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Time Projection Chamber

Time Projection Chamber. Ron Settles, MPI-Munich Pere 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.

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Time Projection Chamber

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

  2. 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

  3. 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 Pere Mato/CERN, Ron Settles/MPI-Munich

  4. 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

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

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

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

  8. Drift velocity Drift of electrons in E- and B-fields (Langevin) Typically ~5 cm/ms for gases like Ar(90%) + CH4(10%) Electrons tend to follow the magnetic field lines (vt) >> 1 mean drift time between collisions particle mobility cyclotron frequency Vd along E-field lines Vd along B-field lines Pere Mato/CERN, Ron Settles/MPI-Munich

  9. 3-D coordinates • Z coordinate from drift time • X coordinate from wire number • Y coordinate? • along wire direction • need cathode pads z track projected track y wire plane x Pere Mato/CERN, Ron Settles/MPI-Munich

  10. 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

  11. 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

  12. 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

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

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

  15. 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

  16. 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

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

  18. E-field produced by a Field Cage y • 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 wires at ground potential planar HV electrode E HV potential strips encircle gas volume Pere Mato/CERN, Ron Settles/MPI-Munich

  19. 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

  20. 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

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

  22. 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

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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

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

  34. 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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