US scientists help solve stellar mystery of how strontium is formed in the stars

Scientists at the Facility for Rare Isotope Beams (FRIB) and Argonne Tandem Linac Accelerator System (ATLAS) in the US have helped resolve a key uncertainty in the nuclear physics reaction that helps understand how strontium is formed in the stars. The achievement involved 12 other participating institutions across Canada and Europe and used an indirect approach: measuring how the isotope of a separate element, krypton, behaves to determine strontium. Most of the elements that are found in the universe were formed as a result of stellar activity. Since the 1950s, scientists have relied on three established processes to explain the formation of elements heavier than iron. These are called the r-process (rapid neutron-capture process), s-process (slow neutron-capture process), and p-process, which forms more proton-rich heavy isotopes that cannot be made by the neutron-capture process. Each process involves the capture or removal of neutrons and worked well to explain these elements until the 1990s. This changed when scientists began observing the abundance of certain elements in old stars, which did not match these processes. One among them was strontium. Explaining strontium abundance Strontium is an alkaline earth metal used in fireworks and glow-in-the-dark paints. It can also be used for archaeological analysis, where isotopes help scientists determine a specimen’s age and place of origin. For astrophysicists, understanding the formation of strontium can help explain nucleosynthesis, or the formation of elements by stars. A proposed solution for explaining the abundance of strontium is the i-process. This occurs under conditions between the s-process and the r-process, where the neutron densities and timescales are on the order of minutes. While the r-process is the rapid neutron-capture process and the s-process is the slow neutron-capture process, the i-process rapidly absorbs neutrons and creates heavier elements. Although the i-process model can explain multiple elemental abundances, it does not hold true for strontium since it produces too little of this element. “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, who was involved in the work. Clearing the uncertainty Neutron capture reactions are difficult to measure directly because recreating stellar conditions in the laboratory is hard; the reactions are infrequent, and the nuclei involved are short-lived. The researchers took an indirect approach to overcome this and produced krypton-89, a higher isotope, and allowed it to decay to a lower energy state. Using FRIB’s Summing Nal (SuN) detector at ATLAS, the researchers were able to measure krypton-89’s gamma-ray emissions and indirectly infer the krypton-88 neutron-capture rate. The researchers found that the newly determined krypton-88 neutron-capture rate is consistently lower than theory predictions. When they used this rate in i-process models, the results showed higher amounts of strontium generated, much closer to astronomical observations. “Now that we know this reaction rate, the next step is again on us as modelers,” added Herwig in the press release. ​”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 research findings were published in the journal Communications Physics. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.