Ion Beam Damage in CaF_2

The change in lattice parameter and the induced damage are studied in single crystal CaF_2 bombarded by a 15 MeV Cl ion beam. The lattice parameter change (strain) and the damage for increasing ion beam dose (5 x 10^(12) /cm^2 to 7 x 10^(15)/cm^2) is observed via x-ray rocking curve analysis using a double-crystal diffractometer and x-ray reflection topography. The ion beam energy (range = ~ 4.5 µm in CaF_2) is such that both the electronic region and the nuclear cascade region of energy loss show up in the diffraction signal. By kinematical x-ray diffraction theory analysis, the progress of strain/damage depth profile with increasing beam dose is shown explicitly. The increase in strain is nonlinear with beam dose for the dose range studied. For increasing beam dose, the strain level in the electronic energy loss region is fixed, while that in the nuclear collision loss region increases effectively until that region becomes completely amorphous.