Thermoluminesence dosimetry

TLD

1. THERMOLUMINESCENCE DOSIMETRY DR KIRAN KUMAR BR

2. A Thermoluminescent dosimeter , or TLD , is a type of radiation dosimeter . PRINCIPLE THERMOLUMINESCENCE

3. Thermoluminescence Thermoluminescent (TL) means emitting light when heated. Many crystalline materials exhibit the phenomenon of thermoluminescence .

4. THERMOLUMINESCENCE PROCESS

5. In an individual atom, electrons occupy discrete energy levels. In a crystal lattice, on the other hand, electronic energy levels are perturbed by mutual interactions between atoms and give rise to energy bands: the “allowed” energy bands and the forbidden energy bands . In addition, the presence of impurities in the crystal creates energy traps in the forbidden region, providing metastable states for the electrons.  

6. When the material is irradiated, some of the electrons in the valence band (ground state) receive sufficient energy to be raised to the conduction band. The vacancy thus created in the valence band is called a positive hole. The electron and the hole move independently through their respective bands until they recombine (electron returning to the ground state) or until they fall into a trap ( metastable state).

7. If there is instantaneous emission of light owing to these transitions, the phenomenon is called fluorescence . If an electron in the trap requires energy to get out of the trap and fall to the valence band, the emission of light in this case is called phosphorescence (delayed fluorescence). If phosphorescence at room temperature is very slow. But can be speeded up significantly with a moderate amount of heating (~300°C) The phenomenon is called thermoluminescence .

8. TLD Thermoluminescent dosimeters (TLD) are often used instead of the film badge. A TLD badge based on CaSO 4 : Dy Teflon discs has been designed and is in  regular Personnel Monitoring use since 1975. 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 .

9. A very minute fraction of the absorbed energy is stored in the crystal lattice. Materials exhibiting thermoluminescence in response to ionizing radiation.

10. MEASURING

11. The irradiated material is placed in a heater cup or planchet. Where it is heated for a reproducible heating cycle. The emitted light is measured by a photomultiplier tube (PMT). Which converts light into an electrical current. The current is then amplified and measured by a recorder or a counter.

12. The complete Personnel Monitoring TLD badge consists of a TLD card. A plastic cassette for holding the TLD card. The badge is affixed to the clothing of a person with the help of a crocodile clip attached to the badge. There are several TL phosphors available, but the most noteworthy are lithium fluoride ( LiF ), lithium borate (Li 2 B 4 O 7 ), and calcium fluoride (CaF 2 ).

13. There are three filters in the cassette corresponding to each disc namely, Cu + Al. Perspex. Open. The Al should face the radiation and Cu should face the TLD disc.

14. The metallic filter is for gamma radiation. The perspex is for beta radiation. The filter are mainly used to make the TLD discs energy independent. The card enclosed by a paper wrapper in which users name is written. The discs are reusable after proper annealing. The badge can cover a wide range of dose from 10mR to 10,000R with accuracy of (+-)10 percent.

15. ADVANTAGES OF TLD No need to processing. Insensitive to temperature, humidity, pressure. More accurate. Linearity of response to dose. Its relative energy independence Sensitive to low doses. Reusable. Automated readout. Small physical size.

16. Its application areas are : Measurement  of output from Co-60 units and accelerators used in medicine and industry.  Area  survey  of  medical  (diagnostic  and  therapeutic) and industrial radiographic installations. Measurement of stray and leakage radiation around X-ray tubes and source containers  

17. Medical radiographic exposure measurement and population exposure survey studies. Estimation  of activities of various radionuclides used in brachytherapy and nuclear medicine. Personnel Monitoring.  

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