Time Projection Chamber

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

Pere Mato/CERN, Ron Settles/MPI-Munich
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

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 wire chamber to detect projected tracks Pere Mato/CERN, Ron Settles/MPI-Munich

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 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 Pere Mato/CERN, Ron Settles/MPI-Munich

Detector with TPC Pere Mato/CERN, Ron Settles/MPI-Munich

ALEPH Event Pere Mato/CERN, Ron Settles/MPI-Munich

NA49 Event Pad charge in one of the main TPCs for a Pb-Pb collision (event slice) Pere Mato/CERN, Ron Settles/MPI-Munich

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

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 Pere Mato/CERN, Ron Settles/MPI-Munich

Cathode Pads x y Amplitude on ith pad avalanche position projected track position of center of ith pad z pad response width drifting electrons y avalanche pads Measure Ai Invert equation to get y Pere Mato/CERN, Ron Settles/MPI-Munich

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 Pere Mato/CERN, Ron Settles/MPI-Munich

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 -> longer pulses -> worse resolution Pere Mato/CERN, Ron Settles/MPI-Munich

Coordinate Resolutions: ALEPH TPC
Pere Mato/CERN, Ron Settles/MPI-Munich

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 Pere Mato/CERN, Ron Settles/MPI-Munich

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

TPC ingredients Field cage Gas system Wire chambers Gating Laser system Electronics Pere Mato/CERN, Ron Settles/MPI-Munich

E-field produced by a Field Cage
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 Pere Mato/CERN, Ron Settles/MPI-Munich

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

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

ALICE Field Cage prototype
Pere Mato/CERN, Ron Settles/MPI-Munich

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 (<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

Influence of Gas Parameters (*)
(*) from ALEPH handbook (1995) Pere Mato/CERN, Ron Settles/MPI-Munich

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 V=0 sense wire z pad plane x field wire Pere Mato/CERN, Ron Settles/MPI-Munich

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 pads readout pads and wires Pere Mato/CERN, Ron Settles/MPI-Munich

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 mm x 40 mm ~4000 per module total pads readout Pere Mato/CERN, Ron Settles/MPI-Munich

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

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

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 Pere Mato/CERN, Ron Settles/MPI-Munich

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 Pere Mato/CERN, Ron Settles/MPI-Munich

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 Pere Mato/CERN, Ron Settles/MPI-Munich

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

Some TPC examples Pere Mato/CERN, Ron Settles/MPI-Munich

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

Time Projection Chamber

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