Ion-Track Annealing Kinetics in Apatite

Fission tracks in minerals, such as apatite and zircon, generated by energetic nuclear fragments that result from spontaneous fission of U, are used for determining the age and thermal history of Earth’s crust. Taken together with other techniques, the data infer rates of tectonic uplift and landscape evolution [1]. At elevated temperatures, these tracks shrink in size and fragment into sections until they are completely annealed. The current fission-track dating technique utilizes chemical etching, which preferentially attacks the radiation-damaged volume in the undamaged bulk. The etching procedure dissolves the nanometer-sized latent track and thus enlarges the structure such that it can easily be observed by optical microscopy. The number-density and length distribution of an etched-track population correlated with the present uranium content are then used to determine the age and thermal histories of archaeological and geological samples. The etching, however, completely erases the initial damage structure that essential information on the atomic scale of the underlying radiation damage is irrevocably lost. Little is known about the primary damage track, and how its morphology depends on geological parameters such as pressure, temperature, and mineral composition.