Researchers at CERN have made a great step forward in revealing how atomic nuclei behave after they mapped the edge of a mysterious ‘island of inversion’ where the standard rules of nuclear physics do not apply.
The ISOLDE team at the European Organization for Nuclear Research (CERN) used high-precision measurements to pinpoint the western border of an exotic region known as the 40-neutron island of inversion.
The 40-neutron island of inversion is reportedly a region of the nuclear chart where protons and neutrons no longer obey the usual shell structure seen in stable elements.
Led by Louis Lalanne, a researcher at the French National Centre for Scientific Research (CNRS), the team focused on a little-studied isotope called chromium-61, which contains 24 protons and 37 neutrons.
“The ultimate goal is to understand how nuclear structure emerges and evolves across the nuclear landscape,” Lalanne revealed.
Edge of inversion revealed
The unstable nucleus of chromium-61 was long believed to be positioned close to the edge of the inversion island. New measurements now confirm that it stands as a clear and definitive marker for the island’s boundary,
In the classic nuclear shell model, protons and neutrons arrange themselves in levels of increasing energy, which are also known as shells. At the same time, certain ‘magic numbers’ of nucleons, whether protons or neutrons, lead to particularly stable configurations.
However, in certain isotopes with unusual proton-to-neutron ratios, this ordering collapses as part of a phenomenon that gives rise to so-called islands of inversion, where the nuclei exhibit strange shapes and unanticipated properties.
According to the researchers, the 40-neutron island of inversion is one of only a few small islands of exotic nuclei scattered across the neutron-rich edge of the nuclear chart, which is dominated by more conventional, stable configurations.
The standard rules of nuclear shell filling in these isolated regions no longer apply. Neutrons occupy unexpected shells and give rise to nuclei with distorted shapes and unusual properties compared to their more stable neighbors.
Breaking structural norms
To make the measurement, the team used ISOLDE’s collinear resonance ionization spectroscopy (CRIS) apparatus, which is one of the few instruments in the world capable of resolving fine details in neutron-rich nuclei.
This enabled them to study neutron-rich nuclei with high precision and determine two properties of chromium-61 known as spin and magnetic dipole moment, which reveal how protons and neutrons are distributed inside the nucleus.
When compared with theoretical calculations, the experimental data revealed that chromium-61 has a unique shell-filling configuration. It appears to sit in a state of transition, both the one expected for nuclei located outside the 40-neutron island of inversion and that expected for nuclei that lie within it.
This hybrid configuration allowed scientists to pinpoint the western border of the 40-neutron island of inversion, the point where the traditional shell model begins to break down.
“Islands of inversion are important because they represent regions of rapid evolution that challenge our understanding,” Lalanne stated in a press release. “This result is helping us to build a clearer picture of the mechanism driving this evolution.”