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Commissioning and performance of the ALICE TPC. Adam Matyja for the ALICE TPC co l laboration Subatech, Nantes & INP PAN Kraków. Outline Components of TPC Calibration Performance results Summary. The ALICE detector. *.

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commissioning and performance of the alice tpc
Commissioning and performance of the ALICE TPC

Adam Matyja

for the ALICE TPC collaboration

Subatech, Nantes & INP PAN Kraków

Outline

  • Components of TPC
  • Calibration
  • Performance results
  • Summary
the alice detector
The ALICE detector

*

* ALICE TPC Collaboration, J. Alme et al., "The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events.", Physics. Ins-Det/10011950 (2010)

the principle of the t ime p rojection c hamber
The principle of the Time Projection Chamber

→ Based on multi-wire proportional chamber

  • Charged particles ionize working gas atoms
  • Ionization electrons drift with the constant velocity (v) in the direction of readout chambers due to the electric field
  • Near the anode plane the strong electric field causes ionization avalanche
  • It induces the signal on the pad plane
  • It allows to obtain the information in two coordinates - (x,y)
  • The third coordinate is inferred from the measurement of the electrons drift time measurement (z=vt)
  • 3 coordinates give the space point on the track
  • Collected charge gives the information about the energy loss
  • Main tracking device
  • Allows to distinguish the charged particle species
alice time projection chamber
ALICE Time Projection Chamber
  • General features:
    • Diameter  Length: 5 m  5 m
    • Azimuth angle coverage: 2
    • Pseudo-rapidity interval: ||<0.9
    • Readout chambers: 72
    • Drift field: 400 V/cm
    • Maximum drift time: 94 s
    • Central electrode HV: 100 kV
  • Gas:
    • Active volume: 90 m3
    • Ne-CO2-N2: 85.7% - 9.5% - 4.8%
    • Cold gas - low diffusion
    • Non-saturated drift velocity

 temperature stability and homogeneity  0.1 K

5 m

2.5 m

2.5 m

  • Data readout:
    • Pads (3 types): 557 568
    • Samples in time direction: 1000
    • Data taking rate:
      • ~ 2.8 kHz for p-p
      • ~ 300 Hz for Pb-Pb
readout chambers

Components

ReadOut Chambers
  • 2 sides with 18 sectors
  • Sector consists of:
    • Inner chamber (IROC)
    • Outer chamber (OROC)

 72 readout chambers

  • Pad readout
    • 3 sizes
  • Stable operation
drift voltage system

Components

Drift voltage system

Voltage dividers' network

Provide constant electric field

  • Water cooled voltage dividers

→ remove dissipated power

  • Control of water conductivity
  • Radiation length X / X0of Field Cage
    • 1.367 % - inner FC
    • 0.607 % - gas
    • 2.153 % - outer FC

Resistor rods

  • Very stable operation

Few well understood trips (beam loss)

  • Tomography of FC in good agreement with MC
recirculating gas system

Components

Recirculating gas system

Precise control of gas mixture

  • O2 and H2O contamination removed by Cu catalyser
  • To minimize signal loss (e- attachment)
    • Contamination:

~ 1 ppm O2 (design < 5)

  • Humidity kept at fixed level

→ to avoid aging of components

  • In operation since 2006
cooling system

FEC with its cooling envelope

Components

Cooling system

Provide temperature stability

  • ~ 500 temperature sensors
  • Leakless underpressure system with ~ 60 adjustable cooling circuits
  • Thermal screening towards ITS and TRD
  • Copper shields of service support wheel
  • Cooling of ROC bodies
  • Water cooling of FEE in copper envelope (~27 kW)
  • Result: Temperature homogenity:T = 0.046 K

Good agreement with design specifications

detector control system

Components

Detector Control System

Ensure a safe and correct operation of TPC

  • Integrated into Experiment Control System
  • Hardware architecture
    • Supervisory layer: user interface (PC) + databases
    • Control layer: hub - collect & process information from supervisory and field layers
    • Field layer: electronics to control equipment (power supplies, FEE, …)
  • TPC is fully controlled by ALICE shifter
noise measurements

Calibration

Noise measurements
  • Noise level improved during commissioning
  • Mean noise level:
    • 0.7 ADC count (700 e)
    • Designed - 1 ADC count (1000 e)
  • Data volume of empty event:
    • non-zero suppressed (ZS): ~ 700MB
    • ZS event: ~ 30kB
  • Typical size of the event with data:
    • 0.1 - 0.2 MB (p - p)
      • 360 kB TPC @ 7 TeV
    • ~ 30 MB (Pb - Pb, dN/dy = 2000)

Current noise

gain calibration using 83 kr

Calibration

Gain calibration using 83Kr
  • Resolution of main peak:
  • 4.0 % for IROCs
  • 4.3 % for OROCs

OROC

main peak (41.6 keV)

Determine gain for each pad

  • Procedure:
    • Inject radioactive 83Kr
    • Characteristic decay spectrum
    • Dedicated clusterizer
    • Fit the main peak (41.6 keV)
    • Parabolic fit
    • Calibration constants
  • 3 different HV settings (gains)
  • High statistics: several 108 Kr events
  • Accuracy of peak position: << 1% (design: 1.5%)
  • Repeated after electronic maintenance or every year

Gain variation

Result:

Relative gain variation C-side

kr calibration systematics

Calibration

Kr calibration - systematics

Azimuth

Radial

OROC

IROC

long

short

mid

Sector-by-sector

Edge effect well visible

→ Parabolic fit applied to avoid it

The shape reflects a mechanical deformation of the pad plane

laser system

Calibration

Laser system

The principle of laser system for the TPC

  • 336 laser beams
  • Used for:
    • E  B effect
    • Drift velocity measurements
    • Alignment

Laser features:

  •  = 266 nm or E = h = 4.66 eV
  • Energy: 100 mJ/pulse
  • Duration of pulse: 5 ns

The ionization in the gas volume along the laser path occurs via two photon absorption by organic impurities.

Reconstructed laser tracks

e b effect

Laser system Calibration

E  B effect

Caused by:

  • Mechanical or electrical imperfections
  • Imperfect B field

Correction map from laser tracks

  • Measure r
  • For each laser track
  • For several magnetic field settings
  • r  7 mm

→ for longest drift and nominal field

  • Corrected to ~ 0.3 mm
  • Detailed studies ongoing
drift velocity measurements

Calibration

Drift velocity measurements
  • d = d(E, B, (Tin, Patm), CCO2, CN2)
  • Crucial for track matching with other detectors
  • How to obtain drift velocity correction factor:
    • Matching laser tracks and mirror positions
    • Matching TPC and ITS tracks
    • Matching tracks from two halves of TPC
    • Drift velocity monitor
  • Required accuracy: 10-4  update every 1h
  • Photo electrons from central electrode

monitor top-bottom arrival time offset caused by T and P gradients

drift velocity measurements1

Calibration

Drift velocity measurements
  • d = d(E, B, (Tin, Patm), CCO2, CN2)
  • Crucial for track matching with other detectors
  • How to obtain drift velocity correction factor:
    • Matching laser tracks and mirror positions
    • Matching TPC and ITS tracks
    • Matching tracks from two halves of TPC
    • Drift velocity monitor
  • Required accuracy: 10-4  update every 1h

Corrected spectrum

  • Photo electrons from central electrode

monitor top-bottom arrival time offset caused by T and P gradients

space point resolution

Performance

Space point resolution
  • Depends on:
    • Drift length
    • Inclination angle
    • Charge deposited on the anode wire
  • In r direction: Y = 300 - 800 m
    • for small inclination angles (high momentum tracks)
  • In drift direction: Z = 300 - 800 m
  • Good agreement with simulations
momentum resolution

Performance

Momentum resolution
  • High momentum tracks
    • Cosmic muon tracks treated independently in two halves of TPC
    • Comparison of pT at vertex gives resolution
    • Statistics: ~ 5  106 events
  • Low momentum tracks
    • Deduced from the width of K0S mass peak
  • Current status (w/o many corrections):

(pT/pT)2 = (0.01)2 + (0.007pT)2

  • Achieved: ~ 7 % @ 10 GeV/c

~ 1 % below 1 GeV/c

  • Place for improvement:

~ 4 % @ 10 GeV/c

de dx resolution cosmics

Performance

dE/dx resolution - cosmics

TPC cosmic data

d

p

Allows particle identification up to 50 GeV/c

  • Statistics: 8.3106 cosmic tracks in 2008
  • Design goal: 5.5 %
  • Measured: < 5 %

e

500 < p < 550 MeV

e

p

material budget

Performance

Material budget

Radial distribution

Vertices from conversion photons

TPC begins here

Agreement between MC and DATA: 5 ~ 15 %

life of tpc
Life of TPC
  • 2006 - first tests at the surface
    • No ZS
    • Only 2 sectors powered at a time
  • 2007 - first commissioning underground
    • C-side successfully operated
    • Online ZS
  • 2008 - commissioning with cosmics
    • Full TPC
    • Different run types implemented
    • Extensive calibration data taken

_

  • 06. 12. 2009 - first s = 900 GeV collisions

_

  • 30. 03. 2010 - first s = 7 TeV collisions
summary
Summary
  • Physics results are well visible:
    • "Midrapidity antiproton-to-proton ratio in pp collisons at s = 0.9 and 7 TeV measured by the ALICE experiment." - Phys. Rev. Lett. 105.072002 (2010)
    • "Two-pion Bose-Einstein correlations in pp collisions at s = 900 GeV." - Phys. Rev. D 82, 052001 (2010)
    • "Transverse momentum spectra of charged particles in proton-proton collisions at s = 900 GeV with ALICE at the LHC." - submitted to PLB (arXiv:1007.0719)
    • Other papers to be submitted soon:
      • J/ production
      • Particle spectra
      • Strangeness
      • D(*)-mesons
  • ALICE TPC works stably during p-p data taking
  • Calibration done → working on improvements
  • Very good performance, close to specifications
  • We are waiting for Heavy Ions
the alice tpc collaboration
The ALICE TPC collaboration

Harald Appelshaeuser6, Peter Braun-Munzinger7, Peter Christiansen9, Panagiota Foka7, Ulrich Frankenfeld7, Chilo Garabatos7, Peter Glassel8, Hans-Ake Gustafsson9, Haavard Helstrup1, Marian Ivanov7, Rudolf Janik2, Alexander Kalweit5, Ralf Keidel11, Marek Kowalski10, Dag Toppe Larsen1, Christian Lippmann3, Magnus Mager3, Adam Matyja10,12 , Luciano Musa3, Borge Svane Nielsen4, Helmut Oeschler5, Miro Pikna2, Attiq Ur Rehman3, Rainer Renfordt6, Stefan Rossegger3, Dieter Roehrich1, Hans-Rudolf Schmidt7, Martin Siska2, Brano Sitar2, Carsten Soegaard4, Johanna Stachel8, Peter Strmen2, Imrich Szarka2, Danilo Vranic7, Jens Wiechula8

  • Department of Physics and Technology, University of Bergen, Bergen, Norway.
  • Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia.
  • European Organization for Nuclear Research (CERN), Geneva.
  • Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
  • Institut für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany.
  • Institut für Kernphysik, Johann-Wolfgang-Goethe Universität Frankfurt, Frankfurt, Germany.
  • Gesellschaft für Schwerionenforschung mbH (GSI), Darmstadt, Germany.
  • Physikalisches Institut, Ruprecht-Katls-Universität Heidelberg, Heidelberg, Germany.
  • Division of Experimental High Energy Physics, University of Lund, Lund, Sweden.
  • The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland.
  • Zentrum fur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany.
  • Now at SUBATECH, Ecole des Mines de Nantes, Universite de Nantes, CNRS-IN2P3, Nantes, France.
cluster finder
Cluster finder

Track

Krypton

Pad-row

Pad-row

Pad

Pad

Decay point

Track

Fired pad

Fired pad

Cluster

Cluster