pere mato/cern, ron settles/mpi-munich1 time projection chamber ron settles, mpi-munich pere mato,...

Pere Mato/CERN, Ron Settles/MPI-Munich 1

Time Projection Chamber

Ron Settles, MPI-MunichPere Mato, CERN


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


Time Projection Chamber

Ingredients:– Gas

E.g.: Ar + 10 to 20 % CH4

– E-fieldE ~ 100 to 200 V/cm

– B-fieldas big as possible to measure momentumto limit electron diffusion

– Wire chamberto detect projected tracks

y

z

x

E

B drift

chargedtrack

wire chamber to detect projected tracks

gas volume with E & B fields


TPC Characteristics

– Only gas in active volumeLittle material

– Very long drift ( > 2 m ) slow detector (~40 s)no impurities in gasuniform E-fieldstrong & uniform B-field

– Track points recorded in 3-D(x, y, z)

– Particle Identification by dE/dx

– Large track densities possible

y

z

x

E

B drift

chargedtrack

0BE


Detector with TPC


ALEPH Event


NA49 Event

Pad charge in one of the main TPCs for a Pb-Pb collision (event slice)


Drift velocity

22

2

)()()(

)(1 B

BBE

B

BEEvd

Drift of electrons in E- and B-fields (Langevin) mean drift time between collisions

me particle mobility

mceB cyclotron

frequency1)( Vd along E-field lines

1)( Vd along B-field lines

Typically ~5 cm/s for gases like Ar(90%) + CH4(10%)

Electrons tend to follow the magnetic field lines () >> 1


3-D coordinates

z

x

y

wire plane

track

projected track

– Z coordinate from drift time– X coordinate from wire number– Y coordinate?

» along wire direction» need cathode pads


Cathode Pads

projected track

pads

drifting electrons

avalanche

y

x

y

z

– Measure Ai

– Invert equation to get y

)222)(( prwi

iyyAeA

Amplitude on ith pad

y avalanche position

iy position of center of ith pad

prw pad response width


TPC Coordinates: Pad Response Width

Normalized PRW:

2

2

2ˆ

prw

Distance between pads

is a function of:– the pad crossing angle

» spread in r

– the wire crossing angle » ExB effect, lorentz angle

– the drift distance» diffusion

2̂

22 tan~ˆ

cos)tan(tan~ˆ 22

z~ˆ 2


TPC coordinates: Resolutions

Same effects as for PRW are expected but statistics of drifting electrons must be now considered

zz

z

D

r

)(

cos)tan(tan

tan

),,(

2

222

22

2

0

2

electronics, calibration

angular pad effect (dominant for small momentum tracks)

angular wire effect

forward tracks -> longer pulses -> worse resolution

)_(22 angledipzZ

“diffusion” term


Coordinate Resolutions: ALEPH TPC


Particle Identification by dE/dx

– 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

2)1(

2ln

1 22

2

22

J

mv

A

ZKz

dx

dE

Energy loss (Bethe-Bloch)

m mass of electron

vz, charge and velocity of incident particle

J mean ionization energy density effect term


dE/dx: Results

Good dE/dx resolution requireslong track lengthlarge number of samples/trackgood calibration, no noise, ...

ALEPH resolutionup to 334 wire samples/tracktruncated (60%) mean of samples5% (330 samples)

NA49 resolutiontruncated (50%) mean of clusters38%/sqtr(number of clusters)from 3% for the longest tracks to 6% measured with a single TPC


TPC ingredients

Field cage Gas system Wire chambers Gating Laser system Electronics


E-field produced by a Field Cage

HV

Ewires at ground potential

planar HV electrode

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

z

y


Field cage: ALEPH example

Dimensionscylinder 4.7 x 1.8 m

Drift length2x2.2 m

Electric field110 V/cm

E-field toleranceV < 6V

Electrodescopper strips (35 m & 19 m thickness, 10.1 mm pitch, 1.5 mm gap) on Kapton

Insulatorwound Mylar foil (75m)

Resistor chains2.004 M (0.2%)

Nucl. Instr. and Meth. A294 (1990) 121


Field cage: NA49 (MTPC)Dimensions

box 3.9x3.9x1.8 m3

Drift length1.1 m

Electric field175 V/cm

Tolerances< 100 m geometrical precision

Electrodesaluminized Mylar strips (25 m thickness, 0.5 in width, 2 mm gap) suspended on ceramic tubes

InsulatorGas envelope

Nucl. Instr. and Meth. A430 (1999) 210


ALICE Field Cage prototype


Gas system

Properties:Drift velocity (~5cm/s)Gas amplification (~7000)Signal attenuation my electron attachment (<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)

Typical mixtures: Ar(91%)+CH4(9%), Ar(90%)+CH4(5%)+CO2(5%)

Operation at atmospheric pressure


Influence of Gas Parameters (*)

Parameterchange

Drif t velocity, vd Eff ect on gasamplifi cation, A

Signal ettenuation byelectron attachment

0.1% CH4 0.4 % -2.5% f or A = 1x104

10 ppm O2 Negligible up to 100 ppm Negligible up to 100 ppm 0.15%/ m of drif t

10 ppm H2O 0.5 % Negligible at 100 ppm < 0.03% / m of drif t

1 mbar Negligible if at max. -(0.5%-0.7%)

(*) from ALEPH handbook (1995)


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

pad plane

field wire

sense wire

gating grid

Drift region

cathode plane

V=0

x

z


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

readoutpads and wires


Wire Chambers: NA49

62 chambers in totaleach 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

readoutpads


ALICE Ring cathode chambers

Cathode pads are folded around sense wiresBetter coupling (factor 4 better)

Integrated gating elementEasier to construct than the 3 wire planes


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


Laser Calibration System

PurposeMeasurement 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 lineCompare laser tracks with and without B-field

Laser tracks in the ALEPH TPC


Laser Calibration System (2)

LasersNd-YAG with 2 frequency doublers UV at 266 nm 4 mJ per pulse

Laser beamsUp to 200 beams at precisely defined positions can be produced

IngredientsBeam splittersPosition-sensitive diodesstepping-motors

etc.

NA49 Laser system


Electronics: from pad to storageTPC pad

amp

FADC

zerosuppression

featureextraction

DAQ

Pre-amplifiercharge 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


Analog Electronics

ALEPH analog electronics chain

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


Some TPC examples

TPC ReferencePEP4 PEP-PROPOSAL-004, Dec 1976TOPAZ Nucl. Instr. and Meth. A252 (1986) 423ALEPH Nucl. Instr. and Meth. A294 (1990) 121DELPHI Nucl. Instr. and Meth. A323 (1992) 209-212NA49 Nucl. Instr. and Meth. A430 (1999) 210STAR IEEE Trans. on Nucl. Sci. Vol. 44, No. 3 (1997)


Summary

TPC is a 3-D imaging chamber– Large dimensions. Little material– Slow device (~50 s)

– 3-D coordinate measurement (xy 170 m, z 740 m)

– 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-munich1 time projection chamber ron settles, mpi-munich pere mato,...

Documents

pere matocern

track slide

ron settlesmpimunich9

aleph tpc slide

efield e

cern slide

drift distance diffusion

drift time x