3 days ago
Five-dimensional physics solves decades-old mystery of mercury fission
An international team of researchers, including scientists from Science Tokyo, has developed a five-dimensional Langevin model that accurately reproduces the complex fission fragment distributions and kinetic energies of medium-mass mercury isotopes like 180Hg and 190Hg.
Unlike previous models that struggled to explain mercury's asymmetric fission, this approach captures the unusual double-humped mass distribution seen in mercury-180, revealing how nuclear shell effects continue to shape fission dynamics even at higher excitation energies than previously assumed.
By demonstrating that these structural effects persist beyond heavy elements like uranium and plutonium, the findings enhance the understanding of nuclear fission processes and could improve predictive models for unexplored isotopes across the nuclear chart.
Aiming to uncover the reasons behind mercury's unusual fission behavior, Associate Professor Chikako Ishizuka and her international team at the Institute of Zero-Carbon Energy, Science Tokyo, developed a five-dimensional Langevin model. Published online in Physical Review C on May 20, 2025, their study offers precise predictions of fragment distributions and total kinetic energy, earning recognition as an Editor's Suggestion by the journal.
Unlike the well-studied fission of heavy elements such as uranium and plutonium, the way lighter nuclei like mercury split remains poorly understood.
Experiments have revealed that mercury-180 undergoes an unusual asymmetric fission, producing fragments of very different sizes. These surprising results challenge current theories and highlight the need to understand how nuclear structure influences fission in elements with atomic numbers below 82.
The Langevin model tracks the changing shape of the nucleus in real time, from its initial equilibrium state to the point of scission when it splits into smaller fragments. Developing consistent models for these lighter elements is crucial, as they often behave very differently from well-studied heavy isotopes.
In their study, the team focused on two mercury isotopes: 180Hg, created by colliding 36Ar with 144Sm, and 190Hg, formed from 36Ar and 154Sm. They calculated how the fission fragments split and their total kinetic energies.
One major improvement in the model was the introduction of a soft wall at the edges of the deformation space, allowing it to more accurately simulate how the nucleus changes shape during fission. The researchers also included the way shell effects evolve with rising excitation energy, a factor often oversimplified in earlier models.
Additionally, the simulation closely matched experimental results for both the fragment mass distributions and total kinetic energy. For 180Hg, it successfully recreated the unusual double-peaked mass pattern observed in experiments. The study also revealed that shell effects remain important even at higher excitation energies of 40–50 MeV, contradicting earlier assumptions that they disappear.
The researchers also included multichance fission, where the nucleus releases neutrons before splitting. They found this has little impact on fragment masses at low energies but strongly affects the total kinetic energy, making TKE a useful way to study multichance fission.
According to Ishizuka, these findings offer valuable new insights into the fission process, deepening our fundamental understanding of nuclear behavior, and they confirm that the 5D Langevin approach is a reliable and effective tool for accurately predicting key fission observables.
