Tracking 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 32Mg 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 g9/2 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 68Ni indicate that the impact of the N = 40 neutron shell closure is rather modest.