Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN Pere Mato/CERN, Ron Settles/MPI-Munich
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Transcript Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN Pere Mato/CERN, Ron Settles/MPI-Munich
Time Projection Chamber
Ron Settles, MPI-Munich
Pere Mato, CERN
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
momentum
to limit electron diffusion
– Wire chamber
to detect projected tracks
gas volume with E
& B fields
B
y
drift
E
x
z
charged
track
Pere Mato/CERN, Ron Settles/MPI-Munich
wire chamber
to detect
projected tracks
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TPC Characteristics
– Only gas in active volume
Little material
– Very long drift ( > 2 m )
slow detector (~40 ms)
no impurities in gas
uniform E-field
strong & uniform B-field
EB 0
B
y
E
x
z
– Track points recorded in 3-D
(x, y, z)
– Particle Identification by dE/dx
– Large track densities possible
drift
charged
track
Pere Mato/CERN, Ron Settles/MPI-Munich
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Detector with TPC
Pere Mato/CERN, Ron Settles/MPI-Munich
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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)
m
E
B
(
E
B
)
B
2
E ( ) ( )
vd
e
2
2
m
1 ( )
B
B
m
eB
( ) 1 Vd along E-field lines
mc
( ) 1
mean drift time
between collisions
particle mobility
cyclotron
frequency
Vd along B-field lines
Typically ~5 cm/ms for gases like Ar(90%) + CH4(10%)
Electrons tend to follow the magnetic field lines (v) >> 1
Pere Mato/CERN, Ron Settles/MPI-Munich
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3-D coordinates
z
– Z coordinate from drift time
– X coordinate from wire number
– Y coordinate?
» along wire direction
» need cathode pads
track
projected track
y
wire plane
x
Pere Mato/CERN, Ron Settles/MPI-Munich
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Cathode Pads
x
y
Amplitude on ith pad
( ( y yi ) 2 2 prw 2 )
Ai Ae
y avalanche position
yi
position of center of ith pad
prw pad response width
projected track
drifting
electrons
z
y
avalanche
pads
– Measure Ai
– Invert equation to get y
Pere Mato/CERN, Ron Settles/MPI-Munich
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TPC Coordinates: Pad Response Width
Normalized PRW:
ˆ 2
ˆ 2 is a function of:
2
prw
Distance between pads
2
– the pad crossing angle b
» spread in rf
ˆ 2 ~ tan 2 b
– the wire crossing angle a
» ExB effect, lorentz angle
ˆ 2 ~ (tan a tan ) 2 cos a
– the drift distance
» diffusion
ˆ 2 ~ z
Pere Mato/CERN, Ron Settles/MPI-Munich
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TPC coordinates: Resolutions
Same effects as for PRW are expected but statistics of
drifting electrons must be now considered
r2f ( b ,a , z ) 02
b2 tan 2 b
a2 (tan a tan ) 2 cos 2 a
D2 ( z ) z
Z2 z2 (dip _ angle)
electronics, calibration
angular pad effect (dominant for
small momentum tracks)
angular wire effect
“diffusion” term
forward tracks -> longer pulses ->
worse resolution
Pere Mato/CERN, Ron Settles/MPI-Munich
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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)
dE
2mv2
2 Z 1
2
Kz
ln
b
dx
A b 2 J (1 b 2 )
2
m
z, v
J
mass of electron
charge and velocity of
incident particle
mean ionization energy
density effect term
– 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 nonrelativistic 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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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TPC ingredients
Field cage
Gas system
Wire chambers
Gating
Laser system
Electronics
Pere Mato/CERN, Ron Settles/MPI-Munich
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E-field produced by a Field Cage
y
z
wires at
ground
potential
E
planar HV
electrode
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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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 < 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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
< 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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Gas amplification
Signal attenuation my electron
attachment
(~5cm/ms)
(~7000)
(<1%/m)
Parameters to control and monitor:
Mixture quality (change in amplification)
O2 (electron attachment, attenuation)
H2O (change in drift velocity, attenuation)
Other contaminants (attenuation)
Pere Mato/CERN, Ron Settles/MPI-Munich
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Influence of Gas Parameters (*)
Parameter
change
0.1% CH4
Drift velocity, vd
Effect on gas
amplification, A
-2.5% for A = 1x104
Signal ettenuation by
electron attachment
0.4 %
10 ppm O2
Negligible up to 100 ppm Negligible up to 100 ppm 0.15%/m of drift
10 ppm H2O
0.5 %
Negligible at 100 ppm
1 mbar
Negligible if at max.
-(0.5%-0.7%)
< 0.03% /m of drift
(*) from ALEPH handbook (1995)
Pere Mato/CERN, Ron Settles/MPI-Munich
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Wire Chambers
3 planes of wires
– gating grid
– cathode plane (Frisch grid)
– sense and field wire plane
Drift region
cathode plane
gating grid
– cathode and field wires at
zero potential
V=0
pad size
– various sizes & densities
– typically few cm2
gas gain
– typically 3-5x103
sense wire
pad plane
z
x
field wire
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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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 Bfield
Laser tracks in the ALEPH TPC
Pere Mato/CERN, Ron Settles/MPI-Munich
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Laser Calibration System (2)
Lasers
NA49 Laser system
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.
Pere Mato/CERN, Ron Settles/MPI-Munich
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Electronics: from pad to storage
TPC pad
amp
FADC
zero
suppression
feature
extraction
DAQ
Pre-amplifier
charge sensitive, mounted on wire chamber
Shaping amplifier:
pole/zero compensation. Typical FWHM ~200ns
Flash ADC:
8-9 bit resolution. 10 MHz. 512 time buckets
Multi-event buffer
Digital data processing: zero-suppression.
Pulse charge and time estimates
Data acquisition and recording system
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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Some TPC examples
TPC
PEP4
TOPAZ
ALEPH
DELPHI
NA49
STAR
Reference
PEP-PROPOSAL-004, Dec 1976
Nucl. Instr. and Meth. A252 (1986) 423
Nucl. Instr. and Meth. A294 (1990) 121
Nucl. Instr. and Meth. A323 (1992) 209-212
Nucl. Instr. and Meth. A430 (1999) 210
IEEE Trans. on Nucl. Sci. Vol. 44, No. 3 (1997)
Pere Mato/CERN, Ron Settles/MPI-Munich
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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
Pere Mato/CERN, Ron Settles/MPI-Munich
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