Characterization of Un-etched And Etched Ion Tracks in Mineral Apatite

Abstract

Ion tracks consist of narrow (~10 nm), long (~10-100 micrometer) 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 initial defect structure in the tracks is lost.

Synchrotron small angle x-ray scattering (SAXS) is a non-destructive technique yielding information on the geometry and size distribution of nanometer sized objects. SAXS was used as the primary characterization technique in this work. 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 ~15 micrometer and requires only a minimum of sample preparation.

The key goal of this work is to investigate how the un-etched track structure depends on geological conditions such as the mineral composition and track orientation as well as it investigates the influence of these conditions on the annealing behavior. It also investigates the detailed structure of etched tracks and attempts to relate the results to those of the un-etched tracks to bridge the gap between the fundamental research into track formation and their application in fission track analysis. Natural apatite samples were irradiated with 185 MeV Au and 2.3 GeV Bi ions to simulate fission tracks. Following irradiation, the samples were polished and prepared for SAXS characterization for both un-etched and etched tracks. For the etched tracks, apart from SAXS measurement, real space techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM) were also used to complement SAXS data. 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. Un-etched tracks show comparable trends as etched tracks suggesting that the damage production and etching kinetics are controlled by similar parameters.

The tracks size or radius (prior to annealing) as well as the annealing kinetics of un-etched tracks in different compositions and orientations of apatite were also investigated using SAXS. Results indicate that tracks size parallel to the c-axis are slightly larger than those of tracks perpendicular to the c-axis for all apatite compositions. The results also show a dependence on the composition; tracks in chlorine-rich apatite are more resistant to annealing than in other compositions whereas for the orientations dependence, it was observed that tracks parallel to the c-axis anneal slower than tracks perpendicular to the c-axis. 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. Show more