Wierbik, Jessica2026-09-102026-09-10https://hdl.handle.net/1885/733815352Trace amounts of actinides incorporated into minerals such as apatite, mica, and zircon undergo spontaneous fission, producing energetic fragments that form narrow cylindrical damage zones a few nanometres in diameter known as fission tracks (FTs). Thermal annealing of these tracks is widely used in FT thermochronology to reconstruct low-temperature thermal histories of rocks and estimate rates of geological processes within the upper crust. Current FT dating relies on chemical etching to enlarge tracks to the micron scale, but this removes the primary radiation damage, obscuring information about the actual damaged region. A detailed understanding of unetched track damage and its evolution under geologically relevant conditions is therefore essential for improving the reliability of FT thermochronology. When crystalline solids are irradiated with swift heavy ions (SHIs), the formation of ion tracks is similar to that of FTs, making them well suited as a proxy to study the parameters controlling FT morphology. Ion tracks are conventionally described as cylindrical damage zones with circular cross-sections. This view, however, largely reflects limitations of existing characterisation techniques in resolving fine cross-sectional features. The structural anisotropy of single-crystals suggests deviations from circular track geometries should be expected, and variations in cross-sectional shape and annealing behaviour for differently oriented tracks are likely governed by the host crystal's properties. In this work, synchrotron-based small-angle X-ray scattering (SAXS) is used to resolve cross-sectional anisotropy of ion tracks at the nanometre scale. SAXS measurements using a rotational sample stage enabled determination of the full cross-sections of differently oriented tracks in single-crystalline fluorapatite, elbaite tourmaline, and synthetic alpha-quartz irradiated with 185 MeV Au ions along the [0001] and <10-10> directions. The results reveal pronounced anisotropy in both track shape and size. Tracks formed parallel to [0001] show near-circular cross-sections, whereas tracks oriented perpendicular to the c-axis exhibit distinctly non-circular cross-sections. The observed anisotropy correlates with the direction-dependent elastic properties of the host crystals, with smaller radii occurring along elastically stiffer directions. The rotational SAXS setup was further combined with ex situ thermal annealing to investigate track evolution in fluorapatite irradiated with 1.64 GeV Au ions. Annealing results in reduction of track size for both orientations. <10-10>-tracks initially show non-circular cross-sections that become increasingly anisotropic with temperature, indicating heterogeneous recovery. The strongest shrinkage occurs at the position of the hexagonal axis within the plane, where lower activation energies are required to initiate recrystallisation. Tracks parallel to [0001] retain their circular geometry and display more isotropic recovery. In situ annealing of ion tracks in tourmaline over 300-640 C shows that tracks, initially characterised by a core and surrounding transition zone, exhibit progressive narrowing of the transition zone above 460 C, indicating a sharpening of the track-matrix interface. Above 520 C, reliable density profiles can no longer be determined for either orientation. Raman measurements indicate that irradiation induces lattice disorder exceeding that expected from track overlap alone, suggesting additional damage mechanisms. Moderate annealing promotes partial lattice recovery, but above 500 C crystallinity decreases and cylindrical track models break down, indicating that host-lattice metamictisation directly affects track annealing behaviour. The results highlight the fundamental role of host crystal structure in ion-track formation and annealing, with implications extending to nuclear materials design, planetary science, nanoscale fabrication, and thermochronology.en-AU2026