11. September at 15.15 in the Senat Hall of the University of Tartu (Ülikooli 18) will defend Anna Šugai her doctoral theses in physics "Creation of structural defects under superhigh-dense irradiation of wide-gap metal oxides"
Superviser: Prof. Aleksandr Luštšik
Oponent: Dr .Anatoli Popov (Latvian University).
"Creation of structural defects under superhigh-dense irradiation of wide-gap metal oxides "
Insufficient radiation resistance is a serious limitation for wide-gap materials possessing other necessary properties to be used as fast scintillation detectors, selective dosimeters for medical and safety purposes, laser hosts and optical materials, spectral transformers for luminescent lamps and displays, etc. The goal of the thesis was to experimentally reveal the peculiarities of the processes of the creation and thermal annealing of radiation damage Frenkel defects, extended and 3D defects under irradiation of metal oxides with swift heavy ions (~2 GeV, 197Au, 238U) that spent more than 99.9% of their energy on ionization losses and provide a superhigh excitation density (> 20 keV/nm) within cylindrical ion tracks. A comparative study of virgin and irradiated -Al2O3, Lu3Al5O12 and Gd2SiO5 single crystals was performed using spectroscopic and thermoactivation methods in a temperature region of 61425 K. Special attention was paid to the influence of heavy luminescent impurity ions on the efficiency of radiation defect creation. It was shown that Frenkel defects are created at the periphery of ion tracks via recombination of nonrelaxed conduction electrons and valence holes. The manifestations of the creation of 3D defects (involving many host ions) were revealed in complex metal oxides under irradiation providing the overlapping of ion tracks. According to theoretical predictions, the collapse of supersonic discrete solitons causes the creation of these 3D defects, which are temperature-stable and facilitate brittle destruction of the crystal. Frenkel defects creation via hot eh recombination can be attenuated by a solid-state analogue of the Franck-Hertz effect in slightly cerium-doped metal oxides, where single impurity centers are dominant. However, the resistance against irradiation decreases in complex oxides containing pairs of heavy impurity ions, where at least one of impurities substitutes for a light host metal. Such pair centers cause mass disbalance, strong local anharmonicity and can serve as places for a collapse of discrete solitons.