Pure and magnesium doped (2000 ppm) LiF crystals have been irradiated with &ggr; rays from241Amand60Cosources and the thermal stability of the resulting defects has been studied using photoluminescence (PL) and optical absorption (OA). The composite nature of theMband at 440 nm is highlighted by PL data which reveal emission attributed toF3+defects otherwise hidden in OA measurements. The following bands appear in excitation spectra;Z3(222 nm),Z2(285 nm),R1(329 nm),R2(375 nm),F3+(440 nm), andF2(440 nm). Notable differences are observed between pure and doped samples during thermal annealing relating to theF2andF3+bands in particular. Results support the idea ofZcenters as beingFandF−centers trapped at impurity ions in the crystal. ©1997 American Institute of Physics.