The atomic nucleus was discovered over a century ago, yet many questions remain about the force that keeps its constituent protons and neutrons together and the way in which these particles pack themselves together within it.
In the classic nuclear shell model, protons and neutrons arrange themselves in shells of increasing energy, and completely filled outer shells of protons or neutrons result in particularly stable "magic" nuclei. But the model only works for nuclei with the right mix of protons and neutrons. Get the wrong mix and the model breaks down.
Identifying the regions on the chart of nuclei where this breakdown occurs is keeping nuclear physicists busy worldwide. The goal? To develop a model that applies to all nuclei and leads to a deeper understanding of their internal structure.
In a paper just published in Physical Review C, Louis Lalanne and his colleagues report data from CERN's ISOLDE facility that allowed them to determine the western border of one such region - the "island of inversion" associated with the neutron number 40.
The 40-neutron island of inversion is one of only a few small islands of unusual nuclei in a sea of mostly "normal" nuclei at the neutron-rich edge of the nuclear chart. In these insular regions, the usual order of nuclear shell filling breaks down and neutrons occupy shells other than those where we expect to find them. This uncommon shell filling gives these nuclei unusual shapes and properties compared to their neighbours.
To explore the 40-neutron island of inversion, Lalanne and his co-workers used ISOLDE, a unique facility for the production and study of nuclei that have too many or too few neutrons to be stable. Specifically, they created and investigated the little-studied chromium-61 nucleus, which has 24 protons and 37 neutrons and was thought to be located right at the western shore of the 40-neutron island of inversion.
Using measurements taken with the facility's collinear resonance ionisation spectroscopy (CRIS) apparatus, which allows neutron-rich nuclei to be studied with high precision, the researchers determined two properties of chromium-61 known as spin and magnetic dipole moment.
Paired with theoretical calculations, these measurements showed that chromium-61 has a shell-filling configuration that lies between the one expected for nuclei located outside the 40-neutron island of inversion and that expected for nuclei that lie within it - thus determining the western border of the 40-neutron island of inversion.
"The ultimate goal is to understand how nuclear structure emerges and evolves across the nuclear landscape," says Louis Lalanne. "Islands of inversion are important because they represent regions of rapid evolution that challenge our understanding. This result is helping us to build a clearer picture of the mechanism driving this evolution."
