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An international team of scientists has identified an unexpected region of heavy, neutron-deficient isotopes in the nuclear chart where nuclear fission is predominantly governed by an asymmetric mode. The experiment was conducted by the R3B-SOFIA collaboration at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, within the FAIR Phase 0 program. The results are published in the journal Nature.
The research team investigated the fission properties of 100 different neutron-deficient exotic isotopes, ranging from iridium (atomic number Z = 77) to thorium (Z = 90). These isotopes with a low number of neutrons relative to the number of protons were produced via the fragmentation of a relativistic primary beam of uranium-238 at 87.6 percent of the speed of light, and subsequently separated and identified individually using the GSI/FAIR Fragment Separator FRS.
In the GSI/FAIR experimental setup R3B (Reactions with Relativistic Radioactive Beams), extended by a set of specialized systems developed for the unique pattern of fission experiments, the isotopes were directed onto a segmented lead target. There, the excitation to a few megaelectronvolt above their ground state energy induced the fission into two lighter fragments. The double ionization chamber TWIN-MUSIC enabled the measurement of the charges of both fission products. Additionally, the large superconducting dipole magnet GLAD, cooled with Helium, separated the fission fragments according to their momentum-to-charge ratio, bending them toward large-area detector arrays for tracking and time of flight measurement to reconstruct the reaction dynamics.
Terabytes of data collected during ten days of experiment, reveal a transition toward increasingly asymmetric fission in neutron-deficient heavy nuclei. This marks the discovery of a new “island of asymmetric fission” in the nuclear chart, characterized by a surprising dominance of light fission fragments of krypton (Z = 36).
“Beyond mapping this novel phenomenon, our findings enhance our understanding of both terrestrial and cosmic fission processes,” says Pierre Morfouace from CEA, France, first author of the Nature publication. “Moreover, they offer valuable benchmarks for theoretical models, significantly improving their predictive power for fission fragment distributions in neutron-rich systems, relevant for example in r-process nucleosynthesis in the cosmos.” The discovery is a major step forward for our understanding of the fission recycling expected in supernova explosions feeding the element production in our Galaxy and a start toward identifying the extent of a newly observed region in the nuclear chart where asymmetric fission dominates.
“In addition, the results are an impressive demonstration of the R3B setup’s performance and give an outlook on FAIR in the future,” adds Dr. Haik Simon, head of the GSI/FAIR department “Super Fragment Separator” and deputy spokesman of the R3B collaboration. “The combination of the Super Fragment Separator, the successor to the FRS, and the planned NUSTAR experiment program at FAIR will offer unique possibilities for the production and selection of even rarer and more exotic isotopes to address open research questions in this area.”
A series of follow-up experiments is planned at the international accelerator facility FAIR (Facility for Antiproton and Ion Research), which is currently under construction at GSI. The new superconducting fragment separator Super-FRSwill be key to mapping the phenomenon of asymmetrical fission in greater detail and to revealing fundamental aspects of nuclear matter under extreme conditions.
https://doi.org/10.1038/s41586-025-08882-7
https://www.gsi.de/en/start/news/details/2025/05/14/asymmetrische-kernspaltung
Dr. Haik Simon at the R3B-Magnet GLAD
Photo: J. Hosan, GSI/FAIR
Visualization of the future NUSTAR experiment area at FAIR
Picture: GSI/FAIR, Zeitrausch
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Dr. Haik Simon at the R3B-Magnet GLAD
Photo: J. Hosan, GSI/FAIR
Visualization of the future NUSTAR experiment area at FAIR
Picture: GSI/FAIR, Zeitrausch
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