FRIB-led team identifies magnetic source of zinc-70 low-energy gamma rays
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An international team led by the Facility for Rare Isotope Beams (FRIB) has determined that magnetic transitions inside the zinc-70 nucleus generate a puzzling abundance of low-energy gamma rays. The findings, published July 31 in Nature, resolve a decades-old mystery in nuclear physics and could refine models of how heavy elements are forged in stars and neutron star mergers.
The Gamma-Ray Anomaly
For decades, nuclear physicists have observed an unexplained surge of low-energy gamma rays from certain atomic nuclei, a feature known as the low-energy enhancement (LEE). This phenomenon appears in the gamma-ray strength function, which measures how often excited nuclei release energy at different wavelengths. While nuclear transitions are classified as electric or magnetic depending on proton-neutron rearrangements, the exact origin of the LEE remained elusive. The zinc-70 isotope showed a particularly strong LEE signal, making it an ideal target for investigation.
Magnetic Origin Confirmed
An international collaboration of 25 institutions from seven countries, led by Michigan State University's Facility for Rare Isotope Beams (FRIB), used a beam of rare isotopes to excite zinc-70 nuclei and then analyzed the emitted gamma rays. The team detected a telltale pattern indicating magnetic dipole transitions, not electric ones, as the source of the excess. The findings, published July 31 in Nature, resolve the long-standing question. Key U.S. national laboratories, including Lawrence Livermore, Los Alamos, Lawrence Berkeley and Pacific Northwest, participated in the experiment.
Astrophysical Impact
The discovery directly refines the nuclear reaction networks that simulate how stars, supernovae and neutron star mergers produce elements heavier than iron. Accurate gamma-ray strength functions are essential for calculating rates of neutron capture and decay in the rapid neutron-capture process (r-process). With the magnetic origin established, theorists can update models of element synthesis, potentially improving predictions of cosmic abundances. The work also demonstrates FRIB's dual role in fundamental science and national security applications.
What's Next
FRIB and its partners plan to extend the measurements to other isotopes to build a more complete library of strength functions for astrophysical simulations. It remains uncertain how the magnetic LEE influences the final yields of the heaviest elements, such as gold and uranium, in cataclysmic cosmic events.
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FRIB-led team identifies magnetic source of zinc-70 low-energy gamma rays
