Thermoluminsence dosimeter

TLD radiation dosimeter

1. Contents :- Definition of TLD. Invention. Basic principle of TLD. TLD material used. TLD material features. TLD structure and function. Advantages and disadvantages. Guidelines.

2. Definition:- A thermoluminescence dosimeter abbreviated as TLD is a passive radiation dosimeter, that measure ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated after radiation exposure (2019- 05 – 22 Nick Connor)

3. Invention of TLD TLD was invented in 1954 by professor Harrington Daniel the university of Wisconsin. TLDs are applicable to situation where real time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurement or for assessing the potential long term health effects . Principle of TLD TLD is based up on the principle of thermoluminescence i.e the emission of light when certain materials are heated after radiation.

4. TLD material used • Lithium fuloride (LiF) is most Widely used TLD material. Its Z= 8.2 and exhibit x-ray absorption property similar to that of tissue . Measure dose as low as 50 micro Gy(5mrad). •CaF2:Mn has higher effective Z (Z=16.3) than LiF this makes it considerablely more sensitive to ionizing radiation. It can measure radiation dose less than 10 micro Gy(1mrad) with moderate accuracy other types are seen in table on next slice…

5. Characteristic of TLD material High Thermoluminescence sensitivity i.e more light out put per unit of dose Emission spectrum is visible, preferably in the range 400- 500nm. Negligible thermal fading (loss of TL signal due to ambient conditions like temperature etc.) Glow peak preferably around 200 °C. Should be tissue equivalent. Should be cheap, easy to manufacture and annealing procedure. Linearity between dose and light output over wide range.

6. In India Caso4:Day (1:3) embedded Teflon TLD disk are used are used for personal monitoring. Characteristics of Caso4:Dy Effective atomic no. = 15 Glow peak 200°C. Emission maximum 480 – 570 nm. Fading – less than 5% per month (at 25°C). Linearity in response - up to 30Gy. Colour of emitting light – yellowish- white.

7. Glow curve The of intensity of emission of light by TLD material with temperature can be study with the help of curve know as glow curve. Specific to a particular TLD material. The height of highest peak and area under the curve is directly proportional to energy deposited in the TLD by ionizing radiation. The glow curve of LiF are shown .. As temperature increases light emission also increases in irregular manner. Several prominent peaks can be seen on the graph.

8. Structure of TLD TLD consists of Caso4: Dy (0.05%).Teflon is mixed with Caso4: Dy uniformly to give rigidity. A typical TLD badge consists of: • plastic TLD cassette With filters • TLD Card with 3 TLD discs • thin paper • polythene pouch

9. TLD card • Three Caso4:Dy-teflon discs of TLD ( 0.8mm thick and 13.2mm dia each) clipped over three symmetrical circular holes each dia 12mm on Nicle coated Al plate (52.5mm x 29.9mm x 1mm). An asymmetrical v cut is provided at one end to ensure fixed orientation of card inside TLD cassette. The three TLD discs (D1, D2,D3) are positioned such that D1 comes under metal filter ( 1mm Al + 0.9mm Cu) D2 is sandwiched b/w pair of plastic filter 1 mm thick, D3 is positioned Under open window. TLD discs are radiation recording devices which are sensitive to x-rays,gamma rays and beta radiations.

10. • Card is enclosed by paper wrapper in which user’s personal data and period of use is written. Thickness of wrapper (12mg/cm2) makes measurement equivalent to 10 mm depth below skin surface. •To protect the TLD discs from mishandling card along with wrapper is sealed in a thin plastic (polythene) pouch which protects it from radioactive contamination while working with open sources. • TLD card is used in dose range of 10mR – 10,000 mR .

11. TLD Cassette TLD cassette ia made of high impact plastic.three filters are present corresponding to each disc namely : Metallic filter (1mm Al & 0.9mm Cu) 1000mg/cm2 thichness . It is used for gamma radiation. Plastic filters (1.5mm ) 180 mg/cm2 . It is used for beta radiation. Open filter is used for x- rays . • A clip attachment affixes the badge to users clothing or to the wrist.

12. Function of TLD When TLD disc is exposed to radiation the electrons in the crystal are excited and move from valence band to conduction band and form a trap just below the conduction band. No. of trapped electrons is directly proportional to radiation exposure and thus it stores absorbed radiation energy in the crystal lattice. After radiation exposure , dose measurements are made by using a TLD reader. The reader has heater, PMT , amplifier and a recorder. TLD disc is placed in the heater cup ( Planchet )  where it is heated for reproducible heating cycle.

13. While heating ,electrons return to their ground state with emission of light. this emitted light is measured by PMT which convert light into electric signal which is then amplified and measured by a recorder,. the reader is then calibrated in terms of mSv or mR, so that one can get directly dose estimation. • Nowadays windows based computer controlled TLD reader is available. They are capable of analyzing TLD, chips, ribbons ,powder, discs, rods and microtubes. They display digital glow curve and temperature profile and can handle one or more planchets at a time either with manual drawer function. Programmable annealing oven is also available along with the system. • The disks are reusable after proper annealing, up to 300 times. A typical annealing cycle consists of 40 °C for 1hr , followed 300  °C for 3 hrs. • This badge cover dose range from 10 mR to 10,000 R with accuracy 10%. • LiF can also be used as TLD phosphor, which has dose response , 10 mSv – 1000mSv. Its Z- effective close to tissue with accuracy 2%.

14. TLD badge are normal worn at chest level, that is expected to receive the maximum radiation exposure. During fluoroscopy, it is preferable for the for radiologist to wear at the collar level inside lead appron to measure the dose to the thyroid and lens of the eye, since most st of the body is shielded from radiation exposure. Pregnant radiation workers should wear a second badge at the waist level under lead appron to assess fetal dose. Additional wrist badge is advised for procedures involving nuclear medicine, interventional radiology etc.

15. Advantages Able to measure a greater range of doses. Small size. Available in various forms. Some are reasonably tissue equivalent. Inexpensive. Reusable. Less likely to give false reading due to environmental conditions like heat , light, temperature etc. Fast reading capability For emergency situations.

16. Disadvantages Lack of uniformity. Storage instability. Light sensitivity. No permanent record.

17. Guidelines TLD badges are to be used only by persons directly working in radiations. TLD badge is used to measure the radiation dose not to protect user from the radiation. TLD badge once issued to a person should not be used by other. The name, personal number, type of radiation ,period of use,location on the body etc should be written in block letters on front side of badge. TLD badge should be worn at chest level. It represents whole body dose equivalent. If Lead apron is used, TLD badge should be worn under it.

18. While leaving the premises of institute, workers should deposite their badges in the places control TLD is kept. A badge without filter or damaged filter should not be used , it should be replaced with new holder. Every radiation worker must ensure that badge is not left in the radiation field or near hot plates, ovens,furnaces,burners etc. All the used or unused TLD badges should be returned after every service period.

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