PHYS 3446, Fall 2006

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PHYS 3446 Lecture 13
Monday, Oct. 23, 2006 Dr. Jae Yu

• Particle Detection
• Scintillation Counters
• Time of Flight
• Cerenkov Counter
• Calorimeters
• 2. Particle Accelerators
• Electrostatic Accelerators

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Announcements

• Research Day tomorrow Rio Grande
• 9am 4pm poster presentations
• Our cloud chamber prototype will be in display
  from noon 4pm
• Need presenters to operate the chamber and man
  the poster
• Next LPCC Workshop
• Preparation work
• Lists of goals and items to purchase?
• 10am 5pm, Saturday, Nov. 4
• CPB303 and HEP experimental areas
• Quiz results
• Class average 68.5
• Top score96/90
• Assignments
• Derive Eq. 7.10
• Carry out computations for Eq. 7.14 and 7.17
• Due for these assignments is Monday, Oct. 30

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Paper Template
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Scintillation Counters

• Two types of scintillators
• Organic or plastic
• Tend to emit ultra-violate
• Wavelength shifters are needed to reduce
  attenuation
• Faster decay time (10-8s)
• More appropriate for high flux environment
• Inorganic or crystalline (NaI or CsI)
• Doped with activators that can be excited by
  electron-hole pairs produced by charged particles
  in the crystal lattice
• These dopants can then be de-excited through
  photon emission
• Decay time of order 10-6sec
• Used in low energy detection

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Scintillation Counters Photo-multiplier Tube

• The light produced by scintillators are usually
  too weak to see
• Photon signal needs amplification through
  photomultiplier tubes
• Gets the light from scintillator directly or
  through light guide
• Photocathode Made of material in which valence
  electrons are loosely bound and are easy to cause
  photo-electric effect (2 12 cm diameter)
• Series of multiple dynodes that are made of
  material with relatively low work-function
• Operating at an increasing potential difference
  (100 200 V) difference between dynodes

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Scintillation Counters Photo-multiplier Tube

• The dynodes accelerate the electrons to the next
  stage, amplifying the signal to a factor of 104
  107
• Quantum conversion efficiency of photocathode is
  typically on the order of 0.25
• Output signal is proportional to the amount of
  the incident light except for the statistical
  fluctuation
• Takes only a few nano-seconds for signal
  processing
• Used in as trigger or in an environment that
  requires fast response
• ScintillatorPMT good detector for charged
  particles or photons or neutrons

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Some PMTs
Super-Kamiokande detector
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Scintillation Detector Structure
HV PS
Scintillation Counter
Light Guide/ Wavelength Shifter
PMT
Readout Electronics
Scope
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Time of Flight

• Scintillator PMT can provide time resolution of
  0.1 ns.
• What position resolution does this corresponds
  to?
• 3cm
• Array of scintillation counters can be used to
  measure the time of flight (TOF) of particles and
  obtain their velocities
• What can this be used for?
• Can use this to distinguish particles with about
  the same momentum but with different mass
• How?
• Measure
• the momentum (p) of a particle in the magnetic
  field
• its time of flight (t) for reaching some
  scintillation counter at a distance L from the
  point of origin of the particle
• Determine the velocity of the particle and its
  mass

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Time of Flight (TOF)

• TOF is the distance traveled divided by the speed
  of the particle, tL/v.
• Thus Dt in flight time of the two particle with
  m1 and m2 is
• For known momentum, p,
• Since
• In non-relativistic limit,
• Mass resolution of 1 is achievable for low
  energies

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Cerenkov Detectors

• What is the Cerenkov radiation?
• Emission of coherent radiation from the
  excitation of atoms and molecules
• When does this occur?
• If a charged particle enters a dielectric medium
  with a speed faster than light in the medium
• How is this possible?
• Since the speed of light is c/n in a medium with
  index of refraction n, if the particles bgt1/n,
  its speed is larger than the speed of light
• Cerenkov light has various frequencies but blue
  and ultraviolet band are most interesting
• Blue can be directly detected w/ standard PMTs
• Ultraviolet can be converted to electrons using
  photosensitive molecules mixed in with some gas
  in an ionization chamber

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Cerenkov Detectors

• The angle of emission is given by
• The intensity of the produced radiation per unit
  length of the radiator is proportional to sin2qc.
• For bngt1, light can be emitted while for bnlt1, no
  light can be observed.
• Thus, Cerenkov effect provides a means for
  distinguishing particles with the same momentum
• One can use multiple chambers of various indices
  of refraction to detect Cerenkov radiation from
  particles of different mass but with the same
  momentum

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Cerenkov Detectors

• Threshold counters
• Particles with the same momentum but with
  different mass will start emitting Cerenkov light
  when the index of refraction is above a certain
  threshold
• These counters have one type of gas but could
  vary the pressure in the chamber to change the
  index of refraction to distinguish particles
• Large proton decay experiments use Cerenkov
  detector to detect the final state particles,
  such as p ? ep0
• Differential counters
• Measure the angle of emission for the given index
  of refraction since the emission angle for
  lighter particles will be larger than heavier ones

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Super Kamiokande A Differential Water Cerenkov
Detector

• Kamioka zinc mine, Japan
• 1000m underground
• 40 m (d) x 40m(h) SS
• 50,000 tons of ultra pure H2O
• 11200(inner)1800(outer) 50cm PMTs
• Originally for proton decay experiment
• Accident in Nov. 2001, destroyed 7000 PMTs
• Dec. 2002 resumed data taking

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Super-K Event Displays
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Cerenkov Detectors

• Ring-imaging Cerenkov Counters (RICH)
• Use UV emissions
• An energetic charged particle can produce
  multiple UV distributed about the direction of
  the particle
• These UV photons can then be put through a
  photo-sensitive medium creating a ring of
  electrons
• These electrons then can be detected in an
  ionization chamber forming a ring
• Babar experiment at SLAC uses this detector

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Semiconductor Detectors

• Semiconductors can produce large signal
  (electron-hole pairs) for relatively small energy
  deposit (3eV)
• Advantageous in measuring low energy at high
  resolution
• Silicon strip and pixel detectors are widely used
  for high precision position measurements
• Due to large electron-hole pair production, thin
  layers (200 300 mm) of wafers sufficient for
  measurements
• Output signal proportional to the ionization loss
• Low bias voltages sufficient to operate
• Can be deposit in thin stripes (20 50 mm) on
  thin electrode
• High position resolution achievable
• Can be used to distinguish particles in multiple
  detector configurations
• So what is the catch?
• Very expensive ? On the order of 30k/m2

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DØ Silicon Vertex Detector
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Calorimeters

• Magnetic measurement of momentum is not
  sufficient for physics, why?
• The precision for angular measurements gets worse
  as particles momenta increases
• Increasing magnetic field or increasing precision
  of the tracking device will help but will be
  expensive
• Cannot measure neutral particle momenta
• How do we solve this problem?
• Use a device that measures kinetic energies of
  particles
• Calorimeter
• A device that absorbs full kinetic energy of a
  particle
• Provides signal proportional to deposited energy

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Calorimeters

• Large scale calorimeter were developed during
  1960s
• For energetic cosmic rays
• For particles produced in accelerator experiments
• How do high energy EM (photons and electrons) and
  Hadronic particles deposit their energies?
• Electrons via bremsstrahlung
• Photons via electron-positron conversion,
  followed by bremsstrahlung of electrons and
  positrons
• These processes continue occurring in the
  secondary particles causing an electromagnetic
  shower losing all of its energy

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Electron Shower Process
Photon, g
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Calorimeters

• Hadrons are massive thus their energy deposit via
  brem is small
• They lose their energies through multiple nuclear
  collisions
• Incident hadron produces multiple pions and other
  secondary hadrons in the first collision
• The secondary hadrons then successively undergo
  nuclear collisions
• Mean free path for nuclear collisions is called
  nuclear interaction lengths and is substantially
  larger than that of EM particles
• Hadronic shower processes are therefore more
  erratic than EM shower processes

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Sampling Calorimeters

• High energy particles require large calorimeters
  to absorb all of their energies and measure them
  fully in the device (called total absorption
  calorimeters)
• Since the number of shower particles is
  proportional to the energy of the incident
  particles
• One can deduce the total energy of the particle
  by measuring only the fraction of their energy,
  as long as the fraction is known ? Called
  sampling calorimeters
• Most the high energy experiments use sampling
  calorimeters

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How particle showers look in detectors