Anisotropic fine structure of ion tracks in single crystals

Narrow nanometer-sized damage trails created by swift heavy ions, so-called "ion tracks", reflect a material's response to intense local electronic excitation. Ion tracks are commonly described as cylindrical damage zones with circular cross-sections. This assumption largely results from limitations of current characterization techniques to resolve ion track morphologies with sufficient detail. Here we present measurements of the cross-sectional morphology of ion tracks with angstrom-level precision using synchrotron-based small-angle x-ray scattering. We analyzed the track shape in single-crystalline fluorapatite, tourmaline, and synthetic alpha-quartz irradiated with 185 MeV 197Au ions along different crystallographic directions. Our results reveal a clear anisotropy in the track cross-sections: while [0001]-oriented tracks have a largely circular cross-section, tracks along (1010) show a distinct anisotropy. This anisotropy cannot be explained solely by electronic energy loss, but instead reflects the influence of the intrinsic physical properties of the crystals. The track dimensions correlate with the elastic properties in different crystallographic directions showing smaller cross-sections along directions of higher elastic stiffness. Furthermore, crystals with predominantly covalent bonding and higher thermal conductivity exhibit significantly smaller tracks. These findings highlight how anisotropic physical characteristics of single-crystal govern ion track formation, providing insight into the interplay between irradiation effects and crystal anisotropy.