The Angular Momentum of Fission Fragments and its Effects on Neutron-Gamma Emission
Marin, Stefano
2022
Abstract
The emission of neutrons and gamma rays accompanying nuclear fission is a process that has been intensely studied since the discovery of fission. Surprisingly, some of the aspects of n-gamma emission are still largely unknown and thus hard to model. The obstacle that hinders our efforts to model n-gamma emission is the role played by the fragments' angular momenta. The characterization of this intrinsic property of the fragments is experimentally challenging and, as a result, the experimental evidence is riddled with inconsistent observations. The purpose of this dissertation is to work towards a resolution of the issue of angular momentum in fission. We proceed to do so by presenting experimental and theoretical evidence of the effects of angular momentum on n-gamma multiplicities, energy, and angular distribution, which are the observables of interest in fission modeling. We analyze data from three experiments, looking at three different observables regarding the correlations of fragment angular momenta with excitation energy. The first result is an event-by-event analysis of n-γ correlations in 252Cf(sf), measured with the Chi-Nu array at Los Alamos National Laboratory. The second result, also collected with Chi-Nu, is an analysis of gamma-ray emission with incident neutron energy in 239Pu(n,f). Lastly, we have performed an experiment using an array of organic scintillators, the FS-3, in coincidence with a twin Frisch-gridded ionization chamber, a sensitive fragment detector. The results of this dissertation have revealed a relationship between angular momentum and excitation energy that is more complicated than previously suspected. In fact, we find evidence of a saturated angular momentum mechanism; the angular momentum depends on the excitation energy of the nucleus only up to a certain energy, above which the angular momentum remains constant. We have also determined that while the angular momentum is usually polarized in a plane perpendicular to the fission axis, there exist significant depolarization effects when one of the fragments approaches sphericity. Finally, we explain the observations made in this dissertation by introducing a theoretical model of angular momentum generation based on longitudinal vibrational modes of fissioning systems. The mechanism we propose leads to fragment angular momenta that are predominantly parallel to one another, an observable that will be pursued in future work. Furthermore, we also explain how in induced fission larger values of fragment angular momenta can be reached. The results of this dissertation shed light on the importance of the scission process in determining the fragment angular momenta, an important theoretical challenge. Throughout, we also show the impact that angular momentum has on the emission and measurement of neutrons and gamma rays, an important practical and technological challenge.Deep Blue DOI
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Nuclear Fission Angular Momentum Event-by-Event Correlations
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