Exotic nuclear shapes with radioactive ion beams

Atomic nuclei, made up of protons and neutrons, are fundamental building blocks of nature that have been known to exist for more than 100 years. Yet, the structure of the nucleus is not even close to being understood, either in terms of its collective behaviour or individual nucleon interactions. We most often picture atomic nuclei as a spherical ball at the centre of orbiting electrons, completing the textbook version of the atom. However, what we do know is that this complex quantum system of protons and neutrons at the centre is almost never spherical, but deformed. Nuclei have shapes, from rugby-balls (prolate) and Smarties (oblate) to pears (octupole).

Predicting shapes is difficult, and nuclear theory cannot always predict the exotic shape phenomena that arise in experimental discoveries. The shape turns out to be a very sensitive test of nuclear-structure models. Shape can help distinguish which models perform best, and which ones are most reliable at predicting experimental findings in the heaviest (and newest!) elements, for example.
The most interesting shapes are often to be found in radioactive isotopes with very short half-lives. These exotic nuclei can be studied though, at facilities such as ISOLDE at CERN. Here, high-energy and high-intensity proton beams bombard targets of materials such as uranium carbide, to produce almost a thousand different isotopes. These are then filtered by mass and element before being transported and accelerated to various experimental stations where they can be studied. In this research, the radioactive beams need to be energetic so that secondary reactions can be induced. This is done by re-accelerating the extracted isotopes to around 10% of the speed of light and focusing the beam onto thin foils of isotopically pure materials.

Nuclear shape is important not only for nuclear-structure physics, but also for the study of fundamental symmetries in physics. The understanding of pear-shaped nuclei has strong implications in the search for atomic electric dipole moments (EDMs), where the reflection-asymmetric charge distribution in the nucleus enhances the EDM. A non-zero EDM would imply a violation of the charge-parity symmetry (CP-violation) and herald a new era of testing physics beyond the current standard model. The race is on for such a discovery, particularly in the odd-mass radon and radium isotopes, which form part of this study. Indeed, understanding CP-violation is seen as a route to the understanding of the observed matter/anti-matter asymmetry in the early universe, which led to our existence today.