Singh, Jagjit (2016) New approaches to the physics of weakly-bound nuclei : treatment of continuum in 6He. [Tesi di dottorato]
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Nowadays in low energy nuclear physics, the study of properties of the neutron dripline nuclei is a highly interesting topic. The study of the structure of 2n-halo Borromean nuclei demands for a three-body description with proper treatment of continuum, the conventional shell-model assumptions being insufficient. This thesis is focused on the development of a simple nuclear structure model for ground and continuum states of Borromean nuclei. Initially it is tested for 6 He, the states of which are built by starting from the continuum single-particle p-states of 5 He. The quadrupole response (0+ → 2+ ) for 6He has been investigated, finding two resonances, the narrow low-lying 2+ and a broader 2+ at 2.9 MeV above threshold with a width of about 1.8 MeV. With the aim of studying also negative parity states of 6He, the model space has been extended with inclusion of sd-continuum waves of 5 He. The extension of model space is a computationally challenging problem. The role of different continuum components in the weakly bound nucleus 6He is studied by coupling unbound spd-waves of 5 He by using simple pairing contact-delta inter-action. Our results show that the 6He ground state 0+ displays collective nature by taking contribution from five different oscillating continuum states i.e. (s1/2 )2 , (p1/2 )2 , (p3/2 )2 , (d3/2 )2 and (d5/2 )2 , that sum up to give an exponentially decaying bound wavefunction Other ground state properties of 6 He have been calculated and compared with other theories and experiments. The two-particle density as a function of the radius r1 = r2 ≡ r and the angle θ12, is also calculated, finding two peaks, one at angles smaller than 90o and one at larger angles, referred to as “di-neutron”and “cigar-like”configurations respectively. Di-neutron component has higher density and it has a longer radial tail, which confirms the halo structure of 6He, while the cigar-like component has a very compact structure comparatively. We have also investigated electric transitions to the continuum : the monopole(five configurations), dipole (five configurations), quadrupole (seven configurations) and octupole (three configurations) response of the system and contributions of different configurations to these electric multipoles has been discussed. The shapes and total integrated strengths of monopole, dipole and quadrupole response functions are in well agreement with previous studies.
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Oggigiorno lo studio delle proprietà dei nuclei vicini alla dripline neutronica è un argomento di grande interesse in fisica nucleare delle basse energie. Lo studio della struttura dei nuclei Borromeani con alone di 2 neutroni richiede una descrizione a tre corpi con un’opportuna trattazione del continuo che il modello a shell tradizionale non fornisce.
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