PHYSICS 225, 2ND YEAR LAB

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PHYSICS 225, 2ND YEAR LAB

• NUCLEAR
• RADIATION DETECTORS

G.F. West
Thurs, Jan. 19
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INTRODUCTION, -1

• Radiation here refers to ionizing radiation
  such as a, ß, ? nuclear emanations, not low
  energy electromagnetic (photonic) radiation.
• Typically arising from spontaneous or stimulated
  nuclear decay, e.g., neutron, ? or X-ray
  irradiation of atoms.
• Kinetic energy (non rest mass component) gtgt 10 eV
  , typically gt 1 keV.
• But not HEP energies gt 100MeV.

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INTRODUCTION, - 2

• EM SPECTRUM

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INTRO - 3

• EM spectrum
• with
• photon energies

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INTRODUCTION, - 4

• X and ? rays are pure EM radiation of
  sufficiently high energy that they exhibit
  particle-like behaviour.
• a, (He nucleii), ß, (electrons), ß, (positrons)
  radiation are massive particles. Obviously, they
  behave differently, but they may often be
  detected by similar methods.
• Other emissions in this energy range, (e.g.,
  neutrons) need separate discussion.

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WHAT IS A PARTICLE DETECTOR ?

• An apparatus to detect a radiation flux, usually
  as a stream of separate events
• i.e., by counting the individual particles as
  they pass through a defined aperture.
• Thus, the particle must interact with the
  detector and deposit some, or all, of its energy
  into it.
• The detector can therefore be thought of as a
  target body, having a cross-section (a
  probability) for interaction with the radiation.
• Some radiation may go through the detector
  without significant interaction, some may
  interact and be absorbed or altered and thereby
  detected.

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PARTICLE DETECTORS , continued

• Possible functions-
• Simple detection (counting),
• Energy measurement (spectroscopy),
• Path tracking.
• Basic types-
• Ionization chamber
• Scintillation detector
• Solid state electronic detector
• Track imager

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INTERACTION PHYSICS

• Effect of an incoming ? ray
• Photoelectric Effect (PE) - knocks out an
  electron (and may continue on to another event).
• Pair Production (PP) - converts to
  electron-positron pair.
• Compton scattering (C) - elastic collisions with
  free electrons (partial energy absorption in each
  collision).
• I Io exp(-µx), where µ µPE µPP µC
• µPE Z5, µPP Z2, µC   Z .

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IONIZATION CHAMBERSDosimeter, proportional
counter, geiger counter

• Chamber filled with gas or insulating liquid.
• Some of the radiation produces ion-electron pairs
  in the medium. Most passes through unaffected.
• A voltage gradient is established in the gas,
  usually by applying a few hundred volts between a
  central wire and an outer cylindrical conductor.
  These electrodes collect any charges produced in
  the medium.

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IONIZATION CHAMBER Voltage dependence
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DOSIMETER
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USES OF IONIZATION CHAMBERS

• Dosimetry (safety and radiation therapy)
• Proportional and geiger counters for a, ß
  counting, where sample can be in the chamber, or
  outside next to an ultra thin window.
• Particle tracking chambers.

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SCINTILLATION DETECTORS

• Much larger capture cross section due to use of
  solid target volume.
• Particle-target interaction produces ions and
  ions give off optical flashes when the ions
  return to ground state.
• Captured optical radiation is observed with
  photomultiplier tube or photo diode layer.
• Classic scintillator is NaI crystal doped with
  thallium impurity. Many others.

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PHOTO-MULTIPLIER (PMT)

• Need for a forepump.

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NaI SCINTILLATOR
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ABSORPTION IN DETECTOR
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SOLID STATE DETECTORS

• Use semiconductor materials, and construction
  techniques.
• Faster and much more precise energy analysis.
• Low capture cross-section.
• Most need liquid nitrogen cooling.

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SOLID STATE DETECTORS, - 3

• Note logarithmic count scales on both graphs

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SOLID STATE DETECTORS - 2
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TRACKING METHODSUsually used with magnetic field
for path analysis

• Wilson cloud chamber (historical)
• Bubble chambers
• Wire ion chambers
• Spark chambers

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TRACKING METHODS

• Bubble chamber

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TRACKING METHODS

• Wire chambers (spark, or ionization)

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DOSIMETRY

• Quantities and Units