Thermoluminescence dosimeter

Monday, August 3, 2009

Thermoluminescence is the emission of light when a material that has been exposed to ionizing radiation is heated. This light results from the release of electrons that were excited and trapped when the material was irradiated, and the amount of light released is directly related to the radiation dose received by the material.
Advantages of use of TLD for radiation dose level monitoring:
The advantages of TLDs are summarized as; they are Small in size, point dose measurements are possible, many TLDs can be exposed in single exposure, available in various forms, some are reasonably tissue equivalent and they are not expensive.
Disadvantages of Thermoluminescence dosimeter:
The disadvantages of TLDs are summarized as; they are signal erased during readout, easy to lose reading, no instant readout, accurate results require care, readout and calibration time consuming, not recommended for beam calibration.
Principles of Thermoluminescence Dosimetry:
Some materials, upon absorption of radiation, retain part of the absorbed energy in metastable states. When this energy is subsequently released in the form of ultraviolet, visible or infrared light, the phenomenon is called as luminescence. There are two types of luminescence: fluorescence and phosphorescence.
Fluorescence and Phosphorescence
The difference between two is of time delay. Fluorescence is emission of light during or immediately after irradiation of the phosphor. Phosphorescence is the emission of light after the irradiation period. The delay time can be from a few seconds to weeks or months. Fluorescence occurs with a time delay between Exp-10 to exp-8 seconds and phosphorescence with a time delay exceeding exp-8 seconds. Fluorescence is not a particularly useful reaction for TLD. So, phosphorescence is used in thermoluminescence dosimetry. The process of phosphorescence can be accelerated with a suitable excitation in the form of heat or light.
Thermoluminscence phenomenon
When thermoluminescence phosphor is exited to ionization, at sufficient low temperature, many freed electrons or holes become trapped at lattice imperfections in the crystalline solid. They remain trapped for long periods of time when stored at that temperature (or other lower temperatures). If the temperature is raised the probability of escape is increased and electrons (or holes) are released from traps, subsequently returning to stable energy state with emission of light. If the light intensity is measured and plotted as a function of temperature or the time during the period while the temperature is being increased, the result is called a glow curve. Typical glow curve exhibit one or more maxima as traps of various energy “depth” are emptied; the relative amplitudes of the peaks indicate approximately the relative population of trapped electrons in several trap species (provided the heating rate is constant through the glow curve). Faster heating rate results in peaks of greater amplitude, and with the maxima shifting to the higher temperatures.
Either the total light emitted during part or the entire glow curve, or the light of one or more of the peaks, may serve as the measure of the absorbed dose in phosphor, or gamma ray exposure it received. When the peak height is used, the heating cycle must be sufficiently reducible to avoid causing peak height fluctuations. A uniform heating throughout the glow curve is, however not required. After the trapped has been emptied by heating at high temperatures for a sufficient length of time, and the phosphor has subsequently cooled, it normally returns to its original condition and is ready to register another exposure. The random release of trapped electrons before readout is called fading, and may result from thermally or optically stimulated release of the electrons. In thermoluminescent dosimetry (TLD), the relationship between the relevant signal and the dose equivalent to be measured must be determined by calibration. This technique is relatively simple, required a suitable dosimeter material, a controllable heating mechanism and a light collection and recording system. In using this phenomenon for dosimetry, the thermoluminescent material is observed by a photomultiplier or other light sensitive device during the heating process. A plot of the luminescent light output against temperature is called the ‘glow curve’. The shape of the glow curve depends on the type and amount of impurities and lattice defects present in the material, as well as on the thermal history and treatment of the material. The photomultiplier tube has high sensitivity, a high signal to noise ratio and a large dynamic range. The area under the glow curve is used as a measure of dose. The thermoluminescent material is discharged by the reading process and is then ready to register a new exposure (although some materials must be annealed before reissue).
TLD has found increasing application with the progress made in the development of solid thermoluminescent dosimeters and instrumentation for reading them. TLD is now commercially available, and is widely used in routine personal dosimetry, environmental monitoring and clinical radiation dosimetry.


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