The LHC Experiment ALICE

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The LHC Experiment ALICE

• TPC Challenges at High Particle Multiplicities

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Talk dedicated to contributions and the memory of
Hans Sann
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Outline

• Heavy-Ion Physics at the LHC
• Initial conditions
• The ALICE Experiment
• The ALICE Time Projection Chamber
• Design Considerations
• Challenges at High Multiplicities and
  Luminosities
• Solutions

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Heavy Ions at the Large Hadron Collider

• 4 approved experiments Atlas, CMS, LHCb, ALICE
• spring 2007 start up LHC
• fall 2007 PbPb
• vsNN 5.5 TeV ? Ecm (PbPb) 1148 TeV ( 0.2
  mJ 1 g (0.5 m/s)2 )

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General Conditions at LHC for Heavy-Ion Collisions

• 5.5 TeV CM-energy (NN)
• Pb Pb 1248 TeV
• rapidity interval ?y17 units
• Luminosity (max.)
• Pb Pb 1.0 1027 cm-2 s-1
• 8 kHz inelastic collision rate
• event rate 100 200 Hz
• Ar Ar 0.6 1029 cm-2 s-1
• p p 30 1030 cm-2 s-1
• Rapidity density predictions
• dN/dy 2000 - 8000 (model dependent)
• what can we learn from RHIC?

educated extrapolation (employing the
saturation model, Eskola et al.) gives dN/dy ?
3000 at ? 0
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Fireball Evolution of PbPb Collisions at the LHC

• high energy densities
• ei 1000 GeV/fm3
• et1 fm/c 40 GeV/fm3
• long life times
• tQGP gt10 fm/c
• tfreeze 70 fm/c
• large volumes
• dNch/dy 8000
• Vfreeze(Dy1) 105 fm3

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The ALICE Experiment
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ALICE TPC Layout
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Field Cage Assembly
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Simulations - Event Display
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Challenges at High dN/dy

• high granularity
• tracking efficiency pt dE/dx resolution
• stability of readout chambers at high load
• space charge problems
• ageing problems
• cooling
• zero suppression/readout electronics

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TPC Principle

• space charge distortions from
• primary ionizations
• ion feed back
• overlapping clusters

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TPC Occupancy in Pad-Time Space- Simulation
occupancy at inner pad rows 40-50 gtcluster
finding tracking very involved
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How to measure in a high density environment?

• Increase granularity
• R-f (pad direction) smaller pads
• limitations
• of channels (cost!)
• HV-GND gets critical
• few primary electrons/pad
• signal/noise intolerable
• PRF is diffusion limited
• oversampling

• Z (time direction) smaller shaping time
• limitations
• ?shapinglt 200 ns
• signal/noise gets critical
• temporal signal is diffusion limited
• oversampling

gas choice is a critical issue
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Example Pad Plane
pad size 4 x 7.5 mm2
570 000 pads (36 sectors)
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Gas Choice

• TPCs operate typically with Ar as main gas (e.g.
  90) and CH4 (e.g. 10) as quencher
• Good (momentum) resolution at high multiplicity
  operatation and high luminosity dictates NeCO2, a
  very unpleseant mixture....
• Why?

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Hans Sanns ALICE Projects

• project leader of the ALICE TPC gas system
• build gas system
• program to measure the properties of NeCO2 with
  very high precision
• HV, CO2, pressure temperature dependence of
  drift velocity
• CO2, pressure temperature dependence of gas
  gain
• ageing properties
• electron attachment
• ....
• project leader of TPC (electronics) cooling
  system
• electronics cooling
• resistor rod cooling

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Diffusion

• To minimize lateral and temporal size of the
  electron cloud after the long drift to the
  readoaut chambers a cold gas, i.e., CO2 is
  chosen as quencher

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Momentum Resolution

• space charge from ionization of high Z gases
  (like Ar) causes drift field distortions
• varies with luminosity gt rate dependent
  corrections
• choice of low Z gas (Ne)

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Gas Properties - Drift Velocity
ALICE (400 V/Ccm)
STAR, 150 V/cm
J. Wiechula, U. Frankenfeld, H. Sann, C.
Garabatos and H.R. Schmidt
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Gas Properties Temperature Dependence
0.35 change in vdrift/K ?? Dz ? 1
cm temperature stabilization DT ? 0.1 K of TPC
volume (88 m3) very challenging! ? 300 kW heat
dissipated by the detectors (TPC, TRD, TOF, ITS)
J. Wiechula, U. Frankenfeld, H. Sann, C.
Garabatos and H.R. Schmidt
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NeCO2 Pros and Cons
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The TPC Gas System
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TPC Cooling Scheme
6 cooling circuits forseen to achieve temperature
stability lt 0.1 K
outer resistor rod
TPC gas volume
inner resistor rod
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Front-End Electronics Cooling
U. Frankenfeld, S. Popescu, H. Sann, H.R. Schmidt
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Voltage Divider Chain (Resistor Rod)

• A minimum current (?100 ?A) through resistor
  chain is required to ensure drift field stability
  
• dissipation of 4 x 8 Watt as heat into drift
  space
• cooling needed

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Resistor Rod Cooling

• Solution
• removable, water cooled resistor rod
• serviceable in case of resistor failure
• leakless cooling
• high cooling efficiency
• ultra-pure water 18 µS/cm

GND
105 V
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Exchangeable, Water-cooled Resistor Rod(Hans
Prototype)
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Exchangeable, Water-cooled Resistor Rod (in
existence)
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High Muliplicity - Baseline Shift
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Frontend Electronics Architecture
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ALTRO - digital tail cancellation and baseline
restoration
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