Fission Track Dating

A chronometric dating technique based on the natural, spontaneous nuclear fission of Uranium 238 and its byproduct, linear atomic displacements/tracks. The basis for this technique is that a uranium isotope, U 238, as well as decaying to a stable lead isotope, also undergoes spontaneous fission. One in every two million atoms decays in this way. Fission is accompanied by an energy release which sends the resulting two nuclei into the surrounding material, the tracks causing damage to the crystal lattice. These tracks can be counted under a microscope after the polished surface of the sample has been etched with acid. The concentration of uranium can be determined by the induced fission of U 235 by neutron irradiation of the sample. Since the ratio of U 235 to U 238 is known, and is constant, a comparison of the number of tracks from natural fission and the number from induced fission will give the age of the sample. Though the method has been limited in its archaeological use so far, it has already proved a useful check method for potassium-argon dating for volcanic deposits at Olduvai Gorge, Tanzania, and obsidian, tephra beds, mineral inclusions in pottery, and some man-made glasses have also been dated. A further use of the method is based on the fact that fission tracks disappear if the substance is heated about 500? or so: thus a date achieved for clay (like a hearth), pottery, or obsidian that had been burnt gives the date of burning or firing, since previous fission tracks would have disappeared.

Radiometric dating method for mineral materials. Principle. One of the isotopes of uranium, 238U, spontaneously undergoes nuclear fission to produce two heavy ‘fragment’ nuclei. This happens at a very slow, but known, rate. The fission fragments are released with great energy and create tracks of damage through the mineral in which the uranium is contained. Such fission tracks are very small, but can be identified by etching and microscopic examination. A count of the tracks is made for a unit area of mineral, and since this represents the number of fissions that has occurred, an age can be calculated from the known rate of uranium fission. The method depends on there being enough uranium and a large enough piece of mineral for the tracks to be counted successfully. In archaeology, it is also essential that the mineral has been heated, annealing any previous tracks, otherwise the date produced will only be that of the original formation of the mineral. Fission track dating is therefore confined to pottery or the rare occasions where stone objects can be proved to have been heated. Range. Maximum age is almost unlimited; minimum age depends on the uranium content, the area of mineral available for examination and the time available for counting tracks. Accuracy. Again dependent upon uranium content and the size of the piece of mineral. For most practical purposes the method is limited to samples that are very old or have a high uranium content. Materials. Obsidian contains relatively large quantities of uranium and can provide a large area for counting, but it must have been reheated in antiquity for the date to be useful. Pottery in general has a low uranium content, but some of its mineral inclusions contain adequate uranium for a reasonable dating accuracy. Zircon is the most common of these, having a uranium content between 1000 ppm and 1 per cent. Grains of zircon can be separated from crushed pottery by standard heavy mineral flotation techniques. The grain must, however be large enough for there to be a chance of counting tracks over a large enough area for reasonable dating accuracy. The size of this area depends on the uranium content, the age of the sample and the accuracy required. Small grains may be ground down in stages to provide a whole series of surfaces for counting, to make up the required area. Problems. Fission track dating has proved very useful for geology, but its time-consuming nature for many archaeological applications has resulted in little work being done with it. Where pottery contains sufficient zircon, fission track is potentially very useful as a cheap, reliable and independent method of dating. It could easily be combined, for example, with a petrological study of pottery inclusions.