THERMOLUMINESCENT DOSIMETER

THERMOLUMINESCENT DOSIMETER 

Film badge or thermoluminecect dosimeters (TLD) are used as personnel monitoring devices. The film badge has some disadvantages such as fading at high temperatures and humidity, high sensitivity to light, pressure and chemicals, complex darkroom procedure and limited self-life, etc. Hence, TLD badges are used currently in India, instead of film badges. It 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 radiations. It gives very reliable results since no fading is observed under extreme climatic conditions. The typical TLD badge consists of a plastic cassette in which a nickel coated aluminum (Al).

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 (52.5 mm × 29.9 mm × 1 mm). An asymmetric V cut provided at one end of the card ensures a fixed orientation of the card in the TLD cassette. The card is enclosed by a paper wrapper in which user’s personnel data and period of use is written. The thickness of the wrapper (12 mg/cm2) makes the measurements equivalent to 10 mm depth below the skin surface. To protect the TLD discs from mishandling, the card along with its wrapper is sealed in a thin plastic (polythene) pouch. The pouch also protects the card from radioactive contamination while working with open sources.

TLD cassette:

TLD cassette is made of 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 (thick:1000 mg/cm2). 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 1.5 mm thick plastic filters (180 mg/cm2). The third disk (D3) is positioned under a circular open window. A clip attachment affixes the badge to the users clothing or to the wrist

  The metallic filter is meant for gamma radiation, and the Perspex is for beta radiation. The filters are mainly used to make the TLD discs energy independent. When the TLD disk is exposed to radiation, the electrons in the crystal lattice are excited and move from the valence band to conduction band. There they form a trap just below the 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 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 reproducible heating cycle. While heating, the electron returns to their ground state with emission of light. This emitted light is measured by the PMT, which converts light into an electrical current (signal). The PMT signal is then amplified and measured by a recorder. The reader is calibrated in terms of mR or mSv, so that one can get direct dose estimation.

Nowadays, windows 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. They can handle one or more planchets at a time either with manual drawer or computer controlled drawer function. Programmable annealing oven is also available along with the system. In India, Ca SO4:Dy-teflon disks are used in country wide personnel monitoring, with an accuracy of ± 10%.

The disks are reusable after proper annealing, up to 300 times. The annealing process release the residual energy stored from the earlier exposure. A typical annealing cycle consists of 400°C for 1 hour, followed by 300°C for 3 hours. This badge can cover a wide range of dose from 10 mR to 10,000 R with a accuracy of ± 10%.

TLD badges do not provide a permanent record and it is available for extremity dosimetry and finger dosimetry (ring). LiF can also be used as TLD phosphor, which has wide dose response,10 mSv to 1000 Sv. Its effective atomic number is close to that of tissue with an accuracy of ± 2%.

TLD badges are normally worn at the chest level, that is expected to receive the maximum radiation exposure. Most of the radiation workers used to wear the badge at the waist level which is not correct. During fluoroscopy, it is preferable for the radiologist to wear at the collar level inside the lead apron to measure the dose to the thyroid and lens of the eye, since most of the body is shielded from the radiation exposure. Pregnant radiation workers should wear a second badge at waist level (under the lead apron) to assess the fetal dose. Additional wrist badge is advised for procedures involving nuclear medicine, brachytherapy source handling and interventional radiology.