Intrinsic luminescence in CdF2 crystals?

The luminescence stimulated in pure CdF2 crystals by photons with energy higher than 6.5 eV is studied in the temperature range 10K-100K. The emission spectrum, extending from 300nm to 500nm, is independent on the temperature and excitation energy. The excitation spectrum up to 41 eV shows a maximum at about 7.2 eV and a shoulder at 7.6 eV. The dependence of the intensity on the temperature and the decay time of the luminescence are also examined