Credit: Image courtesy of Gaute Hagen
Scientists predict nickel-78 to be doubly magic (meaning it has an ultra-stable number of protons and neutrons) because its first excited state is significantly higher in energy than for neighboring nuclei. As shown in the figure, the energy of the first excited collective state, experimentally (Exp), is higher for the doubly magic nucleus nickel-78. Thick horizontal bars (connected by dashed lines) mark the energies of the first excited states, and the theoretical uncertainty for nickel-78 is shown as a pink band. As indicated by the depicted shapes, nuclei with a low-energy collective state exhibit a softness to surface vibrations, while doubly magic nuclei are stiffer.