The Time Projection Chamber for the CERNLHC HeavyIon Experiment ALICE

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The Time Projection Chamber for the CERN-LHC
Heavy-Ion Experiment ALICE

• ALICE Detector overview
• TPC overview
• TPC components
• Field Cage
• Readout Chambers
• Front End Electronics
• Simulated Performance

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Alice Setup
TOF
TRD
TPC
RICH
ITS
L3-Magnet
PMD
PHOS
µ-ARM
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TPC Comparison
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TPC challenges

• ALICE PbPb _at_ vs5.5 TeV/A ?6000-16000 charged
  tracks in TPC acceptance ( 30 BGND)
• problem 1 high track density
• problem 2 high track density
• problem 3 high track density
• space charge problem
• stability of operation (sparks, breakdown)
• accelerated ageing
• high occupancy
• deteriorated tracking efficiency
• deteriorated dE/dx and momentum resolution
• large number of channels
• huge data volume per event
• cost of front-end electronics of channels

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ALICE TPC Layout
500 cm
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TPC Field Cage- Structural Layout

• TPC vessels central electrode end plates
  chambers supplies
• 7600kg
• Four separate vessels
• 1800kg (flanges incl.)
• Outer containment vessel
• Inner containment vessel
• Outer field cage vessel
• Inner field cage vessel
• Two end plates for readout
• 1500kg

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Test-Setup at SPS Beamline
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Exposure to secondaries from Pb beam

• Global irradiation of the FC led to charge
  accumulation on the insulating surface
• Injection of water vapor reduced the tendency of
  discharges (increase of surface conductance)
  stable up to 100 kV.
• The test was thus repeated at lower primary Pb
  beam rates (no water). Field cage stable up to
  100 kV!

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Beam Tests with GSI Heavy-Ion Beams

• Investigate chamber operation at high gain and/or
  particle load
• Result
• Stable operation with NA35 chamber,
• ALICE full size chamber will tested this fall

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Short Term Test of Long Term Behavior
Total accumulated charge in 2000 hrs 60
mC/cm ALICE PbPb 1.1 mC/cm year
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TPC working principle - 3D-imaging
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TPC cccupancy pad-time space
occupancy (pad-time) _at_ inner chambers up to 50
(!) ?cluster finding tracking very involved
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How to measure in a high track/cluster density
environment?

• R-f (pad direction)smaller pads
• limitations
• of channels (cost!)
• HV-GND gets critial
• PRF is diffusion limited
• oversampling
• Z (time direction) fewer time bins
• limitations
• signal/noise gets critical for ????????lt200 ns
• temporal signal is diffusion limited
• oversampling
• Conclusion
• choose the time/pad area which yields still
  reasonable signal (S/N gt 20)
• for a give pad area optimize aspect ratio
• minimize diffusion cold gas, use high drift
  field
• 400 V/m, central electrode at 100 kV (!)

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Example Pad Plane
570 000 pads (36 sectors)
pad size 4 ? 7.5 mm2
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Space Charge Problem
Get field line plot from blumet al or tdr

• Two source of positive charge in drift volume
• direct ionization by charged particle
• mion 4 cm2/Vs gtgt melectron gt quasi stationary
  positive charge in drift
• ion feed back from amplification region
• depends on gating ineffeciency gain 104
  requires at least 10-4 gating inefficiency!
• varies with luminosity gt rate dependent
  corrections

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Gating Inefficiency for Positive Ions

• primary ionization 20 ions/cm
• gas gain 104 ? 2 ? 105 ions/cm track from
  amplification
• egate lt 0.5 ? 10-4 ? lt 10 ions/cm from
  amplification

Gating ineff. lt 0.5 ? 10-4 (2s)
gate open/close
measurement sensitivity limit
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TPC FEE Basic Components
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TPC FEE Mounting
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FEE the Altro-16 Chip
Digital Filter Control Logic
8 ADCs
8 ADCs
Data Pedestal Memories
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FEE Adaptive Zero Suppression

• Zero suppression with adaptive threshold
• Cluster finding
• masking the cluster neighborhood
• updating the threshold with the average over a
  number of samples

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FEE Digital Tail Cancellation Filter
Experimental results
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Occupancy Tracking
occupancy clear side 51
absorber side 42
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Projected Performance- Tracking

• ?egood 88
• ?efake 2

• ltDpt/ptgt 2.4 (1.8))
• 5 GeV/c
• Dpt/pt 14 (7))

)with vertex cut
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Projected Performance- dE/dx

• dE/dx resolution 9 _at_ 8300 primary tracks

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Summary and Conclusion

• ALICE TPC
• conventional TPC (design similar to NA49, STAR)
• optimized for extreme particle densities
• tests show
• chamber operational and stable under high
  particle load
• simulations show
• acceptable performance even at highest
  anticipated multiplicities