Electron-Spin Paramagnetic Resonance Studies of Neutron-Irradiated LiF

The electron-spin resonance spectra of $F$ centers in neutron-irradiated crystals of ${\mathrm{Li}}^6$F, ${\mathrm{Li}}^7$F, and LiF of normal isotopic content have been studied in order to test existing theories of $F$-center resonance absorption. The spectrum for ${\mathrm{Li}}^6$F has several interesting features and confirms the ENDOR measurements of Holton, Blum, and Slichter for crystals of normal isotopic content. Specimens of the latter composition have been subjected to a wide range of neutron doses in the present work and their spectra examined. For doses smaller than about ${10}^{17}$n/${\mathrm{cm}}^2$ the spectra exhibit a resolved hyperfine structure on a Gaussian envelope. For doses exceeding ${10}^{17}$n/${\mathrm{cm}}^2$, changes are observed in resonance linewidth, shape, and spin-lattice relaxation time. The observations appear to be consistent with the view that the radiation-induced defects are not uniformly distributed in the crystals. Exchange interactions between paramagnetic defects in regions of high defect concentration appear to be responsible for the observed changes in the resonance spectra as the neutron dose is increased. Crystals which received doses in excess of ${10}^{19}$n/${\mathrm{cm}}^2$ exhibit resonance absorption characteristic of conduction electrons in small particles of lithium metal. Annealing and saturation studies give some information regarding the spin-lattice relaxation of the conduction electrons in the metal particles.