Lab measurement of krypton-88 solves decades-old strontium puzzle in ancient stars

For decades, astronomers studying the oldest stars in the universe have encountered a stubborn inconsistency. The chemical fingerprints they observe in these ancient objects don't match what nuclear physics models predict should be there. Strontium, in particular, appears in quantities that existing theories cannot fully account for. An international research team has now narrowed the gap by measuring, for the first time, a specific nuclear reaction that had eluded direct observation: how the isotope krypton-88 captures neutrons in the extreme conditions of stellar interiors.

The measurement, published in June in Communications Physics, represents a breakthrough in understanding element formation across cosmic time. Led by Caley Harris, a former graduate student at the Facility for Rare Isotope Beams at Michigan State University, the team reduced the uncertainty in krypton-88's neutron-capture rate from a factor of at least eight down to roughly three. More significantly, they discovered that the actual rate is consistently lower than theoretical models had predicted. When researchers fed this corrected measurement back into their simulations of how stars forge heavy elements—a process called the intermediate neutron-capture process, or i-process—the models suddenly produced much more strontium, bringing theory and observation into alignment.

The puzzle these scientists were solving traces back to the 1990s. For forty years before that, physicists had relied on three established nuclear processes to explain how elements heavier than iron form inside stars: the rapid neutron-capture process, the slow neutron-capture process, and a gamma-ray-driven process. This framework seemed complete until astronomers began analyzing very old stars and found elemental abundances that none of these three mechanisms could explain. Strontium was among the most glaring discrepancies. The i-process, occurring under conditions between the rapid and slow processes, was proposed as a solution, but even those models consistently underpredicted strontium production. Something was missing from the nuclear data.

Measuring neutron-capture reactions directly is extraordinarily difficult. The isotopes involved are often short-lived, the reactions happen rarely, and recreating stellar conditions in a laboratory is nearly impossible. To overcome these obstacles, the team employed an indirect approach. They produced krypton-89—krypton-88 with one additional neutron—at the Argonne Tandem Linac Accelerator System, a Department of Energy facility in Illinois. As the krypton-89 decayed to lower energy states, it emitted gamma rays. A specialized detector called the Summing NaI detector, brought from Michigan State's FRIB facility, captured these gamma-ray signatures. By analyzing the pattern of emissions, the researchers could reconstruct what had happened during the original neutron-capture event and extract the rate they were seeking.

The team, which drew researchers from twelve institutions across the United States, Canada, and Europe, found that krypton-88 absorbs neutrons at a lower rate than predictions suggested. When they incorporated this finding into various i-process models and ran new simulations, strontium production increased across the board, moving the theoretical predictions closer to what astronomers actually observe in ancient stars. Artemis Spyrou, a professor of physics at Michigan State and part of the research group, noted that explaining elemental abundances in the universe has proven slightly more intricate than previously understood. Falk Herwig, a co-author from the University of Victoria, emphasized that the measurement now clears the way for the next phase of investigation: examining the astrophysical factors that remain uncertain, such as neutron densities and the timescales of nuclear burning inside stars.

Strontium itself has practical importance on Earth—it appears in glow-in-the-dark paints, fireworks, and archaeological analysis, where scientists use its isotopic ratios to determine the origin, diet, or age of specimens. But in astrophysics, understanding strontium's formation is crucial for reading the chemical history written into the light of the universe's oldest objects. These ancient stars preserve a record of how the cosmos manufactured its elements in the first place. With the main nuclear uncertainty now resolved, researchers can focus on refining the astrophysical models themselves, working to close the remaining gap between what theory predicts and what the oldest stars reveal.