Every atom of gold in a wedding band, every trace of platinum in electronics, and even the heavy metals deep inside the Earth were forged in the cataclysmic fires of dying stars. These cosmic explosions release enormous amounts of energy, including gamma rays. Gamma rays play a crucial role in the formation of heavy elements. For decades, nuclear astrophysicists understood that cosmic explosions create these elements. However, a nagging mathematical discrepancy made it impossible to accurately model the process.
Now, an international collaboration of researchers led by Michigan State University’s Facility for Rare Isotope Beams (FRIB) has solved a decades-old nuclear physics mystery. Their findings were published in Nature. Importantly, the breakthrough pinpointed the subatomic origin of a mysterious gamma-ray surge. This surge alters how stars forge heavy elements across the cosmos.
The Mystery: The “Low-Energy Enhancement” Puzzle
When massive stars collapse into supernovae or collide as neutron stars, they trigger rapid neutron-capture reactions (the r-process). These extreme environments blast lighter atomic nuclei with free neutrons, building up heavy elements up to uranium.
However, when physicists measured gamma-ray emissions from unstable isotopes involved in these reactions, they noticed an unpredicted surge of low-energy radiation. This phenomenon was dubbed Low-Energy Enhancement (LEE).
“This low-energy enhancement wasn’t predicted by theory, so it was kind of a shock to the community when it was first observed,”
-Eleanor Ronning, lead author of the study and research fellow at the National Institute for Nuclear Physics in Padova, Italy.
Because LEE boosts nuclear reaction rates when compounded across hundreds of short-lived isotopes, scientists couldn’t build accurate computer models of cosmic element creation.
The Breakthrough Experiment at FRIB
Isolating the cause of LEE required subatomic precision. The research team focused on zinc-70, a key nucleus known to emit this puzzling gamma-ray signal.
Using FRIB’s Low Energy Beam and Ion Trap (LEBIT) mass spectrometer, the team engineered a novel experimental technique:
- Purifying Isomers: They produced ultra-pure beams of the parent nucleus, copper-70. They isolated two distinct quantum states—its ground state and an excited isomeric state.
- Dual Entry Pathways: By observing how each copper-70 state decayed into zinc-70, researchers gained two independent, high-resolution views of zinc-70’s internal energy levels.
- Detection & Analysis: Using the specialized Summing NaI (SuN) detector, the team captured the precise energy spectra of the emitted gamma rays.
What the Findings Mean for Astrophysics
The dual-pathway analysis conclusively proved that the low-energy gamma-ray surge is produced by magnetic transitions inside the atomic nucleus. Protons and neutrons shifting their magnetic orientations release tiny, high-frequency bursts of energy that boost reaction rates.