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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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The Nobel Prize in
Physics 1950 - Cecil Frank Powell
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