Thermoluminescent dosimeter

A thermoluminescent dosimeter (TLD)

A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD measures ionizing radiation exposure by measuring the intensity of visible light emitted from a crystal in the detector when the crystal is heated. The intensity 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.

In Radiometallurgy laboratories, TLD movement inside a nuclear facility is carried out by a radiation worker. However, there is a chance that the TLDs may get lost in laboratories or some places without the knowledge of concerned TLD user or health physicist. It may lead to a false effective dose later. A RFID based TLD monitoring system like TLDguard which acknowledges the use of TLDs through self operating software can prevent the misuse of detector. This device can be used both for environmental monitoring and for staff personnel in facilities involving radiation exposure, among other applications.

Types

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). Other types include beryllium oxide,^[1] calcium sulfate doped with Tm.^[2] As the radiation interacts with the crystal it causes electrons 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,^[3] 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.

References

1. Tochilin, E., N. Goldstein, and W. G. Miller. "Beryllium oxide as a thermoluminescent dosimeter." Health physics 16.1 (1969): 1-7.
2. Yamashita, T., et al. "Calcium sulfate activated by thulium or dysprosium for thermoluminescence dosimetry." Health physics 21.2 (1971): 295-300.
3. Faiz M. Khan (2003). "The Physics of Radiation Therapy". Lippincott Williams & Wilkins.