In a groundbreaking first for astrophysics and nuclear science, an international team of researchers has directly measured a nuclear reaction that plays a key role in the creation of the universe’s heaviest elements. This elusive process, which occurs during violent neutron star collisions, had only ever been predicted through theoretical models—until now. Led by physicists at the University of Surrey, the experiment offers unprecedented insight into how elements like gold, platinum, and uranium are forged in cosmic crucibles, while also holding promise for advancements in nuclear reactor technology here on Earth.
Their findings, published in the prestigious journal Physical Review Letters, mark the first experimental measurement of a weak r-process reaction cross-section using a radioactive ion beam. Specifically, they observed the reaction 94Sr(α,n)97Zr, in which a radioactive isotope of strontium (strontium-94) captures an alpha particle (two protons and two neutrons—the nucleus of a helium atom), emits a neutron, and transforms into zirconium-97. Though deceptively simple in description, the reaction itself unlocks deep mysteries about element formation in the most violent events in the universe.
Cracking the Cosmic Code: What is the r-process?
To grasp the significance of this experiment, it helps to understand the “r-process,” or rapid neutron-capture process. This sequence of reactions occurs when atomic nuclei rapidly absorb neutrons in environments rich with them—like supernovae or the merging of neutron stars. There are two primary flavors of the r-process: the strong and the weak.
The strong r-process builds the heaviest elements known to science, such as gold and uranium, and has been well-documented as occurring in neutron star mergers. The weak r-process, however, fills in the gaps by creating many of the elements just below iron on the periodic table, contributing to the vast elemental diversity observed in the universe. Until now, though, our understanding of this weaker but no less vital process was largely hypothetical.
The First Direct Measurement: A Breakthrough in Nuclear Astrophysics
“This is a major achievement for astrophysics and nuclear physics,” said Dr. Matthew Williams, lead author of the study and physicist at the University of Surrey. “Until now, our understanding of how these elements form has relied entirely on theoretical predictions. Our experiment provides the first real-world data to test those models that involve radioactive nuclei.”
The team’s success hinged on a remarkable feat of experimental physics. Working with collaborators from the University of York, the University of Seville, and Canada’s national particle accelerator center, TRIUMF, the researchers had to recreate conditions similar to those found in neutron star mergers—one of the most extreme environments in the cosmos.
At the heart of the experiment was a challenge: how to introduce helium, a gas, into the reaction environment as an effective target. Traditional methods were impossible, given helium’s chemical inertness and gaseous state. But scientists at the University of Seville found an elegant solution. By embedding helium gas inside ultra-thin silicon films, they created nano-material targets, containing billions of microscopic helium bubbles—each just a few dozen nanometers wide.
“These nano-targets were critical,” said Dr. Williams. “They allowed us to fire a beam of highly radioactive strontium-94 ions, accelerated by TRIUMF’s world-leading facilities, into the helium environment and measure the reaction products directly.”
From Cosmic Collisions to Nuclear Reactors
While the astrophysical implications of this research are profound, the results have the potential to reverberate in the field of nuclear energy technology.
Radioactive nuclei, similar to those involved in the team’s experiments, are constantly produced inside nuclear reactors. Yet the lack of direct experimental data on their behavior has long stymied efforts to improve reactor designs.
“Reactor physics depends on accurate nuclear reaction data to predict how long components will last, how often they need replacing, and how to design better, more efficient systems,” explained Dr. Williams. “This study represents a new frontier, where advances in astrophysics and nanotechnology are poised to revolutionize reactor science.”
By studying the reactions of radioactive isotopes under controlled laboratory conditions, researchers hope to develop safer, longer-lasting nuclear reactors. Better understanding these nuclear reactions may lead to innovations in waste management, reducing the environmental impact of nuclear power, and potentially enabling the development of advanced reactor types that are more efficient and sustainable.
What Happens Next? Mapping the Origins of the Heaviest Elements
With the first direct measurements of a weak r-process reaction cross-section under their belt, the researchers are already looking toward the next phase. Their findings will be applied to refine astrophysical models of heavy element production, helping explain why ancient stars—some of the oldest objects in the universe—contain the distinct fingerprints of these cosmic forges.
Many of these so-called “stellar fossils” have elemental compositions that suggest they were enriched by just one or two ancient neutron star mergers or supernova explosions. “Understanding how these events seeded the universe with heavy elements gives us a window into the birth of the cosmos,” said Dr. Williams.
The Big Picture: How Neutron Star Collisions Shape the Universe
Neutron star mergers are among the most powerful events in the universe. When two ultra-dense remnants of massive stars collide, they unleash gravitational waves, blasts of energy, and clouds of neutron-rich matter. This matter undergoes nuclear reactions that synthesize heavy elements in seconds—processes scientists have long sought to unravel.
Until the first detection of gravitational waves from a neutron star merger in 2017 (the famous GW170817 event), many aspects of these violent encounters were purely speculative. That historic observation not only confirmed the existence of gravitational waves but also provided the first definitive proof that neutron star mergers are responsible for producing a significant portion of the universe’s gold and other heavy elements.
But what about the less massive, but no less critical, elements forged in the weak r-process? This new experiment at TRIUMF represents a crucial piece of that puzzle.
A New Era in Nuclear Science and Astrophysics
“This work is just the beginning,” emphasized Dr. Williams. “By combining advanced particle accelerators, cutting-edge nanotechnology, and international collaboration, we’re opening up new ways to explore the most extreme environments in nature. Whether we’re asking how the universe makes its heaviest elements or designing better nuclear reactors, the answers may come from the same fundamental science.”
As researchers continue their investigations into nuclear reactions under extreme conditions, both cosmic and terrestrial, the boundaries between astrophysics and nuclear engineering grow ever blurrier. What unites them is a quest to understand and harness the fundamental forces that shape our universe—from the heart of a neutron star collision to the core of tomorrow’s nuclear power plants.
In the end, this research reminds us that the story of the universe isn’t just written in the stars; it’s forged in fire, framed by physics, and unlocked by human curiosity.
Reference: M. Williams et al, First Measurement of a Weak r -Process Reaction on a Radioactive Nucleus, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.112701