Scintillation Detector

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Scintillation Detector
The Radiology Physics Laboratory

• October 28, 2003
• Prof. Tsi-chian Chao

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Scintillation Light

• Fluorescence
• Phosphorescence
• Delayed Fluorescence

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Types of Detector

• Inorganic material
• Alkali halide crystals (Ex. NaI)
• Best light output linearity
• Slower in response time
• Favor for gamma-ray spectroscopy
• High Z and high density
• Organic-based liquids and plastics
• Generally faster but yield less light
• Preferred for beta spectroscopy fast neutron
  detection

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Ideal Scintillation Material

• High scintillation efficiency
• Linear conversion
• Transparent
• Short decay time
• Good optical quality
• Index of refraction should be near that of glass

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Organic Scintillators
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Emission Spectrum

• Self-absorption
• Scintillation efficiency

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Inorganic Scintillators
The band gap of a typical NaI(Tl) crystal is
about 3eV The scintillation efficient of NaI(Tl)
is about 13
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Photomultiplier (PM) Tubes
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Photocathode

• Transfer photons to electrons by the
  photoemissive material
• Quantum Efficiency (QE)
• Practical photocathodes show maximum quantum
  efficiencies of 20-30

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Dynode

• Convert one electron to many electrons
• Overall multiplication factor

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Photomultiplier (PM) Tubes

• Some common types of PM tubes

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Photomultiplier (PM) Tubes

• Some common types of PM tubes-continued

Continuous channel electron multiplier
Microchannel plate electron multiplication
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Spectroscopy with Scintillators

• Significant Interaction Mechanisms for Gamma-ray
  Spectroscopy
• Photoelectric absorption
• Predominante for low-energy gamma rays
• Compton scattering
• Most probable process over the range of energies
  between two extremes
• Pair production
• Predominate for high-energy gamma rays

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Spectroscopy with Scintillators

• Photoelectric absorption
• Liberation of a photoelectron which carries most
  of the gamma-ray energy
• Energy gamma-ray energy original binding
  energy of the photoelectron
• Differential distribution

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Spectroscopy with Scintillators

• Compton scattering
• Creation of a recoil electron and scattered
  gamma-ray photon
• Division of energy between the two dependent on
  the scattering angle

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Spectroscopy with Scintillators

• Compton scattering-continued
• A continuum of energies can be transferred to the
  electron, ranging from 0 to the maximum (? ?).
• As ? ?
• Differential distribution

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Spectroscopy with Scintillators

• Pair production
• Creation of an electron-positron pair at the
  point of complete disappearance of the incident
  gamma-ray photon.
• Differential distribution

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Spectroscopy with Scintillators

• Measured pulse height spectrum for 60Co