Understanding Angular Momentum in Nuclear Fission

Aaron Gallant | 20-LW-045

Project Overview

While many basic properties of nuclear fission have been known for more than 80 years, the generation of angular momentum during fission remains a mystery. We conducted a combined theoretical and experimental effort to address this lack of understanding by focusing on the spontaneous fission of 252Cf. This was accomplished by developing a novel ion-counting technique at the Canadian Penning Trap (CPT) at Argonne National Laboratory. This novel technique enables mass-resolved, single-ion counting of both the isomeric and ground states of relaxed fission fragments. In many ways, this new technique is superior to conventional techniques. For example, an entire measurement cycle can occur within a few hundred milliseconds, eliminating the need for fast radiochemical-separation techniques, and, as the fragments are counted directly, well characterized nuclear data is not required. This novel ion-counting technique was used to measure the isomer-to-ground-state-population ratio of 102Nb. Using time-dependent nuclear density functional theory, the first microscopic calculations of angular momentum distributions in fission fragments for a wide range of fragment masses were completed for the benchmark case of neutron induced fission of 239Pu and the impact of restoring broken symmetries was explored in the benchmark case of neutron induced fission of 240Pu. These results highlight the need to use predictive fission models to guide both the evaluation of nuclear data and nuclear-structure experimental studies at radioactive beam facilities.

Mission Impact

This effort positions LLNL to continue the program of isomer yield ratio measurements with planned upgrade to nuCARIBU at Argonne National Laboratory, allowing for studies on neutron-induced fission on uranium and plutonium. With this effort we were able to: (1) develop the novel ion-counting techniques required to make isomer to ground state yield ratio measurements using the Canadian Penning Trap and (2) performed the first microscopic calculation of angular-momentum distributions for a wide range of fission-fragment masses, revealing the large impact of the underlying shell structure of the fission fragments at scission on their angular momentum distributions.

Publications, Presentations, and Patents

Marevic, P. and N. Schunck. 2020. "Fission of 240Pu with Symmetry-Restored Density Functional Theory." Physical Review Letters 125, 102504 (2020). IM# 1014647.

Verriere, M. et al. 2020. "Initial Conditions for the Deexcitation of Fission Fragments." CSWEG 2020. Virtual. December 2020. LLNL-PRES-817101.