Method of determining the age of a mineral that utilizes the damage done by the spontaneous fission of uranium-238, the most abundant isotope of uranium .
The fission results in radiation damage, or fission tracks, that can be made visible by preferential leaching (removal of material by solution) of the host substance with a suitable chemical reagent; the leaching process allows the etched fission-track pits to be viewed and counted under a microscope. The amount of uranium present can be determined by irradiation that produces thermal fission of uranium-235, which produces another population of tracks, related to the uranium concentration of the mineral. Thus, the ratio of naturally produced, spontaneous fission tracks to induced fission tracks is a measure of the age of the sample. See also dating .