fission-track dating

In geology, a dating method based on the natural and spontaneous nuclear fission of uranium-238 and its physical product, linear atomic displacements (tracks) created along the trajectory of released energized fission fragments. By knowing the rate of fission (a constant) and the uranium content of the material, and counting the number of fission tracks, the scientist can determine the geological age of the material back to the point when it was last heated to a sufficient temperature to eliminate any previous tracks. The method is most widely used to date volcanic deposits adjacent to archaeological material.

Although invisible even under an optical microscope, the track's core can be enlarged by selective chemical etching as the surrounding strained regions dissolve more rapidly than the bulk matrix. Minerals studied in this way include obsidian, tektites, zircon, apatite, sphene, mica, and hornblende. Most applications concentrate on the era over 10 million years ago; exceptions include the dating of volcanic pumice (or tuff) at Olduvai Gorge, Tanzania, to 1.8 million years to support studies of the hominid fossil Homo habilis. Results for the H. habilis skull unearthed by the British archaeologist Richard Leakey at East Turkana, Kenya, 1972 suggested a similar date after fission-track techniques employed on pumice were rerun. An earlier test, cross-checked with geomagnetic reversal had put the date as 2 million years ago.

Fission-track dating considered archaeological is limited to geological material that has been fired, destroying its geological tracks and leaving only an archaeological track record today. Some baked zircons from Japanese hearths dating from about AD 670, and obsidian implements fired by chance, have been studied using this dating method. Its use for recent material is usually limited because of the time-consuming nature of the technique.

The technique developed out of the manufacture of a controlled vacuum leak system, consisting of holes etched in mica, at General Electrics research centre in Schenectady, New York, 1963.