Michigan State University researchers, led by graduate student Caley M. Harris, solved a decades-old stellar chemistry mystery by determining the neutron-capture rate of radioactive Krypton-88. Published in Nature Communications Physics, the breakthrough explains unexpected strontium abundance ratios in ancient Carbon-Enhanced Metal-Poor stars using exotic isotope experiments at Argonne National Laboratory.
Decoding the Cosmic Recipe for Strontium
Everything from the gold in your smartphone to the calcium in your bones originates inside stars. Yet, astrophysicists spent decades wrestling with an anomaly in how heavy elements form. Traditional models pointed to two distinct pathways: the slow “s-process” in dying stars and the rapid “r-process” in catastrophic explosions like supernovae. When scientists examined Carbon-Enhanced Metal-Poor (CEMP) stars—ancient stellar bodies born shortly after the universe formed—the elemental ratios refused to match either framework. This discrepancy forced researchers to model a third pathway: the intermediate neutron-capture process, or “i-process.”
Even the i-process struggled to account for real-world observations. Strontium, the element responsible for vibrant red fireworks and glow-in-the-dark paint, appeared far more frequently in actual stellar observations than computer simulations predicted. According to the research team, the missing link was a lack of hard data regarding the neutron-capture rate of Krypton-88.
The Radioactive Bottleneck of Krypton-88
Krypton-88 is a highly unstable, radioactive isotope with a half-life of just 2.8 hours. During its brief existence, it faces a fork in the nuclear road. It can absorb a neutron to become Krypton-89, eventually decaying into heavier elements like Yttrium-89. Alternatively, it can undergo beta decay into Rubidium-88, which quickly decays into – you guessed it – Strontium-88.
Because scientists previously had no empirical way to measure how effectively Krypton-88 absorbs neutrons, astrophysical models relied entirely on guesswork. Measuring the neutron absorption rate of an isotope that exists for only a few hours presented a formidable experimental barrier. Direct bombardment of a purified Krypton-88 sample with neutrons proved logistically impractical.
Reverse-Engineering Nuclear Physics at Argonne National Laboratory
To circumvent the short half-life problem, the research team adopted an indirect approach at the Californium Rare Isotope Breeder Upgrade facility at Argonne National Laboratory. Instead of targeting Krypton-88 directly, researchers fired a particle beam at Bromine-89, an even shorter-lived isotope with a half-life of just 4.357 seconds. Bromine-89 naturally decays into Krypton-89, allowing physicists to study that nucleus to infer the properties of its slightly lighter sibling.
The experimental setup routed the resulting sample into a specialized Summing NaI (SuN) detector. This instrument tracked gamma-rays emitted as the newly formed Krypton-89 cooled, extracting two precise metrics: the Nuclear Level Density and the gamma-ray Strength Function. Armed with these values, the team mathematically reverse-engineered the exact neutron-capture rate of Krypton-88.
Validating the Intermediate Neutron-Capture Models
When the team plugged this experimentally derived value into stellar evolution models, the theoretical outputs finally aligned with observational data. The calculated isotope ratios in CEMP stars matched reality, showing significantly higher amounts of strontium. Specifically, the ratio between strontium and yttrium—another long-lived product of this nucleosynthesis chain—fell into place with striking precision.
Laboratory instrumentation on Earth successfully resolved a cosmic discrepancy that baffled astrophysicists for generations. By replacing estimated parameters with rigorous empirical data from specialized particle facilities, stellar nucleosynthesis models can now accurately map how the early universe forged the heavy elements observed today.
