Post on 16-Dec-2015
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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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