Final PhD Seminar

Determining the Impact of Shell Structure on Nuclear Reactions

Mr Jacob Buete
Nuclear Reaction Dynamics Group

To understand observed and predicted differences in cross-section between 48Ca and 50Ti for forming superheavy elements (SHEs) --- and inform the choice of a suitable replacement for 48Ca for the next generation of SHEs --- I have developed a novel model for energy dissipation in fusion reactions based on the Landau-Zener Transition Model (LZTM). The LZTM enables the probabilistic treatment of the single nucleon transitions which occur in the fusion process and enables the creation of the Monte-Carlo-based dissipation model. These transitions may prove key to understanding the role of diabatic transitions and shell structure in suppressing or enhancing the formation of a compound nucleus. A key part of this model is the development of a new implementation of the Asymmetric Two-Centre Shell Model (ATCSM) which contains corrections to the original publication which have hindered its usage over the past 50 years. Novel improvements to both parameter selection for the basis elements and the calculation of the non-integer principle quantum numbers — a unique feature of the ATCSM — have also been explored.

Recent experimental efforts to measure multimodal mass-asymmetric fission --- led by the presence of multiple shells structure effects --- in the preactinides necessitated the development of a new analysis method, Panther, and a method of pseudodata generation. Panther provides a data-driven, iterative approach to determining the minimum number of fissionmodes present in a given measurement. Pseudodata generation may be used to benchmark the accuracy and precision of the results of general analysis methods. Initial results from pseudodata analysis on the effect of measurement size on the precision and accuracy of fitted results are presented. The effectiveness of a subsampling-based approach to determining uncertainties from fitting procedures for is also discussed for fission mass distributions.

I also present an analysis of the fission of 220Ra using low- and high-statistics measurements. The combination of Panther and pseudodata-based robustness testing shows clear evidence of both the standard-I and standard-II fission modes from the actinides. The result agrees with historic measurements of the same reaction but disagrees on the yield of the asymmetric fission. Using generated pseudodata based on the results of this work, the discrepancy in yields was found to be a result of the differing sizes of the two measurements.

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