Identifying Mass-Asymmetric Fission Across the Nuclide Chart

Abstract

greatly extended the known location of mass-asymmetric fission observed across the nuclide chart. To determine what shell effects drive fission in the pre-actinide region --- and how they relate to fission in the actinides --- it is imperative that extensive and detailed experimental measurements of fission mass and kinetic energy distributions are made. In this thesis, I present two detailed experimental investigations of fission distributions in the pre-actinides. In order to determine how fission mass distributions evolve with excitation energy in this region, I have measured mass-asymmetric fission of 205, 207, 209Bi at energies close to the fission barrier using proton bombardment reactions on 204, 206, 208Pb. Through fitting the measured mass distributions, I present conclusive evidence of multi-modal fission in 205, 207, 209Bi, with the identification of a mass-symmetric and a mass-asymmetric fission mode. The measured mass distributions show a rapid evolution over a 10 MeV range of excitation energy, with the probability of symmetric fission increasing from from 40\% to 70\%. This evolution is much more rapid than was expected in this region, and highlights the importance of low excitation energy studies in determining the presence of multiple fission modes. Through analysis of a high-statistics measurement of the fission of 178Pt, I demonstrate the necessity of fitting the two-dimensional mass and kinetic energy distribution to unambiguously identify the presence of multiple fission modes in this region. Through this investigation I resolved discrepancies between published works on the number of fission modes in 178Pt, determining conclusively that three fission modes are present. To determine the extent of the new region of mass-asymmetric fission in sub-lead nuclides, I performed a large experimental systematic study of fission mass-kinetic energy distributions forming compound nuclei ranging from 144Gd through to 212Th. Using the newly developed two-dimensional fitting procedure, multi-modal mass-asymmetric fission is observed in every system, dramatically expanding the known region of nuclei that fission asymmetrically. It is concluded that quadrupole deformed proton shell gaps at Z=34, 36 and Z=42, 44 are responsible for the inner mass-asymmetric fission mode, whilst the outer asymmetric mode is found to correlate strongly with both Z=28 and N=38. This thesis provides robust evidence and methods to interrogate the plethora of quantum shell effects driving fission across the chart of the nuclides. Show more