Gasfilled Detectors

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Gas-filled Detectors
The Radiology Physics Laboratory

• September 23, 2003
• Prof. Tsi-chian Chao

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Radiation Detectors
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Gas Filled Detectors

• Ion chamber
• Proportional counter
• GM tube

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Hypothetical Detector
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Active Volume
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Radiation passes energy to a detector and yields
ion pairs.
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W-value

• The average energy lost to form an ion pair
  (Typical 30-35 eV)

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Ionization Processes in Gas

• Ionization and excitation of gas molecules
  happened along the particle track and resulting
  positive ion and free electron
• If an external electric field is applied, the
  charges would move to anode and cathode to yield
  electronic output
• Diffusion
• Charge transfer
• Recombination

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Ionization Chamber

• Pulse height discriminator can be used with ion
  chamber to measure different radiations.
• Low output

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Proportional Counter

• Secondary ionization - avalanche
• Higher output
• Can still tell different radiations

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Gas Multiplication and Avalanche
anode
I am an electron
cathode
The avalanche will stop after the electric field
reduced to a threshold caused by the space charge
of accumulated positive ions in the gas.
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Choice of Geometry
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Geiger-Mueller (GM) Tube

• Entire avalanche full ionization
• Cannot still tell different radiations
• Quenching is necessary
• Electrical quenching reduce electrical voltage
  after avalanche
• Chemical quenching add a little halogen gas

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Resolving Time
R True counting rate R0 Observed counting
rate t Resolving time