Solodovnyk K. Photon strength functions and their usage for averaged description of electromagnetic transitions in atomic nuclei

The thesis is the result of search for the simplified microscopically based expression for the photon strength functions (PSF) for the averaged description of the electric dipole transitions (Е1) in atomic nuclei. The processes of photoabsorption and gamma-decay were considered. The analytical models for E1 PSF were analyzed, namely the standard Lotentzian model (SLO), generalized Lorentzian model (GLO, EGLO), triple Lorentzian model (TLO) and simplified modified Lorentzian approach (SMLO). For testing of PSF for photoabsorption and for their further usage, the database with experimental values of E1 PSF with systematical error less than 10% were prepared for the first time using all data for photoabsorption cross section from the international EXFOR database. The PSF database contains 475 datasets on PSF for photoabsorption for 144 isotopes from to and data for 19 elements of natural isotopic composition. The updated experimental values of the giant dipole resonance (GDR) characteristics (energy, width and strength) were obtained for 144 isotopes from to using the least-square method. This new database extends and updates all previously published data. The updated values of systematics of the GDR parameters were obtained and compared. For energy-dependent PSF models simple consistent with energy weighted sum rule was given (SMLOe model). Theoretical models were compared with experimental data on photoabsorption and gamma-decay. With the use of mathematical criteria, it was shown that the models based on the modified Lorentzian approach (SMLO, SMLOe) are the most accurate for the description of experimental data as well as for the prediction of the PSF in their absence. For activation technique, the isomeric ratios were measured for the first time for the gamma-ray energies 10.5, 12.0, 18 МeV for the nuclei , , and for the photofission reaction of . With these results, the spin distribution functions of the investigated fragments were calculated and the mean angular momentum were obtained. It was demonstrated that the use of different PSF and nuclear level densities has only a slight effect on mean angular momenta of fragments.