Time Projection Chamber

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Time Projection Chamber
Ron Settles, MPI-Munich Pere Mato, CERN
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Outline

• TPC principle of operation
• Drift velocity, Coordinates, dE/dx
• TPC ingredients
• Field cage, gas system, wire chambers, gating
  grid, laser calibration system, electronics
• Summary

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Time Projection Chamber

• Ingredients
• Gas
• E.g. Ar 10 to 20 CH4
• E-field
• E 100 to 200 V/cm
• B-field
• as big as possible to measure momentumto limit
  electron diffusion
• Wire chamber
• to detect projected tracks

gas volume with E B fields
B
y
drift
E
x
z
charged track
wire chamber to detect projected tracks
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TPC Characteristics

• Only gas in active volume
• Little material
• Very long drift ( gt 2 m )
• slow detector (40 ms)
• no impurities in gas
• uniform E-field
• strong uniform B-field
• Track points recorded in 3-D(x, y, z)
• Particle Identification by dE/dx
• Large track densities possible

B
drift
y
E
x
z
charged track
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Detector with TPC
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ALEPH Event
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NA49 Event
Pad charge in one of the main TPCs for a Pb-Pb
collision (event slice)
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Drift velocity
Drift of electrons in E- and B-fields (Langevin)
mean drift time between collisions
particle mobility
cyclotron frequency
Vd along E-field lines
Vd along B-field lines

• Typically 5 cm/ms for gases like Ar(90)
  CH4(10)
• Electrons tend to follow the magnetic field lines
  (vt) gtgt 1

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3-D coordinates
z
track

• Z coordinate from drift time
• X coordinate from wire number
• Y coordinate?
• along wire direction
• need cathode pads

projected track
y
wire plane
x
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Cathode Pads
x
y
Amplitude on ith pad
avalanche position
projected track
position of center of ith pad
z
drifting electrons
pad response width
y
avalanche
pads

• Measure Ai
• Invert equation to get y

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TPC Coordinates Pad Response Width
Distance between pads
Normalized PRW

• is a function of
• the pad crossing angle b
• spread in rf
• the wire crossing angle a
• ExB effect, lorentz angle ?
• the drift distance
• diffusion

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TPC coordinates Resolutions
Same effects as for PRW are expected but
statistics of drifting electrons must be now
considered
electronics, calibration
angular pad effect (dominant for small momentum
tracks)
angular wire effect
diffusion term
forward tracks -gt longer pulses -gt worse
resolution
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Coordinate Resolutions ALEPH TPC
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Coordinate Resolutions ALEPH TPC
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Particle Identification by dE/dx
Energy loss (Bethe-Bloch)

• Energy loss (dE/dx) depends on the particle
  velocity.
• The mass of the particle can be identified by
  measuring simultaneously momentum and dE/dx (ion
  pairs produced)
• Particle identification possible in the
  non-relativistic region (large ionization
  differences)
• Major problem is the large Landau fluctuations on
  a single dE/dx sample.
• 60 for 4 cm track
• 120 for 4 mm track

mass of electron
charge and velocity of incident particle
mean ionization energy
density effect term
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dE/dx Results

• Good dE/dx resolution requires
• long track length
• large number of samples/track
• good calibration, no noise, ...
• ALEPH resolution
• up to 334 wire samples/track
• truncated (60) mean of samples
• 5 (330 samples)
• NA49 resolution
• truncated (50) mean of clusters
• 38/sqtr(number of clusters)
• from 3 for the longest tracks to 6 measured
  with a single TPC

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TPC ingredients

• Field cage
• Gas system
• Wire chambers
• Gating
• Laser system
• Electronics

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E-field produced by a Field Cage
y
z
wires at ground potential
planar HV electrode
E
HV
potential strips encircle gas volume

• chain of precision resistors with small current
  flowing provides uniform voltage drop in z
  direction
• non uniformity due to finite spacing of strips
  falls exponentially into active volume

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Field cage ALEPH example

• Dimensions
• cylinder 4.7 x 1.8 m
• Drift length
• 2x2.2 m
• Electric field
• 110 V/cm
• E-field tolerance
• ?V lt 6V
• Electrodes
• copper strips (35 mm 19 mm thickness, 10.1 mm
  pitch, 1.5 mm gap) on Kapton
• Insulator
• wound Mylar foil (75mm)
• Resistor chains
• 2.004 M? (?0.2)

Nucl. Instr. and Meth. A294 (1990) 121
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Field cage NA49 (MTPC)

• Dimensions
• box 3.9x3.9x1.8 m3
• Drift length
• 1.1 m
• Electric field
• 175 V/cm
• Tolerances
• lt 100 mm geometrical precision
• Electrodes
• aluminized Mylar strips (25 mm thickness, 0.5 in
  width, 2 mm gap) suspended on ceramic tubes
• Insulator
• Gas envelope

Nucl. Instr. and Meth. A430 (1999) 210
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ALICE Field Cage prototype
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Gas system
Typical mixtures Ar(91)CH4(9),
Ar(90)CH4(5)CO2(5) Operation at
atmospheric pressure

• Properties
• Drift velocity (5cm/ms)
• Gas amplification (7000)
• Signal attenuation my electron
• attachment (lt1/m)

• Parameters to control and monitor
• Mixture quality (change in amplification)
• O2 (electron attachment, attenuation)
• H2O (change in drift velocity, attenuation)
• Other contaminants (attenuation)

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Influence of Gas Parameters ()
() from ALEPH handbook (1995)
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Wire Chambers

• 3 planes of wires
• gating grid
• cathode plane (Frisch grid)
• sense and field wire plane
• cathode and field wires at zero potential
• pad size
• various sizes densities
• typically few cm2
• gas gain
• typically 3-5x103

Drift region
gating grid
cathode plane
V0
sense wire
z
pad plane
x
field wire
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Wire Chambers ALEPH

• 36 sectors, 3 types
• no gaps extend full radius
• wires
• gating spaced 2 mm
• cathode spaced 1 mm
• sense field spaced 4 mm
• pads
• 6.2 mm x 30 mm
• 1200 per sector
• total 41004 pads
• readout
• pads and wires

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Wire Chambers NA49

• 62 chambers in total
• each 72x72 cm2
• wires
• gating spaced 2 mm
• cathode spaced 1 mm
• sense field spaced 4 mm
• pads
• 3.6-5.5 mm x 40 mm
• 4000 per module
• total 182000 pads
• readout
• pads

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ALICE Ring cathode chambers

• Cathode pads are folded around sense wires
• Better coupling (factor 4 better)
• Integrated gating element
• Easier to construct than the 3 wire planes

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Gating

• Problem Build-up of space charge in the drift
  region by ions.
• Grid of wires to prevent positive ions from
  entering the drift region
• Gating grid is either in the open or closed
  state
• Dipole fields render the gate opaque
• Operating modes
• Switching mode (synch.)
• Diode mode

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Laser Calibration System

• Purpose
• Measurement of drift velocity
• Determination of E- and B-field distortions

• Drift velocity
• Measurement of time arrival difference of
  ionization from 2 laser tracks with known
  position

• ExB Distortions
• Compensate residuals of straight line
• Compare laser tracks with and without B-field

Laser tracks in the ALEPH TPC
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Laser Calibration System (2)

• Lasers
• Nd-YAG with 2 frequency doublers
• UV at 266 nm
• 4 mJ per pulse
• Laser beams
• Up to 200 beams at precisely defined positions
  can be produced
• Ingredients
• Beam splitters
• Position-sensitive diodes
• stepping-motors
• etc.

NA49 Laser system
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Electronics from pad to storage
TPC pad
Pre-amplifier charge sensitive, mounted on wire
chamber
Shaping amplifier pole/zero compensation.
Typical FWHM 200ns
amp
FADC
Flash ADC 8-9 bit resolution. 10 MHz. 512 time
buckets
Multi-event buffer
zero suppression
Digital data processing zero-suppression.
feature extraction
Pulse charge and time estimates
DAQ
Data acquisition and recording system
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Analog Electronics
ALEPH analog electronics chain

• Large number of channels O(105)
• Large channel densities
• Integration in wire chamber
• Power dissipation
• Low noise

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Some TPC examples
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Summary

• TPC is a 3-D imaging chamber
• Large dimensions. Little material
• Slow device (50 ms)
• 3-D coordinate measurement (?xy ? 170 mm, ?z ?
  740 mm)
• Momentum measurement if inside a magnetic field
• Reviewed some the main ingredients
• Field cage, gas, wire chambers, gating grid,
  laser calibration, electronics, etc.
• History
• First proposed in 1976 (PEP4-TPC)
• Used in many experiments
• Well established detecting technique