Not all atomic nuclei are created equal. ‘Doubly magic’ nuclei have fully occupied shells of protons and neutrons, the subatomic particles known generically as nucleons. Such nuclei are therefore more strongly bound together and more difficult to excite than their neighbours on the Segrè chart — a 2D grid in which nuclei are arranged by their proton and neutron numbers. The simple structure of doubly magic nuclei makes them the cornerstone of our understanding of nuclear physics. This is because their neighbours can be described in terms of a few extra interacting nucleons, therefore turning a quantum many-body problem into a few-body problem. Only a handful of doubly magic nuclei are known. Writing in Nature, Taniuchi et al.1 identify the newest member of this elite club: the nickel nucleus 78Ni, which consists of 28 protons and 50 neutrons.
Naturally occurring nickel — used in coins, rechargeable batteries and plumbing fixtures — typically has only 30 neutrons in its nucleus. By comparison, 78Ni is extremely rich in neutrons, which makes it difficult to produce and study. This rare isotope of nickel was first observed2 in 1995. Apart from its half-life, which was measured3 in 2005 to be only about 0.1 seconds, little was known about its properties.
Taniuchi and colleagues made and investigated 78Ni by knocking one and two protons out of beams of rare isotopes of copper (79Cu) and zinc (80Zn), respectively. These beams were produced at the Radioactive Isotope Beam Factory in Wako, Japan. Coinciding with the proton-knockout reactions, the authors observed γ-rays that were emitted when 78Ni transitioned from various excited states to the ground state. Key challenges that Taniuchi et al. had to contend with included low rates of 78Ni production from the rare-isotope beams and the need to measure a substantial number of associated γ-rays.
The authors detected a strong emission of γ-rays when 78Ni transitioned from an excited state at an energy of 2.6 megaelectronvolts (MeV) to the ground state (Fig. 1). The relatively large energy of this excited state identified 78Ni as a doubly magic nucleus. Taniuchi et al. suggest that this excited state is a 2+ state of 78Ni that is associated with a spherical nucleus — the ‘2’ refers to the intrinsic angular momentum of the nucleus and the ‘+’ indicates that the nucleus has a property known as even parity. This assignment is corroborated by theoretical calculations made by the authors, and by earlier predictions4,5.
The identification of 78Ni as a doubly magic nucleus provides us with a beachhead from which to explore neutron-rich nuclei in the vicinity of 78Ni. Such exploration is valuable because the synthesis of heavy elements in the Universe proceeds — by means of a nuclear reaction called neutron capture — along pathways that include neutron-rich nuclei that are close to nickel on the Segrè chart. In the neighbourhood of 78Ni, a radioactive process known as β-decay is dominant over neutron capture, which leads to enhanced natural abundances of nuclei that have about 80 nucleons. Taniuchi and colleagues’ work will enable nuclear physicists to advance understanding of the neutron-rich nuclei that lie between 78Ni and the next cornerstone on the Segrè chart: the tin nucleus 132Sn, which was shown6 to be doubly magic in 2010.
Among the other transitions observed in their experiment, Taniuchi et al. tentatively identified a second 2+ state at an energy of about 2.9 MeV that corresponds to a deformed nucleus (Fig. 1). This assignment, which is supported by the authors’ theoretical calculations, indicates that excited 78Ni has competing spherical and deformed shapes. The results raise further questions. Is there a deformed 0+ excited state, as predicted by the nuclear-shell model? And why is the spherical 2+ state seen in only the one-proton knockout reaction and the deformed 2+ state seen in only the two-proton reaction?
A coexistence of spherical and deformed shapes is rare in general, but not that unusual in neutron-rich nuclei. It will be interesting to see how these structures change and evolve as a few nucleons are added or removed from 78Ni. The competition of the spherical and deformed shapes in 78Ni suggests that nuclei in this region of the Segrè chart — such as neutron-rich isotopes of copper or iron — will be deformed in their ground states. In this sense, 78Ni is a stronghold against deformation in this region. Such a region is called an island of inversion5,7 because the usual rules about the structure of nuclei, derived from the nuclear-shell model, do not apply.
Investigating and understanding this island of inversion is an exciting yet challenging prospect for nuclear theory and its associated experiments. Deformation sets in when protons and neutrons can occupy spherical shells that are extremely close in energy. In neutron-rich nuclei, the nucleons in the corresponding shells are weakly bound to the nucleus, or even unbound. As a result, accurate modelling is required, for instance, to discover the limits of the nuclear landscape — in other words, to determine the maximum number of neutrons that can be bound together by the strong nuclear force in the isotopes of a given chemical element8.
For the purposes of theory, it is important to account for nuclear rotation — a collective motion of an entire nucleus. However, describing nuclear rotation is a challenge when one tries to build the nucleus from scratch on the basis of individual nucleons, and when, as in the ab initio computations of the present work, one starts with two-nucleon and three-nucleon interactions that were constrained solely from the properties of light nuclei (isotopes of hydrogen and helium). The experimental identification of nuclear deformation also requires the measurement of transitions between the energy levels of a sequence of states called a rotational band. Such measurements need relatively high production rates of rare-isotope beams. Taniuchi and colleagues’ work is both a milestone in the Segrè chart and an entrance to a previously unexplored region of it. For theory and experiments, the best is yet to come.
Nature 569, 49-50 (2019)