This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
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.
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: 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.
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