Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF 2 crystal: Evidence for two thresholds of damage creation

CaF₂ crystals as representatives of the class of ionic nonamorphizable insulators were irradiated with many different swift heavy ions of energy above 0.5 MeV/u providing a broad range of electronic energy losses (Sₑ). Beam-induced modifications were characterized by Channeling Rutherford Backscattering Spectrometry (C-RBS) and x-ray diffraction (XRD), complemented by transmission electron microscopy (TEM). Results from C-RBS give evidence of significant damage appearing above a Sₑ threshold of 5 ± 2 keV/nm. A second critical Sₑ appears around 18 ± 3 keV/nm; below this value the damage as function of ion fluence saturates at 20$%$, while above this the damage saturation level increases with Sₑ, reaching ~60$%$ for ions of Sₑ $=$ 30 keV/nm. XRD measurements also show effects indicating two threshold values. Above 5 keV/nm, the widths of the XRD reflection peaks increase due to the formation of nanograins, as seen by TEM, while a significant decrease of the peak areas only occurs above 18 keV/nm. The track radii deduced from C-RBS measurements are in agreement with those extracted from the fluence evolution of the widths of the XRD peaks. Moreover, track radii deduced from the peak area analysis are slightly smaller but in agreement with previous track observations by high resolution electron microscopy. Calculations based on the inelastic thermal spike model suggest that the lower threshold at 5 keV/nm is linked to the quenching of the molten phase, whereas the threshold at 18 keV/nm can be interpreted as quenching of the boiling phase. The results of CaF₂ are compared with other nonamorphizable materials such as LiF and UO₂.