Two neutron transfer to the continuum

This work is mainly dedicated to the study of a two neutron process taking place in the reaction 13C(18O,16O)15C at 84 MeV incident beam energy, realized using the large acceptance magnetic spectrometer MAGNEX, at Catania's LNS ("Laboratori Nazionali del Sud") laboratories. The study of light neutron-rich nuclei has a great importance in the description of the evolution of nuclear structure from the beta-stability valley towards the drip lines. Such nuclei can be experimentally investigated via multi-neutron transfer reactions using intense stable beams, a technique that allows the collection of accurate and statistically reasonable data; otherwise, their investigation can be performed by the utilization of the so-called "Radioactive Ion Beams" (RIBs). From the theoretical point of view, the description of these nuclei represents a crucial step in the understanding of the nuclear interaction which binds them. The aim of the experiment discussed in this work is the study of the structure of the weakly bound nucleus 15C, whose one-neutron separation energy is S_{n}=1.218 MeV. The analysis is made through the measurement of the transfer reaction of two neutrons, that could have been influenced by a pairing interaction between the nucleons themselves. During the same experiment, also one neutron transfer data have been collected which helps us to test the reliability of the theoretical model we apply. It has been predicted that the pairing force generates in some nuclei a certain type of excitations, named as "pairing vibrations'': these excitations have a well defined spin and parity (0+) and present collective characteristics. They have been studied through transfer reactions of a pair of nucleons considering nuclei close to the 208Pb, which is a doubly magic nucleus. If the pair excitations take place among primary shells, pairing vibrational states are expected at high energy and this kind of states are named "Giant Pairing Vibrations" (GPV). Hypothetically, all the transfer reactions of two correlated nucleons could excite these collective modes, nevertheless GPVs have never been observed, even though many research work has been done thanks to different (t,p) experiment on heavy targets (such as lead or tin). Two large bumps have been observed in the experiment, centered at excitation energies of E_{x}=11.45 MeV and E_{x}=14.05 MeV with widths Gamma=3.22 MeV and Gamma=2.67 MeV, respectively, and one hypothesis is that one of these structures could correspond to a first evidence of the Giant Pairing Vibrations expected by Broglia and Bes. Notice that these giant resonances have never been observed before, even in (t,p) experiments and thus the explanation of their existence could be different. The first chapter of this thesis contains a general introduction to the type of nuclei of interest: the so-called "exotic nuclei". The peculiarity of exotic nuclei relies in the fact that they are located away from the "stability valley" and having an unusual N/Z ratio they have protons or neutrons in excess with respect to the stable ones. Furthermore, their binding energy per nucleon decreases as the number of nucleons in excess increases, generating several nuclear phenomena. After the description of this kind of nuclei and a general resume of the phenomena related to them, such as nuclear $halo$ and $skin$, the chapter develops describing the experimental technique and setting used to investigate exotic nuclei, consisting in the "Radioactive Ion Beams Production". The second chapter clarifies the characteristics of the pairing interactions between nucleons. Such interaction can influence the mechanism of transfer reactions of a pair because collective excitation modes are introduced and they are related to the possible correlation between the coupled nucleons. The third chapter deals with the reaction, how the experiment was performed and what are the results. To introduce the subject, a review of the present knowledge of the nucleus of 15C is given: its structure and spin and parity assignments that were obtained in previous experiments and measurements. Then, a brief outline of MAGNEX spectrometer is given and the details of the experiment performed at LNS in Catania are explained. The theoretical framework, in which the reaction is interpreted, is discussed in the fourth chapter considering either the transfer to bound states or to continuum states formalism, since the inclusive spectra studied show the presence of both situations. Besides, the different potentials used in the calculations are described: this is a crucial part of the work, since several potentials have been tested. Finally in the fifth chapter the reaction studied is analyzed in details, through several comparisons between theoretical calculations and experimental data. In this chapter, the consistency of the formalism introduced to describe bound states is established, thanks to the really good agreement of the theoretical and the experimental ratio between cross sections of the first excited state (d5/2) and the ground state (s1/2) of 15C . The analysis of the one neutron transfer data helps us fixing the details of the model potential to be used. Comparisons with free neutron-carbon total cross section's spectra show the consistency of our method. Using our model we are also able to interpret accurately the inclusive experimental spectrum of 15C: the energy region between the one and two neutron thresholds, where all the narrow resonances are located, requires a good knowledge of the energy-dependent potential but the optical model cannot reproduce in details all those excited states. Nevertheless, we are able to describe the background that lays below the resonances by considering only the elastic part of the transfer to the continuum reaction: 14C(17O,16O)15C. Furthermore, when the threshold for the emission of second neutron is reached, our analysis shows that the first bump is the result of the combination of the tail of the first neutron energy distribution plus the distribution of the second emitted neutron in the continuum. Finally, the origin of the second bump is not clearly understood in the framework of our final-state interaction model and could represent a starting point for future research work