Timing Properties of T0 Detectors At PHOBOS

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Timing Properties of T0 Detectors At PHOBOS

• Saba Zuberi,
• Erik Johnson, Nazim Khan, Frank Wolfs, Wojtek
  Skulski
• University of Rochester

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OUTLINE

• Brief introduction to PHOBOS
• Particle ID and why the timing properties of T0s
  are important?
• Time of Flight (TOF) Detector
• Dependence of timing resolution on voltage and
  position of incident particles
• Results
• Summary

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RHIC Little Big Bang on Long Island

• Two rings 2.4 miles long.
• Collisions of Au-Au, d-Au and p-p studied.
• Ions have energies of up to 100GeV/nucleon for Au
  and 250GeV/nucleon for p.
• Bunches of billions of particles travel with
  speeds up to 0.99995c.
• Matter formed in head on collisions reaches
  temperatures of 1012 oC in region of diameter
  10-14m.

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RHIC Physics Goals

• At everyday energies the basic building blocks
  of matter, quarks and gluons are confined in
  hadronic matter.
• At RHIC energies, quarks and gluons become
  deconfined and the formation of Quark Gluon
  Plasma (QGP) is potentially possible. QGP is
  believed to have existed microseconds after the
  Big Bang.
• New phase of matter is expected to last for
  10-23sec.
• Hadrons, such as K, p, p, produced in collision
  are detected.
• Properties of expected phase transition of matter
  inferred from particles produced.
• PHOBOS Physics Aim To investigate properties of
  hadron
• production at extreme energy densities.

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PHOBOS Detector
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Particle ID and Time Of Flight

• Low momentum particles (eg.pplt55MeV/c,
  pKlt135MeV/c)
• Stopped in Spectrometer arms.
• Identified from dE/dx and ETOT.
• Higher momentum particles
• Enter magnetic field and momentum measured.
• Identified from dE/dx and p
• (eg. p/K separation 0.6GeV/c)
• High momentum particles
• Reach the TOF walls.
• Momentum and TOF allows mass of particle to be
  calculated.

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Extension of Particle ID to High Momentum Region
TOF detector extends particle ID to higher
momentum region.

• Timing Resolution, sTOF, of TOF must be smaller
  than the difference in the time-of-flight in
  order to distinguish between particles of
  different mass with same momentum.
• Timing resolution of TOF, sTOF, limits momentum
  range of particle identification.
• Require timing resolution of TOF, sTOF100ps.

With TOF p/K separation 0.6GeV/c to 1.2GeV,
K/p separation 1.2Gev/c to 2GeV
(L1.7m,sTOF100ps)
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Determining Time-of-Flight

• T0 counters determine time of collision
• TOF Wall determines time of arrival.
• Accuracy of arrival time and collision time
  measurements determine sTOF.

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TOF Walls
T0 Detectors

• Cerenkov radiators, 50mm in diameter, coupled to
  fast PMTs.
• Cerenkov radiation is produced when charged
  particles in a medium have a velocity faster than
  the speed of light in that medium.
• Ten detectors in a ring on either side of
  collision point.
• Can determine the collision point to within 50mm.

• Each TOF wall consists of 120 scintillation slats
  0.8x0.8x20cm3, coupled to two PMTs (top and
  bottom).
• Position of incident particles is determined and
  allows spectrometer tracks to be matched with TOF
  hits.

T0 Counters installed at RHIC
TOF Wall installed at RHIC
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Photomultiplier Tubes

• PMT
• Photoelectron is emitted when cathode is struck
  by photon.
• Dynode chain accelerates and amplifies current as
  secondary electrons are emitted
• Final signal collected from anode.
• PMT Time Resolution depends on
• Variation in transit time of electrons through
  tube
• Fluctuations due to statistical noise.
• Gain is the amplification factor of the PMT.
• Gain depends on number of dynodes in chain and a
  secondary emission factor. GK?V?n

http//laxmi.nuc.ucla.edu8248/M248_99/autorad/Sci
nt/pmt.html
T0 detector
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Dependence of T0 Timing Resolution on Supply
Voltage
Dependence of T0 Timing Resolution on Position of
Incident Particles

• For larger supply voltage, T0 detectors have
  larger gain and provide larger signals.
• Expect timing resolution, sTO to improve with
  increased voltage.
• To avoid saturation of PMTs operate at as low a
  voltage as possible without sacrificing sTO.
• Determine dependence of T0 timing resolution, sTO
  on supply voltage and optimum operating voltage.

• Possible non-uniformity due to
• Cerenkov radiator
• Optical grease coupling the radiator to the PMT
• Geometry of T0 detector
• This may lead to variation in sTO depending on
  position of incidence.

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Experimental Setup

• Timing properties were studied using cosmic rays.
• Two 0.8x0.8x0.8cm3 plastic scintillating
  detectors used to confine position of incidence
  and provide start time in timing measurements.
• Supply voltage of the start detectors is kept
  constant and T0 voltage is varied.

• Position scan conducted in two ways
• Diameter Scan
• Circumference Scan
• The vertical distance between the detectors
  remained constant.

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Result Timing Resolution vs. Supply Voltage

• Timing resolution improves with increased voltage
  up to -2700V.
• At -2700V achieved a 55ps resolution.
• The testing system has an intrinsic resolution of
  50ps.

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Result Timing Resolution vs. Position of
Incident particles

• No significant variation in T0 timing resolution
  was observed either across the diameter of the
  detector, or around its circumference.

Circumference Scan
Diameter Scan
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Summary

• The timing resolution of the TOF subsystem limits
  the particle id capabilities in high momentum
  region.
• The T0 timing resolution is found to have little
  dependence on the position of incident particles.
• Verified that the most effective operating
  voltage for the T0s is -2700V, where sT0 55ps.
• Beyond 2700V the supply voltage does not
  influence the timing resolution of the T0
  counters.