Scientists just recreated a rare cosmic reaction never seen before
Scientists have reported a major experimental advance in understanding how some of the rarest elements in the universe are
Scientists have reported a major experimental advance in understanding how some of the rarest elements in the universe are formed. These unusual atoms, known as p-nuclei, are proton-rich isotopes heavier than iron that have long puzzled researchers.
The new study, led by Artemis Tsantiri, who conducted the work as a graduate student at the Facility for Rare Isotope Beams (FRIB) and is now a postdoctoral fellow at the University of Regina in Canada, achieved a milestone. For the first time, researchers directly measured how arsenic-73 captures a proton to form selenium-74 using a rare isotope beam. This result places new limits on how the lightest p-nucleus is created and destroyed in space.
The findings were published in Physical Review Letters (“Constraining the Synthesis of the Lightest 𝑝 Nucleus 74Se”) and involved more than 45 scientists from 20 institutions across the United States, Canada, and Europe.
Why Some Elements Remain a Mystery
A key goal in nuclear astrophysics is to understand where the elements come from. Many elements heavier than iron are formed through slow and rapid neutron-capture processes. In these reactions, atomic nuclei repeatedly absorb neutrons and then undergo radioactive decay until they reach stable forms.
However, this explanation does not apply to a special group of proton-rich isotopes. These p-nuclei cannot be produced through neutron capture. They span a range from selenium-74, the lightest, to mercury-196, the heaviest, and their origin has remained unclear for decades.
Supernova Explosions and the Gamma Process
One leading explanation for the creation of p-nuclei is the gamma process, which takes place in certain types of supernova explosions. In these extreme environments, intense heat produces gamma rays that strip neutrons and other particles from existing heavy nuclei.
After this process, the remaining nuclei contain more protons than neutrons. Over time, some of these nuclei convert protons into neutrons, moving toward a more stable balance and eventually forming p-nuclei.
Many of the isotopes involved in this process are short-lived and difficult to produce in the lab. Because of this, scientists have had to rely heavily on theoretical models rather than direct measurements.
“Even though the origin of the p-nuclei has been a topic of study for over 60 years, measurements of important reactions on short-lived isotopes are almost non-existent,” said Tsantiri. “Experiments of this kind are only now possible with facilities like FRIB.”
Recreating a Stellar Reaction in the Lab
In this study, researchers successfully recreated a key step in the process by observing proton capture on radioactive arsenic-73 for the first time. To do this, they generated a beam of arsenic-73 specifically for the experiment and directed it into a chamber filled with hydrogen gas. The hydrogen served as a source of protons and was positioned at the center of the Summing Nal (SuN) detector.
The team produced the arsenic-73 using FRIB’s ReA accelerator, which they operated in a standalone configuration rather than relying on the main linear accelerator. The radiochemistry group, led by Katharina Domnanich, prepared the material in a form suitable for use in the experiment. The isotope was then placed into a batch-mode ion source, where it was ionized, accelerated to high energies, and delivered to the target. This setup demonstrated the flexibility of ReA for producing and studying rare isotopes.
Tracking How Selenium-74 Is Formed and Destroyed
During the reaction, arsenic-73 absorbs a proton and becomes selenium-74 in an excited state. It then releases a gamma ray to reach a stable state. The researchers focused on the reverse reaction because it plays a key role in the gamma process inside stars. By measuring the forward reaction, they could determine how quickly the reverse process occurs.
To understand how much selenium-74 exists in the solar system, scientists must consider both its creation and its destruction. One of the biggest remaining uncertainties has been how often selenium-74 is broken apart by gamma rays during stellar explosions.
Improved Models but New Questions Remain
When the researchers incorporated their measurements into astrophysical models, they reduced the uncertainty in the predicted abundance of selenium-74 by half. This marks a significant improvement in understanding how this isotope is produced.
Even so, the updated models still do not fully match what is observed in nature. This gap suggests that scientists may need to refine their assumptions about the conditions inside supernova explosions.
“These results bring us a step closer to understanding the origins of some of the rarest isotopes in the universe,” said Artemis Spyrou, professor of physics at FRIB and in the Michigan State University Department of Physics and Astronomy, research advisor to Tsantiri, and original architect of the experiment. “Tsantiri’s work is a nice example of the multidisciplinary collaborations needed for advancing the field, and of the kind of professional development opportunities for early career researchers at FRIB.”
Collaboration and Support
This research was supported in part by the U.S. Department of Energy Office of Science Office of Nuclear Physics; the U.S. National Science Foundation; the U.S. National Nuclear Security Administration; and the Natural Sciences and Engineering Research Council of Canada.
The isotope(s) used in this research was supplied by the U.S. Department of Energy Isotope Program, managed by the Office of Isotope R&D and Production.
