Researchers from the Facility for Rare Isotope Beams and 24 other institutions across six countries have resolved a long-standing mystery in nuclear physics. The collaboration, involving scientists from national laboratories and universities, identified the origin of an unexpected abundance of low-energy gamma rays released by the zinc-70 nucleus. Their findings appear in Nature in a study titled “Magnetic Character of the Low-Energy Enhancement in 70Zn.”
The research addressed a phenomenon called the low-energy enhancement, or LEE, which has been observed in the gamma-ray strength function of certain nuclei for decades without explanation. When an excited atomic nucleus transitions to a lower energy state, it releases energy as gamma rays. Transitions between nuclear energy states can be classified as either electric or magnetic, each reflecting different ways that protons and neutrons reorganize within the nucleus. Scientists had detected an unexplained rise in low-energy gamma emissions but could not determine its cause.
The team used an innovative experimental approach, examining the beta decay of two separate states of copper-70 to observe zinc-70 from complementary perspectives. Using FRIB’s Low Energy Beam and Ion Trap spectrometer, researchers created exceptionally pure beams of both the ground and excited states of copper-70, providing two distinct pathways into zinc-70. The gamma rays emitted were recorded with the Summing NaI detector, and two analytical methods were applied to determine the gamma-ray strength function for each pathway.
Analysis of the data provided conclusive evidence that magnetic transitions inside the nucleus are responsible for the low-energy enhancement. Beyond the theoretical significance of explaining nuclear structure, this discovery has important implications for astrophysics. The enhancement can increase the frequency of neutron-capture reactions that play a central role in producing heavy elements during extreme cosmic events such as supernovae and neutron star mergers. Changes in reaction rates resulting from LEE could substantially affect models of nuclear activity in stars and other astrophysical systems.
The research demonstrates collaborative benefits and provides training opportunities for early-career scientists and students. The project involved staff scientists from Lawrence Livermore National Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory, and Pacific Northwest National Laboratory, representing a partnership between fundamental research and national security applications. The successful methodology opens pathways for investigating the low-energy enhancement phenomenon in additional nuclei and improving models of element creation in astrophysical environments.
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