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The Aleph Time Projection Chamber

The Aleph Time Projection Chamber. Ron Settles, MPI-Munich/DESY TPC Symposium@LBNL 17 October 2003. Summary. TPC is a 3-D imaging chamber Large volume, small amount of material. Slow device (~50 m s) 3-D ‘continuous’ tracking (  xy  170 m m,  z  600 m m for Aleph)

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The Aleph Time Projection Chamber

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  1. The Aleph Time Projection Chamber Ron Settles, MPI-Munich/DESY TPC Symposium@LBNL 17 October 2003 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  2. Summary • TPC is a 3-D imaging chamber • Large volume, small amount of material. • Slow device (~50 ms) • 3-D ‘continuous’ tracking (xy 170 mm, z 600 mm for Aleph) • Review some of the main ingredients • History • First proposed in 1976 (PEP4-TPC) • Used in many experiments • Aleph as an example here • Now a well-established detection technique that is still in the process of evolution… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  3. Outline • Examples • TPC principles of operation • Drift velocity, Coordinates, dE/dx • TPC hardware ingredients • Field cage, gas system, wire chambers, gating, laser calibration system, electronics • The Aleph TPC • From the drawing board to the gadget • Performance • Some ‘features’ (i.e. trouble shooting…) • Conclusion TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  4. Some TPC examples STAR FTPC, ALICE, LC, … Grand-daddy/mama of all TPCs TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  5. TPC principles of operation A TPC contains: • Gas E.g.: Ar + 10-20 % CH4 • E-field E ~ few x 100 V/cm • B-field as large as possible to measure momentum, to limit electron diffusion • Wire chamber (those days) to detect projected tracks gas volume with E & B fields • B y electron drift E x z charged track wire chamber to detect projected tracks Now trying out new techniques--► TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  6. TPC Characteristics • Only gas in active volume, small amount of material • Long drift ( > 2 m ) therefore slow detector (~50 ms) want 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 • TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  7. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  8. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  9. Coordinate from 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  10. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  11. TPC coordinate resolution Same effects as for PRW are expected but statistics of • drifting electrons must be considered electronics, calibration angular pad effect (dominant for small momentum tracks) angular wire effect (…disappears with new technologies…) “diffusion” term forward tracks -> longer pulses -> degrades resolution TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  12. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  13. TPC ingredients (Aleph example) • Wire chambers • Gating • Cooling • Mechanics • Field cage • Gas system • Laser system • Electronics TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  14. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  15. Wire Chambers: ALEPH 36 sectors, 3 types • • no gaps extend full radius wires • gating spaced 2 mm • cathode spaced 1 mm • sense & field spaced 2 mm, interleaved pads • 6.2 mm x 30 mm • ~1200 per sector • total 41004 pads readout pads and wires • TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  16. 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 (Aleph) • Diode mode TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  17. Cooling, Mechanics • Terribly mundane but terribly important (everything is important) • Cooling: • Combined air and water cooling to completely insulate the gas volume • Mechanics: • 25% X_0 for sectors, preamps, cooling (but before cables) TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  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 • TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  20. Laser Calibration System Purpose Measurement of drift velocity Determination of E- and B-field distortions • Drift velocity Laser system  ∂(v_drift) ~ 1‰ Hookup tracks to Vdet  ∂(v_drift)~a few times 0.01‰ …used after Vdet installation • ExB Distortions Laser used only in early days to get first corrections. After, tracks (mostly μ pairs from Z decays) used exclusively (read on…) Laser tracks in the ALEPH TPC • TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  21. Gas system Typical mixtures: Ar91%+CH49%, Ar90%+CH45%+CO25% 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) • TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  22. Influence of Gas Parameters (*) (*) from ALEPH handbook (1995) TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  23. 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 TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  24. Analog Electronics ALEPH analog electronics chain • Large number of channels O(105) • Large channel densities • Integration in wire chamber • Power dissipation • Low noise TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  25. More details about Aleph… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  26. Wire Chambers: ALEPH Long pads for better coordinate precision TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  27. After 3 man-centuries(or more, depending on how you count)… …as usual, lots of meetings…↓ From TPC90…↑ TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  28. From the drawing board to the gadget… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  29. A Detector with TPC you end up with-----------► …where… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  30. Thanks to many people… (and to Pere Mato and Werner Wiedenmann for help on these slides) …you need a few cables, cooling, etc… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  31. It finally started working… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  32. ALEPH Event, early days… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  33. And towards the end… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  34. Coordinate Resolution(1): ALEPH TPC TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  35. Coordinate Resolution(2): ALEPH TPC TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  36. 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 4.5% (330 samples) TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  37. But, there were ‘FEATURES’… Werner’s talk contains many details, see alephwww.mppmu.mpg.de/~settles/tpc  • here a few examples… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  38. Historical Development (1) • LEP start-up: 1989-1990 • Failure of magnet compensating power supplies in 1989 required development of field-corrections methods • derived from 2 special laser runs (B on/off) • correction methods described in NIM A306(1991)446 • Later, high statistics Z->μμ events give main calibration sample • LEP 1: 1991-1994 • VDET 1 becomes operational in 1991 • Development of common alignment procedures for all three tracking detectors • Incidents affect large portions of collected statistics and require correction methods based directly on data • 1991-1993, seven shorts on field cage affect 24% of data • 1994, disconnected gating grids on 2 sectors affect 20% of data • All data finally recuperated with data-based correction methods TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  39. Historical Development (2) • LEP 1/2: 1994-1996 • Tracking-upgrade program (LEP 1 data reprocessed) • Improved coordinate determination requires better understanding of systematic effects • Combined calculations for field and alignment distortions, reevaluation of B-field map • All methods for distortion corrections now based directly on data • Development of “few”-parameter correction models to cope with drastically reduced calibration samples at LEP 2 • LEP 2: 1995-2000 • New VDET with larger acceptance • Calibrations@Z at beginning of run periods have limited statistics • Frequent beam losses cause charge-up effects and new FC shorts • Superimposed distortions • Short-corrections with Z -> μμ;time-dep. effects tracked with hadrons TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  40. Examples from Werner’s slides… TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  41. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  42. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  43. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  44. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  45. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  46. e.g. (see Werner’s slides…) TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  47. e.g., non-linear F.C. potential TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  48. TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  49. e.g., disconnected gating grids TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

  50. e.g., field-cage shorts TPC Symposium@LBNL ALEPH TPC Ron Settles, MPI-Munich/DESY

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