Principles of Thermoluminescent Dosimetry
Thermoluminescent dosimetry is based on imperfections in crystal lattice structures and their ability to capture electrons released by ionising radiation.
The most commonly used crystal lattice for dosimetry is lithium flouride, doped with magnesium and titanium. This is written as LiF:Mg,Ti. Personal radiation monitoring devices typically use calcium sulfate crystals which are more sensitive to lower doses.
When the crystal is exposed to ionising radiation, electrons may be liberated from the structure. These electrons are 'trapped' by the first impurity (magnesium). The electron may remain trapped for a long period of time (years). The number of electrons trapped in this way is proportional to the amount of ionising radiation absorbed by the crystal.
When the crystal is heated, the electrons are freed from the impurity but recaptured by the second impurity (titanium). As they are captured by the titanium, they release their excess energy as a light photon. This light photon is captured by a photoamplifying tube, which is able to amplify the energy in the light photon to readable levels (in an electrometer).
The TLD reading device is able to calculate the amount of light released during heating, which can then be correlated with known values to determine the absorbed dose the TLD received.
TLDs are relatively inexpensive and reusable, and can also be small.
Use of thermoluminescent dosimetry
TLDs are the dosimeter of choice for most in vivo dosimetry measurements due to their small size and tissue equivalence. They can be used in most other dosimetry applications.
TLDs are also used in radiation protection as personal radiation monitors. TLDs are also useful for long term area survey functions (eg. in a brachytherapy suite)