Reaction and Structure Models for Nuclei far from Stability

This thesis is concerned with spectroscopic studies of exotic nuclei. We mainly concentrate on the structure and reactions of two-neutron halo systems such as 11Li and 14Be. Our main goal is to establish the single particle shell ordering in a series of cases through the use of pp-RPA structure calculations, appropriate reaction mechanism models and comparison with existing data. Two different formalisms are developed in order to deal with the transfer to continuum and projectile fragmentation type of experiment. The goal of our work is to establish the form and parameters of the interaction potentials necessary to reproduce the shell ordering deduced from experimental spectra. It will be shown that the potentials found are strongly angular momentum dependent trought a surface term coming from particle-vibration couplings. The thesis contains four main parts. In a first part, we study transfer to the the continuum reaction. By studying neutron transfer from a series of targets (deuteron, 9Be, 12C) to a 9Li beam, we show that the theory of transfer reactions from bound to continuum states is well suited to extract structure information on 10Li from data obtained by performing ``spectroscopy in the continuum". The next chapter deals with a simple time dependent model for the excitation of a nucleon from a bound state to a continuum resonant state in a neutron-core complex potential which acts as a final state interaction. The final state is described by an optical model S-matrix so that both resonant and non-resonant states of any continuum energy can be studied as well as deeply bound initial states. It is shown that, due to the coupling between the initial and final states, the neutron-core free particle phase shifts are modified, in the exit channel, by an additional phase. The effect of the additional phase on the breakup spectra is clarified and comparisons with the sudden approximations are made. As an example the population of the low energy resonances of 11Be and of the unbound 13Be and 10Li are discussed. It is also suggested that the excitation energy spectra of an unbound nucleus might reflect the structure of the parent nucleus from whose fragmentation they are obtained. In the third chapter the nuclear structure of halo nuclei is studied by applying the particle-particle Random Phase Approximation (RPA) to study a range of Beryllium isotopes from 8Be to the neutron-halo nucleus 14Be. There is quick review of the RPA formalism and then the one- and two-body densities of the valence neutrons of halo nuclei in the particle-particle RPA formalism are derived. These densities are used to calculate quantities such as rms, transition rates or average distance between the neutrons of the halo. Results obtained with our method are compared with available experimental data with success. The level ordering in 13Be is established on a firm basis. Finally in the fourth part, we present some coupled-channels calculation related to transfer experiments faisible at the forthcoming facility EURISOL. We are mainly concerned with tests of sensitivity of the results to several microscopic or phenomenological optical potentials in order to see what can be expected from the comparison of theoretical models and experimental results for heavy nuclei at the limits of stability.