Nuclear emulsion
A Nuclear Emulsion plate is a type of particle detector first used in Nuclear and Particle physics experiments in the early decades of the 20th century.[1][2][3] It is a type of photographic plate, but coated with a thicker photographic emulsion of gelatine containing a higher concentration of very fine silver halide grains; the exact composition of the emulsion being optimised for particle detection. Nuclear emulsion can be used to record and investigate fast charged particles like alpha-particles, nucleons or mesons. After exposing and developing the emulsion, single particle tracks can be observed and measured using a microscope.
It has the primary advantage of extremely high spatial precision, limited only by the size of the silver halide grains (a few microns), a precision that surpasses even the very best of modern particle detectors. A stack of emulsion plates can record and preserve the interactions of particles so that their trajectories literally exist in 3-dimensional space as a trail of silver-halide grains, which can be viewed from any aspect on a microscopic scale.[3] In addition, the emulsion plate is an integrating device that can be exposed or irradiated until the required amount of data have been accumulated. It is compact, with no associated read-out cables or electronics, allowing the plates to be installed in very confined spaces and, compared to other detector technologies, it is significantly less expensive to manufacture, operate and maintain. These features were decisive in enabling the high-altitude, mountain and balloon based studies of cosmic rays that led to the discovery of the Pi-meson[4][5] and Parity violation in K-meson decays;[6] the opening of the subnuclear "Particle zoo" and the path to modern Experimental Particle Physics[1]
The chief disadvantage of Nuclear Emulsion is that it is a dense and complex material (silver, bromine, carbon, nitrogen, oxygen) which potentially impedes the flight of particles to other detector components through multiple scattering and ionising energy loss. Finally, the photographic development and scanning of large volumes of emulsion, to obtain useful digitised output data, is a very slow and labour intensive process.
These disadvantages, coupled with the emergence of new particle detector and particle accelerator technologies, led to a decline in use of Nuclear Emulsion plates in Particle Physics towards the end of the 20th century.[1] However there remains a continuing use of the method in the study of rare processes and in other branches of science, such as autoradiography in medicine and biology.
For a comprehensive and technically detailed account of the subject refer to the books by Barkas[3] and by Powell, Fowler and Perkins.[2] For an extensive review of the history and wider scientific context of the Nuclear Emulsion method, refer to the book by Galison.[7]
History[]
Following the 1896 discovery of radioactivity by Henri Becquerel[8] using photographic emulsion, Ernest Rutherford, working first at McGill University in Canada, then at the University of Manchester in England, was one of the first physicists to use that method to study in detail the radiation emitted by radioactive materials.[9] In 1905 he was using commercially available photographic plates to continue his research into the properties of the recently discovered alpha rays produced in the radioactive decay of some atomic nuclei.[9] This involved analysing the darkening of photographic plates caused by irradiation with the alpha rays. This darkening was enabled by the interaction of the many charged alpha particles, making up the rays, with silver halide grains in the photographic emulsion that were made visible by photographic development. Rutherford encouraged his research colleague at Manchester, Kinoshita Suekiti,[10] to investigate in more detail the photographic action of the alpha-particles.
Kinoshita included in his objectives “to see whether a single