Pure and Mg-doped (2000ppm) crystals were irradiated with γ-rays, from 241Am and 60Co sources, and the thermal stability of the resultant defects was studied by using photoluminescence and optical absorption methods. A composite nature of the M band at 440nm was revealed by photoluminescence data which included an emission that could be attributed to F3+ defects and was otherwise hidden in optical absorption measurements. Various bands appeared in the excitation spectra: Z3 (222nm), Z2 (285nm), R1 (329nm), R2 (375nm), F3+ (440nm) and F2 (440nm). Marked differences were observed between pure and doped samples, during thermal annealing, that were related to the F2 and F3+ bands in particular. The results supported the concept of Z centers being F and F– centers that were trapped at impurity ions in the crystal.

Photoluminescence and the Thermal Stability of Color Centers in γ-Irradiated LiF and LiF(Mg). A.T.Davidson, A.G.Kozakiewicz, J.D.Comins: Journal of Applied Physics, 1997, 82[8], 3722-9