Wikipedia
In particle and nuclear physics, a nuclear emulsion plate is a photographic plate with a particularly thick emulsion layer and with a very uniform grain size. Like bubble chambers, cloud chambers, and wire chambers nuclear emulsion plates record the tracks of charged particles passing through. They are compact, have high density and produce a cumulative record, but have the disadvantage that the plates must be developed before the tracks can be observed.
Nuclear emulsions can be used to record and investigate fast charged particles like nucleons or mesons. After exposing and developing the plate, single particle tracks can be observed and measured using a microscope.
In 1937, Marietta Blau and Hertha Wambacher discovered nuclear disintegration stars due to spallation in nuclear emulsions that had been exposed to cosmic radiation at a height of 2,300 metres (≈7,500 feet) above sea level.
Using nuclear emulsions exposed on high mountains, Cecil Frank Powell and colleagues discovered the charged pion in 1947. Thus discovery won them a Nobel Prize in Physics in 1950.
In biology and medicine, nuclear emulsion is used in autoradiography to locate radioactive labels in samples of cells and tissues. Emulsion detectors are still used by several modern particle physics experiments (for example, the OPERA experiment).