Radiation-Damage, Cosmogenic, and Atom-Counting Methods

Abstract

Fission-track dating, one of the more recent techniques involving the use of radioactivity, has developed one of the widest ranges of applications. Dates of objects have been obtained ranging from 6 months to 109 years BP. Volcanic tephra, obsidian, man-made and basaltic glass, meteorites, and mica have been dated. A more apt term is nuclear-track dating because fissionable elements do not have to be present in the material. Fission, which produces one form of nuclear track, is a rare mode of radioactive decay. A more common decay is alpha decay, which produces a different type of track. Uranium 238 fissions spontaneously and has a well-defined half-life. It also fissions in the presence of neutrons such as are produced by reactors, accelerators, or neutron "howitzers." About 99.27% of all uranium is uranium 238. Robert L. Fleischer, Paul B. Price, and Robert M. Walker, who have done most of the original work in this field, have determined that most minerals contain this isotope in amounts from a few parts per billion (ppb) to many parts per million (ppm). These researchers devised a chart which characterizes the ease of use of this technique as a function of the uranium concentration. A high uranium concentration allows an "easily measured" age where the observer spends an hour at the microscope counting chemically etched fission tracks. For "considerable labor," 40 hours of such work is assumed. Ancient synthetic glass typically contains 1-2 ppm of uranium, so most glasses older than 8,000 years are datable. Most pottery clay contains about 5 ppm of uranium in either the clay itself or other minerals that occur as inclusions. It is very probable that some pottery clays or the mineral inclusions, such as zircon, might contain higher concentrations than this, which would make the age measurement lie between "easily" and "with considerable labor." It is important to point out that mineral inclusions such as zircons or micas act as solid-state detectors in that they register fissions as a track on the surface in contact with the pottery clay. Both fission and alpha events can do this.