Thermoluminescent Dosimeter
Encyclopedia
A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter
. 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. 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
and feldspar
.
The two most common types of TLDs are calcium fluoride
and lithium fluoride
, with one or more impurities to produce trap states for energetic electrons. The former is used to record gamma exposure, the latter for gamma and neutron
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). As the radiation interacts with the crystal it causes electron
s in the crystal's atoms to jump to higher energy states, where they stay trapped due to intentionally introduced impurities
(usually manganese
or magnesium
) in the crystal, until heated. Heating the crystal causes the electrons to drop back to their ground state, releasing a photon
of energy equal to the energy difference between the trap state and the ground state.
The electrons can also drop back to ground state after a long period of time; this effect is called fading and is dependent on the incident radiation energy and intrinsic properties of the TLD material. As a result, each material possesses a limited shelf life after which dosimetric information can no longer be obtained. This varies from several weeks in calcium fluoride to up to two years for lithium fluoride.
It can be used both for environmental monitoring and for staff personnel in facilities involving radiation exposure, among other applications.
Dosimeter
Dosimeters measure an individual's or an object'sexposure to something in the environment — particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime...
. A TLD measures ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
exposure by measuring the amount of visible light emitted from a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
in the detector when the crystal is heated. The amount of light emitted is dependent upon the radiation exposure. Materials exhibiting thermoluminescence in response to ionizing radiation include but are not limited to calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
, lithium fluoride
Lithium fluoride
Lithium fluoride is an inorganic compound with the formula LiF. It is the lithium salt of hydrofluoric acid. This white solid is a simple ionic compound. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten...
, calcium sulfate
Calcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
, lithium borate, calcium borate
Calcium borate
Calcium borate , also called Gerstley borate is a bluish white crystal with a very defined structure. It can be prepared by reacting calcium metal with boric acid. The resulting precipitate is calcium borate...
, potassium bromide
Potassium bromide
Potassium bromide is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the United States. Its action is due to the bromide ion...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
.
The two most common types of TLDs are calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
and lithium fluoride
Lithium fluoride
Lithium fluoride is an inorganic compound with the formula LiF. It is the lithium salt of hydrofluoric acid. This white solid is a simple ionic compound. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten...
, with one or more impurities to produce trap states for energetic electrons. The former is used to record gamma exposure, the latter for gamma and neutron
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
exposure (indirectly, using the Li-6 (n,alpha) nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
; for this reason, LiF dosimeters may be enriched in lithium-6 to enhance this effect or enriched in lithium-7 to reduce it). As the radiation interacts with the crystal it causes electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in the crystal's atoms to jump to higher energy states, where they stay trapped due to intentionally introduced impurities
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
(usually manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
or magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
) in the crystal, until heated. Heating the crystal causes the electrons to drop back to their ground state, releasing a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
of energy equal to the energy difference between the trap state and the ground state.
The electrons can also drop back to ground state after a long period of time; this effect is called fading and is dependent on the incident radiation energy and intrinsic properties of the TLD material. As a result, each material possesses a limited shelf life after which dosimetric information can no longer be obtained. This varies from several weeks in calcium fluoride to up to two years for lithium fluoride.
It can be used both for environmental monitoring and for staff personnel in facilities involving radiation exposure, among other applications.