Advancing nuclear fission models for lighter sub-lead nuclei

A five-dimensional (5D) Langevin approach developed by an international team of researchers, including members from Science Tokyo, accurately reproduces complex fission fragment distributions and kinetic energies in medium-mass mercury isotopes (¹⁸⁰Hg and ¹⁹⁰Hg). The model successfully captures the unusual “double-humped” fragment mass distribution observed in mercury-¹⁸⁰ and offers new insights into how nuclear shell effects influence fission dynamics—even at higher excitation energies than previously thought—advancing our understanding of fission in the sub-lead region.

Nuclear fission, the process by which an atomic nucleus splits into smaller parts, is a fundamental process in nuclear physics. While the fission of heavy elements like uranium and plutonium is well studied, lighter nuclei such as mercury (Hg) behave in unexpected ways. Experiments have shown that ¹⁸⁰Hg undergoes an unexpected form of asymmetric fission, producing fragments of very different sizes. These findings challenge existing models and highlight the need to better understand how nuclear structure affects fission in the sub-lead region, which includes elements with atomic numbers below 82.

To better understand unusual fission behaviors in Hg isotopes, an international research team led by Associate Professor Chikako Ishizuka from Institute of Zero-Carbon Energy at Institute of Science Tokyo (Science Tokyo), Japan developed a five-dimensional (5D) Langevin model. Their work, published online in Physical Review C on May 20, 2025, provides accurate predictions of fragment distributions and total kinetic energy (TKE) and was recognized as an Editor’s Suggestion by the journal.

The study was a collaborative effort and included Dr. F. A. Ivanyuk from the Institute for Nuclear Research Ukraine; Professor C. Schmitt from the University of Strasbourg, France; and Researcher Satoshi Chiba from NAT Co., Ltd., Japan, as co-authors.

Unlike static models, the Langevin model dynamically tracks the shape evolution of the nucleus from its equilibrium state all the way to scission, the point where it splits into smaller fission fragments. “Describing the fission of actinides like uranium is not enough to understand the full picture,” says Ishizuka. “We need consistent models that work for other nuclei too, especially lighter ones like Hg that behave differently.”

The team successfully modeled the fission of two Hg isotopes: ¹⁸⁰Hg produced from the collision of ³⁶Ar and ¹⁴⁴Sm, and ¹⁹⁰Hg from ³⁶Ar and ¹⁵⁴Sm. For both, they calculated the distribution of fission fragment masses and their TKEs. The model was designed to simulate nuclear fission more realistically by considering both the macroscopic, large-scale, liquid-drop-like motion and microscopic shell structure effects.

A key improvement was the use of a “soft wall” at the boundaries of the deformation space, which accurately simulates how the shape of the nucleus evolves during fission. The model also accounted for how shell effects change with increasing excitation energy, an aspect that previous models often oversimplified.

The simulation results showed strong agreement with experimental data for both the mass number distributions and the TKE of fission fragments. In the case of ¹⁸⁰Hg, the model reproduced the unusual double-humped mass pattern seen in experiments. A key finding of the study is that shell effects persist even at higher excitation energies of 40–50 MeV, where they were previously thought to vanish.

The model also accounted for multichance fission, a process where the nucleus emits neutrons before it undergoes fission. They found that while this process has only a minor effect on fragment mass distributions at low excitation energies, it significantly impacts the TKE, making it a valuable indicator for studying multichance fission.

Overall, these findings provide new insights into the fission process with important implications for fundamental research. “The calculations presented in this work confirm that our 5D Langevin approach is a reliable tool for the theoretical predictions of the fission process observables,” concludes Ishizuka.

Authors:

F. A. Ivanyuk¹, C. Schmitt², C. Ishizuka³, and S. Chiba⁴

Title:

Shell effects and multichance fission in the sub-lead region

Journal:

Physical Review C

DOI:

10.1103/PhysRevC.111.054620

Affiliations:

¹Institute for Nuclear Research, Ukraine
²IPHC, University of Strasbourg, France
³Institute of Science Tokyo, Japan
⁴NAT Corporation, Japan

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• Chikako Ishizuka | Researcher Finder - Science Tokyo STAR Search
• Chikako ISHIZUKA Laboratory (Japanese)
• Laboratory for Zero-Carbon Energy
• Department of Transdisciplinary Science and Engineering, School of Environment and Society
• Institute of Integrated Research

• NAT Corporation
• University of Strasbourg