Tracking changes in shell structure in neutron-rich nuclei as a function of spin

Abstract

PAPERTracking changes in shell structure in neutron-rich nuclei as a function of spin Abstract Taking advantage of the resolving power of modern gamma-ray spectrometers in combination with different types of nuclear reactions, it has been possible to investigate to fairly high-spin neutron-rich nuclei in a number of regions of the nuclear chart. The primary motivations for such studies are to characterize changes in shell structure as a function of the neutron-to-proton ratio and to document the impact of a large neutron excess on global properties, such as the nuclear shape. Recent data on nuclei close to the 'island of inversion' near 32 Mg are discussed first. They highlight challenges in describing the observations with modern effective interactions. Subsequently, the nature of excitations in neutron-rich fpg-shell nuclei between Ca and Ni is reviewed. A neutron sub-shell closure at N = 32 has been attributed to the monopole tensor force. Furthermore, the presence of collective excitations at moderate spin in neutron-rich Cr and Fe isotopes illustrates the role of the g 9/2 orbital in driving the nuclear shape. The observations suggest that mixing between 'deformed' and 'shell-model' states needs to be considered. Finally, recent results in the direct vicinity of 68 Ni indicate that the impact of the N = 40 neutron shell closure is rather modest.

Time-of-flight mass measurements of neutron-rich chromium isotopes up toN=40and implications for the accreted neutron star crust

“…The evolution of nuclear structure away from the valley of β-stability is a direct consequence of the forces at work in nuclei [1,2]. Neutron-rich nuclides are of particular interest, since much of the neutron-rich nuclear landscape has yet to be explored [3].…”

Shell structure from nuclear observables

“…For 32 Mg, a two particle-two hole configuration occurs eliminating the N = 20 shell as discussed in Refs. [2], [3] and references therein. As a consequence, the deformed ground-state of this nuclide results in a comparatively low 2 + 1 state.…”

Shell Model calculations of nuclear structure in ʺIsland of inversionʺ N=20 region: 32-36Mg isotopes

“…The other structure having the same positive parity was discovered using proton inelastic scattering and Coulomb excitation [8,9]. Similarly, a new state with unknown (J) has been observed in 34 Mg isotope by two separate works: P. Doornenbal [24] and S. Michimasa [71]. According to the fragmentation experiments carried out and via the laser spectroscopy technique, the last structure of 35 Mg isotopes revealed a negative parity for the ground and first excited states [10,11].…”

“…A common attribute discovered in many previous theoretical studies has been the application of mixed configurations, namely, (0p − 0h), (1p − 1h), (2p − 2h) and (3p − 3h), in the full model space [22]. Accordingly, this technique (mixed configurations) revealed the following deficiencies for SDPF-M and SDPF-U-MIX interactions: (1) non-observance of the intruder states' presence could have an impact at high excitation energy; (2) the higher excitation energy of a negative party state, which could be ascribed to the over-prediction of the excitation energy for the (2p − 2h) configurations or the overestimation of the (0p − 0h) and (2p − 2h) mix as in the Hamiltonian SDPF-M [24].…”