Radiation Dosimetry

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Radiation Dosimetry
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Types of Dosimeters

• Delayed read
• Film Badges,
• TLD (Thermo Luminescent Dosimeters)
• Immediate Read
• Pocket Ionization Chambers (pencil)
• GM detectors with dose accumulation function
  (electronic)

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Film Badges

• Most common type of radiation dosimeter used.
• Works by darkening of x-ray film in proportion to
  radiation absorbed.
• Cheap
• Short monitoring period per badge (3 months or
  less)

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Film Badges . Cont.

• The film badge dosimeter, or film badge, is a
  dosimeter used for monitoring cumulative exposure
  to ionizing radiation.
• The badge consists of two parts photographic
  film, and a holder.
• The film is removed and developed to measure
  exposure.

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window
Al
Pb/Sn
Cd/Pb
Al
Pb/Sn
Cd/Pb
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Thermoluminescence Dosimeter (TLD)

• Some materials glow when heated. This is referred
  to as thermoluminesecence.
• Some materials will glow brightly after exposure
  to ionizing radiation and subsequently heated.
• This is the principle of operation of the TLD.
  Discovered in the 1960 at the Wisconsin
  University.

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TLD

• After irradiation, the TLD material is placed on
  a special dish for analysis in the analyzer.
• The analyzer is light tight and temperature
  controlled.
• A PM tube is used to read the exposure.

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TLD cont.

• A TLD measures ionizing radiation exposure by
  measuring the amount of visible light emitted
  from a crystal in the detector when the crystal
  is heated.
• The amount of light emitted is dependent upon the
  radiation exposure

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TLD materials

• Materials exhibiting thermoluminescence in
  response to ionizing radiation include but are
  not limited to
• calcium fluoride
• lithium fluoride
• calcium sulfate
• lithium borate
• calcium borate
• potassium bromide

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TL Process
Conduction Band
Electron trap (metastable state)
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Valence Band
Incident radiation
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TL Process cont.
Conduction Band
Thermoluminescent photon
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Heat Applied
Valence Band
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Glow Curve
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TLD Reader Construction
DC Amp
To High Voltage
To ground
PMT
Recorder or meter
Filter
TL material
Heated Cup
Power Supply
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TLD Card
242 mg / cm 2 ABS plastic 91 mg/cm 2 copper
17 mg/cm 2 (open window)
600 mg/cm 2 ABS plastic
TLD chips
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Advantages

• Advantages (as compared to film dosimeter badges)
  includes
• Able to measure a greater range of doses
• Doses may be easily obtained
• They can be read on site instead of being sent
  away for developing
• Quicker turnaround time for readout
• Reusable
• Small size
• Low cost

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TLD Disadvantages

• Lack of uniformity batch calibration needed
• Light sensitivity
• Fading
• Reader instability
• No permanent record

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Pocket Ionization Chambers

• Pocket chambers and pocket dosimeters are small
  ionization chambers that, as the name implies,
  are usually worn in the pocket.
• They were designed to measure x-rays and gamma
  ray exposures, they would also respond to betas
  above 1 MeV.

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GM detectors with dose accumulation function
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