RADIOGRAPHY
Formation of the Latent Image in the Film-Screen System
After exposure, no image can be observed on the film, this invisible image is called Latent Image. A proper chemical processing is needed to make the image visible.
The photons that reach the emulsion are primarily light photons from the intensifying screen that are in contact with the film. However, x-ray photons are also involved in the production of image. This deposits energy from the photon within the lattice of the silver halide crystal.
The interaction between photons and silver halide crystal has already been established and is well understood, however, the formation of the latent image or photographic effect is not yet well explored and still a subject of considerable research. There are many theories describing the formation of latent image, however, it is the Gurney-Mott Theory which is clearly simplified and is widely accepted.
Before interaction, the surface of an unexposed crystal is surrounded by a negative charge.
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Radiation interaction releases electrons. The process begins when the incident photon (light or x-ray) interacts with one of the halides (bromine or iodine). The interaction may be Photoelectric, wherein x-rays are totally absorbed; or Compton, wherein x-rays are partially absorbed.
2. These electrons migrate to the sensitivity center. The ejected electron is freed to wander and may eventually attract and trapped in the
sensitivity center or sensitivity speck which will turn the speck into a negative charge.
3. At the sensitivity center, atomic silver is formed by attraction of an interstitial silver ion. Because the sensitivity center becomes electrified it
causes to attract interstitial silver ions (silver is a positive ion). As a result, the halides (negative ions) were removed from the surface, leaving the
silver ions.
4. The silver ion neutralizes the sensitivity speck. The process is known as ionic stage.
5. The process is repeated several times resulting in build up of silver atoms in the speck.
A single incident photon may free thousand of electrons for deposition at the sensitivity specks. At least three silver atoms must be deposited for a visible clump of black metallic silver to be formed by chemical processing of the image.
6. The remaining silver halide is converted to black metallic silver during processing.
7. The silver grain results.