Fission track dating methods

Determination of a fission track age requires several further experimental steps to measure the uranium concentration. The uranium concentration is not measured directly, but a second set of fission tracks is created artificially in the sample by a thermal neutron irradiation. This irradiation induces fission in a tiny fraction of the atoms, which are present in a constant ratio to U in natural uranium. Knowing the total neutron fluence received during irradiation, the number of induced tracks provides a measure of the uranium concentration of the grain. Because the induced tracks are derived from a different isotope of uranium than the spontaneous tracks an important consideration in fission track dating is the assumption that the isotopic ratio of the two major isotopes of uranium, and is constant in nature. With the notable exception of the unique natural nuclear reactors of Oklo in Gabon (Bros et al. 1998), where this isotopic ratio is disturbed, this is a very safe assumption. Numerous measurements have shown that and are always present in their natural abundances of 0.73% and 99.27%, respectively. 

The fission track age, t, is then calculated from the ratio of spontaneous (ps) to induced (pi) track densities according to the standard fission track age equation (Fleischer and Price 1964, Naeser 1967)  

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FIGURE 24 Obsidian knife. A knife made of obsidian in Mesolithic times and found in Elmenteita, Kenya, which was heated in the distant past. The time of heating (3700 + 900 years ago) was determined by the fission tracks dating method (see Fig. 25). 

Methode der fossilen Bahnspuren (Fission-Track-Dating)... 

Macdougall (1971) used fission-track dating of glass shards to determine the ages of volcanic layers in deep-sea sediments. He compared his results to K/Ar dates and showed that both methods gave identical results (Table 2). 

After the separation and purification stages, isotopic abundances are measured in most cases by mass spectrometry. Exceptions include shortlived radioactive isotopes, where decay rates are measured by fission track dating that measures the abundance of 238U by induced fission. Ionisation is carried out using a thermal method for a solid source or electron bombardment for a gas source. Efficiency of ionisation determines the amount of sample required for analysis, and efficiency may range from almost 100%... 

Figure 6. Schematic representation of different fission track dating procedures (after Hurford and Green 1982). Of these, only population and external detector methods have gained wide currency.

Figure 7. The sequence of steps involved in the external detector method of fission track dating. This method is now the dominant procedure used in most fission track dating laboratories for apatite because of its ease of handling, suitability for automation and its provision of single grain age information.

The temperature is an important variable because uranium-bearing minerals anneal fission tracks at elevated temperatures and retain them quantitatively only after the temperature has decreased below the level at which all tracks are preserved. Therefore, the fission track date of a mineral or glass is the time that has elapsed since the specimen cooled through the track-retention temperature for the last time. In other words, fission track dates are equal to the age of a specimen of mineral or glass only in cases when the specimen cooled to the track-retention temperature immediately after it was formed. The fission-track method appears to be straightforward in principle but is, in fact, quite labor intensive and demanding (Faure and Mensing 2005). 

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