What is Thermoluminescent dosimeter ( TLD ) ?
Thermoluminescent dosimeter ( TLD ) is a radiation dose measuring device, which are used in personnel monitoring and patient dose estimation. All occupational radiation workers need to be monitored to safe guard them from the deleterious somatic and genetic effects of ionizing radiations.
The Thermoluminescent dosimeter ( TLD ) badge is based on the phenomenon of Thermoluminescence, the emission of light when certain materials are heated after radiation exposure. It is used to measure individual doses from X, beta and gamma radiation. The Thermoluminescent dosimeter ( TLD ) badge consists of a plastic cassette in which a nickel coated aluminum (Al) card is placed.
TLD card:- Three CaSO4: Dy Teflon disks of the TLD card consists of 0.8 mm thick and 13.2 mm diameter each, which are mechanically clipped over three symmetrical circular holes, each of diameter 12 mm, on a nickel plated aluminum plate. An asymptomatic V cut provided at on end of the card ensure a fixed orientation of the card in the TLD cassette. The TLD card is enclosed by a paper wrapper in which user’s personnel data and period of use is written.
TLD cassette:- TLD cassette is made oh high impact plastic. There are three filters in the cassette corresponding to each disk, namely Cu+Al, Perspex and open. When the TLD card is inserted properly in the cassette, the first disk (D1) is sandwiched between a pair of filter combination of 1 mm Al and 0.9 mm Cu. The copper filter is nearer to the TLD disk and the Al should face the radiation. The second disc (D2) is sandwiched between a pair of 1.5mm thick plastic filter. The third disk (D3) is positioned under a circular open window. A clip attachment affixes the badge to the users clothing or the wrist.
When theThermoluminescent dosimeter ( TLD ) disk is exposed to radiation, the electrons in the crystal lattice are excited and move from the valence band to conduction band. The number of electrons in the trap is proportional to the radiation exposure and thus it stores the absorbed radiation energy in the crystal lattice.
After radiation exposure, the dose measurements are made by using a Thermoluminescent dosimeter ( TLD ) reader. The reader has heater, photomultiplier tube(PMT) , amplifier and a recorder. The TLD disk is placed in the heater cup or planchet, where it is heated for a a reproduce heating cycle. While heating, the electrons returns to their ground state with emission of light. This emitted light is measured by the PMT which converts light into an electrical current. The PMT signal is then amplified and measured by a recorder. The reader is calibrated in terms of mR or mSv, so that I e can get direct dose estimation.
But now a days window based computer controlled TLD readers are available. They are capable of analyzing TLD chips, ribbons, powder, discs, pellets, rods and microcubes. They display digital glow curve and temperature profile.
TLD badges do not provide permanent record and it is available for extremely dosimetry and finger dosimetry (ring). LiF can also be used in TLD phosphor which has wide dose response, 10 mSv to 1000 Sv.
Guideline For Using Thermoluminescent dosimeter ( TLD )
1) TLD badges are to be used only by persons directly working in radiation.
2) TLD is used to measure radiation dose. It doesn’t protect the user from radiation.
3)The name, personnel number, types of radiation, period of use, location on the body ( chest or wrist) etc., Should be written in block letter.
4) A TLD badge issued to a person should not be used any other person.
5) TLD badge should be worn compulsory at chest level. It represents the whole body dose equivalent.
6) if lead apron is used, TLD badge should be worn under the lead apron.
7) While leaving the department, workers should kept their badge in the place where control TLD is kept.
8) Every radiation worker must ensure that the badge is not left in the radiation field or near plates, ovens , furnaces, burners etc.
9) Every new radiation worker hat to fill up the personnel data form and should be to the BARC accredited agency.
10) All the used or unused TLD badges should be return , after every service period (quarterly) in one lot so to reach 10th of next month.
Book link :- https://amzn.to/3FQQMki