Composition and orientation dependent annealing of ion tracks in apatite - Implications for fission track thermochronology

The annealing behaviour of swift heavy-ion tracks in apatite from different origins is studied as a function of their crystallographic orientation and the mineral composition. The tracks were generated by irradiating the apatite samples with 2.3 GeV Bi ions, which have a comparable rate of energy loss to fission tracks in this mineral. The track radius was investigated using synchrotron-based small-angle x-ray scattering (SAXS) combined with ex situ annealing. Results indicate that tracks parallel to the c-axis are initially larger and anneal slower than those perpendicular to the c-axis. Natural variation in the mineral composition shows stronger annealing resistance of ion tracks with higher chlorine content. The SAXS results are consistent with previous studies on etched tracks and provide evidence that the orientation and composition effects are directly linked to the property of the un-etched track and not to preferential etchability. The study helps to connect the empirical studies on etched fission tracks to more fundamental solid-state processes.