Scientists have reported major experimental advances in understanding how some of the rarest elements in the universe are formed. These unusual atoms, known as p-nuclei, are isotopes rich in protons, which are heavier than iron, and have long puzzled researchers.
The new research was led by Artemis Tsantilli, who conducted the research as a graduate student at the Rare Isotope Beam Facility (FRIB) and is now a postdoctoral fellow at the University of Regina in Canada, and achieved the breakthrough results. Researchers used a rare isotope beam to directly measure for the first time how arsenic-73 captures protons to form selenium-74. This result imposes new constraints on how the lightest p-nuclei in the universe are created and destroyed.
The survey results are physical review letter (“Suppression of the synthesis of the lightest 𝑝nuclear 74Se”) involved more than 45 scientists from 20 institutions in the United States, Canada, and Europe.
Why some elements remain a mystery
An important goal of nuclear astrophysics is to understand where elements come from. Many elements heavier than iron are formed through slow and rapid neutron capture processes. In these reactions, the nucleus repeatedly absorbs neutrons and undergoes radioactive decay until a stable form is reached.
However, this explanation does not apply to a special group of proton-rich isotopes. These beta nuclei cannot be produced by neutron capture. They range from the lightest, selenium-74, to the heaviest, mercury-196, but their origins have remained unknown for decades.
Supernova explosion and gamma process
One of the main explanations for the formation of p-nuclei is the gamma process that occurs in certain types of supernova explosions. In these extreme environments, intense heat produces gamma rays that strip neutrons and other particles from existing heavy atomic nuclei.
After this process, the remaining nucleus contains more protons than neutrons. Over time, some of these nuclei convert protons to neutrons and approach a more stable balance, eventually forming p-nuclei.
Many of the isotopes involved in this process are short-lived and difficult to produce in the laboratory. For this reason, scientists have had to rely heavily on theoretical models rather than direct measurements.
“Despite the origin of the beta nucleus being a topic of research for more than 60 years, very few measurements of important reactions involving short-lived isotopes exist,” Tsantilli said. “This type of experiment is currently only possible at facilities like FRIB.”
Reproducing stellar reactions in the laboratory
In this study, researchers were able to reproduce a key step in the process by observing for the first time the capture of protons on radioactive arsenic-73. To do this, they generated a beam of arsenic-73 specifically for the experiment and directed it into a chamber filled with hydrogen gas. Hydrogen served as the proton source and was placed at the center of the Summing Nal (SuN) detector.
The team produced arsenic-73 using FRIB’s ReA accelerator. This accelerator 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 experiments. The isotopes are then placed in a batch-mode ion source where they are ionized, accelerated to high energies, and delivered to the target. This setup demonstrated the flexibility of ReA for the production and study of rare isotopes.
Track how selenium-74 is formed and destroyed
During the reaction, arsenic-73 absorbs a proton and becomes selenium-74 in the excited state. It then emits gamma rays and reaches a steady state. The researchers focused on back reactions because they play an important role in the gamma process inside stars. By measuring the forward reaction, it may be possible to determine how quickly the reverse process occurs.
To understand how much selenium-74 is present in the solar system, scientists need to consider both selenium-74 production and destruction. One of the biggest remaining uncertainties is how often selenium-74 is broken down by gamma rays during star explosions.
The model has improved, but new questions remain
When the researchers incorporated their measurements into an astrophysical model, the uncertainty in the predicted abundance of selenium-74 was cut in half. This represents a significant advance in understanding how this isotope is produced.
Still, the updated model still doesn’t perfectly match what’s observed in nature. This gap suggests that scientists may need to refine their assumptions about the conditions inside supernova explosions.
“These results bring us one step closer to understanding the origins of some of the rarest isotopes in the universe,” said Artemis Spirou, professor of physics at FRIB and Michigan State University’s Department of Physics and Astronomy, Tsantilli’s research advisor and the original architect of the experiment. “Mr. Tsantiri’s research is a great example of the multidisciplinary collaboration needed to advance the field and the 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. National Science Foundation. U.S. National Nuclear Security Administration. and the Natural Sciences and Engineering Research Council of Canada.
The isotopes used in this study were supplied by the U.S. Department of Energy Isotope Program managed by the Isotope Research, Development and Production Office.
