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An emulsion is made, as for photographic film, of a silver salt,
usually bromide, embedded in gelatine and spread thinly on a substrate.
Multiple layers of emulsion were historically the first means of visualizing charged particle tracks; emulsion stacks are still used today to record, with very high positional precision,
very short tracks (e.g. tau leptons, which have a track length of less than a millimetre), or in other circumstances demanding very high precision.
Emulsions are permanently sensitive and cause nontrivial data acquisition work by microscopic methods; usually, emulsions are left in place for long runs, and hence are restricted to applications in areas of small particle flux or in low-cross-section experiments, like neutrino physics.
Data acquisition by automated means (e.g. by scanning the film with a CCD camera) has been found possible in some circumstances.
Rudolf K. Bock, 9 April 1998