1. 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

2. 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

3. 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

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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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 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