TECH Signal 245
FRIB-led Nature study suggests magnetic transitions behind unexplained low-energy gamma ray excess in nuclei
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A Nature study led by the Facility for Rare Isotope Beams with Lawrence Livermore National Laboratory researchers provides evidence that magnetic transitions inside atomic nuclei, where protons and neutrons flip their internal magnets, are responsible for the long-puzzling excess of low-energy gamma rays seen in some nuclei.
This is the first consistent mechanism connecting a decades-old empirical anomaly in nuclear decay to a specific physical process, magnetic transitions rather than the standard electric ones. The finding may sharpen nuclear models used in stockpile stewardship, nuclear forensics, and simulations of how heavy elements form in stars and neutron star mergers, all of which depend on accurate transition data for unstable isotopes.
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Researchers at FRIB measured gamma rays from a radioactive copper isotope decaying into zinc and separated two competing decay channels with different underlying physics.
Only the magnetic transition state, in which protons and neutrons flipped their internal magnets, produced the low-energy gamma ray excess, providing direct evidence for a magnetic origin.
The study, published in Nature and co-authored by LLNL researchers, is expected to feed back into nuclear models used for stockpile assessment, nuclear forensics, and astrophysical nucleosynthesis.
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The finding pins a long-standing empirical anomaly in nuclear decay to a specific physical mechanism. For decades physicists have seen some nuclei emit an unexpectedly large number of low-energy gamma rays, an effect called low-energy enhancement that standard nuclear models did not predict and could not anticipate which nuclei would show it. The FRIB-led result reframes the phenomenon as a property of magnetic transitions: cases where protons and neutrons flip their internal magnets during de-excitation, rather than rearranging their positions as they do in the better-understood electric transitions. That distinction is the conceptual payload of the paper, not just an incremental measurement. The experimental design earned this conclusion by isolating the two transition types in the same isotope. The team produced a radioactive copper isotope that decays into zinc, then used FRIB's instruments to populate two distinct excited states of the daughter nucleus: one that must shed energy through an electric transition and one that must shed energy through a magnetic transition. By counting low-energy gamma rays in each pathway separately, they obtained a direct comparison rather than
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