Lithium Fluoride

Thermoluminescent materials such as lithium fluoride (LiF) provide a simple inexpensive method of measuring radiation dose over an extended period of time. Normally when energy is applied to an electron, it will move up to a higher energy state (orbital), then drop back to its ground state releasing the excess energy. 

Crystals of LiF have meta stable energy state in which excited electrons may be trapped for periods up to 80 years.  Heat applied to the crystal will raise the electron out of the meta-stable trap, allowing it to revert back to it ground state by emitting the excess energy as photons.  

 The amount of light emitted is proportional to the amount of radiation absorbed and can be quantified using a photo-multiplier tube.

Lithium Fluoride Response to Radiation

Thermoluminescent Dosimeter (TLD)

Construction of a TLD consists of two Lithium Fluoride crystals of different thickness mounted on a metal plaque.  These crystals are shielded by either a metal foil or an aluminum planchet.  The difference in the shielding and thickness of the crystals allows the differentiation between whole body dose and skin dose.

Use

1. TLD badges should be worn at the chest position (if worn at waist bench tops will shield the badge from radiation giving low results) 
   
2. The TLD badge should be stored away from light, radiation and dust when it is not used. 
   
3. TLDs are changed every three months except for badges worn by pregnant women.  These badges change every two weeks.

Response

LiF is considered to be approximately tissue equivalent, with little energy dependence for photon energy in excess of 100 keV.  

Beta (MAX) 0.766 MeV - 5 MeV
Photon 5 keV - 6 MeV
Neutron Thermal to 6 MeV

Low energy beta (tritium or C-14) and alpha radiation are not detected by TLD's because the beta emissions cannot penetrate the holder.

Lithium Fluoride TLDs will detect a minimum of 0.2mSv dose for the wearing period (usually 3 months).

Ring Badge

Ring badges consist of a single Lithium Fluoride crystal inside a plastic holder.  This type of badge is used to measure extremity dose and is mandatory for work using 32P in excess of 50 MBq. 

Ring Dosimeter

Use

1. Ring badges should be worn on the dominant hand inside the glove to prevent contamination.
   
2. The Ring badge should be stored away from light, radiation and dust when it is used. 
   
3. Ring badges are changed every three months. 

Storage of Ring and TLD badges

It is imperative that Neutron,Ring and TLD badges be stored away from sources of radiation, dust and light to ensure the results reflect the actual dose recieved by the worker.

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