Phys. Rev. Lett. 123, 142502 (2019)

Nuclei come in various shapes — from spheres to deformed ellipsoids, which can coexist, so that a nucleus with a spherical ground state may possess a deformed excited state, or vice versa. Shape coexistence is mostly observed for nuclei with proton or neutron numbers that almost fill up a nuclear shell, such as in cadmium-110 with 48 protons.

Now, Paul Garrett and colleagues have reported spectroscopic studies of the structure of the stable cadmium-110 and cadmium-112 isotopes combined with beyond-mean-field calculations. The results suggested that at least three different shapes coexist in these nuclei. In contrast to the prevalent view that low-energy excitations in cadmium nuclei are caused by vibrations around a spherical nucleus, the authors found that these excitations are deformed states, which do not possess vibrational wave functions. If confirmed, this result would challenge our understanding of nuclear structure because deformation might already appear at closed shells.