Molybdenum-84, a nucleus with equal numbers of protons and neutrons, has been found to violate a foundational assumption of nuclear physics — that so-called “Islands of Inversion” exist only in neutron-rich isotopes far from stability.
The finding, produced by an international team including physicists from the Institute for Basic Science (IBS), University of Padova, Michigan State University, and University of Strasbourg, identifies a new Island of Inversion along the N=Z line, where proton and neutron counts are equal. According to the announcement, every previously known example of this phenomenon — including beryllium-12, magnesium-32, and chromium-64 — occurred in highly unstable, neutron-heavy nuclei.
Islands of Inversion are regions of the nuclear chart where normal ordering breaks down. Magic numbers disappear, spherical nuclear shapes collapse, and nuclei shift into heavily distorted configurations. That this could happen in a symmetrical, balanced nucleus was not anticipated.
Two Neutrons, Wildly Different Behavior
The team studied two molybdenum isotopes: Mo-84 (Z=N=42) and Mo-86 (Z=42, N=44). The pair differ by just two neutrons, yet their nuclear behavior diverges sharply. Mo-84 displays an unusually large degree of collective motion — many protons and neutrons moving together across a major shell gap — behavior the researchers say places it squarely within an Island of Inversion. Mo-86 shows no such anomaly.
Producing these isotopes required significant experimental effort. Scientists accelerated Mo-92 ions into a beryllium target at Michigan State University, generating fast-moving Mo-86 nuclei, which were isolated using an A1900 separator. That beam was then directed at a secondary target. Some nuclei became excited; others shed two neutrons and became Mo-84. Both species emitted gamma rays as they returned to ground state.
Precision Measured in Trillionths of a Second
Those gamma rays were captured by GRETINA, a high-resolution germanium detector array, alongside TRIPLEX, an instrument built to measure nuclear lifetimes lasting mere picoseconds. The team cross-referenced results against GEANT4 Monte Carlo simulations to extract the lifetimes of first excited nuclear states and calculate how far each nucleus deviated from a spherical shape.
The contrast between the two isotopes was unambiguous. Mo-84’s collective motion across a shell gap is the hallmark signature of inversion behavior — a structural shift the standard nuclear shell model does not predict for nuclei in this region of the chart.
The discovery forces a reassessment of where Islands of Inversion can form. The prevailing framework had treated neutron excess as a precondition for this kind of nuclear deformation. The behavior of Mo-84 shows that assumption was incomplete — balanced nuclei sitting well within the stable region of the chart can exhibit the same exotic structural breakdown previously reserved for the most unstable corners of nuclear physics.
The study was conducted under the auspices of the IBS Center for Exotic Nuclear Studies, with contributions from several additional institutions. The researchers say the results suggest similar anomalies may exist in other N=Z nuclei that have not yet been examined with equivalent precision.
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