Zinc-70’s Hidden Magnetic Glow Explained

For decades, nuclear physicists have been puzzled by an unexplained surge of low-energy gamma rays emitted by certain atomic nuclei—a phenomenon known as the low-energy enhancement, or LEE. Now an international collaboration led by scientists at Michigan State University’s Facility for Rare Isotope Beams (FRIB) has traced the source of this emission in zinc‑70 to magnetic transitions occurring deep inside the nucleus.

The team isolated two different forms of the parent nucleus copper‑70—its ground state and an excited isomeric state—using FRIB’s Low Energy Beam and Ion Trap (LEBIT). Each state provided a distinct pathway into zinc‑70, allowing researchers to observe the nucleus from two complementary angles. The gamma rays released were recorded with the Summing NaI detector, and two independent analytical methods—the beta‑Oslo and Shape techniques—confirmed that magnetic, rather than electric, transitions drive the LEE.

The findings, published in Nature, draw on resources from 25 institutions across the U.S., Canada, Italy, Germany, Norway and South Korea, including three U.S. national laboratories. The result not only resolves a long‑standing question in nuclear structure but also promises to sharpen models of how heavy elements are forged in extreme cosmic events like supernovae and neutron star mergers.

What This Means for Nuclear Astrophysics

Magnetic Transitions Finally Explain the LEE

Until now, the low‑energy enhancement was an experimental fact without a clear theoretical cause. It had been detected in several nuclei since the early 2000s but was largely unpredictable—scientists could not say which nuclei would exhibit it or why. The new measurements show conclusively that, at least in zinc‑70, the enhancement stems from magnetic transitions among excited states. This hands nuclear theorists a concrete benchmark for testing models of nuclear structure and for predicting LEE in other isotopes.

Why It Matters for Heavy Element Synthesis

LEE is not merely a curiosity of the nucleus. It can significantly boost neutron‑capture reaction rates beyond standard predictions. Because neutron‑capture processes are the engine that builds heavy elements in stellar interiors, supernova explosions, and neutron‑star collisions, an accurate description of the enhancement across many nuclei is essential for realistic astrophysical models. The study underscores that magnetic contributions—often overlooked in early reaction‑rate calculations—must now be incorporated if we are to understand where the universe’s silver, gold, and uranium actually come from.

A Novel Experimental Technique Unlocks Future Work

The experiment’s success hinged on a first‑of‑its‑kind use of LEBIT to produce exceptionally pure beams of copper‑70 in two distinct nuclear states. This technique, called isomer separation, let the team disentangle competing decay paths within zinc‑70. The method is generalizable: the collaboration explicitly states it opens the door to studying LEE in many more nuclei. As FRIB scales up its beam capabilities, this approach could become a standard tool for mapping the gamma‑ray strength function across the nuclear chart—a development that would directly benefit both fundamental science and the nuclear‑data needs of national security applications.