A new experiment using rare-isotope beams has provided new insight into the origin of proton-rich isotopes known as p-nuclei.
Researchers have taken an important step toward solving one of astrophysics’ oldest isotope mysteries: where the rare proton-rich atomic species known as p-nuclei come from.
The study was 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.
The team carried out the first rare isotope beam measurement of proton capture on arsenic-73, a reaction that produces selenium-74. Their findings place new limits on how the lightest p-nucleus forms and is destroyed in space. The results were published in Physical Review Letters. More than 45 researchers from 20 institutions across the United States, Canada, and Europe contributed to the project.
One of the central questions in nuclear astrophysics is how the elements in the universe are created. Many elements heavier than iron form through two well-known processes. In both cases, atomic nuclei repeatedly capture neutrons and then undergo radioactive decay until they reach stable isotopes.
However, these neutron-capture processes cannot explain a group of proton-rich isotopes known as p-nuclei. These rare isotopes range from selenium-74, the lightest, to mercury-196, the heaviest.
Supernova Gamma Process
Scientists have proposed several possible environments where p-nuclei might form. The leading explanation is the gamma process, which occurs during certain types of supernova explosions.
During these events, extremely high temperatures generate intense gamma radiation that strips neutrons and other particles from existing heavy nuclei. Afterward, the remaining nuclei contain more protons than neutrons. Over time, nuclear transformations convert some protons into neutrons, shifting the nucleus toward a more stable neutron-to-proton balance and ultimately producing a p-nucleus.
Many of the nuclei involved in the gamma process are rare isotopes. Because they are difficult to create in the laboratory, many of their properties remain poorly measured. As a result, models of the gamma process often rely heavily on theoretical estimates.
“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.”
Measuring a Key Reaction
In the new experiment, researchers directly studied how the radioactive isotope arsenic-73 captures a proton to form selenium-74. This measurement was achieved in the laboratory for the first time.
Scientists produced a beam of arsenic-73 specifically for the experiment and directed it into a small chamber filled with hydrogen gas. The hydrogen served as the proton target and was positioned at the center of the Summing NaI (SuN) detector.
To perform the measurement, the team obtained radioactive arsenic-73 and operated FRIB’s ReA accelerator in standalone mode rather than receiving beams from the main FRIB linear accelerator. The radiochemistry group, led by Katharina Domnanich, assistant professor of chemistry at FRIB and in MSU’s Department of Chemistry, prepared the isotope in a suitable chemical form.
The material was placed into FRIB’s batch-mode ion source. There, arsenic-73 ions were extracted, accelerated to high energies, and delivered to the experiment. The work demonstrated that the ReA accelerator can generate arsenic-73 beams in offline mode, providing researchers with greater flexibility for future studies.
Insights Into Selenium-74
In the reaction observed in the experiment, arsenic-73 absorbs a proton and forms selenium-74 in an excited state. The nucleus then emits a gamma ray as it settles into a more stable configuration.
The researchers focused on the inverse reaction, which occurs during the gamma process in stars. Its rate can be determined by measuring the direct proton-capture reaction in the laboratory. To explain the observed abundance of any isotope, scientists must consider both the processes that produce it and those that destroy it.
For selenium-74, the largest remaining nuclear physics uncertainty affecting its estimated abundance in the solar system involves its destruction by gamma rays during the gamma process.
When the researchers incorporated their experimental data into an astrophysical model of the gamma process, the uncertainty in the predicted relative abundance of selenium-74 decreased by a factor of two. Despite this improvement, the model still does not fully reproduce the observed abundance of selenium-74. The result suggests that existing models of the astrophysical conditions driving the gamma process may need revision.
“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.”
Reference: “Constraining the Synthesis of the Lightest p Nucleus Se74” by A. Tsantiri, A. Spyrou, E. C. Good, K. Bosmpotinis, P. Giuliani, H. Arora, G. Balk, L. Balliet, H. C. Berg, J. M. Berkman, C. Dembski, P. DeYoung, Pavel A. Denissenkov, N. Dimitrakopoulos, A. Doetsch, T. Gaballah, R. Garg, A. Henriques, R. Jain, S. N. Liddick, S. Lyons, R. S. Lubna, B. Monteagudo Godoy, F. Montes, S. Nash, G. U. Ogudoro, J. Owens-Fryar, A. Palmisano-Kyle, J. Pereira, A. Psaltis, A. L. Richard, L. Roberti, E. K. Ronning, H. Schatz, A. Sebastian, M. Smith, M. K. Smith, C. S. Sumithrarachchi, C. Tinson, P. Tsintari, N. Tubaro, S. Uthayakumaar, A. C. C. Villari, E. Weissling and R. G. T. Zegers, 20 November 2025, Physical Review Letters.
The work was supported in part by funding from 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.
