A team led by Caley M. Harris used rare-isotope beams and gamma-ray detection to derive the neutron-capture rate of Krypton-88, resolving a long-standing mismatch between stellar observations and nucleosynthesis models for strontium in Carbon-Enhanced Metal-Poor stars.
Researchers produced Bromine-89, observed its decay to Krypton-89 with a Summing NaI (SuN) detector, and extracted nuclear level density and gamma-strength values to infer Krypton-88’s neutron-capture rate.
Using the experimentally derived rate in nucleosynthesis models aligns predicted strontium-to-yttrium ratios with observations, supporting the intermediate neutron-capture (i-process) explanation for these stars.
# The problem: strontium levels in very old stars didn't fit models Astronomers studying Carbon-Enhanced Metal-Poor (CEMP) stars — some of the oldest stars in the Galaxy — found element ratios that didn't match the two standard nucleosynthesis pathways: the slow s-process and the rapid r-process. To better reproduce the observed ratios, researchers invoked a third mechanism called the intermediate neutron-capture process, or i-process. The i-process improved model fits for many isotopes, but strontium still appeared more abundant in observations than in simulations.
# Why Krypton-88 mattered The composition mismatch traced back to the nuclear physics of an unstable isotope: Krypton-88. Krypton-88 has a half-life of about 2.8 hours and during nucleosynthesis can either capture a neutron (leading toward heavier isotopes like yttrium-89) or beta-decay into rubidium-88 and then to strontium-88. Models lacked an experimentally measured neutron-capture rate for Krypton-88, so they relied on estimates. Those estimates turned out to be the main source of the discrepancy for strontium abundances.
# The experimental approach
NaI (SuN) detector, a device designed to capture and sum gamma-ray energy with high efficiency. From those measurements the team derived NLD and gSF for the krypton isotopes and used them to reverse-engineer the neutron-capture cross section for Krypton-88.
# How the new number fixes the models With the experimentally constrained neutron-capture rate for Krypton-88 plugged into nucleosynthesis calculations for i-process conditions, the predicted abundances of strontium rose. The new calculations brought the strontium-to-yttrium ratio into close agreement with the observed ratios in CEMP stars, removing the long-standing mismatch. The results are reported in a Nature Communications Physics paper led by Caley M. Harris of Michigan State University.
# Why this matters for stellar chemistry This is a concrete example of laboratory nuclear physics removing a specific uncertainty in stellar nucleosynthesis models. The paper shows that one previously missing or poorly known nuclear rate can change predicted elemental abundances in old stars and brings models and observations into alignment. The experiment used rare-isotope beams and sophisticated gamma-ray detection instead of attempting to bombard Krypton-88 directly with neutrons, a practical solution given the isotope's half-life.
# Bottom line A targeted laboratory measurement of nuclear properties tied to Krypton-88 resolved why strontium appeared overly abundant in some ancient stars. The result strengthens the case that the i-process can explain these stars' chemistries when nuclear physics inputs are measured rather than assumed.
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