Nuclear physicists have long believed that “inversion islands” are primarily found in isotopes packed with extra neutrons. These unusual regions of the nuclear diagram are places where the normal structure of an atomic nucleus suddenly no longer follows the expected rules. In such cases, the well-known magic number disappears, the round nucleus shape collapses, and the nucleus can transform into a highly distorted shape.
All known examples so far have occurred in highly unstable, neutron-rich atomic nuclei. Examples include beryllium-12 (N = 8), magnesium-32 (N = 20), chromium 64 (N = 40). All of these are far from the stable elements commonly found in nature.
Scientists discover surprising nuclear island
A new study by an international research team has revealed something unexpected. Scientists from the Center for Exotic Nuclear Research, the Institute of Basic Sciences (IBS), the University of Padova, Michigan State University, the University of Strasbourg, and several other institutions identified an inverted island in a location no one expected.
Rather than appearing in neutron-heavy nuclei, the newly discovered region exists in one of the most symmetrical parts of the nuclear diagram. In this region, the number of protons and neutrons are equal.
Research on rare molybdenum isotopes
Researchers discovered two isotopes of molybdenum, molybdenum 84 (Z = N = 42) and molybdenum-86 (Z = 42, N = 44). both lie along N = Z This is especially important in nuclear physics. However, these isotopes are difficult to create in laboratory experiments, making them very difficult to study.
Using rare isotope beams and highly sensitive gamma-ray detectors at Michigan State University, the team measured the lifetimes of excited nuclear states with picosecond-scale precision.
To create the necessary beam, scientists accelerated Mo-92 ions and fired them at a beryllium target, creating fast-moving Mo-86 nuclei. An A1900 separator was used to separate the desired debris from the many particles produced during the collision. The Mo-86 beam was then directed at the second target. During this step, some nuclei were excited and others lost two neutrons and changed to Mo-84.
When these nuclei returned to their lowest energy state, they emitted gamma rays that provided clues about their internal structure.
Nuclear structure revealed by gamma ray measurements
The emitted gamma rays were detected with GRETINA, a high-resolution germanium detector array that can track individual gamma-ray interactions. The scientists also used TRIPLEX, an instrument designed to measure extremely short lifetimes, lasting only a few trillionths of a second.
The researchers compared their measurements to GEANT4 Monte Carlo simulations. This allowed us to determine the lifetime of the initially excited nuclear state and estimate how much the nucleus is distorted from its spherical shape.
Big difference between Mo-84 and Mo-86
The results showed a significant contrast between the two isotopes. Mo-84 and Mo-86 differ in only two neutrons, but their behavior is very different.
Mo-84 exhibits an unusually large amount of collective motion. This means that many protons and neutrons move together across the large shell gap. Nuclear physicists describe this phenomenon as “particle Hall excitation.” In this process, some nucleons jump to higher energy orbits and become particles, but leave empty spaces or holes in lower energy orbits.
When many nucleons participate in these cooperative transitions, the nucleus deforms strongly.
Particle hole excitation and nucleus deformation
Detailed theoretical calculations helped explain why the two isotopes behave so differently. In Mo-84, protons and neutrons simultaneously undergo very large particle-hole excitations. In fact, the nucleus undergoes an essentially 8-particle, 8-hole rearrangement. This extensive reorganization produces a highly deformed nuclear shape.
This effect arises from the interaction between the proton-neutron symmetry and the narrowing of the shell gap. N = Z = 40. This combination makes it easy for many nucleons to jump across the gap at the same time.
The researchers also found that they could not reproduce these results without considering the three nucleon forces. In these interactions, three nucleons affect each other simultaneously. Traditional models that include only dinucleon interactions cannot produce the observed structure.
A new type of inversion island
Mo-86 works completely differently. It exhibits a more modest 4p-4h excitation and therefore remains much less deformed.
Taken together, these findings indicate that Mo-84 is located inside the newly identified “inversion island” and Mo-86 is located outside this region.
This newly discovered “isospin symmetry inversion island” is N = Z The Mo-84 nucleus represents the first known example of an inversion island in a proton-neutron symmetry system. The discovery challenges long-held assumptions about where these unusual nuclear regions form and provides new insights into the fundamental forces that hold atomic nuclei together.
