New Insights into Asymmetric Nuclear Fission from GSI/FAIR Experiments

New Insights into Asymmetric Nuclear Fission from GSI/FAIR Experiments
by Robert Schreiber
Berlin, Germany (SPX) May 15, 2025

An international research team has uncovered a previously unknown region in the nuclear chart characterized by a unique pattern of asymmetric fission among heavy, neutron-deficient isotopes. The findings, part of the FAIR Phase 0 program at GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt, Germany, reveal surprising behavior in the fission of exotic isotopes, marking a significant advancement in the field of nuclear physics.

The experiments, led by the R3B-SOFIA collaboration, examined around 100 neutron-deficient isotopes, spanning elements from iridium (Z = 77) to thorium (Z = 90). These isotopes, produced by fragmenting a uranium-238 beam accelerated to 87.6 percent of the speed of light, were isolated using the GSI/FAIR Fragment Separator (FRS). Once separated, the isotopes were directed onto a segmented lead target, where interactions elevated their energy by a few megaelectronvolts above their ground states, prompting fission into lighter fragments.

Key to these measurements was the TWIN-MUSIC double ionization chamber, which precisely captured the charges of the resulting fission fragments. The superconducting dipole magnet GLAD, cooled with helium, further separated the fragments by their momentum-to-charge ratio, enabling precise tracking and time-of-flight measurements to reconstruct the fission dynamics.

The data, collected over ten days, revealed a striking trend: a pronounced shift towards increasingly asymmetric fission among neutron-deficient isotopes, with a surprising dominance of light krypton (Z = 36) fragments. This marks the identification of a new "island of asymmetric fission" in the nuclear chart, offering critical benchmarks for improving theoretical models of fission, including those related to r-process nucleosynthesis in astrophysical settings.

"Beyond mapping this novel phenomenon, our findings enhance our understanding of both terrestrial and cosmic fission processes," said 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."

Dr. Haik Simon, head of the GSI/FAIR department "Super Fragment Separator" and deputy spokesman of the R3B collaboration, added, "The results are an impressive demonstration of the R3B setup's performance and give an outlook on FAIR in the future. 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."

Future experiments at the under-construction FAIR (Facility for Antiproton and Ion Research) are expected to further explore this asymmetric fission region, using the next-generation Super-FRS to map these phenomena in greater detail and uncover fundamental properties of nuclear matter under extreme conditions.

Research Report:An asymmetric fission island driven by shell effects in light fragments