Physicists at the University of Surrey measured nuclear reactions that can happen in neutron star collisions, giving direct experimental evidence for a process that had previously only been theorized. The study offers fresh insight into the formation of the universe’s heaviest elements and may even spur improvements in the physics of nuclear reactors. The study was published in Physical Review Letters.
The experimental setup at TRIUMF. Image Credit: University of Surrey
In collaboration with the Materials Science Institute of Seville (CSIC-Univ. Seville), the University of York, and TRIUMF, Canada's national particle accelerator center, the research has led to the first-ever measurement of a weak r-process reaction cross-section using a radioactive ion beam. The 94Sr (α,n) 97Zr reaction was the subject of the study. Here an alpha particle (a helium nucleus) is absorbed by strontium-94, a radioactive type of strontium, which then releases a neutron and changes into zirconium-97.
The weak r-process plays a crucial role in the formation of heavy elements, which astronomers have observed in ancient stars – celestial fossils that carry the chemical fingerprints of perhaps only one prior cataclysmic event, like a supernovae or neutron star merger. Until now, our understanding of how these elements form has relied on theoretical predictions, but this experiment provides the first real-world data to test those models that involve radioactive nuclei.
Dr. Matthew Williams, STFC Ernest Rutherford Fellow, University of Surrey
New helium targets were used to make the experiment possible. Since helium is a noble gas -that is, neither solid nor reactive - researchers at the University of Seville created a novel nano-material target by embedding helium in extremely thin silicon sheets, creating billions of tiny helium bubbles that are only a few tens of nanometers across.
The scientists measured the nuclear reaction at conditions akin to those encountered in intense cosmic settings by accelerating short-lived strontium-94 isotopes into these targets using TRIUMF's sophisticated radioactive ion beam technology.
This is a major achievement for astrophysics and nuclear physics, and the first-time nanomaterials have been used in this way, opening exciting new possibilities for nuclear research.
Dr. Matthew Williams, STFC Ernest Rutherford Fellow, University of Surrey
Dr. Matthew Williams added, “Beyond astrophysics, understanding how radioactive nuclei behave is crucial for improving nuclear reactor design. These types of nuclei are constantly produced in nuclear reactors, but until recently, studying their reactions has been extremely difficult. Reactor physics depends on this kind of data to predict how often components need replacing, how long they’ll last and how to design more efficient, modern systems.
Scientists will be able to better comprehend the origins of the heaviest known elements by applying the study to astrophysical models in the next phase of research. As scientists investigate these processes further, the study may contribute to a better understanding of the extreme physics of neutron star collisions as well as useful nuclear technology applications.
Journal Reference:
Williams, M., et al. (2025) First Measurement of a Weak r-Process Reaction on a Radioactive Nucleus. Physical Review Letters. doi.org/10.1103/PhysRevLett.134.112701