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Murthy S. Ganti, VEC CentreMurthy S. Ganti, VEC Centre

Detector Choice

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Highest hit density expected at various muon stations(URQMD, central)

Effective rate ~10MHz/Cm2

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Comparison of detectors..

MWPC GEM Micromegas

Rate capability 10^4Hz/mm^2 >5x10^5Hz/mm^2 10^6Hz/mm^2

Gain High 10^6 low 10^3 (single)

> 10^5 (multi GEM)

High > 10^5

Gain stability Drops at 10^4Hz/mm^2

Stable over 5*10^5Hz/mm^2

Stable over 10^6Hz/mm^2

2D Readout ? Yes Yes and flexible Yes, not flexible

Position resolution > 200 µm (analog) 50 µm (analog) Good < 80 µm

Time resolution < 100 ns < 100 ns < 100 ns

Magnetic Field effect High Low Low

Cost Expensive, fragile Expensive(?), robust

Cheap, robust

Both GEM & MICROMEGAS are suitable for high rate applications

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Typical geometry:5 µm Cu on 50 µm Kapton70 µm holes at 140 mm pitch

Manufactured with technology developed at CERN

100-150 µm

Gas Electron Multiplier - GEM

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GEM..

F. Sauli, Nucl. Instrum. Methods A386(1997)531

Thin metal-coated polyimide foil chemically etched to form high density of holes.

On application of a voltage gradient, electrons released on the top side drift into the hole, multiply in avalanche and transfer to the other side.

Proportional gains above 103 are obtained in most common gases.

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Multi GEM configurations..

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Multiple structures provide equal gain at lower voltageThe discharge probability on exposure to a particles is strongly reduced

For a gain of 8000 (required for full efficiency on minimum ionizing tracks) in the TGEM the discharge probability is not measurable.

Single-Double-Triple GEM

S. Bachmann et al, Nucl. Instr. and Meth. A479 (2002) 294

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GEM..

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GEM..

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GEM..

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Narrow charge distribution problem..

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GEM…

• High (100 High (100 m) pitch m) pitch small pad response functionsmall pad response function• No ExB effects No ExB effects better resolutionbetter resolution• Direct electron signal Direct electron signal no lossesno losses

• Efficient ion collectionEfficient ion collection

• Easy to build Easy to build

• Robust to aging Robust to aging insensitive to LHC backgroundsinsensitive to LHC backgrounds

• Multi-stage structuresMulti-stage structures large gains (10large gains (1033-10-1044)) • Low mass construction Low mass construction no wire framesno wire frames

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Issues of implementing GEM in Large area detectors..

Only a few sources of supply

Large area GEM foils are difficult to fabricate. Small foils leave large dead areas in the tracking plane

Single stage GEM gain is low

Expensive (relative)

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MICROMEGAS..

High (50 mm) pitch High (50 mm) pitch small pad response functionsmall pad response function

No ExB effects No ExB effects better resolution better resolution

Direct electron signal Direct electron signal no lossesno losses

Funnel effect Funnel effect very efficient ion collectionvery efficient ion collection

Electron amplification independent of the gap to first Electron amplification independent of the gap to first

order order promising dE/dxpromising dE/dx

Easy to build Easy to build dead zones potentially smalldead zones potentially small

Robust to aging Robust to aging insensitive to LHC backgroundsinsensitive to LHC backgrounds

Good electro-mechanical stability Good electro-mechanical stability large gains (10large gains (1033 -10 -1044))

Low mass construction Low mass construction no wire framesno wire frames

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MICROMEGAS – discharges ..

Detailed investigations have shown that the rate capability of the gaseous detector strongly depends of the type of the incident particle beam and the nature of the gas mixture. A test in a high energy muon beam at a rate of 108/s showed that Micromegas can cope with very high flux of these particles.

However, an undesirable effect has been observed when the incident beam is composed by high- energy hadrons: a discharge rate proportional to the incident hadron rate. It is believed that sparks are triggered by large charge deposits in the drift space from recoil nuclei produced by charged particles, especially hadrons, traversing the detector. In the case of muons the corresponding cross section is several orders of magnitudes lower, therefore the probability to induce sparks is negligible.

Several investigations in the PS beam have shown a continuous decrease of the discharge probability from heavy to light gas fillings. The most promising are Helium mixtures and the effect is illustrated in Fig.1. With a gas mixture of He + 10% Isobutane the discharge probability decreases with the cathode mesh voltage of the detector; at 420 Volts, the lowest required voltage for full detection efficiency, the probability reaches a value (<108) that is suitable for safe operation of the detector in high particle environment.

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Mesh Choice for MICROMEGAS..

1. they exist in rolls of 4 m x 40 m and are quite inexpensive,2. they are commonly produced by several companies over the world,3. there are many metals available: Fe, Cu, Ti, Ni, Au,4. they are more robust for stretching and handling.

Electroformed mesh

Copper with integrated polyimide pillars (by etching technology

SS woven mesh

Advantages of Woven mesh

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Mass produced MICROMEGAS

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S. Kane et al./NIM A 505(2003)215-218

Purdue University

Double mesh MICROMEGAS

Protection of Front end electronics from discharges

Cathode and readout pad plane are separated

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MICROMEGAS : Some practical questions

Choice of wire mesh : Woven vs. electro-formed vs. etched copper clad kapton

Bulk MICROMEGAS – pillars of photo resist. Also other spacers like fish lineHow to optimize ion backflow? Practical limitations of mesh aperture(LPI)Minimizing discharges due to heavily ionizing particles ( nuclear recoils)

Choice of gas mixturesMuon tracking at high rates: how to widen pad response function?

Need of resistive coating ( Cermet, graphite)Other readout schemes : Second anode mesh and a separate readout pad

plane Practical construction :

How to paste mesh to frame? how to protect mesh edges? How much Minimum frame width? How to tap HV connection? Frame material, rigidity Mesh sag due to temperature fluctuations Invar mesh (shadow mask of CRTs)? Effect of pad ridges on field uniformity

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Thick GEM.. Worth investigating further for CBM Muon Tracker..

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Design concepts..

Wheel type design of planes with 8 sector type chambers in each plane

Each sector with a single woven mesh supported on insulating pillars or THGEM

Readout pad granularity to vary from 3mm to 7mm pads radially in 3 zones - to keep occupancy within 10% level

(needs further optimization study)

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