For many years, nuclear physicists believed that "Islands of Inversion" were found mainly in isotopes packed with extra neutrons. These unusual regions of the nuclear chart are places where the normal structure of atomic nuclei suddenly stops following the expected rules. In these cases, the well known magic numbers vanish, round nuclear shapes break down, and the nucleus can shift into a highly distorted form.
Until now, every known example occurred in very unstable, neutron rich nuclei. Examples include beryllium-12 (N = 8), magnesium-32 (N = 20), and chromium-64 (N = 40). All of these lie far from the stable elements commonly found in nature.
A new study by an international research team has uncovered something unexpected. Scientists from the Center for Exotic Nuclear Studies, Institute for Basic Science (IBS), University of Padova, Michigan State University, University of Strasbourg, and several other institutions have identified an Island of Inversion in a place no one anticipated.
Instead of appearing in neutron heavy nuclei, the newly discovered region exists in one of the most symmetrical parts of the nuclear chart. In this region, the number of protons and neutrons is equal.
The researchers focused on two isotopes of molybdenum: molybdenum-84 (Z = N = 42) and molybdenum-86 (Z = 42, N = 44). Both lie along the N = Z line, which is especially important in nuclear physics. However, these isotopes are extremely difficult to study because they are challenging to create in laboratory experiments.
Using rare isotope beams at Michigan State University and highly sensitive gamma ray detectors, the team measured the lifetimes of excited nuclear states with precision on the scale of picoseconds.
To generate the required beam, scientists accelerated Mo-92 ions and fired them at a beryllium target, producing fast moving Mo-86 nuclei. An A1900 separator was used to isolate the desired fragments from the many particles produced during the collision. The Mo-86 beam was then directed at a second target. During this step, some nuclei became excited, while others lost two neutrons and transformed into Mo-84.
As these nuclei returned to their lowest energy states, they emitted gamma rays that provided clues about their internal structure.
The emitted gamma rays were detected with GRETINA, a high resolution germanium detector array capable of tracking individual gamma ray interactions. Scientists also used TRIPLEX, an instrument designed to measure extremely short lifetimes that last only trillionths of a second.
Researchers compared the measurements with GEANT4 Monte Carlo simulations. This allowed them to determine the lifetimes of the first excited nuclear states and estimate how much the nuclei were distorted from a spherical shape.
The results showed a striking contrast between the two isotopes. Although Mo-84 and Mo-86 differ by only two neutrons, their behavior is very different.
Mo-84 displays an unusually large amount of collective motion. This means that many protons and neutrons move together across a major shell gap. Nuclear physicists describe this phenomenon as a "particle-hole excitation." In this process, some nucleons jump to higher energy orbitals, becoming particles, while leaving empty spaces, or holes, in lower energy orbitals.
When many nucleons participate in these coordinated transitions, the nucleus becomes strongly deformed.
Detailed theoretical calculations helped explain why the two isotopes behave so differently. In Mo-84, protons and neutrons undergo very large simultaneous particle hole excitations. In fact, the nucleus effectively experiences an 8-particle-8-hole rearrangement. This extensive reorganization produces a highly deformed nuclear shape.
The effect arises from the interaction between proton neutron symmetry and a narrowing of the shell gap at N = Z = 40. This combination makes it easier for many nucleons to jump across the gap at the same time.
The researchers also found that these results cannot be reproduced without accounting for three nucleon forces. In these interactions, three nucleons influence each other simultaneously. Models that include only traditional two nucleon interactions fail to produce the observed structure.
Mo-86 behaves quite differently. It exhibits more modest 4p-4h excitations and therefore remains far less deformed.
Taken together, the findings show that Mo-84 sits inside a newly identified "Island of Inversion," while Mo-86 lies outside this region.
This newly discovered "Isospin-Symmetric Island of Inversion" in the N = Z nucleus Mo-84 represents the first known example of an Island of Inversion in a proton neutron symmetric system. The discovery challenges long standing assumptions about where these unusual nuclear regions can form and offers new insight into the fundamental forces that hold atomic nuclei together.
