Scientists have solved a decades-old mystery about the way some atomic nuclei release energy, a discovery that could improve our understanding of how stars create many of the heavy elements found throughout the universe.
The international research team, led by the Facility for Rare Isotope Beams (FRIB) at Michigan State University, found that an unusual burst of low-energy gamma rays from the atomic nucleus of zinc-70 is caused by magnetic changes inside the nucleus.
Their findings were published in the journal Nature.
The study involved researchers from 25 institutions across the United States, Canada, Italy, Germany, Norway, and South Korea.
Gamma rays are a very high-energy form of light. Unlike the light people can see with their eyes, gamma rays are produced by atomic nuclei.
When a nucleus has extra energy, it releases that energy by emitting gamma rays as it settles into a more stable state.
Scientists have long studied how often nuclei emit gamma rays at different energy levels because these emissions reveal what is happening inside atoms. However, for many years researchers noticed something unexpected. Certain nuclei produced far more low-energy gamma rays than existing theories predicted.
This unusual effect, known as low-energy enhancement (LEE), has puzzled nuclear physicists for decades. Although scientists observed it in several different nuclei, they could not explain why it happened or predict which nuclei would show the effect.
To investigate the mystery, the research team focused on zinc-70, an atomic nucleus that had shown signs of this unusual behavior. They studied how zinc-70 formed after the radioactive decay of copper-70, allowing them to examine the nucleus from two different starting conditions.
Producing these special forms of copper-70 required highly advanced equipment at FRIB. The researchers used the facility’s Low Energy Beam and Ion Trap (LEBIT) to create exceptionally pure samples of the two different nuclear states. This was the first time the instrument had been used in this way.
After creating zinc-70, the scientists measured the gamma rays it emitted using a highly sensitive detector called SuN. They then applied two advanced analysis methods to carefully examine the data and separate the different types of gamma-ray emissions.
The results provided a clear answer to the long-standing mystery. The extra low-energy gamma rays were produced by magnetic transitions inside the nucleus rather than electric ones. In other words, the protons and neutrons inside the nucleus were rearranging themselves through magnetic interactions, creating the unexpected increase in low-energy gamma rays.
This discovery is important because it helps scientists better understand how atomic nuclei behave. It also gives researchers a much stronger foundation for improving theories that describe nuclear structure.
The findings have significance far beyond nuclear physics. Low-energy enhancement affects how easily atomic nuclei capture neutrons, an important process that occurs inside exploding stars called supernovae and during collisions between neutron stars.
These violent cosmic events are responsible for creating many of the heavy elements found throughout the universe, including elements that eventually become part of planets and even living organisms.
If low-energy enhancement increases neutron capture more often than expected, it can change scientists’ calculations of how quickly heavy elements form in stars. Better understanding this effect will help researchers build more accurate models of how the universe produces elements over billions of years.
The discovery may also benefit other fields, including nuclear energy research and national security, where accurate nuclear reaction models are essential.
The researchers say this achievement was only possible because of newly developed experimental techniques and improved data analysis methods that did not exist when they first began investigating the mystery more than a decade ago.
The team now plans to use the same approach to study many more atomic nuclei. By identifying which nuclei display low-energy enhancement, scientists hope to further improve nuclear theories and gain a clearer picture of how stars forge the elements that make up the world around us.
