A nuclear emulsion is a photographic plate used in the early days of cosmic rays studies to detect charged particles using the trace left in the plate by the energy generated in the interaction with the surface of the emulsion. The effect of the radiation on a photographic plate was first observed by A. H. Becquerel, who detected the radioactivity of uranium salts in 1896 from the fogging the salts caused on a photographic emulsion.
After the Becquerel discovery, in 1910 the Japanese physicist S. Kinoshita demonstrated that silver halide grains in an ordinary photographic emulsion become capable of development if even a single alpha particle passes through them. As a result nuclear emulsions are composed by gelatine and silver halide molecules that can interact with the charged particles passing through it. The molecules are excited by the passage for an indefinite period of time and can be converted to metallic silver by a chemical process, increasing the number of affected molecules to the point that the developed grain is visible. Then, the emulsion is fixed by dissolving away the undeveloped silver halide grains. The emulsions can record individual tracks visible under a microscope.
In 1927, L.V. Mysovskii and other russians scientists prepared plates with an emulsion layer 50 micrometers thick and used them to observe scattering of alpha particles. By 1937 Marietta Blau an Austrian physicist also performed experiments for the detections of alpha-particles and protons exposing emulsion plates at the top of mountains. However, the emulsion technique had its grand evolution at the end of the 1940s thanks to the work of scientists like Cecil F. Powell, head of the Bristol Research group, the Italian physicist Giuseppe Occhialini, and their collaborators. They made the emulsions thicker and more sensitive, in order to record tracks of particles traveling in other directions than parallel to the emulsion, and to capture traces of less ionizing -faster- particles. Also, the Occhialini group developed a special technique known as the temperature method, to reveal the tracks of the particles in those ticker emulsion plates. Powell affirmed that the development of improved emulsions opened a new window into nature through which man was able to see for the first time strange and unexpected tracks previously unknown to physicists.
The best method to expose the emulsion plates to cosmic radiation high and long enough to capture particles that due to atmospheric attenuation, less frequently reach the sea level allways has been to launch them onboard stratospheric balloons.
In early 1950s several groups around the world started to use stripped emulsions on balloon flights. Among them were the Bristol group, the Bombay group at the Tata Institute for Fundamental Research from India, and the Naval Research Laboratory. The emulsions were stripped off and packed together to form a solid block, so as to form a continuously sensitive medium of arbitrarily large volume. Registration of one emulsion against the neighbours was made by fine X-ray marks. After exposure, the emulsion sheets were separated, attached again to glass plates and then processed in the usual way. The highlight of the use of the nuclear emulsions would come in 1955 with the so called G-Stack experiment which would be the culmination of the development of the technique, and also would mark one of the highest points in terms of collaboration between different European research institutes.
Althought the introduction of the particles accelerators at the end of the 1950s rendered almost useless the continuation of the massive balloon expeditions to expose nuclear emulsions at high altitudes, this was not the end of their use in balloon experiments. Nuclear emulsions were part of the equipment carried during manned balloon missions in 1950s and 1960s, often attached to the body of the aeronauts to record the incidence of cosmic rays in each part of their bodies. Also animals were subjects of experimentation on this field. Also, during the following decades, and even today, several experimenters around the world made use of the great advantage of the longer exposure times obtained during long duration balloon flights to try to detect less abundant particles, which can only be detected this way. These experiments were based on the use of nuclear emulsions, but interleaved with suitable converters and detectors, to enrich the measurements. As usual, the payloads had to be recovered and the energy and charge information had to be extracted from the analysis of the emulsion data.
Among the most famous experiments performed this way we can mention the International Cooperative Emulsion Flights (1960), the Japanese-American Collaborative Emulsion Experiment (1979~1995) and the RUssia-Nippon JOint Balloon program (1995~1999).
Nowadays, nuclear emulsions are used in nuclear physics, elementary particle physics, and cosmic ray physics; for autoradiography and in nuclear radiation dosimetry.