Fission track dating method and its applications. (Answer in 150 words)

- Fission track dating method and its applications. (Answer in 150 words)
Introduction
Fission track dating counts physical damage rather than an isotope ratio, and it dates the last cooling of a mineral or glass.

Body
1. The principle: uranium-238 decays by spontaneous fission at a slow known rate, its two fragments tearing damage trails ten to twenty micrometres long through the lattice. Track density therefore rises with age and with uranium content.
2. The procedure: a polished surface is etched — hydrofluoric acid for glass, sodium hydroxide for zircon — until tracks are optically countable. Uranium content is measured by irradiating the sample with thermal neutrons to induce fission of uranium-235 and counting induced tracks in an attached mica detector. Age follows from the ratio of the two densities.
3. Materials, limits, uses: zircon, apatite, sphene, obsidian and tektites, from a few hundred to a billion years. Reheating anneals tracks and yields underestimates. It dated Olduvai Bed I at about two million years, confirming the potassium-argon age.

Conclusion
Its value lies in cross-checking potassium-argon ages on the same tuffs and in dating heated glass.