An international research team has reported the first experimental investigation of the neutron-capture reaction on krypton-88, a key missing piece in models of how stars produce strontium. The study, published June 8 in Communications Physics, used indirect techniques at Argonne National Laboratory's ATLAS facility to measure the rate at which krypton-88 captures neutrons, reducing the uncertainty from at least a factor of eight to about a factor of three.
The work was led by Caley Harris, a former graduate student at the Facility for Rare Isotope Beams (FRIB) at Michigan State University, and involved researchers from 12 institutions in the United States, Canada, and Europe. By installing FRIB's Summing NaI (SuN) detector at the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at ATLAS, the team produced krypton-89 and measured its gamma-ray decay to infer the krypton-88 neutron-capture rate. The experimentally determined rate is consistently lower than theoretical predictions, and when fed into intermediate neutron-capture process (i-process) models, it raises the predicted strontium output, bringing simulations into better agreement with observations of very old stars.
Confirmed
- Study published June 8, 2026 in Communications Physics (DOI: 10.1038/s42005-026-02713-5).
- First experimental constraint on the krypton-88 neutron-capture rate, a long-standing uncertainty in i-process nucleosynthesis.
- Uncertainty reduced from ≥8× to ~3×; measured rate is lower than theory.
- Updated i-process models with the new rate produce more strontium, matching astronomical data on metal-poor stars more closely.
- Experiment performed at ATLAS (Argonne) using FRIB's SuN detector at the CARIBU facility; 12-institution collaboration led by Caley Harris (FRIB/MSU).
- Funding: U.S. NSF, DOE Office of Science, DOE/NNSA Nuclear Science and Security Consortium, Stewardship Science Academic Alliances, Research Council of Norway, Norwegian Nuclear Research Center, U.S. Nuclear Data Program, PNNL LDRD, NSERC Canada, Canada Foundation for Innovation.
Unknown
- Exact numerical value of the new neutron-capture rate (cross section) and its energy dependence — the article reports the uncertainty reduction factor but not the absolute rate.
- Whether the remaining factor-of-three uncertainty is dominated by experimental statistics, systematic effects, or the indirect method's model dependence.
- Timeline for independent replication or direct measurement of the krypton-88(n,γ) reaction.
- How much of the residual strontium discrepancy in the oldest stars is now attributable to astrophysical parameters (neutron density, burning timescales) versus other nuclear physics inputs.
- Whether the same indirect technique will be applied to other key i-process branch points (e.g., rubidium-87, strontium-89).
Our take
This result shows how a single, well-targeted nuclear measurement can shift an entire class of stellar models. The team has moved the i-process from "nuclear physics limited" to "astrophysics limited" — future progress now depends on constraining neutron densities and burning timescales inside stars, not more lab time. That handoff from experimentalists to modelers is exactly where the field should be heading.
