1 MUON TRACKER FOR CBM experiment Murthy S. Ganti, VEC Centre Detector Choice

2 Highest hit density expected at various muon stations (URQMD, central) Effective rate ~10MHz/Cm 2

3 Comparison of detectors.. MWPCGEMMicromegas Rate capability10^4Hz/mm^2>5x10^5Hz/mm^210^6Hz/mm^2 GainHigh 10^6low 10^3 (single) > 10^5 (multi GEM) High > 10^5 Gain stabilityDrops at 10^4Hz/mm^2 Stable over 5*10^5Hz/mm^2 Stable over 10^6Hz/mm^2 2D Readout ?YesYes and flexibleYes, not flexible Position resolution> 200 µm (analog) 50 µm (analog)Good < 80 µm Time resolution< 100 ns Magnetic Field effectHighLow CostExpensive, fragileExpensive(?), robust Cheap, robust Both GEM & MICROMEGAS are suitable for high rate applications

4 Typical geometry: 5 µm Cu on 50 µm Kapton 70 µm holes at 140 mm pitch Manufactured with technology developed at CERN µm Gas Electron Multiplier - GEM

5 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 10 3 are obtained in most common gases.

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10 Multi GEM configurations..

11 Multiple structures provide equal gain at lower voltage The 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

12 GEM..

13 GEM..

14 GEM..

15 Narrow charge distribution problem..

16 GEM… High (100  m) pitch small pad response functionHigh (100  m) pitch small pad response function No ExB effects better resolutionNo ExB effects better resolution Direct electron signal no lossesDirect electron signal no losses Efficient ion collectionEfficient ion collection Easy to buildEasy to build Robust to aging insensitive to LHC backgroundsRobust to aging insensitive to LHC backgrounds Multi-stage structures large gains ( )Multi-stage structures large gains ( ) Low mass construction no wire framesLow mass construction no wire frames

17 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)

18 MICROMEGAS.. High (50 mm) pitch small pad response function No ExB effects better resolution Direct electron signal no losses Funnel effect very efficient ion collection Electron amplification independent of the gap to first order promising dE/dx Easy to build dead zones potentially small Robust to aging insensitive to LHC backgrounds Good electro-mechanical stability large gains ( ) Low mass construction no wire frames

19 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 10 8/ 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 (<10 8 ) that is suitable for safe operation of the detector in high particle environment.

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22 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

23 Mass produced MICROMEGAS

24 S. Kane et al./NIM A 505(2003) Purdue University Double mesh MICROMEGAS Protection of Front end electronics from discharges Cathode and readout pad plane are separated

25 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 line How to optimize ion backflow? Practical limitations of mesh aperture(LPI) Minimizing discharges due to heavily ionizing particles ( nuclear recoils) Choice of gas mixtures Muon 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

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

27 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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