TLD – Thermoluminescent Dosimeter
A thermoluminescent dosimeter, abbreviated as TLD, is a passive radiation dosimeter, that measures ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure. Thermoluminescent dosimeters was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison. TLD dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).
Advantages and Disadvantages of TLDs
Advantages of TLDs
- TLDs are able to measure a greater range of doses in comparison with film badges.
- Doses from TLDs may be easily obtained.
- TLDs can be read on site instead of being sent away for developing.
- TLDs are easily reusable.
Disadvantages of TLDs
- Each dose cannot be read out more than once.
- The readout process effectively “zeroes” the TLD.
The OSL dosimetry (Optically Stimulated Luminescence) is a method that has established itself in the whole-body dosimetry. As can be deduced, this method is based on optically stimulated luminescence. The OSL dosimeter provides a very high degree of sensitivity by giving an accurate reading as low as 1 mrem for x-ray and gamma ray photons with energies ranging from 5 keV to greater than 40 MeV. OSL dosimeters are designed to provide X, gamma, beta and neutron radiation monitoring using OSL technology. OSL dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In diagnostic imaging the increased sensitivity of the OSL dosimeter makes it ideal for monitoring employees working in low-radiation environments and for pregnant workers. OSL dosimeters offer advantages that include the ability to be re-read and a high sensitivity (low minimum measurable dose), and they have become popular because of these favourable properties.
OSL materials (e.g. beryllium oxide ceramic) contain defects in their crystal structure that trap electrons released by exposure to radiation. In TLDs, the trapped electrons are subsequently freed by stimulation with heat, while OSL uses stimulation with light. After stimulation by light, the detector releases the stored energy in the form of light, i.e., it is stimulated to emit light. The light output measured with photomultipliers is a measure unit for the dose. In comparison with TLDs, their major difference is that luminescence is produced by a light beam, rather than by heat.
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