An international team of scientists has identified an unexpected Island of Inversion in molybdenum-84, a nucleus with equal numbers of protons and neutrons. This discovery challenges previous beliefs that such regions occur only in neutron-rich isotopes. The finding reveals new insights into nuclear deformation and fundamental forces.
Nuclear physicists have long understood Islands of Inversion as regions where atomic nuclei deviate from expected structures, becoming highly deformed due to the breakdown of magic numbers. These areas were previously observed only in unstable, neutron-rich isotopes such as beryllium-12, magnesium-32, and chromium-64, all far from natural stability.
In a recent study, researchers from institutions including the Center for Exotic Nuclear Studies at the Institute for Basic Science, University of Padova, Michigan State University, and University of Strasbourg examined molybdenum isotopes along the N=Z line. They focused on molybdenum-84 (Z=42, N=42) and molybdenum-86 (Z=42, N=44), which are difficult to produce and study.
Using rare isotope beams at Michigan State University, the team accelerated molybdenum-92 ions and collided them with a beryllium target to generate molybdenum-86 fragments. An A1900 separator isolated these, and the beam was then directed at a second target, where some nuclei shed two neutrons to form molybdenum-84. As the excited nuclei relaxed, they emitted gamma rays captured by the GRETINA detector array and the TRIPLEX instrument, measuring lifetimes in picoseconds.
Analysis, supported by GEANT4 simulations, revealed stark differences. Molybdenum-84 exhibits extensive collective motion through an 8-particle-8-hole excitation, leading to strong deformation. This arises from proton-neutron symmetry and a narrowed shell gap at N=Z=40, facilitated by three-nucleon forces—interactions not replicable with two-nucleon models alone. In contrast, molybdenum-86 shows milder 4-particle-4-hole excitations and remains less deformed.
This positions molybdenum-84 within a new Isospin-Symmetric Island of Inversion, the first in a proton-neutron symmetric system. The results, published in Nature Communications in 2025, underscore the role of multi-nucleon interactions in nuclear structure.
