Unresolved puzzles in exotic nuclei

Research into the origin of elements is still of great interest. Many unstable atomic nuclei live long enough to be able to serve as targets for further nuclear reactions—especially in hot environments like the interior of stars. And some of the research with exotic nuclei is, for instance, related to nuclear astrophysics. In this review published in EPJ A, Terry Fortune from the University of Pennsylvania, in Philadelphia, USA, discusses the structure of unstable and unbound forms of Helium, Lithium, and Beryllium nuclei that have unusually large neutron to proton ratios—dubbed 'exotic' light nuclei. The author offers an account of historical milestones in measurements and the interpretation of results pertaining to these nuclei.

Each chemical element is composed of atoms. At the centre of each atom is a nucleus containing nucleons, namely neutrons and protons. Some nuclei are unstable and are prone to emitting an electron, via beta decay, particularly when they have a large number of neutrons compared to protons. For example, Helium-8, with six neutrons and two protons, is unstable. It beta decays into a form of lithium with 3 protons and 5 neutrons, dubbed Lithium-8. Eventually, as more and more neutrons are added, the nucleus becomes unbound to neutron emission. But the properties of these unbound nuclei can still be investigated by producing them in a nuclear reaction and detecting their decay products.

In this review, the author outlines the available experimental information and the models that have been applied to 'exotic' nuclei. The laws of physics relating to the nuclear properties of these nuclei prevail even though some of them are not typically observed in normal nuclei. The author also delineates some of the unresolved puzzles concerning the connection between microscopic structure and the values of quantities that are observable experimentally— particularly the interplay between energies, widths or strengths and microscopic structure. For example, physicists have yet to resolve what is the occupancy of an orbital, called 2s1/2, in the ground state of beryllium-12? Or what is the nature of the unbound ground state of helium-10?