Nuclei further and further from the line of stability are being widely studied using beams of unstable nuclei. The shell model provides an excellent description of nuclear structure, but its predictions are most powerful when it includes many active nuclear orbits and a realistic description of the effective interaction between the active nucleons. This requires time-consuming computations that can handle huge amounts of data.
Writing in Physical Review C, Silvia Lenzi at the National Institute of Nuclear Physics (INFN) in Padova, Italy, and colleagues in France and Spain, report a significant computational advance—involving matrix dimensions reaching ten billion—in shell-model calculations. Starting with a core nucleus of calcium-, they include the remaining proton and neutron orbits from the major shell, and the and orbits for neutrons from the next major shell, to study neutron-rich nuclei that are centered around chromium-. In particular, they explore a possible “island of inversion,” where the neutron orbits are filled in preference to the ones.
Lenzi et al. also calculate the allowed states of the nuclei as pairs of protons are removed from nickel- and find a rapid onset of deformation (changes in the nuclear shape). The deformation is signaled by a decrease in the excitation energy of the first excited state (denoted by ) and a concomitant increase in the transition strength to the ground state. Their calculations point to an island of inversion that is similar to the one seen near magnesium-. Lenzi et al. are able to reproduce the evolution of deformation along various isotopic chains, suggesting they have achieved a comprehensive description in terms of the shell model. – John Millener