Researchers at the Facility for Rare Isotope Beams (FRIB) at Michigan State University have identified the source of a puzzling abundance of low-energy gamma rays emitted by the zinc-70 nucleus: magnetic transitions occurring inside the nucleus itself.
The finding, published in Nature as 'Magnetic Character of the Low-Energy Enhancement in 70Zn,' resolves a decades-old puzzle in nuclear physics. Known as the low-energy enhancement (LEE), the unexplained rise in low-energy gamma rays appears in the gamma-ray strength function of certain nuclei — and its origin had eluded physicists since it was first observed.
'This low-energy enhancement wasn't predicted by theory, so it was kind of a shock to the community,' said Eleanor Ronning, lead author of the study. The team, spanning 25 institutions in the United States, Canada, Italy, Germany, Norway and South Korea, studied two states of copper-70 — its ground state and an excited isomeric state — using FRIB's high-precision LEBIT mass spectrometer, creating two complementary pathways into zinc-70. Gamma rays were recorded with the SuN detector, and two analytical methods confirmed that magnetic transitions produce the enhancement.
The results matter far beyond nuclear physics. The enhancement can increase the rate of neutron-capture reactions — the processes that build heavy elements during supernovae and neutron star mergers — beyond what standard models predict. 'We now have a consistent explanation that connects experimental observations with theory,' said co-lead Andrea Richard.
