Thermoluminescent dosimeter

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

Parts

• 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
  • 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 . The light output is proportional to the radiation exposure .

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 .

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) . TLDs can measure doses from 0.01 mGy to 10 Gy .