16.5 Track annealing

From the very beginning of fission track studies (Silk and Barnes, 1959) it has been known that fission tracks can fade under certain conditions. This was first seen as a result of electron bombardment during microscopy. However, elevated temperatures are the most important cause of track fading or ‘annealing’. During this process the displaced ions within the damage track lose their charge and return to their normal lattice positions, after which the track is no longer susceptible to preferential acid attack.

Following experiments on track annealing in mica, Fleischer et al. (1964) claimed that track annealing progressed by the accumulated ‘healing up’ of short segments at random points along the length of tracks. However, subsequent work on other materials (e.g. on glass by Storzer and Wagner, 1969) has shown that the healing process occurs principally at the ends of the each track, causing a regular and progressive shortening. As the length of tracks is diminished by healing, they have a smaller probability of intersecting the free surface during the etching treatment. Hence, fewer tracks become etched and the apparent track density decreases (Fig. 16.12). This correlation between track length and track density is termed the ‘random line segment model’ (Fleischer et al., 1975).

Fig. 16.12. Schematic illustration of the effect of track shortening on the observed density of etched tracks. Short and long tracks are of equal abundance, but the latter have a higher probability of becoming etched. After Laslett et al. (1982).

Early studies showed that different materials have different degrees of resistance to fission track annealing (Fleischer and Price, 1964a). In addition, however, a temperature)time relationship is found for the annealing process. The higher the temperature, the shorter the time required for complete annealing of tracks in any given material. To examine this behaviour, Fleischer and Price (1964b) performed laboratory annealing experiments on the mineral indochinite and found that annealing obeyed a Boltzmann’s law relation:

t = A e^E/kT [16.8]

where t is the time for track fading, A is a constant, E is the activation energy, k is Boltzmann’s constant and T is absolute temperature. Much of the work since this time has been devoted to determining accurate Boltzmann relation annealing curves for different materials, both by laboratory and well-constrained geological studies.

Detailed laboratory experiments were performed on apatite and sphene by Naeser and Faul (1969) and on tektite glass by Storzer and Wagner (1969). These studies showed that annealing is a progressive process. Different degrees of track annealing in different materials each define their own Boltzmann’s relation lines when shown on Arrhenius plots of time against reciprocal temperature (Fig. 16.13). The fan of annealing lines in Fig. 16.13 is evidence for the existence of a range of activation energies for track annealing within a single type of material. This implies that as annealing progresses (as measured by the fraction of tracks lost) it also becomes progressively more difficult (Storzer and Wagner, 1969). Hence, when comparing the annealing properties of different minerals it is necessary to compare equal fractions of track loss, such as 50% (Fig. 16.13).

Fig. 16.13. Arrhenius plot to show the coherent progress of annealing in sphene (on the left) and apatite (on the right). After Naeser and Faul (1969).

Following this line of investigation, Storzer and Poupeau (1973) compared laboratory annealing rates (in the same material) for freshly induced tracks and spontaneous tracks which had been partially annealed in nature. They found that as temperature was raised the fresh tracks were initially lost at a much higher rate, but that at a certain ‘plateau’ temperature the rates of annealing became equal.

Storzer and Poupeau argued that if both spontaneous and induced tracks were subjected to a heat treatment before counting then fission track ages could be corrected for partial annealing in the environment. Track counting must be by the population method; therefore the sample must have a uniform distribution of uranium. After irradiation of the induced-track sample, track counting analysis is performed by stepwise annealing of both spontaneous- and induced-track samples in the laboratory. After each heating step a new surface of both samples is polished, etched and counted.

Results from this procedure are shown in Fig. 16.14 for a North American tektite. Above a certain threshold temperature (ca. 100 ^oC), induced tracks start to fade, but spontaneous tracks are resistant. Therefore the apparent fission track age increases rapidly with temperature. However, as laboratory heating approaches the temperature at which annealing occurred in the environment, spontaneous tracks also start to fade, and the apparent age therefore reaches a plateau (Fig. 16.14). Storzer and Wagner (1982) argued that this ‘plateau-annealing’ technique can yield corrected fission track ages in glasses with a precision of " 10% (2F).

Fig. 16.14. Demonstration of the ‘plateau-annealing’ technique on a North American tektite. a) Apparent fission track age as a function of temperature step; b) fraction of induced and spontaneous tracks remaining at a given temperature (relative to initial density D₀). After Storzer and Wagner (1982).

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