Building a Predictive Fission Rate Model

Abstract

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, and we must therefore rely on model calculations. Yet all fission rates calculated today use a phenomenological (theoretical) model that was formulated in 1939, updated in 1953, and has remained essentially unchanged. Because the model contains many adjustable parameters, it has been successfully used to fit existing data, but in turn has very limited predictive power in cases where the data are poor or nonexistent. The goal of this project was to develop a novel approach that moves away from phenomenology to construct a fission-rate model built from microscopic ingredients: protons, neutrons, and their mutual interactions. The approach adopted in this work relied on the construction of a discrete basis of mean-field states that form paths through scission. The calculations were performed within the framework of the generator coordinate method. The project has produced 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.