A comprehensive theoretical survey of nuclei

In 2018 researchers at the RIKEN Radioactive Isotope Beam Factory in Japan reported the creation of the isotope calcium-60 . The first observation of the neutron-packed nucleus (20 protons, 40 neutrons) highlighted the gap in knowledge when it comes to which nuclei can exist, even for fractions of a second. Experimentalists have confirmed the neutron drip lines—the lightest and heaviest isotopes of a given element—only for species as heavy as neon . On the theory side, predicting whether a prospective isotope can hold on to all its protons and neutrons requires painstaking calculations of the dynamics of nucleons interacting via two- and three-body forces (see the article by David Dean, Physics Today, November 2007, page 48 ).

Now Ragnar Stroberg of the University of Washington and colleagues have used an ab initio method to calculate the binding and ground-state energies of nearly 700 isotopes spanning the periodic table from helium to iron. The calculations, which entailed solving the Schrödinger equation for the many-body system via a recently developed framework, required about 500 CPU hours for each isotope. The survey complements earlier work that plumbed several thousand prospective nuclei using density functional theory, which relies on experimentally derived values for stable nuclei to extrapolate to unstable ones. Stroberg and colleagues’ results, shown in the figure (probability bars), agree well with experimental data (open and filled-in drops) and provide potential targets for searches at rare isotope facilities. For example, the findings support the probable existence of bound calcium nuclei with 48, 50, or even 56 or more neutrons.

Calcium is one of the primary targets at RIKEN and other facilities such as the Facility for Rare Isotope Beams, which is expected to open next year at Michigan State University. Beyond achieving a more complete census of nature’s isotopes, the research could help chart the path of neutron-rich isotope production during neutron-star mergers (see the article by Anna Frebel and Timothy C. Beers, Physics Today, January 2018, page 30 ) and in other extreme astrophysical environments. (S. R. Stroberg et al., Phys. Rev. Lett. 126, 022501, 2021 .)