Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN Pere Mato/CERN, Ron Settles/MPI-Munich
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Time Projection Chamber Ron Settles, MPI-Munich Pere Mato, CERN Pere Mato/CERN, Ron Settles/MPI-Munich 1 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 2 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 Pere Mato/CERN, Ron Settles/MPI-Munich wire chamber to detect projected tracks 3 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 EB 0 B y E x z – Track points recorded in 3-D (x, y, z) – Particle Identification by dE/dx – Large track densities possible drift charged track Pere Mato/CERN, Ron Settles/MPI-Munich 4 Detector with TPC Pere Mato/CERN, Ron Settles/MPI-Munich 5 ALEPH Event Pere Mato/CERN, Ron Settles/MPI-Munich 6 NA49 Event Pad charge in one of the main TPCs for a Pb-Pb collision (event slice) Pere Mato/CERN, Ron Settles/MPI-Munich 7 Drift velocity Drift of electrons in E- and B-fields (Langevin) m E B ( E B ) B 2 E ( ) ( ) vd e 2 2 m 1 ( ) B B m eB ( ) 1 Vd along E-field lines mc ( ) 1 mean drift time between collisions particle mobility cyclotron frequency Vd along B-field lines Typically ~5 cm/ms for gases like Ar(90%) + CH4(10%) Electrons tend to follow the magnetic field lines (v) >> 1 Pere Mato/CERN, Ron Settles/MPI-Munich 8 3-D coordinates z – Z coordinate from drift time – X coordinate from wire number – Y coordinate? » along wire direction » need cathode pads track projected track y wire plane x Pere Mato/CERN, Ron Settles/MPI-Munich 9 Cathode Pads x y Amplitude on ith pad ( ( y yi ) 2 2 prw 2 ) Ai Ae y avalanche position yi position of center of ith pad prw pad response width projected track drifting electrons z y avalanche pads – Measure Ai – Invert equation to get y Pere Mato/CERN, Ron Settles/MPI-Munich 10 TPC Coordinates: Pad Response Width Normalized PRW: ˆ 2 ˆ 2 is a function of: 2 prw Distance between pads 2 – the pad crossing angle b » spread in rf ˆ 2 ~ tan 2 b – the wire crossing angle a » ExB effect, lorentz angle ˆ 2 ~ (tan a tan ) 2 cos a – the drift distance » diffusion ˆ 2 ~ z Pere Mato/CERN, Ron Settles/MPI-Munich 11 TPC coordinates: Resolutions Same effects as for PRW are expected but statistics of drifting electrons must be now considered r2f ( b ,a , z ) 02 b2 tan 2 b a2 (tan a tan ) 2 cos 2 a D2 ( z ) z Z2 z2 (dip _ angle) 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 12 Coordinate Resolutions: ALEPH TPC Pere Mato/CERN, Ron Settles/MPI-Munich 13 Coordinate Resolutions: ALEPH TPC Pere Mato/CERN, Ron Settles/MPI-Munich 14 Particle Identification by dE/dx Energy loss (Bethe-Bloch) dE 2mv2 2 Z 1 2 Kz ln b dx A b 2 J (1 b 2 ) 2 m z, v J mass of electron charge and velocity of incident particle mean ionization energy density effect term – 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 nonrelativistic 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 Pere Mato/CERN, Ron Settles/MPI-Munich 15 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 16 TPC ingredients Field cage Gas system Wire chambers Gating Laser system Electronics Pere Mato/CERN, Ron Settles/MPI-Munich 17 E-field produced by a Field Cage y z wires at ground potential E planar HV electrode 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 18 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 19 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 20 ALICE Field Cage prototype Pere Mato/CERN, Ron Settles/MPI-Munich 21 Gas system Typical mixtures: Ar(91%)+CH4(9%), Ar(90%)+CH4(5%)+CO2(5%) Operation at atmospheric pressure Properties: Drift velocity Gas amplification Signal attenuation my electron attachment (~5cm/ms) (~7000) (<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 22 Influence of Gas Parameters (*) Parameter change 0.1% CH4 Drift velocity, vd Effect on gas amplification, A -2.5% for A = 1x104 Signal ettenuation by electron attachment 0.4 % 10 ppm O2 Negligible up to 100 ppm Negligible up to 100 ppm 0.15%/m of drift 10 ppm H2O 0.5 % Negligible at 100 ppm 1 mbar Negligible if at max. -(0.5%-0.7%) < 0.03% /m of drift (*) from ALEPH handbook (1995) Pere Mato/CERN, Ron Settles/MPI-Munich 23 Wire Chambers 3 planes of wires – gating grid – cathode plane (Frisch grid) – sense and field wire plane Drift region cathode plane gating grid – cathode and field wires at zero potential V=0 pad size – various sizes & densities – typically few cm2 gas gain – typically 3-5x103 sense wire pad plane z x field wire Pere Mato/CERN, Ron Settles/MPI-Munich 24 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 25 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 26 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 27 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 28 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 Bfield Laser tracks in the ALEPH TPC Pere Mato/CERN, Ron Settles/MPI-Munich 29 Laser Calibration System (2) Lasers NA49 Laser system 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. Pere Mato/CERN, Ron Settles/MPI-Munich 30 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 Pere Mato/CERN, Ron Settles/MPI-Munich 31 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 32 Some TPC examples TPC PEP4 TOPAZ ALEPH DELPHI NA49 STAR Reference PEP-PROPOSAL-004, Dec 1976 Nucl. Instr. and Meth. A252 (1986) 423 Nucl. Instr. and Meth. A294 (1990) 121 Nucl. Instr. and Meth. A323 (1992) 209-212 Nucl. Instr. and Meth. A430 (1999) 210 IEEE Trans. on Nucl. Sci. Vol. 44, No. 3 (1997) Pere Mato/CERN, Ron Settles/MPI-Munich 33 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 34