Low energy electron scattering and nuclear structure: a few selected examples of elementary electric and magnetic excitations

Abstract

Notes of three lectures concerned with low energy electron scattering and nuclear structure are presented. The first part of the lectures dealt with a brief reminder of the definition of a giant resonance, its strength and fragmentation and of the method to determine reduced transition strengths which are the testing grounds for any comparison with a model. As an immediate application, the study of the electric quadrupole giant resonance in 28Si by means of single arm (e,e′) experiments, capture reactions and inelastic hadron scattering experiments has been discussed.Preliminary results of (e,e′x) coincidence measurements were also presented. The second and major part of the lectures has been concerned primarily with the present status and the perspectives of ground state magnetic dipole transitions. Selected examples of pure neutron M1 spin-flip excitations, of mixed neutron and proton excitations leading to the interference between spin and orbital magnetization, and of a recently discovered purely orbital mode in heavy deformed nuclei were presented and discussed in the light of recent model predictions as well as in connection with results from Gamow-Teller charge exchange reactions, high-resolution inelastic proton scattering and beta decay. The phenomenon of quenching of the observed transition strengths and magnetic moments has been used to point out a possible intimate relation between nuclear and subnuclear physics. The constituent quark model has been used to bridge the gap between the two fields. Finally, it was pointed out how the measured M1 transition strengths yield directly the paramagnetic susceptibilities of nuclei. After comparing those with values from atomic and nucleonic matter it has been found that they scale with the corresponding Fermi energies.