Abstract
One ambitious goal of nuclear physics is a predictive model of all nuclei, including the ones at the fringes of the nuclear chart which may remain out of experimental reach. Certain regions of the chart are providing formidable testing grounds for nuclear models in this quest as they display rapid structural evolution from one nucleus to another or phenomena such as shape coexistence. Observables measured for such nuclei can confirm or refute our understanding of the driving forces of the evolution of nuclear structure away from stability where textbook nuclear physics has been proven to not apply anymore. This paper briefly reviews the emerging picture for the very neutron-rich Fe, Cr, and Ti isotopes within the so-called N=40 island of inversion as obtained with nucleon knockout reactions. These have provided some of the most detailed nuclear spectroscopy in very neutron-rich nuclei produced at rare-isotope facilities. The results indicate that our current understanding, as encoded in large-scale shell-model calculations, appears correct with exciting predictions for the N=40 island of inversion left to be proven in the experiment. A bright future emerges with predictions of continued shell evolution and shape coexistence out to neutron number N=50, below 78Ni on the chart of nuclei.
Highlights
One of the challenging goals of the field of nuclear structure physics is to model atomic nuclei, including their properties and their reactions—rooted in the fundamental forces at play between protons and neutrons—with predictive power for the shortestlived nuclear species located near the driplines of the chart
The region of rapid structural change of interest in this review is the so-called “N = 40 island of inversion” [4,5], where the neutron-rich Fe and Cr nuclei around neutron number become the most deformed in the region. This is theorized to be caused by the strong quadrupole-quadrupole interaction producing a nuclear shape transition in which highly-correlated many-particle–many-hole configurations become energetically more favored than the normal-order ones [4]. Such islands of inversion are characterized by rapid structural changes and shape coexistence [5,6], providing insight into nuclear structure physics far from stability [7]
For 9 Be- or 12 C-induced one-nucleon knockout reactions, the exit channel of interest is one where—in a single step—one proton or neutron is removed from the fast rareisotope beam and the projectile-like residue with one less nucleon survives in a bound final state
Summary
One of the challenging goals of the field of nuclear structure physics is to model atomic nuclei, including their properties and their reactions—rooted in the fundamental forces at play between protons and neutrons—with predictive power for the shortestlived nuclear species located near the driplines of the chart. The region of rapid structural change of interest in this review is the so-called “N = 40 island of inversion” [4,5], where the neutron-rich Fe and Cr nuclei around neutron number become the most deformed in the region In nuclear models, this is theorized to be caused by the strong quadrupole-quadrupole interaction producing a nuclear shape transition in which highly-correlated many-particle–many-hole configurations become energetically more favored than the normal-order (spherical) ones [4]. A recent prediction extends this island of inversion to N = 50 [5] and includes nuclei that will only be reached at next-generation rare-isotope beam facilities This exciting prospect of extending the island towards the magic neutron number N = 50 is based on extrapolations of calculations using the LNPS shell-model effective interaction and its monopole drifts [4,5].
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