Building a Predictive Fission-Rate Model

Walid Younes | 18-LW-062


Nuclear fission, and specifically the rate at which fission occurs compared to other types of reactions, is a crucial piece in the puzzle of the origin of the elements in the universe. Fission in astrophysical environments involves rare and exotic nuclei that cannot be studied in the laboratory, requiring scientists to rely on model calculations. However, all fission rates calculated today use a phenomenological (theoretical) model that was formulated in 1939 and has remained essentially unchanged since its 1953 update. The model contains many adjustable parameters and has been used successfully to fit existing data. In turn, the model has very limited predictive power in cases where the data are poor or nonexistent.

Our project developed a novel approach that moves away from phenomenology to construct a fission-rate model built from microscopic ingredients: protons, neutrons, and their mutual interactions. Our approach relied on the construction of a discrete basis of mean-field states that form paths through scission. Calculations were performed within the framework of the generator coordinate method. The result is a methodology for constructing the discrete basis states and identifying likely fission paths, as well as estimates of the energy partition between fragments and a formalism to calculate fission rates based on Fermi's golden rule.

Impact on Mission

Our research positions Lawrence Livermore National Laboratory as a leader in nuclear data evaluations, which supports the Laboratory's core competencies in nuclear, chemical, and isotopic science and technology as well as its energy and counterterrorism mission areas. The Laboratory has already taken the lead in the development of a new format for nuclear data libraries, and the model developed under this project could eventually be used to produce the evaluated fission data in those libraries. The formalism developed under this project lays the groundwork for a more predictive approach to fission calculations with applications to r-process nucleosynthesis of the elements, evaluating high-precision measurements such as those performed using the Laboratory-funded fission Time-Projected Chamber, modeling fission reactions on rare actinides that cannot be directly studied in the laboratory, and selecting optimal reactions to create super-heavy elements, such as Livermorium, for which production is inhibited by fission.

Publications, Presentations, Etc.

Bertsch, G. and W. Younes. 2018. "Scission Dynamics with K Partitions." Physical Review C 97 (6): 064619-1-7. doi:10.1103/PhysRevC.97.064619. LLNL-JRNL-748809.

––– . 2019. "Composite-Particle Decay Widths by the Generator Coordinate Method." Annals of Physics 403: 68-81. doi:10.1016/j.aop.2019.01.014. LLNL-JRNL-759641.

Bertsch, G., et al. 2019. "Diabatic Paths Through the Scission Point in Nuclear Fission." Physical Review C, 100, 024607 . doi:10.1103/PhysRevC.100.024607. LLNL-JRNL-771816.

Younes, W. 2018. "A Basis for Scission Dynamics." Conference on Nuclear Reaction Mechanisms, Varenna, Italy. LLNL-PRES-752343.

––– . 2018. "A Grand Tour of Nuclear Fission Physics." 6th International Workshop on Compound-Nuclear Reactions and Related Topics, Berkeley, CA. LLNL-PRES-758573.

––– . 2018. "An Outlook for Fission Cross Section Theory and Experiments on Radioactive Nuclei." Fifth Joint Meeting of the Nuclear Physics Divisions of the APS and the JPS, Waikoloa Village, HI. LLNL-PRES-760016.