The researchers asked whether experimental information about krypton-89 could constrain the otherwise inaccessible neutron capture of krypton-88—and whether a better rate would resolve the persistent underproduction of strontium in models of the intermediate neutron-capture process.
The research question and why it matters
The researchers asked whether experimental information about krypton-89 could constrain the otherwise inaccessible neutron capture of krypton-88—and whether a better rate would resolve the persistent underproduction of strontium in models of the intermediate neutron-capture process.
The slow and rapid neutron-capture processes explain many elements heavier than iron, but some old stars show abundance patterns between those regimes. The proposed i-process can reproduce several of those patterns, yet models repeatedly made too little strontium while matching nearby yttrium and zirconium. Sensitivity studies had singled out the unmeasured krypton-88 capture rate as a major uncertainty; this experiment directly targeted the nuclear information needed to constrain it.
What researchers found
The experimentally informed krypton-88 neutron-capture rate was consistently lower than the tested theoretical predictions. Its uncertainty narrowed from at least a factor of eight to about a factor of three. Across the tested i-process models, the lower rate left more material available to flow toward strontium, raising the predicted abundance and reducing the mismatch with stellar observations.
Key results from the tested systems
rate uncertainty
The experiment substantially narrowed—but did not remove—the krypton-88 capture uncertainty.
institutions
The collaboration combined radioactive beams, detector expertise, nuclear theory and stellar modeling.
capture rate
The experimentally informed rate sat below the tested theoretical predictions.
strontium increased
The revised rate raised predicted strontium across the i-process calculations examined.
How the research worked
At Argonne's ATLAS facility, the team produced radioactive krypton-89 and recorded its beta-decay gamma rays with the SuN total-absorption detector. The spectra constrained krypton-89 level density and gamma-strength information used in reaction calculations for krypton-88 plus a neutron. Researchers then inserted the resulting rate distribution into several i-process nucleosynthesis calculations and compared elemental yields with observations of old stars.
How to interpret this design
The result is conditional on the model structure, inputs, boundary conditions and scenarios chosen by the researchers. Agreement with known observations strengthens confidence, but a projection is not a direct observation of the future or the inaccessible past.
The evidence comes from a controlled physical or chemical system. That control helps establish what happened under the tested conditions, while scale-up, durability, manufacturing and real-world performance remain separate questions.
What strengthens or limits the finding?
A dedicated radioactive-beam experiment constrained nuclear inputs that had previously been theoretical, the authors propagated the result through several stellar models, and data and analysis tools are available. The capture rate was inferred indirectly and still spans roughly a factor of three, while the stellar setting of the i-process remains unsettled.
The result is meaningfully informative, but identifiable limitations could alter the size, reach or causal interpretation of the finding.
Funding and disclosure context
The recorded funding source is: U.S. National Science Foundation; U.S. Department of Energy Office of Science and U.S. Nuclear Data Program; National Nuclear Security Administration Stewardship Science Academic Alliances and Nuclear Science and Security Consortium; Natural Sciences and Engineering Research Council of Canada; Canada Foundation for Innovation; Norwegian Nuclear Research Centre; Research Council of Norway; and Pacific Northwest National Laboratory's Laboratory Directed Research and Development program. The recorded conflict information is: The authors declared no competing interests. Funding or a disclosed relationship does not by itself invalidate a result, but it is relevant when judging design choices, analysis and the need for independent replication.
What it means
The work replaces one influential theoretical input with an experimentally anchored range. That sharpens model predictions for how elements near strontium form and identifies where more nuclear measurements would help most. It supports the i-process as a possible explanation for some abundance patterns, but the process's stellar site and operating conditions are still open questions.
Deeper analysis
A slower capture can make more strontium
In a reaction network, krypton-88 sits near a branching point. If it captures neutrons less readily, more material follows beta-decay paths that feed rubidium and strontium. The counterintuitive result is that lowering one reaction rate can increase a later element's yield.
Indirect does not mean uninformative
Short-lived targets can make direct neutron-capture experiments impractical. Measuring the daughter nucleus's level structure and gamma decay constrains the same statistical ingredients used to calculate the stellar reaction, provided model assumptions and uncertainties remain explicit.
Model agreement is a consistency test
Bringing simulated strontium closer to observed abundances is encouraging because the rate was selected before the experiment as a sensitive uncertainty. It is still not unique confirmation of the i-process: different stellar histories or other reaction changes could also shift the prediction.
The remaining factor of three matters
For a rare-isotope reaction, moving from an order-of-magnitude range to a few-fold range is meaningful progress. Astrophysical conclusions should nevertheless carry that residual nuclear uncertainty alongside the larger uncertainty over the stellar environment itself.
What it does NOT prove
- It does not directly measure krypton-88 capturing a neutron under stellar conditions.
- It does not prove that the i-process is the only—or even the dominant—source of strontium in the observed stars.
- It does not identify which type of star hosts the i-process.
- It does not eliminate nuclear uncertainty; the revised rate still spans about a factor of three.
- It does not measure strontium being produced in a star in real time.
Important limitations
- The neutron-capture rate was inferred from krypton-89 nuclear properties and statistical reaction models rather than measured directly.
- Different level-density, gamma-strength and optical-model choices contribute to the remaining uncertainty.
- The astrophysical calculations depend on uncertain neutron densities, temperatures, mixing histories and candidate stellar sites.
- Better agreement for strontium does not guarantee that the same model reproduces every neighboring element or every chemically peculiar star.
- The institutional report appeared months after the paper's June publication, so the research is newly reported rather than newly published.
How this fits with previous research
The slow and rapid neutron-capture processes explain many elements heavier than iron, but some old stars show abundance patterns between those regimes. The proposed i-process can reproduce several of those patterns, yet models repeatedly made too little strontium while matching nearby yttrium and zirconium. Sensitivity studies had singled out the unmeasured krypton-88 capture rate as a major uncertainty; this experiment directly targeted the nuclear information needed to constrain it.
Questions still unanswered
- Can a future facility measure krypton-88 neutron capture more directly or reduce the rate uncertainty further?
- Which stellar environments actually sustain the required intermediate neutron densities?
- Will the revised rate improve simultaneous fits to strontium, yttrium, zirconium and heavier elements across more stars?
- Which other unstable nuclei now dominate the model uncertainty?
- Do alternative stellar-mixing histories reproduce the same improvement for a different physical reason?
Relevant U.S. government resources
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A krypton experiment narrowed how stars may make strontium
This review was developed from the source record below and, when separately available, the primary paper or government report. The summary and analysis on this page are original editorial writing.
- Source organization
- Facility for Rare Isotope Beams at Michigan State University
- Source type
- University
- Authors
- Caley M. Harris, Artemis Spyrou, Pavel Denissenkov, Falk Herwig, Andrea L. Richard, Sivahami Uthayakumaar, Dennis Muecher, Hannah C. Berg, Paul DeYoung, Alexander C. Dombos, Beau Greaves, Ann-Cecilie Larsen, Sean N. Liddick, Stephanie M. Lyons, Gerald Owens-Fryar, Alicia Palmisano-Kyle, Georgios Perdikakis, Daniel Santiago-Gonzalez, Guy Savard, Sunniva Siem, Mallory K. Smith, William W. VonSeeger and Mathis Wiedeking
- Journal / report
- Communications Physics
- Publication date
- June 8, 2026
- DOI
- 10.1038/s42005-026-02713-5
- PMID
- Not available
- Institution
- Facility for Rare Isotope Beams and Michigan State University; University of Victoria; University of Guelph; Calvin University; Central Michigan University; University of Oslo; Pacific Northwest National Laboratory; University of Tennessee; Argonne National Laboratory; Lawrence Berkeley National Laboratory; and collaborating institutions
- Funding
- U.S. National Science Foundation; U.S. Department of Energy Office of Science and U.S. Nuclear Data Program; National Nuclear Security Administration Stewardship Science Academic Alliances and Nuclear Science and Security Consortium; Natural Sciences and Engineering Research Council of Canada; Canada Foundation for Innovation; Norwegian Nuclear Research Centre; Research Council of Norway; and Pacific Northwest National Laboratory's Laboratory Directed Research and Development program
- Conflicts
- The authors declared no competing interests
- Open access
- Yes
- Reuse approach
- Methods and results summarized independently from FRIB's institutional report and the open peer-reviewed paper; no source wording, detector photographs, figures, tables or diagrams reproduced.
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