Low-Temperature Annealing of the X-Ray-Induced Volume Expansion and Coloration of LiF

It has been shown previously that both $\frac{\Delta V}{V}$ and the density of $F$ centers produced by x irradiation of LiF at 80°K anneal out partially near 130°K. Upon reirradation at 80°K the $F$ band quickly recovers while $\frac{\Delta V}{V}$ does not recover. In the present study, an additional annealing stage of $\frac{\Delta V}{V}$ and the $F$ band was found near 270°K. Upon reirradiation at 80°K after annealing at 270°K, neither $\frac{\Delta V}{V}$ or the $F$ band recover in contrast to the recovery after annealing at 130°K. The irreversibility of the 270°K annealing of the $F$ band indicates vacancy annihilation which apparently does not occur at 130°K. The formation of $M$ centers at the higher temperature suggests vacancy motion. The fractional annealing at 130°K was found to be smaller than in previous work, presumably because of different sample perfection before irradiation.