Unlocking the cosmic recipe for strontium

Key takeaways:

• An international team has reported the first experimental investigation of a nuclear physics reaction essential to explaining how the element strontium is formed in stars.
• Their findings center on reducing uncertainty surrounding the rate at which an isotope of krypton — known as krypton-88 — captures neutrons.
• The research resolves a key discrepancy between observations of stars and leading models explaining how elements are formed in stars, a process known as nucleosynthesis.

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 the 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 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 insight into important nucleosynthesis processes.” — Guy Savard, ATLAS scientific director and Argonne Distinguished Fellow

The team behind the discovery

The project was led by Caley Harris, 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 insight into important nucleosynthesis processes,” said co-author Guy Savard, ATLAS scientific director and Argonne Distinguished Fellow.

Why strontium matters on Earth and in astrophysics

Strontium is an alkaline earth metal widely used in glow-in-the-dark paint, fireworks and archaeological analysis. Scientists use strontium isotopes to determine a specimen’s place of origin, diet or age.

In astrophysics, understanding how strontium forms is crucial for interpreting the chemical signatures of very old stars, which preserve information about early nucleosynthesis — the formation of elements by stars — in the universe.

Why strontium’s formation has been a puzzle

Nearly all chemical elements in the universe were formed by stellar activity. Since the 1950s, scientists have relied on three established processes to explain the formation of elements heavier than iron:

• r-process (rapid neutron-capture process).
• s-process (slow neutron-capture process).
• p-process (gamma‑ray-driven removal of neutrons, protons or a specific combination of protons and neutrons called alpha particles).

This framework appeared largely complete until the 1990s, when scientists observed elemental abundances in very old stars that were not consistent with any of these processes. Strontium was among the elements whose abundance could not be explained.

One proposed solution to explain this observation is the i-process, which occurs in conditions between those of the s- and the r-processes, with neutron densities and timescales on the order of minutes. During the i-process, atomic nuclei rapidly absorb neutrons and create heavier elements.

While i-process models reproduce many observed elemental abundances, they consistently produce too little strontium, pointing to missing or uncertain nuclear data — particularly the neutron‑capture rate of krypton‑88.

How the experiment worked

Neutron-capture reactions are difficult to measure directly because the nuclei involved are often short-lived, the reactions occur infrequently, and the stellar conditions are hard to recreate in the laboratory.

To overcome these challenges, the team used indirect methods:

• They produced krypton‑89 (krypton‑88 plus one additional neutron).
• As krypton‑89 decayed to lower‑energy states, it emitted gamma rays.
• The SuN detector, installed at the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at ATLAS, captured these gamma rays, allowing researchers to reconstruct the reaction pathway and infer the krypton‑88 neutron‑capture rate.

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 what the theory predicted. 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.

This work is supported by the U.S. National Science Foundation; the DOE Office of Science; the DOE/National Nuclear Security Administration’s Nuclear Science and Security Consortium and the Stewardship Science Academic Alliances; the Research Council of Norway; the Norwegian Nuclear Research Center; the U.S. Nuclear Data program; the Laboratory Directed Research and Development Program at the DOE’s Pacific Northwest National Laboratory; the Natural Sciences and Engineering Research Council of Canada; and the Canada Foundation for Innovation.