[en] 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 F_n 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.

[en] Monte-Carlo simulations were applied for investigation of the initial electronic kinetics (≤10^-14 s) in tracks of Ca⁺¹⁹ (11.4 MeV/u) in SiO₂. The spatial and temporal distributions of the volume and excess energy densities of free electrons, electronic vacancies in different atomic shells and the lattice were obtained. It was demonstrated that at 10^-14 s an essential part (∼55%) of the energy deposited by the ion is trapped in electronic vacancies. The energy transferred to the lattice at times shorter than the characteristic time of electron-phonon coupling was determined. It was found that only ∼6% of the excess energy of delocalized electrons near the projectile trajectory (∼6 nm) may be thermalized on the time 10^-14 s from the projectile passage. Ballistic spatial propagation of excess energy cannot be described by thermal diffusion

[en] The event-by-event Monte Carlo model, TREKIS, was developed to describe the excitation of the electron subsystems of various solids by a penetrating swift heavy ion (SHI), the spatial spreading of generated fast electrons, and secondary electron and hole cascades. Complex dielectric function formalism is used to obtain relevant cross sections. This allows the recognition of fundamental effects resulting from the collective response of the electron subsystem of a target for excitation that is not possible within the binary collision approximation of these cross sections, e.g. the differences in the electronic stopping of an ion and in the electron mean free paths for different structures (phases) of a material. A systematic study performed with this model for different materials (insulators, semiconductors and metals) revealed effects which may be important for an ion track: e.g. the appearance of a second front of excess electronic energy propagation outwards from the track core following the primary front of spreading of generated electrons. We also analyze how the initial ballistic spatial spreading of fast electrons generated in a track turns to the diffusion ∼10 fs after ion passage. Detailed time-resolved simulations of electronic subsystem kinetics helped in understanding the reasons behind enhanced silicon resistance to SHI irradiation in contrast to easily produced damage in this material by femtosecond laser pulses. We demonstrate that the fast spreading of excited electrons from the track core on a sub-100 fs timescale prevents the Si lattice from nonthermal melting in a relaxing SHI track. (paper)

[en] We propose a conceptual idea for developing a technique for the monitoring of transient states of the electronic system of materials irradiated with swift heavy ions (SHIs). The method is based on spectroscopic measurements of photon emission due to radiative decay of holes from different inner shells of ionized target atoms. Since a hole in each shell of each element decays with its own characteristic time, it potentially allows the extraction of femtosecond time-resolved information about the excited electronic system in the valence and conduction band of a target. We suggest that prior ion implantation could be used for constructing a selected time grid for this technique. We also discuss the shift of the ionization potentials in atoms multiple-ionized by a direct SHI impact that should allow us to distinguish the track core within a few angstroms around the ion trajectory from the periphery of a track. (paper)

[en] We propose an analytically solvable model of the differential and total inelastic cross sections, mean free paths, and energy losses of charged projectiles (electrons, positrons, protons, ions, etc) within a unified framework. It is applicable within a broad range of incident energies (from 30–50 eV up to GeV in a case of an electron). Only the optical data are used as an input, without any ad-hoc correction terms, splitting into close and distant collisions, or fitting and adjustable parameters. We demonstrate that the derived expressions are in a very good agreement with other more complex numerical models and experimental data. Application of the derived formulae greatly saves computational time of evaluation of cross sections with respect to the numerical integration required by other models. (paper)

[en] A model of wet chemical etching of tracks of swift heavy ions (SHI) decelerated in solids in the electronic stopping regime is presented. This model takes into account both possible etching modes: etching controlled by diffusion of etchant molecules to the etching front, and etching controlled by the rate of a reaction of an etchant with a material. Olivine ((Mg_0.88Fe_0.12)₂SiO₄) crystals were chosen as a system for modeling. Two mechanisms of chemical activation of olivine around the SHI trajectory are considered. The first mechanism is activation stimulated by structural transformations in a nanometric track core, while the second one results from neutralization of metallic atoms by generated electrons spreading over micrometric distances. Monte-Carlo simulations (TREKIS code) form the basis for the description of excitations of the electronic subsystem and the lattice of olivine in an SHI track at times up to 100 fs after the projectile passage. Molecular dynamics supplies the initial conditions for modeling of lattice relaxation for longer times. These simulations enable us to estimate the effects of the chemical activation of olivine governed by both mechanisms. The developed model was applied to describe chemical activation and the etching kinetics of tracks of Au 2.1 GeV ions in olivine. The estimated lengthwise etching rate (38 µ m · h⁻¹) is in reasonable agreement with that detected in the experiments (24 µ m · h⁻¹). (paper)

[en] Structure changes and their formation threshold in swift heavy ion (SHI) tracks in Al₂O₃ are studied using a combined start-to-end numerical model. The hybrid approach consists of the Monte-Carlo code TREKIS, describing kinetics of the electronic subsystem, and classical Molecular Dynamics for lattice atoms. The developed approach is free from a posteriori fitting parameters. Simulations of Xe 167 MeV ion impacts show that relaxation of an excess lattice energy results in formation of a cylindrical discontinuous disordered region of about 2 nm in diameter. Recent transmission electron microscopy observations agree with these results. The threshold of an SHI track formation is estimated to be ∼6.1 keV nm⁻¹. Calculated x-ray diffraction patterns of irradiated material demonstrate more pronounced damage of the Al atoms sublattice near SHI trajectories. Modeling of Xe ion tracks overlapping demonstrates that the damaged area can be restored to a near virgin state. Estimations give 6.5 nm as the minimal distance between the Xe ion trajectories resulting in recovery of the transformed structure produced by the previous ion. (paper)

[en] 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

[en] Investigation of radiation defects induced by the irradiation of LiF crystals with 5- or 10-MeV Au ions (fluences of 10¹¹-2x10¹⁴ ions/cm²; flux varies by 2 orders of magnitude) at room temperature has been performed using the methods of optical absorption and high-temperature (400-750 K) thermoactivation spectroscopy. The creation efficiency of color centers (F,F₂,F₃,...) 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 eV, the broad absorption band at 2.3-3.3 eV related to intrinsic Li colloids is reliably distinguished. The creation efficiency of Li colloids by 5-MeV Au ions is lower than that by 10-MeV ions, which form δ electrons with higher energies sufficient for the creation of cation excitons (∼62 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 s) and also form, after similar intermediate processes, the groups of F centers that participate in the formation of stable agglomerates of several F₃ or even more complex centers, which serve as stable (up to 620 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

[en] Results of nuclear physics research made using track detectors are briefly reviewed. Advantages and prospects of the track detection technique in particle physics, neutrino physics, astrophysics and other fields are discussed on the example of the results of the search for direct origination of tau neutrino in a muon neutrino beam within the framework of the international experiment OPERA (Oscillation Project with Emulsion-tRacking Apparatus) and works on search for superheavy nuclei in nature on base of their tracks in meteoritic olivine crystals. The spectra of superheavy elements in galactic cosmic rays are presented. Prospects of using the track detection technique in fundamental and applied research are reported. (paper)