US scientists resolve nuclear physics uncertainty in stellar barium production

Scientists at the US Facility for Rare Isotope Beams (FRIB) and Argentina's Tandem Accelerator System (ATLAS) have resolved a significant nuclear physics uncertainty in a reaction that helps explain how strontium forms in stars. The achievement involved 12 participating institutions from Canada and Europe and used an indirect approach, measuring the behavior of isotopes of another element, krypton, to infer strontium's formation. Most elements in the universe are produced by stellar activity. Since the 1950s, scientists have relied on three established processes to explain the formation of elements heavier than iron: the r-process (rapid neutron capture), the s-process (slow neutron capture), and the p-process, which produces heavier, proton-rich isotopes.

These isotopes cannot be produced through neutron capture. Each process involves the capture or removal of neutrons and adequately accounted for these elements until the 1990s. As scientists began observing the abundances of certain elements in old stars, these explanations began to fall short, with strontium being one of them. Explaining Strontium Abundance Strontium is an alkaline earth metal commonly used in fireworks and luminous paints. It is also used in archaeological analysis, where its isotopes help scientists determine the age and origin of specimens. For astrophysicists, understanding how strontium forms helps explain nucleosynthesis, the process by which stars produce elements.

The Formation of Strontium and Its Importance

One proposed mechanism for explaining strontium abundance is the i-process, which occurs under conditions between those of the s-process and r-process, with neutron densities and timescales of roughly several minutes. The r-process is a rapid neutron capture process, the s-process is a slow neutron capture process, and the i-process rapidly absorbs neutrons to produce heavier elements. Although i-process models can explain the abundances of many elements, they do not work well for strontium because its production is too low. Falk Herwig, a professor of physics and astronomy at the University of Victoria, said: "Our models identified the neutron capture on krypton-88 as a key unknown for the strontium shortfall."

Removing the Uncertainty Neutron capture reactions are difficult to measure directly because reproducing stellar conditions in the laboratory is challenging; these reactions occur infrequently and involve short-lived nuclei. The researchers overcame this challenge with an indirect approach, producing the heavier isotope krypton-89 and allowing it to decay to lower energy states. Using FRIB's Summing NaI (SuN) detector at ATLAS, they were able to measure the gamma-ray emission of krypton-89 and indirectly infer the neutron capture rate of krypton-88. The results show that the neutron capture rate of krypton-88 is significantly lower than theoretical predictions.

A Major Advance in Eliminating Nuclear Uncertainty

When they used this rate in i-process models, the amount of strontium produced came far closer to astronomical observations. "Now that we know this reaction rate, the next steps are entirely up to us modellers," Herwig added in a press release. "With the major nuclear uncertainty removed, we can turn to the astrophysics—studying neutron densities and burning timescales—and work to narrow the gap with what we observe in the oldest stars." The findings have been published in the journal Communications Physics.

Item	Specification
Primary Research Institutions	Facility for Rare Isotope Beams (FRIB), Argentina's Tandem Accelerator System (ATLAS)
Key Element	Strontium
Research Method	Indirect measurement of gamma-ray emission from krypton-89

The Implications of Strontium Research for Astrophysics

The formation of strontium is closely tied to stellar nucleosynthesis, and understanding its production mechanism is crucial for astrophysicists. This research not only resolves previous discrepancies in explaining strontium abundance but also points the way forward for future studies. By removing a key uncertainty, scientists can more accurately model nuclear reactions inside stars, further explore the effects of neutron density and burning timescales, and thereby deepen our overall understanding of element formation in the universe.