Question

a) Describe the formation of a latent image center in photographic emulsions. b) Name some advantages and disadvantages of other TLD phosphors compared to lithium fluoride.

Answer 1

The invisible image formed when photosensitive material such as a photographic film is exposed to light is called a latent image. This film, on development forms the final image as the area that is exposed to light darkens. The area exposed to light darkens due to the formation of silver metallic atoms clusters formed on reduction of interstitial silver ions by photoelectrons in or on the silver halide crystal. The incoming photon generates an electron from the silver halide crystal which then migrate to a sensitive shallow electron trap where the silver ions reduce to form metallic silver speck. these latent image centers can be as small as a few atoms but a center of atleast 4 atoms is considered effective. The developer emulsions futher acts as an chmical amplifier to the latent image canters with gain factor of up to several billion.

The advantages and disadvantages of TLD phosphors over lithium fluoride are:

• TLD phosphors like calcium fluoride (CaF) is used to record gamma exposure, whereas LiF for gamma and neutron exposure.
• CaF does not have the its effective atomic number equal to the that of tissue or air. whereas LiF has low effective atomic number (8.1), only slightly greater than tissue or air which poses as an advantage in radiation therapy.
• LiF can store energy during exposure to ionizing radiation and subsequently release it as light when heated.
• The major disadvatage of LiF is that it is not re usable, it can only be used once. The process of reading out eliminates the dose effect.
• Dust on detector glows when heated and is recorded as false reading.