Characterisation of surface hillocks formed on CaF2 under ion irradiation

For many amorphizable insulators (silicates, garnets, etc.) track observations by different techniques give coherent results concerning the track size at the surface and in the bulk [1, 2]. The situation is quite different for ionic crystals. Their strong ionic binding structure prevents amorphization, and under ion irradiation they respond by a complex damage mechanism. To date, there exists a large set of track data in particular for LiF and CaF2 [3-7]. The experimental results give indications that tracks in these crystals are larger on the surface than in the bulk. Moreover, in contrast to amorphisable crystals, the diameter of tracks at the surface increases only slightly with the electronic energy loss (Se). In this study, we exposed single crystals of CaF2 cleaved along the (111) plane to various ion projectiles between Ca and U (1-11 MeV/u), delivered from different accelerators (GANIL, Caen, UNILAC, Darmstadt, and VIVITRON, Strasbourg) covering a Se range between 4.6 and 35.5 keV/nm.