The transition in question involves an isomeric state in ^53Fe characterized by an exceptionally high angular momentum of 19/2^− and a half-life of 2.54 minutes, residing at an excitation energy around 3.0 MeV above the ground state. What sets this nuclear isomer apart is the uniquely allowed electromagnetic decay pathway: a single-photon emission carrying a massive angular momentum quantum number of six (6ħ). This kind of hexacontatetrapole transition, identified by the multipolarity E6, is extraordinarily rare—so rare, in fact, that while analogous high-multipole emissions exist in other quantum systems, such as solid hydrogen matrices or atomic rubidium, ^53mFe remains the only known nuclear system to exhibit this phenomenon spontaneously and without external excitation.

Crucial to the success of this study is the application of the valence-space in-medium similarity renormalization group (VS-IMSRG) method. This advanced computational approach allows the nuclear many-body problem to be tackled starting from realistic nuclear forces derived from chiral effective field theory. Unlike traditional models of nuclear electromagnetic transitions, which often rely on effective charges tuned to match experimental data, the VS-IMSRG framework employs bare nucleon charges for all multipolarities, offering a truly predictive and parameter-free description of complex nuclear processes. This lends considerable credibility and robustness to the findings, setting a new standard for theoretical nuclear physics.

The researchers employed two distinct sets of chiral nucleon-nucleon plus three-nucleon interactions to interrogate the structure of ^53Fe and its isomeric states within extensive model spaces reaching up to e_max = 14 and E_3max = 24, enabling the precise calculation of excitation energies and transition probabilities. Such large-scale computational resources provide the convergence necessary to accurately predict the extremely delicate observables linked to high-multipole transitions, which have hitherto been a formidable challenge for nuclear theorists. The convergence of results across different interaction models further underscores the reliability of the ab initio approach.

One of the striking conclusions from the calculations is the confirmation that the 19/2^− isomer in ^53Fe gains its stability and unique decay characteristics primarily from a pure orbital configuration dominated by the 0f_7/2 shell occupancy. This simplification of the nuclear wavefunction contrasts with more complicated configurations found in other nuclei, suggesting a relatively clean testbed for studying extreme electromagnetic processes in nuclear matter. The ability to describe the isomer’s life and decay properties accurately using this orbital framework bolsters confidence in both the computational model and the nuclear shell model foundations.

Beyond merely reproducing known experimental data, the study pioneers the ab initio calculation of transition probabilities for the E6, M5, and E4 multipolarities within the nuclear electromagnetic decay spectrum of ^53Fe. This comprehensive treatment is unprecedented, marking the first time these complex multipolarities have been tackled with bare nucleon charges and realistic nuclear interactions rather than effective parameters. Such predictive power represents a significant leap forward in theoretical nuclear physics, offering a window into the interplay of nuclear forces and quantum electrodynamics within the atomic nucleus.

The implications of this research extend well beyond ^53Fe. By establishing the validity of ab initio methods for describing the highest-multipole electromagnetic transitions, the study paves the way for future investigations of rare nuclear processes that have long eluded precise theoretical interpretation. These advancements could potentially inform our understanding of nuclear structure in exotic isotopes and guide experimental searches for new nuclear isomers with unusual decay properties, some of which might have applications in quantum information sciences or nuclear astrophysics.

From an experimental perspective, the ability to compute precise electromagnetic decay rates without adjustable parameters represents a paradigm shift. Traditionally, nuclear models often required phenomenological tuning to unify theory and experiment, but this study’s success with bare charges demonstrates a clean, first-principles approach. This not only improves the predictive accuracy for unmeasured nuclear states but also builds a stronger theoretical foundation for interpreting future experimental discoveries and designing new experiments with higher sensitivity.

Moreover, this work highlights the power of high-performance computing and algorithmic sophistication in tackling some of the most challenging problems in fundamental physics. The VS-IMSRG technique, combined with modern chiral nuclear forces, emerges as a versatile and highly accurate tool capable of addressing many unresolved questions concerning nuclear electromagnetic phenomena. It is anticipated that ongoing developments in computational physics will enable even larger nuclei and more intricate transitions to be studied in the near future, expanding the frontier of nuclear theoretical capabilities.

The team’s findings emphasize the remarkable uniqueness of ^53Fe’s E6 transition, an emblematic case of how nature’s quantum complexity produces extraordinary nuclear states. The hexacontatetrapole transition involving a single photon emission remains a rare jewel in the nuclear landscape—a process exquisitely sensitive to the microscopic details of nuclear structure and interactions. Through this research, scientists now have a robust theoretical framework to explore these phenomena with unprecedented precision and confidence.

Looking ahead, the ab initio framework demonstrated here could be extended to investigate other high-multipole electromagnetic transitions, some of which may play roles in astrophysical nucleosynthesis or serve as probes into exotic nuclear matter. Understanding such processes enriches our knowledge of elemental formation, nuclear stability, and the fundamental symmetries governing atomic nuclei. Furthermore, the ability to precisely model long-lived nuclear isomers may have practical implications for nuclear technology, including energy storage and radiation shielding.

The lead author of the study, Dr. Siqin Fan, expressed enthusiasm regarding the potential of the ab initio approach, noting that it eliminates the need for empirical effective charges that have historically complicated the theoretical description of nuclear electromagnetic transitions. A senior nuclear theorist involved in the project further underscored how this breakthrough offers new avenues for the exploration of nuclear structure, especially regarding rare and extreme transitions that challenge conventional nuclear models. These developments signify an exciting era in nuclear physics, where theory and computation are intersecting to reveal deeper layers of complexity within the atomic nucleus.

The complete study detailing these findings is published in the journal Nuclear Science and Techniques and can be accessed via DOI: 10.1007/s41365-025-01812-2. This seminal work is expected to stimulate further experimental and theoretical efforts to probe the frontiers of nuclear electromagnetic phenomena and to test the limits of our understanding of nuclear forces and configurations.

Subject of Research: Not applicable
Article Title: Ab initio calculations of the highest-multipole electromagnetic transition ever observed in nuclei
News Publication Date: 12-Sep-2025
Web References: http://dx.doi.org/10.1007/s41365-025-01812-2
Image Credits: Si-Qin Fan

Keywords

Nuclear physics, Electromagnetic spectrum, Isomerization

Nuclear fission, the process by which an atomic nucleus splits into smaller fragments, is foundational to both fundamental physics and practical applications such as nuclear energy production. While the fission behavior of heavy elements like uranium and plutonium has been extensively characterized, lighter nuclei such as mercury have exhibited perplexing phenomena that defy conventional models. Notably, mercury-180 (^180Hg) manifests an unusual asymmetric fission pattern, producing fragment distributions markedly different from expectations. These anomalies highlight significant gaps in existing theoretical frameworks and necessitate a reexamination of the underlying nuclear structure influences governing fission pathways.

The key theoretical advancement presented by Ishizuka and colleagues is their development of a five-dimensional dynamical Langevin approach that simulates the nuanced shape evolution of fissile nuclei beyond static approximations. Unlike earlier models constrained to simplified parameters, this comprehensive 5D framework traces the nucleus’s transformation from near-equilibrium configurations through to scission—the point of actual division into daughter fragments. This method integrates both macroscopic liquid-drop dynamics and microscopic shell effects, offering a realistic portrayal of how nuclear shapes and energies evolve in a coupled, stochastic environment.

Crucially, the research team applied this sophisticated modeling to two mercury isotopes, ^180Hg and ^190Hg. These isotopes were experimentally synthesized via collision reactions involving argon projectiles (^36Ar) with samarium targets (^144Sm for ^180Hg and ^154Sm for ^190Hg). By simulating the entire dynamical fission process, the team calculated fragment mass distributions alongside total kinetic energies (TKE) of emitted fragments, producing results that align strikingly well with experimental datasets. Notably, the model captures the signature “double-humped” mass distribution characteristic of ^180Hg fission—a feature that had eluded robust theoretical reproduction until now.

An innovation underpinning their success is the implementation of a “soft wall” boundary condition within the deformation space governing nuclear shapes. This technique prevents unphysical behaviors at the edges of allowed deformation parameters, enabling smooth and realistic transitions of nuclear configurations as the nucleus elongates, necks, and ultimately cleaves. Such fidelity in shape evolution is key to replicating the subtle interplay between liquid-drop macroscopic forces and quantum-mechanical shell corrections, especially near scission where fragment properties are effectively “frozen in.”

One of the most striking impacts of this study is its revelation about shell effects in the sub-lead region. Traditionally, it was believed that shell corrections—a quantum effect related to the arrangement of protons and neutrons within nuclear potential wells—attenuate rapidly with rising excitation energy, disappearing beyond approximately 30 MeV of internal energy. Contrary to this, the new simulations demonstrate persistent shell influences on fission dynamics up to energies as high as 40 to 50 MeV, maintaining distinct effects on fragment mass splits and kinetic energies. This persistence necessitates a revision of theoretical assumptions and highlights the robustness of nuclear shell structures under energetic conditions.

Moreover, the model integrates the phenomenon of multichance fission, wherein the pre-fission nucleus emits neutrons in successive stages before final division. This sequential neutron evaporation alters the effective excitation energy and nuclear composition at the time of scission, impacting observables in subtle yet measurable ways. Although multichance fission minimally sways the mass distributions at lower excitations, its effect on the total kinetic energy distributions is pronounced. Hence, TKE measurements emerge as sensitive probes of pre-fission neutron emission processes, enriching experimental interpretive capabilities.

The collaborative nature of this endeavor brings together expertise across multiple institutions: from Dr. F. A. Ivanyuk of the Institute for Nuclear Research in Ukraine to Professor C. Schmitt at the University of Strasbourg in France, and researcher Satoshi Chiba of NAT Co., Ltd. in Japan. This international scope underscores the global drive to refine nuclear reaction models and deepen our understanding of fundamental nuclear processes beyond the actinide region, bridging theoretical predictions with experimental nuclear physics.

In her reflections on the study, Associate Professor Ishizuka emphasized the need to transcend the traditional focus on heavy nuclei like uranium and plutonium. “To fully comprehend fission mechanisms, it is essential that theoretical models reliably capture dynamics across the nuclear landscape, including lighter nuclei such as mercury isotopes that defy classical expectations,” she noted. The 5D Langevin framework developed here provides just such a universal tool, with predictive power validated against empirical data.

The findings carry significant implications not only for nuclear physics theory but also for wider fields such as astrophysics, where fission plays a critical role in nucleosynthesis pathways, as well as practical domains including nuclear energy and reactor design. Understanding the intricate balance of forces and shell structures in fission contributes to better predictions of fragment yields, radiation outputs, and energy release—parameters vital for both safety assessments and technological innovations.

Published on May 20, 2025, in the peer-reviewed journal Physical Review C and distinguished as an Editor’s Suggestion, this research marks a milestone in the quest to unlock the complexities of nuclear fission in the sub-lead region. The integration of advanced computational methods with detailed nuclear physics increases confidence in theoretical tools to describe a broad range of isotopes and nuclear conditions.

Institute of Science Tokyo, established recently through the merger of Tokyo Medical and Dental University and Tokyo Institute of Technology, has positioned itself at the forefront of scientific innovation with this contribution, reinforcing its mission to advance science and human wellbeing. This research exemplifies their commitment to pushing boundaries and expanding knowledge in fundamental and applied sciences.

As experimental techniques continue to refine and new isotopes are explored, this five-dimensional Langevin approach stands as a vital model for interpreting future data and guiding exploratory nuclear science. The revelation of enduring shell effects and the nuanced role of multichance fission open fresh avenues for research, inviting further theoretical and experimental collaboration worldwide.

This study not only deepens our understanding of nuclear fission mechanics but also underscores the importance of integrating multidimensional, dynamic perspectives to capture the true essence of nuclear transformations. It presents a compelling case for revisiting nuclear paradigms and embracing complexity as a pathway to clarity in the microscopic realms of atomic nuclei.

Subject of Research: Not applicable

Article Title: Shell effects and multichance fission in the sub-lead region

News Publication Date: 20-May-2025

Web References: https://doi.org/10.1103/PhysRevC.111.054620

References: Physical Review C, DOI: 10.1103/PhysRevC.111.054620

Image Credits: Institute of Science Tokyo

Keywords

Nuclear physics, Fission reactions, Nuclear reactions, Isotopes, Particle physics, Physical sciences