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

Orsay, January 12, 2005 P. Colas - Resistive anode Micromegas 1

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

Point resolution measurements of a Point resolution measurements of a Micromegas with a resistive anode in an X-Micromegas with a resistive anode in an X-

ray source ray source

Point resolution measurements of a Point resolution measurements of a Micromegas with a resistive anode in an X-Micromegas with a resistive anode in an X-

ray source ray source

Using a 3-6 kV X-ray source we test whether the expected resolution improvement from the resistive layer holds for Micromegas


Motivation for a resistive readoutMotivation 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 m (consistent with X-ray beam diameter) already demonstrated (Dixit et al., NIM) for double GEMs with 1.5 mm strips.


The setupThe 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


Charge spreading with a resistive anodeCharge spreading with a resistive anode

or Micromegas


Resistive anode MicromegasResistive anode Micromegas

50 m pillars

Drift Gap

MESHAmplification Gap

Al-Si Cermet on mylar


Micromegas gain with a resistive anode

Micromegas gain with a resistive anode

Argon/Isobutane 90/10

Cr (SiO2)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 >107 .cm2

(see also Fonte et al.)


Charge dispersion signals in MicromegasSingle event (2 mm wide pads)

Charge dispersion signals in MicromegasSingle 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


ResultsResultsResultsResults

The centroid is calculated for each position of the X-ray beam

(reference positions = pad edges obtained by equalizing the signals)



Comparing actual locations to centroid locations allows the bias curve to be determined

(very homogeneous)



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.


Conclusions and future plansConclusions and future plansConclusions and future plansConclusions 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-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.

Documents

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