Researchers reveal nuclear ‘island’ where magic numbers break down

Atomic nuclei are often described as orderly systems. Certain numbers of protons or neutrons, called magic numbers, usually produce especially stable and spherical nuclei. These numbers fill entire energy shells in much the same way filled electron shells stabilize atoms.

Over the past few decades, that tidy picture has started to crack. When scientists examined radioactive nuclei far from stability, they noticed that some magic numbers lost their power. Nuclei once expected to remain round suddenly twisted into strange shapes, and particles began to leap across energy gaps instead of staying put.

These surprising regions are called Islands of Inversion. In them, the normal shell structure appears flipped. Instead of behaving like simple building blocks, protons and neutrons begin to interact strongly and move together. Until now, every known island appeared on the neutron-rich side of the nuclear map.

A new study points to a major shift in that understanding. An international team has found the first Island of Inversion where protons and neutrons exist in nearly equal numbers. The work focused on two rare nuclei, molybdenum-84 and molybdenum-86, and uncovered a sharp change in their internal structure.

These two isotopes sit near a line where the number of protons matches the number of neutrons. This region is important but difficult to study because such nuclei are short-lived and tough to create in large numbers.

In this case, the difference between the two isotopes amounts to only two neutrons. Yet their inner behavior could not be more different.

Molybdenum-84 turns out to be highly distorted, with protons and neutrons moving together in a coordinated way. Its heavier sibling, molybdenum-86, looks far more restrained and closer to the structure predicted by traditional theory.

The experiment took place at Michigan State University’s National Superconducting Cyclotron Laboratory. Scientists started with a beam of molybdenum-92, hurled it at a beryllium target, and filtered out fragments of molybdenum-86. That beam then struck a second target. Some nuclei became excited, while others lost two neutrons and formed molybdenum-84.

As these radioactive atoms settled back to their lowest energy state, they released gamma rays. Those flashes carried vital clues about how the nuclei were shaped inside.

The research team used a powerful detector array known as GRETINA to capture these signals. GRETINA traces the path of each gamma ray and allows for precise energy measurements, even when the emitting nucleus is moving at high speed.

To gauge how long an excited nucleus lasts before releasing energy, the team also relied on a device called TRIPLEX. It measures lifetimes in trillionths of a second. These brief lifespans reveal how strongly particles inside the nucleus move together.

The key value extracted from the data was the probability that energy would escape as a specific type of gamma ray. High values indicate that many protons and neutrons are acting as a group. Low values mean the nucleus behaves more independently.

Molybdenum-84 showed extremely large values, similar to other known deformed nuclei such as zirconium-80. By contrast, molybdenum-86 produced much smaller values, reflecting a more modest structure.

When compared with earlier measurements across the region, the pattern was striking. In lighter elements, isotopes with equal numbers of protons and neutrons usually matched the behavior of those with two extra neutrons. That trend collapsed at molybdenum.

Only molybdenum-84 broke away, standing out as sharply deformed while molybdenum-86 did not. That split marks the edge of a newly discovered island.

To understand the shift, scientists turned to detailed computer simulations. These models track how protons and neutrons occupy energy levels and how they interact.

The results showed that molybdenum-84 undergoes massive rearrangement inside. Four protons and four neutrons leap across a major energy gap into higher orbits. Their movement creates a lopsided shape and intense collective motion.

Physicists label this an eight-particle, eight-hole arrangement. It means eight particles shift upward and leave eight empty spots below. Such large rearrangements usually demand special conditions.

Molybdenum-86, on the other hand, exhibits fewer of these jumps. Its internal structure mixes several shapes but never reaches the same level of distortion.

The team also tested what would happen if only the traditional two-particle forces were allowed in the calculations. When that constraint was imposed, the predicted behavior changed completely.

Without extra forces at work, molybdenum-84 lost its dramatic deformation in the model. The simulated values dropped by more than tenfold and failed to resemble the real data.

This mismatch highlighted another crucial factor: three-body forces.

In these interactions, three particles influence one another at once. Though subtle, they turn out to be essential. They narrow the energy gap that encourages massive rearrangements and help explain why deformation takes hold in some nuclei but not others.

The study’s authors describe the region around molybdenum-84 as an Isospin-Symmetric Island of Inversion. The term reflects how the island appears where protons and neutrons share equal influence.

This discovery extends the idea of inverted structure into new territory. Until now, these islands were tied mainly to excess neutrons. This one arises from a balance instead of an imbalance.

It also underscores how little of the nuclear chart is truly mapped. Entire regions still hide unexpected behavior, waiting to emerge as experimental methods improve.

The findings show that the same forces shaping small atomic nuclei also operate in heavy, deformed ones. Three-body forces, once thought to matter mainly in stable spheres, play a strong role here too.

With this work, nuclear physicists have gained a deeper and more unified view of how matter organizes itself at the smallest scales.

The discovery reshapes how scientists understand atomic structure, which affects many fields including nuclear energy, radiation safety, and medical imaging.

Better models of nuclei improve predictions used in reactors and in cancer treatment planning.

The research also advances tools for studying rare isotopes that may one day contribute to cleaner energy or new materials.

Research findings are available online in the journal Nature Communications.

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