P. Roussel-Chomaz

Collapse of the N=28 shell closure in (42)Si.

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.

Cluster structure of 6He studied by means of 6He + p reaction at 25 MeV/n energy

Abstract Angular distributions for elastic scattering and 1n and 2n transfer reactions were measured for the 6 He + 1 H system at 25 MeV/nucleon. The experimental data were analyzed in the framework of the one-step finite range DWBA model. The results suggest a lower triton spectroscopic amplitude for the 6 He nucleus as compared to the shell model calculation.

16O elastic scattering at Elab = 94 MeV/nucleon

Abstract Elastic scattering of 94 MeV/nucleon 16 O on 40 Ca, 90 Zr, and 208 Pb was measured, completing similar measurements on 12 C and 28 Si. Optical model analysis of the whole set of data shows that the sensitive region where the nuclear potential is determined moves inwards as the mass of the target decreases. This determination is more precise for light systems which present a strong negative angle scattering contribution in the angular distribution. The data have also been analyzed in the framework of the folding model using density independent and density dependent nucleon-nucleon interaction. In both cases, the resulting potentials have to be renormalized to get a good description of the data. The data are consistent with a smooth decrease with energy of both the real and imaginary potentials.

Direct mass measurements of 19 B, 22 C, 29 F, 31 Ne, 34 Na and other light exotic nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

Direct Mass Measurements ofB19,C22,F29,Ne31,Na34and Other Light Exotic Nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

Model-unrestricted nucleus-nucleus scattering potentials from measurement and analysis of 16O + 16O scattering☆

Abstract The elastic scattering cross section for 16 O ions on 16 O targets has been measured with high accuracy over large angular ranges at incident energies from 250 to 704 MeV. From these data which sample both diffractive and refractive scattering processes, we extract the underlying scattering potentials using model-unrestricted analysis methods. The extracted potentials fit very well into the systematics found in light-ion scattering. The real potential parts also compare very favourably with microscopically calculated folding potentials based on a weak density-dependence of the underlying effective nucleon-nucleon interaction at large overlap densities.

Elastic scattering of a secondary 11Li beam on 28Si at 29 MeV/nucleon

Abstract The elastic scattering of a secondary 11 Li beam (29 MeV/nucleon) on a 28 Si target has been measured for the first time. To compensate for the low intensity of the secondary beam, an efficient detecting system permitting one to obtain reliable experimental data has been used. An attempt has been made to reproduce the data in a phenomenological analysis and with coupled-channel calculations with a double-folding optical potential, with energy- and density-dependent effective interaction and realistic densities. In coupled-channel calculations with the folding optical potential a better description is achieved if a neutron halo of 11 Li is taken into account.

Shape coexistence and the N = 28 shell closure far from stability

A mass measurement experiment by a time of flight method with the SPEG spectrometer at GANIL has been performed to investigate the N=20 and N=28 shell closures far from stability. The masses of 31 neutron-rich nuclei in the range A=29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S and P isotopes are seen to exhibit a change in shell structure around N=28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. The evidence of an isomeric state in the 43S and its interpretation by a shell model calculation confirm the analysis of the masses and constitutes the first evidence of the predicted shape coexistence around N=28.

New insight into the low-energy

The spectrum of

^{9}

^{9}\mathrm{He}