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Pere Mato/CERN, Ron Settles/MPI-Munich1 Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN.

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Presentation on theme: "Pere Mato/CERN, Ron Settles/MPI-Munich1 Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN."— Presentation transcript:

1 Pere Mato/CERN, Ron Settles/MPI-Munich1 Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN

2 Pere Mato/CERN, Ron Settles/MPI-Munich2 Outline u TPC principle of operation –Drift velocity, Coordinates, dE/dx u TPC ingredients –Field cage, gas system, wire chambers, gating grid, laser calibration system, electronics u Summary

3 Pere Mato/CERN, Ron Settles/MPI-Munich3 Time Projection Chamber Ingredients: –Gas E.g.: Ar + 10 to 20 % CH 4 –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 y z x E B drift charged track wire chamber to detect projected tracks gas volume with E & B fields

4 Pere Mato/CERN, Ron Settles/MPI-Munich4 TPC Characteristics –Only gas in active volume Little material –Very long drift ( > 2 m ) slow detector (~40  s) 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 y z x E B drift charged track

5 Pere Mato/CERN, Ron Settles/MPI-Munich5 Detector with TPC

6 Pere Mato/CERN, Ron Settles/MPI-Munich6 ALEPH Event

7 Pere Mato/CERN, Ron Settles/MPI-Munich7 NA49 Event Pad charge in one of the main TPCs for a Pb-Pb collision (event slice)

8 Pere Mato/CERN, Ron Settles/MPI-Munich8 Drift velocity Drift of electrons in E- and B-fields (Langevin) mean drift time between collisions particle mobility cyclotron frequency V d along E-field lines V d along B-field lines Typically ~5 cm/  s for gases like Ar(90%) + CH 4 (10%) Electrons tend to follow the magnetic field lines (  ) >> 1

9 Pere Mato/CERN, Ron Settles/MPI-Munich9 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

10 Pere Mato/CERN, Ron Settles/MPI-Munich10 Cathode Pads projected track pads drifting electrons avalanche y x y z –Measure A i –Invert equation to get y Amplitude on ith pad avalanche position position of center of ith pad pad response width

11 Pere Mato/CERN, Ron Settles/MPI-Munich11 TPC Coordinates: Pad Response Width Normalized 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

12 Pere Mato/CERN, Ron Settles/MPI-Munich12 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 forward tracks -> longer pulses -> worse resolution “diffusion” term

13 Pere Mato/CERN, Ron Settles/MPI-Munich13 Coordinate Resolutions: ALEPH TPC

14 Pere Mato/CERN, Ron Settles/MPI-Munich14 Coordinate Resolutions: ALEPH TPC

15 Pere Mato/CERN, Ron Settles/MPI-Munich15 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 Energy loss (Bethe-Bloch) mass of electron charge and velocity of incident particle mean ionization energy density effect term

16 Pere Mato/CERN, Ron Settles/MPI-Munich16 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

17 Pere Mato/CERN, Ron Settles/MPI-Munich17 TPC ingredients u Field cage u Gas system u Wire chambers u Gating u Laser system u Electronics

18 Pere Mato/CERN, Ron Settles/MPI-Munich18 E-field produced by a Field Cage HV E wires 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

19 Pere Mato/CERN, Ron Settles/MPI-Munich19 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  m & 19  m thickness, 10.1 mm pitch, 1.5 mm gap) on Kapton Insulator wound Mylar foil (75  m) Resistor chains M  (  0.2%) Nucl. Instr. and Meth. A294 (1990) 121

20 Pere Mato/CERN, Ron Settles/MPI-Munich20 Field cage: NA49 (MTPC) Dimensions box 3.9x3.9x1.8 m 3 Drift length 1.1 m Electric field 175 V/cm Tolerances < 100  m geometrical precision Electrodes aluminized Mylar strips (25  m thickness, 0.5 in width, 2 mm gap) suspended on ceramic tubes Insulator Gas envelope Nucl. Instr. and Meth. A430 (1999) 210

21 Pere Mato/CERN, Ron Settles/MPI-Munich21 ALICE Field Cage prototype

22 Pere Mato/CERN, Ron Settles/MPI-Munich22 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) O 2 (electron attachment, attenuation) H 2 O (change in drift velocity, attenuation) Other contaminants (attenuation) Typical mixtures: Ar(91%)+CH 4 (9%), Ar(90%)+CH 4 (5%)+CO 2 (5%) Operation at atmospheric pressure

23 Pere Mato/CERN, Ron Settles/MPI-Munich23 Influence of Gas Parameters (*) (*) from ALEPH handbook (1995)

24 Pere Mato/CERN, Ron Settles/MPI-Munich24 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 cm 2 gas gain –typically 3-5x10 3 pad plane field wire sense wire gating grid Drift region cathode plane V=0 x z

25 Pere Mato/CERN, Ron Settles/MPI-Munich25 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

26 Pere Mato/CERN, Ron Settles/MPI-Munich26 Wire Chambers: NA49 62 chambers in total each 72x72 cm 2 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 pads

27 Pere Mato/CERN, Ron Settles/MPI-Munich27 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

28 Pere Mato/CERN, Ron Settles/MPI-Munich28 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

29 Pere Mato/CERN, Ron Settles/MPI-Munich29 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

30 Pere Mato/CERN, Ron Settles/MPI-Munich30 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

31 Pere Mato/CERN, Ron Settles/MPI-Munich31 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

32 Pere Mato/CERN, Ron Settles/MPI-Munich32 Analog Electronics ALEPH analog electronics chain –Large number of channels O(10 5 ) –Large channel densities –Integration in wire chamber –Power dissipation –Low noise

33 Pere Mato/CERN, Ron Settles/MPI-Munich33 Some TPC examples

34 Pere Mato/CERN, Ron Settles/MPI-Munich34 Summary u 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 u Reviewed some the main ingredients –Field cage, gas, wire chambers, gating grid, laser calibration, electronics, etc. u History –First proposed in 1976 (PEP4-TPC) –Used in many experiments –Well established detecting technique


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