fissiontrack

Fission track refers to a microscopic damage trail left in a solid by the passage of a fission fragment produced during the spontaneous fission of heavy nuclei, most commonly uranium-238. The fragment travels a short distance through the mineral, creating a narrow cylindrical defect along its path. After chemical etching, these latent tracks become visible as pits or lines on the mineral surface when viewed under a light microscope or with other imaging techniques. The tracks accumulate over time in proportion to the uranium content and the duration of exposure to fission events.

Fission-track dating uses the natural density of spontaneous fission tracks in uranium-bearing minerals to estimate the time elapsed since the crystal cooled and remained a closed system for uranium diffusion. An essential part of the method is determining the uranium content of the sample, which controls track production. Many protocols also involve irradiation in a controlled neutron flux to produce a known tracer signal, allowing calibration of track density between sample and detector under standardized conditions. The resultant age reflects the time since the crystal last cooled below its closure temperature.

Track length distributions, not just track density, provide information on thermal history. Tracks shorten (anneal) when heated, so mean track length and the spread of lengths inform on cooling rates and heating events.

Common minerals used include apatite and zircon, with others like titanite and volcanic glass employed in certain contexts. FT dating covers a wide time range, typically from about a few hundred thousand years to several hundred million years, depending on uranium content and thermal history.

Limitations include sensitivity to heating events that reset tracks, uncertainty in uranium content, and potential inheritance of primordial tracks. Accurate interpretation requires careful measurement and calibration.