Scientists Directly Measure a Rare Cosmic Reaction for the First Time, Unlocking the Origin of the Universe's Rarest Elements

Scientists have for the first time directly measured a nuclear reaction thought to occur inside exploding stars, taking a key step toward resolving one of astrophysics' most enduring mysteries: how the universe produces the rarest, proton-rich elements that cannot be explained by any known neutron-based process. The experiment, conducted at the Facility for Rare Isotope Beams at Michigan State University, precisely captured the moment arsenic-73 captures a proton to form selenium-74 — a reaction central to the so-called gamma process believed to generate dozens of rare isotopes during supernova explosions.

The elements in question are called p-nuclei, a group of proton-rich isotopes ranging from selenium-74 to mercury-196 that make up less than 1% of the abundances of the elements they accompany in nature. Unlike most heavy elements, which are built up through successive neutron captures in stars, p-nuclei cannot be assembled by that mechanism — the proton-rich side of the nuclear chart is a very different territory, one that requires entirely separate astrophysical processes and laboratory measurements that have historically been nearly impossible to obtain. "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 Artemis Tsantiri, the study's lead researcher and now a postdoctoral fellow at the University of Regina.

The FRIB accelerator, which came online in 2022 and represents a $730 million investment by the U.S. Department of Energy, is specifically designed for experiments like this one. It produces rare isotope beams at intensities and purities unavailable anywhere else in the world, allowing researchers to study extremely short-lived nuclei that mimic the conditions inside dying massive stars. The team, which comprised more than 45 scientists from 20 institutions across the United States, Canada, and Europe, used the facility's capabilities to isolate an arsenic-73 beam and measure its reaction with protons with unprecedented precision.

The results reduced the uncertainty in predictions about how much selenium-74 is created during the gamma process by 50%, a dramatic improvement for a quantity that was previously known only within enormous error bars. The finding provides strong evidence that photodisintegration reactions during supernova explosions — specifically the gamma process, in which high-energy photons blast apart heavier nuclei — are indeed responsible for creating p-nuclei. However, even with the improved measurement, updated stellar models still do not fully reproduce the observed abundances of these elements in nature, suggesting that some additional astrophysical process or unknown nuclear physics remains undiscovered.

The experiment was led by Artemis Spyrou, professor at Michigan State University's FRIB laboratory, with radiochemistry work led by Katharina Domnanich. The study was published in the journal Physical Review Letters. For researchers working to understand the complete nuclear history of the elements, the result represents a rare instance where theory meets experiment on ground that was previously accessible only to calculation — and where the measurement itself changes the landscape of what models must explain.