Fission-track dating is one type of radioactive dating method used by archaeologists to determine the thermal age of artifacts containing uranium-bearing minerals. Fission tracks are created at a constant rate throughout time so that from the number of tracks present it is possible to determine the amount of time that has past since the track accumulation began. Dates from anywhere between twenty to one thousand million years ago can be determined with this particular technique.

There are two types of fission track dating, spontaneous and induced, which involve two different isotopes of Uranium, U-238 and U-235 respectively. Common kinds of artifacts which can be effectively dated using fission-track measurement include natural and manufactured glass, stones once used in hearths or for boiling, and any fired ceramics such as pottery and tile. When not used exclusively, this dating technique is often utilized as a comparative test for the validity of other dating methods, particularly potassium-argon dating.

This was the case of the archaeological site named Bed I at Olduvai Gorge where fission-track data confirmed the validity of the initial potassium-argon results. The fission-track dating method is not susceptible to the same problem of argon contamination in volcanic deposits which may cause a deceptively high age value.

The pontaneous fission of Uranium-238 (4.5 x 10 9 half-life) was first discovered in 1940 by the Russian Scientists K.A. Petrzhak and G.N. Flerov. Spontaneous fission is accomplished by causing the nucleus of one parent uranium atom to split in two with such a force that it leaves a trail (track) of damage on the crystal in a mineral. The tracks produced were not visible to scientists until 1960, when R.L. Fleischen, P.B. Price, and R.M. Wallace of the General Electric Research Laboratory came across the idea of etching. Etching allowed the fission-tracks to be seen for the first time with the aid of a microscope. Etching is accomplished through a process whereby an appropriate solvent, often hydrofluoric acid, is applied directly to the smooth surface of a mineral, dissolving and thus accentuating the fission-tracks (damage to the mineral's crystal lattice). The tracks produced by the etching process are recorded by placing a thin plastic film against the surface of the sample. Fission-track measurements are based upon the number of tracks present since the crystal last cooled below the temperature that was reached in order to bring about the tracks. These naturally occurring fission tracks alone are not enough to provide an age determination. Induced fission must also be used as a comparison.

A fission-track date can be determined based on the ratio of the number of fission-tracks produced during spontaneous versus induced fission. Fission is induced through controlled irradiation with thermal neutrons of Uranium 235 (0.7 x 10 9 half-life) in a nuclear reactor. Due to the phenomenon known as annealing (fission track fading occurring as a result of exposure to high temperatures) induced fission becomes necessary in order to arrive at an accurate artifact date. With the information gained from both spontaneous and induced fission an approximate thermal age can be calculated through this equation as long as the neutron dose is known:

Age=N x Ds/Di x 6x 10 -8

N=Total Neutron dose expressed in neutrons per square cm

Ds=Observed track density for spontaneous fission

Di=Observed track density for induced fission

Even though Fission Track dating has gained the support of the archaeological community, it is not yet a completely finished method. There are still a number of advancements in this dating technique that are being experimented with at this time. One such advancement is being studied by the Australian Geodynamics Cooperative Research Centre, which is currently working on the project called Advanced Fission Track Acquisition System. This system hopes to accomplish three things:

1. to replace the obsolete and failing equipment now in use
2. to increase productivity by extending computer-controlled microscope systems
3. To fully automate the acquisition of fission track data, including track length measurement and track density determination, based on digital image analysis.

SOURCES:

Britannica Online http://www.britannica.com

Brown, Roderick. What are fission tracks?, http://namib.geology.latrobe.edu.au

Faul, Henry. Ages of Rocks, Planets, and Stars,

McGraw-Hill, New York, 1966.

Michels, Joseph W. Dating Methods in Archaeology,

Seminar Press, New York, 1973.

Monash, Mark J. Advanced Fission Track Acquisition System, http://www.agcrc.csiro.au

Tite, M.S. Methods of Physical Examination in Archaeology, Seminar Press, New York, 1972.