This technique measures the spatial distribution of trace amounts of fissile and fissionable materials in a variety of solid materials, at a microscopic scale. Fission of 235U is induced during slow neutron bombardment. Fission fragments emitted from the polished surface of a small sample are recorded as a visible image of fission tracks in a "detector" of mica that is placed over the sample. The density of recorded fission tracks is directly proportional to the uranium concentration in the sample. This low cost spatial–chemical measurement of U can only be duplicated by the most advanced microbeam analytical instruments. Recent applications include screening of suitable samples for dating by U–series or U–Pb methods, measuring U distribution in small particles of coal fly ash, and determining mineral hosts of U in polished thin sections of natural or contaminated rocks and soils. In some cases the distribution of U can be used as a proxy to suggest the residence of other trace elements of environmental concern such as radium, or arsenic.
