Nuclear photographic emulsion

In photographic emulsion, crystalline grains of silver bromide (with a small proportion of silver iodide) are suspended in gelatine. Exposure to light creates the 'latent' image by modifying some of the grains. The chemical process of development turns modified grains into blackened grains of silver. Unmodified grains are washed out using a 'fixing' solution.

In 1945 it was discovered that photographic emulsion could be used for an entirely different purpose. When fast charged particles of nuclear size cross the emulsion, they produce the same effect as light. They leave a trail of modified grains of silver which upon development appears as a track somewhat like a condensation trail of a high flying jet.

The Ilford company, to their credit, put considerable effort into the development of non profit-making photographic emulsion-for fundamental particle research. By producing emulsion which contained about eight times the normal ratio of silver to gelatine, they succeeded in making it so sensitive that the ionisation produced by the passage of a single proton or electron was sufficient to make a track clearly visible under a high power microscope. As we shall see in Chapter 17, the technique played a key role in a number of pioneering experiments in particle physics.

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