This fission-track density is obtained by counting the number of spontaneous tracks intersecting a polished internal surface of a mineral grain viewed under high magnification (1000×–1250×) using an optical microscope.

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The extent of any track shortening (exposure to elevated temperatures) in a sample can be quantified by examining the distribution of fission-track lengths.

The determination of a fission-track age (a number that relates to the observable track density) depends on the same general equation as any radioactive decay scheme: it requires an estimate of the relative abundance of the parent isotope and of the daughter product.

However, unlike most methods of radiometric dating, it measures the effect, rather than the product, of a radioactive decay scheme, that is it refers to the number of U atoms and the number of spontaneous fission tracks per unit volume.

Fission-track and (U–Th–Sm)/He thermochronology on apatites are radiometric dating methods that refer to thermal histories of rocks within the temperature range of 40°–125 °C.

Their introduction into geological research contributed to the development of new concepts to interpreting time-temperature constraints and substantially improved the understanding of cooling processes within the uppermost crust.

Present geological applications of apatite thermochronological methods include absolute dating of rocks and tectonic processes, investigation of denudation histories and long-term landscape evolution of various geological settings, and basin analysis.

Ar and K–Ar, fission track, and (U–Th)/He (Berger & York 1981).

Amongst these different methods, apatite fission track (AFT) and apatite (U–Th–Sm)/He (AHe) are now, perhaps, the most widely used thermochronometers as they are the most sensitive to low temperatures (typically between ) is based on the analysis of radiation damage trails (‘fission tracks’) in uranium-bearing, non-conductive minerals and glasses.

It is routinely applied on the minerals apatite, zircon and titanite.