Samples of CaF2 irradiated for millions of years in nature were studied by several methods, including X-Ray diffraction, positron annihilation spectroscopy, photoluminescence spectroscopy and transmission electron microscopy. It was found that the unexpectedly high density of radiation-induced defects present in the fluorite structure (documented by transmission electron microscopy) caused significant micro-strains. Even annealing up to 450C could not completely remove these micro-strains, which were stabilised by impurities. Positron annihilation spectroscopy and subsequent theoretical calculations revealed the behaviour of the defects during heating. The photoluminescence spectra of irradiated fluorite were also interpreted.

Defects in CaF2 Caused by Long-Time Irradiation and their Response to Annealing. V.Vlček, J.Cížek, J.Drahokoupil, J.Valenta, N.Miyajima, R.Skála: Philosophical Magazine, 2010, 90[20], 2749-69