Ion tracks consist of narrow (~10 nm), long (~10-100 μm) cylindrical defect regions that are generated by high-velocity heavy ions when they pass through a solid. Such tracks are used for determining the age and thermal history of geological material by studying the number and length distribution of chemically etched tracks that result from spontaneous fission of natural inclusions of uranium in minerals such as apatite and zircon. The etching enlarges the original damage area such that the track can be studied using optical microscopy. The downside of the etching process is that it completely erases the initial damage structure and as a result, important information about the track structure is lost. Furthermore, interpretation of the results is difficult as the track dimensions are not only determined by the latent track (un-etched) structure but also by the etching kinetics.
Synchrotron small angle x-ray scattering (SAXS) is a non-destructive technique yielding information on the geometry and size distribution of nanometer sized objects. It can be used to study the morphology of the etched and un-etched ion tracks in natural apatite with high precision as they are nearly identical, aligned, nano-sized objects. It probes the track over its entire length of ~15µm and requires only a minimum of sample preparation.
This work investigates how differences in the un-etched track morphology translate into etched ion track dimensions to bridge the gap between the fundamental research into track formation and their application in fission track dating, in particular the influence of different compositions and thermal annealing. Natural apatite samples were irradiated with 185 MeV Au and 2.3 GeV Bi ions to simulate fission tracks. Subsequently, the samples were chemically etched and the resulting track morphology was investigated using SAXS and complimentary real space techniques such as scanning electron microscopy (SEM). Results indicate that the etching process is highly anisotropic, exhibiting hexagonal etch-pits that depend on the mineral composition as well as the track orientation. Latent tracks show comparable trends as etched tracks suggesting that the damage production and etching kinetics are controlled by similar parameters.
The annealing kinetics of un-etched tracks in different compositions and orientations of apatite were also investigated using SAXS. The results show a dependence on the composition; tracks in chlorine-rich apatite are more resistant to annealing than in other compositions. These results provide important input to develop an understanding of the correlation of etched and un-etched fission tracks and the use of SAXS as a tool for studying etched tracks.