The Study Of Electromagnetic Responses In Nuclei Based On The Relativistic Mean Field Theory

Nowadays,the electromagnetic responses in nuclei become one of the central top-ics in nuclear physics,particle physics and astrophysics.It can provide the accurate information on the spin properties of the in-medium nuclear interaction,and the equation of state of nuclear matter.Furthermore,the basic and critical quantities in nuclear structure,neutron skin thickness,nuclear matter incompressibility,and symmetry energy coefficient can be determined from nuclear electromagnetic excitations.Moreover,the electromagnetic responses in nuclei also contribute to the investigations of particle and astrophysics,such as the neutral current neutrinonucleus scattering on supernova dynamics,the neutroncapture cross section of nuclei lying on the rprocess path,nuclear photodisintegration on ultrahigh energy cosmic rays.In this thesis,based on the relativistic mean field theory(RMF),the magnetic multi-dipole resonance is described in the framework of relativistic(quasiparticle)random phase approximation((Q)RPA)with density-dependent mesonnucleon coupling.Isovector-pseudovector interaction channel,represented by the exchange of?meson and its zero-range counterterm,is included in the residual interaction to describe unnatural parity transitions.Based on the RMF theory,the Kurath sum rule which consists of the isovector,isoscalar,and their interference components has been derived.The validity of our calculation has been certified by producing the Kurath sum rule.This approach is then applied to investigate the following aspects:Relativistic RPA description of M1 excitations In this thesis,the relativistic RPA approach with the inclusion of?meson is applied to investigate nuclear magnetic dipole(M1)resonance distribution in doubly magic nucleus⁴⁸Ca,⁹⁰Zr,and²⁰⁸Pb.Furthermore,by analysing the contribution of each meson-nucleon coupling in Gamow-Teller and M1 resonances,we conclude that the?meson plays a very important role in determining the energy distribution of the M1 resonances.However,it is found that the strength of the counterterm suggested from the Gamow-Teller resonances or adjusting can't simultaneously reproduce the peak energies of M1 transitions in all these three nuclei with the same high accuracy as the GT cases.Such a conclusion generally holds for currently existing density-dependent meson-exchange interactions like DD-ME1,DD-ME2 and PKDD,even with an inclusion of the density-dependent coupling con-stant in?-meson coupling.To explore the underlying reason,we studied the relevant single-particle structure of main ph configurations for GT state and M1 state.It is found that the suitable spin-orbit splitting is the key to simultaneously reproduce the M1 peak energies from light to heavy nuclei.This work provides meaningful insights for the development of new effective interaction.?-ray strength function based on relativistic QRPA method Based on relativis-tic QRPA method,a microscopic?-ray strength function model which fully includes the contributions of E1 and M1 transitions in nuclei is established in this work.The validity of this model has been certified by comparing with the existing experimental data and phenomenological models.Moreover,this model is applied to investigate the basic properties of E1 and M1?-ray strength function and their efforts on(n,?)reactions in Sn isotopes.It is found that the appearance of pygmy dipole resonance will lead to the E1?-ray strength function near the neutron emission threshold significantly enhanced when the number of neutrons outside the stable core of the nucleus is increased.Furthermore,by exploring the(n,?)reaction rate of neutron-rich nuclei at different astrophysical temperatures,it is found that the pygmy dipole resonance will significantly increase the(n,?)reaction rate of strange nuclei in neutron rich region,which shows that it is necessary to consider this effects in the theoretical study of r-process.