Zinc-70 gamma rays traced to magnetic transitions in nuclei

A team led by the Facility for Rare Isotope Beams has identified the cause of an unusual surplus of low-energy zinc-70 gamma rays, Michigan State University said. The finding, published in Nature, links the effect to magnetic transitions inside the atomic nucleus and gives nuclear physicists a clearer way to model reactions that help build heavy elements in space.

The collaboration involved researchers from 25 institutions in the United States, Canada, Italy, Germany, Norway and South Korea. The paper, titled “Magnetic Character of the Low-Energy Enhancement in 70Zn,” addresses a long-running question about why some nuclei emit more low-energy gamma radiation than theory had expected.

What causes the low-energy enhancement in zinc-70?

According to the FRIB-led team, the excess signal in zinc-70 comes from magnetic transitions as protons and neutrons rearrange inside the nucleus. Gamma rays are released when an excited atomic nucleus drops to a lower-energy state, and researchers describe the pattern of those emissions with a measure called the gamma-ray strength function.

Those nuclear transitions can be electric or magnetic. Michigan State said the new comparison of zinc-70 data gave researchers strong evidence that the low-energy enhancement, often shortened to LEE, has a magnetic origin in this nucleus.

Eleanor Ronning, the study’s lead author and a former FRIB graduate student now at the National Institute for Nuclear Physics in Padova, Italy, said the effect was not anticipated by theory when researchers first saw it. Andrea Richard of Ohio University, a co-lead author, said the result gives scientists a consistent link between measurements and theoretical explanations, according to Michigan State.

Why the result matters for element formation

The low-energy enhancement can raise the probability of neutron-capture reactions, Michigan State said. Those reactions are central to the formation of heavy elements in high-energy astrophysical settings, including supernovae and neutron star mergers.

Small changes in reaction rates can accumulate across many nuclei. Michigan State said that can alter calculations used in models of stellar nucleosynthesis, nuclear energy systems and national security applications involving the National Nuclear Security Administration.

The work also gives nuclear theorists a new benchmark. Knowing where LEE appears, and why, can help scientists choose which nuclei to study at FRIB and other facilities.

How the experiment was done

The team studied zinc-70 by observing the beta decay of two different states of copper-70, its parent nucleus. One copper-70 state was the ground state, while the other was an isomeric, or excited, state.

By separating those two states, researchers created two entry routes into zinc-70. Michigan State said the paths filled different sets of energy levels in the same nucleus, giving the team complementary views of its structure.

The separation used FRIB’s Low Energy Beam and Ion Trap, known as LEBIT, a high-precision mass spectrometer. Ryan Ringle, LEBIT group leader and associate professor of physics at FRIB, said the team used the instrument in this way for the first time and that the technique could be applied to other nuclei, according to Michigan State.

After the beta decays, the emitted gamma rays were measured with the Summing NaI detector, or SuN. The team then used the beta-Oslo method and the Shape method to extract gamma-ray strength functions from each starting state and compare them.

Michigan State said the comparison allowed the researchers to identify the magnetic character of the enhancement. Sean Liddick, a FRIB chemistry professor and Ronning’s graduate adviser, said the separated-isomer method can guide future experiments aimed at improving models of how elements form in astrophysical environments.

This story draws on original reporting from Phys.org.