1. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas1 Dan Burke 1, P. Colas 2, M. Dixit 1, I. Giomataris 2, V. Lepeltier 3, A. Rankin 1, K. Sachs 1 1 Carleton University Ottawa 2 CEA-DAPNIA Saclay 3 LAL Orsay Point resolution measurements of a Micromegas with a resistive anode in an X-ray source Using a 3-6 kV X-ray source we test whether the expected resolution improvement from the resistive layer holds for Micromegas

2. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas2 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%CF 4 -> 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  m (consistent with X-ray beam diameter) already demonstrated (Dixit et al., NIM) for double GEMs with 1.5 mm strips.

3. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas3 The setup Micromegas detector with a 6-mm conversion gap. Al-Si Cermet laminated with a glue foil 1 M  /square, excellent quality 3-6 KeV photons from an X-ray gun with a 40  m pinhole collimator producing a 70  m focal spot detector on micromovers. Gas: Ar + 10% Isobutane Gain about 4000 2x6mm pads

4. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas4 Charge spreading with a resistive anode or Micromegas

5. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas5 Resistive anode Micromegas 50  m pillars Drift Gap MESH Amplification Gap Al-Si Cermet on mylar

6. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas6 Micromegas gain with a resistive anode Argon/Isobutane 90/10 Cr (SiO 2 ) n cermet Instead of breaking down, the resistive anode Micromegas enters a new regime (limited streamer?) Same effect re-observed recently with carbon-loaded kapton 1 M  /square .e >10 7 .cm 2 (see also Fonte et al.)

7. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas7 Charge dispersion signals in Micromegas Single event (2 mm wide pads) Primary signal Two 1 st neighbors 2 nd neighbor (note different scale) Ar/CO 2 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

8. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas8 Results The centroid is calculated for each position of the X-ray beam (reference positions = pad edges obtained by equalizing the signals)

9. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas9 Results Comparing actual locations to centroid locations allows the bias curve to be determined (very homogeneous)

10. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas10 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.

11. Orsay, January 12, 2005P. Colas - Resistive anode Micromegas11 Conclusions and future plans The principle of charge dispersion has been demonstrated with a Micromegas detector Resolutions better than 80  m (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- CO 2 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.