Athermoluminescent dosimeter, orTLD, is a type ofradiation dosimeter, consisting of a piece of a thermoluminescent crystalline material inside a radiolucent package.
When a thermoluminescent crystal is exposed toionizing radiation, it absorbs and traps some of the energy of the radiation in its crystal lattice. When heated, the crystal releases the trapped energy in the form of visible light, the intensity of which is proportional to the intensity of the ionizing radiation to which the crystal was exposed. A specialized detector measures the intensity of the emitted light, and this measurement is used to calculate the dose of ionizing radiation the crystal was exposed to. Since the crystal density is similar to human soft tissue density, the dose measurement can be used to calculateabsorbed dose.[1]
Materials exhibitingthermoluminescence in response to ionizing radiation includecalcium fluoride,lithium fluoride,calcium sulfate,lithium borate,calcium borate,potassium bromide, andfeldspar. It was invented in 1954 by ProfessorFarrington Daniels of the University of Wisconsin-Madison.[2]
The two most common types of TLDs arecalcium fluoride andlithium fluoride, with one or more impurities to produce trap states for energetic electrons. The former is used to recordgamma exposure, the latter for gamma andneutron exposure (indirectly, using the Li-6 (n,alpha)nuclear reaction; for this reason, LiF dosimeters may be enriched in lithium-6 to enhance this effect or enriched in lithium-7 to reduce it). Other types includeberyllium oxide,[3] andcalcium sulfate doped withthulium.[4]
As the radiation interacts with the crystal it causeselectrons in the crystal's atoms to jump to higher energy states, where they stay trapped due tointentionally introduced impurities (usuallymanganese ormagnesium) in the crystal,[5] until heated. Heating the crystal causes the electrons to drop back to their ground state, releasing aphoton of energy equal to the energy difference between the trap state and the ground state.[6]