The Nuclear Emulsion

Cecil Powell developed the nuclear emulsion technique to see particles. With this technique the pion, the particle that was assumed to be responsible for the strong nuclear interaction keeping the nucleus together, was discovered in 1947. Powell received the Nobel Prize in 1950. Photographic emulsions had been used earlier, but were not sensitive to high velocity single charge particles. By increasing the content of silver bromide to make it sensitive for singly charged particles, and also by making the emulsion thicker and thereby increasing the active volume, it became a very useful instrument for studying particles, both in cosmic rays and later at accelerators.

It was in an emulsion stack brought to high altitude that the pion was discovered. Stacks of 0.5 mm thick emulsions were also lifted with balloons to very high altitudes to study the cosmic radiation. The spatial resolution of emulsions is very good, about one micron, a feature that is still used today investigating very shortlived particles.

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