Thermoluminescent dosimeter

Last revised by Andrew Murphy on 29 Mar 2020

Thermoluminescent dosimeter (TLD) is a passive radiation detection device that is used for personal dose monitoring or to measure patient dose. 

  • plastic holder
  • nickel-coated aluminum card with TLD discs
    • the discs are made of a thermoluminescent material, commonly calcium sulphate doped with dysprosium (CaSO4:Dy) or lithium fluoride (LiF)
      • nearly tissue equivalent, although not at all x-ray energies 1
    • the discs are 0.8 mm thick and have a 1.35 cm diameter
  • three filters against each disc
    • top: aluminum and copper
    • middle: perspex
    • lower: open

When the radiation falls on TLD, the electrons are excited and store energy. After a varied period (can be one month, bimonthly, quarterly or even biannual), the TLD badges are sent for reading. The TLD reader consists of a heater. On getting heated, the excited electrons again come back to the ground state and emit light which is then read by a photomultiplier 2. The light output is proportional to the radiation exposure 3

TLD has better accuracy than a film dosimeter and do not require an estimate of photon energy. It also has a higher dynamic range and are re-usable 3.

If a radiation worker is wearing a lead apron, the TLD badge can be worn under the lead apron (to reflect body dose) or outside the lead apron (to reflect extremity/thyroid dose) 2. TLDs can measure doses from 0.01 mGy to 10 Gy 3

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Cases and figures

  • Figure 1: TLD badge opened up
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  • Figure 2: TLD card with discs
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