Islands of insight in the nuclear chart

Abstract

Like the shell model for atoms, which identifies which elements are volatile and which are inert, the nuclear shell model has guided our understanding of nuclear properties. Certain “magic number” landmarks associated with “inert” nuclei have dominated the nuclear landscape for over 50 years. As the more exotic neutronrich nuclei are being studied, however, one finds that nuclear properties associated with the traditional landmarks can suddenly disappear. The reason is that both the relative importance of the average (“mean field”) and residual nuclear interactions, as well as the mean field itself, change. One of the regions of the chart of nuclides where this breakdown occurs is at N = 20 (N is the number of neutrons) and it is called the “island of inversion” (Fig. 1). The island of inversion is now known to be part of an archipelago of “islands of shell breaking” associated with the magic neutron numbers 8, 14, 20, 28, and 40. In order to accurately account for the effects of interactions and understand how the shell model breaks down, theorists need high-precision spectroscopic data for the isotopes in these regions. Now, in a paper appearing in Physical Review Letters, an international collaboration (Wimmer et al.) presents precise spectroscopic measurements of the neutron-rich element magnesium-32, which lies in the much explored island of inversion at N = 20 [1]. The measurements, which are the first of their kind and were performed at the REX-ISOLDE radioactive beam facility at CERN, address a long standing question about the low-lying energy spectrum of this nucleus and provide important information about the collective effects of nuclear interactions. A Google search for “island of inversion” yields about 1000 results, almost all of which refer to a group of neutron-rich nuclei in a region of the nuclear chart centered around sodium-31, which has 11 protons and 20 neutrons (stable sodium has 12 neutrons). The term goes back to a paper by Warburton, Becker, and Brown [2] in which they studied the unusual features for nuclei FIG. 1: Nuclear chart showing the stable nuclei (black) and those predicted to lie inside the proton and neutron drip lines (open blue). The archipelago of islands where the shell-model magic numbers are broken is shown in red. The nuclei 30Mg and 32Mg involved in this experiment are shown in green, with their shell-model structure for neutrons in the upper lefthand corner. (Credit: Alan Stonebraker)