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Radiation Dosimetry

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Radiation Dosimetry Types of Dosimeters Delayed read Film Badges, TLD (Thermo Luminescent Dosimeters) Immediate Read Pocket Ionization Chambers (pencil) GM detectors ... – PowerPoint PPT presentation

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Title: Radiation Dosimetry


1
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2
Radiation Dosimetry
3
Types of Dosimeters
  • Delayed read
  • Film Badges,
  • TLD (Thermo Luminescent Dosimeters)
  • Immediate Read
  • Pocket Ionization Chambers (pencil)
  • GM detectors with dose accumulation function
    (electronic)

4
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)

5
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.

6
window
Al
Pb/Sn
Cd/Pb
Al
Pb/Sn
Cd/Pb
7
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8
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.

9
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.

10
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

11
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

12
TL Process
Conduction Band
Electron trap (metastable state)
-
Valence Band
Incident radiation
13
TL Process cont.
Conduction Band
Thermoluminescent photon
-
Heat Applied
Valence Band
14
Glow Curve
15
TLD Reader Construction
DC Amp
To High Voltage
To ground
PMT
Recorder or meter
Filter
TL material
Heated Cup
Power Supply
16
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
17
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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

19
TLD Disadvantages
  • Lack of uniformity batch calibration needed
  • Light sensitivity
  • Fading
  • Reader instability
  • No permanent record

20
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
24
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