MUON TRACKER FOR CBM experiment

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MUON TRACKER FOR CBM experiment
Detector Choice
Murthy S. Ganti, VEC Centre
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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..
Both GEM MICROMEGAS are suitable for high rate
applications
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Gas Electron Multiplier - GEM
Manufactured with technology developed at CERN
100-150 µm
Typical geometry 5 µm Cu on 50 µm Kapton 70 µm
holes at 140 mm pitch
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GEM..
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.
F. Sauli, Nucl. Instrum. Methods A386(1997)531
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Multi GEM configurations..
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Single-Double-Triple GEM
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.
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 mm) pitch small pad response function
• No ExB effects better resolution
• Direct electron signal no losses
• Efficient ion collection
• Easy to build
• Robust to aging insensitive to LHC backgrounds
• Multi-stage structures large gains (103-104)
• Low mass construction no 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 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 (103
-104) Low mass construction no 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 (lt108) that is
  suitable for safe
• operation of the detector in high particle
  environment.

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Mesh Choice for MICROMEGAS..
Copper with integrated polyimide pillars (by
etching technology
Electroformed mesh
SS woven mesh
Advantages of Woven mesh
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.
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Mass produced MICROMEGAS


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Double mesh MICROMEGAS

• Protection of Front end electronics from
  discharges
• Cathode and readout pad plane are separated

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

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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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Lot of Work to do!