focal spot detector on micromovers. Gas: Ar + 10% Isobutane Gain about 4000 2x6mm pads Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

P. Colas - Resistive anode Micromegas Charge spreading with a resistive anode or Micromegas Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

Resistive anode Micromegas 50 m pillars Drift Gap Al-Si Cermet on mylar MESH Amplification Gap Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

Micromegas gain with a resistive anode Instead of breaking down, the resistive anode Micromegas enters a new regime (limited streamer?) Same effect re-observed recently with carbon-loaded kapton 1 MW /square r.e >107 W.cm2 (see also Fonte et al.) Argon/Isobutane 90/10 Cr (SiO2)n cermet Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

Charge dispersion signals in Micromegas Single event (2 mm wide pads) Primary signal Two 1st neighbors 2nd neighbor (note different scale) Ar/CO2 90/10, Gain ~ 3000 1st neighbor peak ~ 100 ns after the primary pulse peak 2 x 4 channel Tektronix X-ray spot centred on one pad Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

P. Colas - Resistive anode Micromegas Results The centroid is calculated for each position of the X-ray beam (reference positions = pad edges obtained by equalizing the signals) Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

P. Colas - Resistive anode Micromegas Results Comparing actual locations to centroid locations allows the bias curve to be determined (very homogeneous) Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

P. Colas - Resistive anode Micromegas Results Correcting for the bias with half of the data allows to determine residuals and resolution for each actual position in the other half of the data. Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas

Conclusions and future plans The principle of charge dispersion has been demonstrated with a Micromegas detector Resolutions better than 80 mm (close to the size of the X-ray beam) have been measured with photons giving 100-200 electrons. Future plans : cosmic test in progress at Carleton with Ar-CO2 10%. (maybe pursued in a magnetic field) Repeat with a new photoelectron source at Orsay (see Thomas Zerguerras’s talk) Bulk Micromegas (one process to include resistive foil, mesh and pillars) have been/will be built and tested. Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas