X-ray film

a photographic film that consists of a radiation sensitive emulsion coated on one or both sides of a thin transparent base (Fig.1). The base consists of a thin plastic sheet which is only necessary to give support to the emulsion. The base is designed to provide a flexible, but strong support such that it can be conveniently and repeatedly snapped into a viewbox. Second, the base must withstand any geometric distortion due to the heat of the developing process. Finally, the base must provide a uniform, highly transparent, optical background.

Historically, photographic glass plates were used as the X-ray film base. After the supply of photographic glass from Belgium was cut off by World War I, cellulose nitrate, used as a base for photographic film was adapted for use with X-ray film. A cellulose triacetate base was developed in 1924 to avoid the highly flammable nature of cellulose nitrate. Finally, a stronger, thinner, more dimensionally stable film base made of polyester was developed in 1960.

The X-ray film emulsion is composed of a form of gelatin and a silver halide, typically silver bromide. The gelatin for X-ray film is made from cattle bone. The gelatin serves as a matrix which keeps the silver halide grains well dispersed and prevents their clumping. The developing and fixing solutions can penetrate the gelatin very rapidly without changing the strength or permanence of the gelatin. Finally gelatin can be obtained in large quantities with uniform quality.

Small crystal grains of silver halide on the order of 1.0 to 1.5 microns in diameter comprise the light sensitive substance in the emulsion. These grains, known as silver-iodo-bromide, are typically between 90 and 99% silver bromide and between 1 and 10% silver iodide. The grains are suspended in the gelatin of the film emulsion. The atoms in the silver-iodo-bromide crystal are arranged in a cubic lattice and each crystal contains many point defects, where a silver ion is displaced and is free to move through the crystal. It is the mobility of these silver ions that contributes to the formation of the latent image. In the process of film exposure, the energy from absorbing a photon of light is sufficient to liberate an electron from a bromide ion in the crystal. The electron travels freely through the crystal until it is trapped at a site of crystal imperfection such as a dislocation defect or a sensitivity speck composed of a AgS molecule. A free silver ion is attracted to the negative charge and combines with the charge (is reduced) to form an atom of metallic silver (which is optically black). The single atom of silver acts as an electron trap for another electron and then attracts another atom of silver which is then reduced to metallic silver. This process continues while the exposure to light continues.

In general, the emulsion and the base do not adhere to each other. For this reason, the emulsion must be attached to the film base using a thin layer of adhesive. To protect the emulsion, which would be easily scratched and damaged by normal handling, a very thin outer protective layer is applied.

Although conventional photographic film is usually coated with emulsion on only one side, radiographic film is usually double coated (coated on each side of the base) to be used with intensifying screens. The reason for this double coating is easy to understand. The great majority of X-ray film exposure comes from optical photons created by interaction of each X-ray photon with the intensifying screen. The thicker the screen, the greater the fraction of the X-ray beam that will be stopped and converted to light photons. Thus, increasing the thickness will increase the sensitivity of the image detection. Unfortunately, the light photons are emitted in all directions from the point of interaction of each X-ray with the atoms of the screen. Thus, the thicker the screen, the greater will be the distance that the light photons can travel before striking the X-ray film. Thus, thick screens can result in a spreading or blurring of detail because of the lateral movement of the light photons before striking the film. For a given screen thickness, the blurring can be minimized by placing the X-ray film in the centre of the intensifying screen. Thus, by placing screen plates above and below the film, the sensitivity of double screen thickness can be obtained while at the same time suffering the blurring equivalent to that of a single screen thickness.

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