LiF and NaCl crystals were irradiated at 8 K and 300 K with various light and heavy ions (C, Ti, Ni, Kr, Sm, Au, Pb, and U) of kinetic energy between about 50 and 2600 MeV, providing electronic energy losses from 0.7 to 26.4 keV/nm. A cryostat installed at the beamline allowed in situ absorption spectroscopy and thermostimulated luminescence (TSL) measurements from 8 K upward. Creation of electron and hole color centers is analyzed as a function of irradiation temperature, fluence, and thermal and optical bleaching. Anion interstitials ($I$ and $H$ centers) were only observed in crystals irradiated at 8 K. These defects are unstable and disappear in the temperature range 10–100 K. For heavy ions (Au, U), the $F$-center accumulation efficiency at low fluences is larger at 8 K than at room temperature. The opposite effect is observed for light ions (C, Ti, Ni). The results are discussed within the frame of transient heating influencing separation or annealing of point defects.

Investigation of radiation defects induced by the irradiation of LiF crystals with 5- or $10\text{−}\mathrm{MeV}$ Au ions (fluences of ${10}^{11}–2\times {10}^{14}\text{ions}∕{\mathrm{cm}}^2$; flux varies by 2 orders of magnitude) at room temperature has been performed using the methods of optical absorption and high-temperature $(400–750\mathrm{K})$ thermoactivation spectroscopy. The creation efficiency of color centers $(F,F_2,F_3,\dots )$ and colloids drastically depends on both the fluence and ion flux (beam current). Besides impurity (magnesium) colloids with the absorption band peaked at $4.4–4.6\mathrm{eV}$, the broad absorption band at $2.3–3.3\mathrm{eV}$ related to intrinsic Li colloids is reliably distinguished. The creation efficiency of Li colloids by $5\text{−}\mathrm{MeV}$ Au ions is lower than that by $10\text{−}\mathrm{MeV}$ ions, which form $\delta$ electrons with higher energies sufficient for the creation of cation excitons $(\sim 62\mathrm{eV})$. The cation exciton decays, in turn, with the formation of a group of spatially close $F$ centers. At a high ion flux, the next bombarding ions hit the same crystal region with a small time delay $(10–100\mathrm{s})$ and also form, after similar intermediate processes, the groups of $F$ centers that participate in the formation of stable agglomerates of several $F_3$ or even more complex centers, which serve as stable (up to $620\mathrm{K}$) seeds for nanosize Li colloids. The peculiarities of the formation, enlargement, and annealing of intrinsic colloids in LiF crystals are considered, invoking a formal analog with the processes in photographic materials based on silver halides.

Absorption spectroscopy were performed to study the effects of thermal annealing on the aggregation of color centers in LiF crystals irradiated with different ions between carbon and uranium of megaelectron volt-gigaelectron volt energy. The beam parameters such as energy, energy loss, and fluence have a pronounced influence on the initial defect composition and concentration as well as their evolution upon thermal annealing. A distinct phenomenon was observed, viz., the enhancement of Fn centers for annealing temperatures between 500 and 700 K, followed by Li colloid formation above 700 K. The phenomenon requires specific irradiation conditions whereas the formation of Mg colloids from Mg impurities occurs in all irradiated crystals. The mechanisms of annealing and colloid formation are discussed.