Evolution of the N=28 shell closure: a test bench for nuclear forces

The evolution of the N=28 shell closure is investigated far from stability. Using the latest results obtained from various experimental techniques, we discuss the main properties of the N=28 isotones, as well as those of the N=27 and N=29 isotones. Experimental results are confronted to various theoretical predictions. These studies pinpoint the effects of several terms of the nucleon-nucleon interaction, such as the central, the spin-orbit, the tensor and the three-body force components, to account for the modification of the N=28 shell gap and spin-orbit splittings. Analogies between the evolution of the N=28 shell closure and other magic numbers originating from the spin-orbit interaction are proposed (N=14,50, 82 and 90). More generally, questions related to the evolution of nuclear forces towards the drip-line, in bubble nuclei, and for nuclei involved in the r-process nucleosynthesis are proposed and discussed.Comment: 40 pages,15 figures, Procceedings Nobel Symposium 2012, accepted for publication in Physica Script

Shell Evolutions and Nuclear Forces

7 pages, 5 figures, Talk given at the 25th International Nuclear Physics Conference (INPC), Firenze, Italy, 2-7 June 2013International audienceDuring the last 30 years, and more specifically during the last 10 years, many experiments have been carried out worldwide using different techniques to study the shell evolution of nuclei far from stability. What seemed not conceivable some decades ago became rather common: all known magic numbers that are present in the valley of stability disappear far from stability and are replaced by new ones at the drip line. By gathering selected experimental results, beautifully consistent pictures emerge, that very likely take root in the properties of the nuclear forces.The present manuscript describes some of these discoveries and proposes an intuitive understanding of these shell evolutions derived from observations. Extrapolations to yet unstudied regions, as where the explosive r-process nucleosynthesis occurs, are proposed. Some remaining challenges and puzzling questions are also addressed

Gamma Spectroscopy of Nuclei far for Stability

The focus of the present review is the study of neutron-rich medium-mass nuclei around major shells or subshells N=20, 28 or 40. Coulomb excitation of secondary beams and in-beam spectroscopy study using the fragmentation of a stable beam have brought a wealth of informations concerning the behaviour of the closed shells when nuclei contain large neutron excesses. Pionneering experiments in this field are presented with special emphasis on the work achieved at the Ganil accelerator

The magic number N=28 and the role of the spin-orbit interaction

International audienceThe N=28 shell closure is the first arising from the strong spin-orbit interaction. Therefore, the study of its evolution in nuclei with large neutron to proton excesses is intimately connected to the evolution of the spin-orbit interaction. From the doubly magic 48Ca nucleus to the close to drip line 42Si nucleus, a wealth of nuclear structure modifications is occurring, starting from spherical, prolate-spherical co-existence to oblate shapes. Using up to date experimental data along the N=28 isotonic chain, the underlying physics origin for these structural modifications will be emphasized. Among these, the action of tensor forces and the central density dependence of the spin-orbit interaction will be invoked

Beta decay studies of neutron-rich nuclei around N=40

Beta decay studies of neutron-rich nuclei at or around N=40 are presented in the Co, Mn and V isotopic chains aiming to aimig to study excited states in Ni, Fe and Cr isotopes respectively. Examples are taken from experimental studies achieved at Louvain la Neuve, CERN/ISOLDE and GANIL/LISE facilities. Increases in production rates in the last five years has brought a dramatic change in the spectroscopic knowledge in the region of mass when the isospin number is increased. If the spherical N=40 subshell is well-established for 68Ni, its effect is steadily decreased when proceeding towards 64Cr which lies at the mid-distance between Z=20 and Z=28 magic shell

Evolution of the N=50 gap from Z=30 to Z=38 and extrapolation towards 78Ni

The evolution of the N=50 gap is analyzed as a function of the occupation of the proton f5/2 and p3/2 orbits. It is based on experimental atomic masses, using three different methods of one or two-neutron separation energies of ground or isomeric states. We show that the effect of correlations, which is maximized at Z=32 could be misleading with respect to the determination of the size of the shell gap, especially when using the method with two-neutron separation energies. From the methods that are the least perturbed by correlations, we estimate the N=50 spherical shell gap in 78Ni. Whether 78Ni would be a rigid spherical or deformed nucleus is discussed in comparison with other nuclei in which similar nucleon-nucleon forces are at play.Comment: 7 pages, 8 figures, accepted for publication PRC (22 december 2011

Shells and Shapes in the N=28 isotones

International audienceNew experimental results on 43S and 44S reveal that these nuclei are located in a transitional region of shape coexistence between the spherical 48Ca and the oblate 42Si. The origin of the deformation is discussed in terms of the evolution of the single particle energy levels leading to the compression of the energy difference of the orbitals in the sd and pf shells for protons and neutrons, respectively. Therefore, due to quadrupole excitations across the Z = 14 and N = 28 gaps, the intruder configuration in the neutron rich S isotopes became the ground state