Microscopic nature of the radiative strength function: Structures, coupling with phonons

The microscopic nature of the radiative strength function, which is the most important characteristic necessary for the description of nuclear reactions involving gamma-ray photons both in astrophysics and in the theory of nuclear reactors, has been discussed. It has been shown that, in contrast to phenomenological approaches based on various modifications of the Lorentzian dependence for this function, the microscopic approach gives structures that are due to the effects both within the standard random phase approximation and of coupling with low-lying collective excitations (phonons), i.e., beyond the standard random phase approximation. Microscopic calculations of the strength function for several Sn and Ni isotopes have been performed within the self-consistent version of the extended theory of finite Fermi systems, where both of these effects are taken into account and the SLy4 Skyrme forces are used to calculate the mean field, effective interaction between nucleons, and characteristics of phonons. Microscopic radiative E1 strength functions have been used in the modern EMPIRE 3.1 code for the calculation of the cross sections for the radiative capture of neutrons and average radiative widths of neutron resonances. Reasonable agreement with the existing experimental data has been obtained with allowance for coupling with phonons. The integral characteristics of the pygmy dipole resonance in the unstable $^{68}$Ni nucleus have been explained. The existence of this resonance has been predicted in the unstable $^{72}$Ni nucleus.