1. Timing Properties of T0 Detectors At PHOBOS Saba Zuberi, Erik Johnson, Nazim Khan, Frank Wolfs, Wojtek Skulski University of Rochester

2. 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

3. 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 10 12 o C in region of diameter 10 -14 m.

4. 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 -23 sec. Hadrons, such as K,  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.

5. PHOBOS Detector

6. Particle ID and Time Of Flight Low momentum particles (eg.p  <55MeV/c, p K <135MeV/c) –Stopped in Spectrometer arms. –Identified from dE/dx and E TOT. Higher momentum particles –Enter magnetic field and momentum measured. –Identified from dE/dx and p (eg.  /K separation: 0.6GeV/c) High momentum particles –Reach the TOF walls. –Momentum and TOF allows mass of particle to be calculated.

7. TOF detector extends particle ID to higher momentum region. Extension of Particle ID to High Momentum Region Timing Resolution,  TOF  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,  TOF  limits momentum range of particle identification. Require timing resolution of TOF,  TOF =100ps. With TOF:  /K separation: 0.6GeV/c to 1.2GeV, K/p separation: 1.2Gev/c to 2GeV (L=1.7m,  TOF =100ps)

8. 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  TOF.

9. TOF Walls Each TOF wall consists of 120 scintillation slats 0.8x0.8x20cm 3, coupled to two PMTs (top and bottom). Position of incident particles is determined and allows spectrometer tracks to be matched with TOF hits. TOF Wall installed at RHIC 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. T0 Counters installed at RHIC

10. Photomultiplier Tubes T0 detector 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. G=K  V  n http://laxmi.nuc.ucla.edu:8248/M248_99/autorad/Scint/pmt.html

11. Dependence of T0 Timing Resolution on Supply Voltage For larger supply voltage, T0 detectors have larger gain and provide larger signals. Expect timing resolution,  TO to improve with increased voltage. To avoid saturation of PMTs operate at as low a voltage as possible without sacrificing  TO. Determine dependence of T0 timing resolution,  TO 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  TO depending on position of incidence. Dependence of T0 Timing Resolution on Position of Incident Particles

12. Experimental Setup Timing properties were studied using cosmic rays. Two 0.8x0.8x0.8cm 3 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.

13. 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.

14. 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. Diameter Scan Circumference Scan

15. 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  T0 = 55ps. Beyond –2700V the supply voltage does not influence the timing resolution of the T0 counters. Summary