New krypton-88 data narrow a key gap in stellar strontium models

An international research team has reported the first experimental investigation of a nuclear physics reaction essential for understanding how the element strontium is produced in stars—specifically in stellar environments where traditional explanations for its formation fall short. The study, published June 8 in Communications Physics, reports that the team used indirect experimental techniques to extract previously inaccessible information about how an isotope of a separate element—krypton—absorbs, or captures, neutrons. Their measurements reduced the uncertainty of the neutron-capture rate of this isotope, krypton-88, from at least a factor of eight to about a factor of three. The team showed that the true rate of neutron capture by krypton-88 is consistently lower than theoretical predictions. When they incorporated these observations into leading models of how stars forge heavy elements—the intermediate neutron-capture process (i-process)—they discovered that the new rate increased the predicted amount of strontium, bringing simulations into better agreement with astronomical observations.

The project was led by Caley Harris, a former graduate student at the Facility for Rare Isotope Beams (FRIB), and included researchers from 12 institutions in the United States, Canada and Europe. "It turns out that explaining the abundances of elements in the universe is slightly more complicated than previously thought," said Artemis Spyrou, professor of physics at FRIB and in Michigan State University's Department of Physics and Astronomy. "Our models had flagged neutron capture on krypton-88 as the key unknown behind the strontium shortfall," said Falk Herwig, professor of physics and astronomy at the University of Victoria and a co-author of the study. "The measurement guides our next simulation and theory steps."

The team installed FRIB's Summing NaI (SuN) detector at the Argonne Tandem Linac Accelerator System (ATLAS), a U.S. Department of Energy (DOE) Office of Science user facility located at the DOE's Argonne National Laboratory, producing krypton-89 (krypton-88 plus one neutron) and measuring its gamma-ray emissions to infer the krypton-88 neutron-capture rate. "The combination of a state-of-the-art instrument such as the SuN detector and the unique high-purity beams provided by ATLAS leads to powerful new insights into important nucleosynthesis processes," said co-author Guy Savard, ATLAS scientific director and Argonne Distinguished Fellow.

The impact of the new measurement and what comes next: The team found that their newly determined krypton-88 neutron-capture rate is consistently lower than theoretical predictions. They explored the impact of the new rate on the production of strontium in old stars using various i-process models. Researchers found that in all models tested with the new rate, strontium was produced in higher amounts, in better agreement with astronomical observations. With the main nuclear uncertainty now addressed, the authors recommend further investigation of i-process models, including the roles of neutron densities and the time evolution of nuclear burning inside stars. "Now that we know this reaction rate, the next step is again on us as modelers," Herwig said. "With the main nuclear uncertainty removed, we can turn to the astrophysics, the neutron densities and the timing of the burning, and work to close the remaining gap with what we see in the oldest stars." "The findings highlight how measurements of rare isotopes in the laboratory can help solve mysteries revealed by observations of very old stars, strengthening the connection between nuclear physics and astronomy," Spyrou said.

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