Transition from asymmetric to symmetric fission in the ²³⁵U(n,f) reaction

Prompt γ rays from the neutron-induced fission of ²³⁵U have been studied using the GEANIE spectrometer situated at the LANSCE/WNR "white" neutron facility. Transitions characteristic of specific fission fragments were identified using a combination of single-fold and coincidence γ-ray analysis. Gamma-ray production cross sections were obtained as a function of incident neutron energy using the time-of-flight technique. Yields were extracted for 206 γ rays produced by 56 distinct fission fragments for 1 ≤ E_n(MeV) ≤ 250. Fission-channel cross sections were deduced for a subset of 22 even-even fragments, where the measured γ-ray yields represent a large fraction of the full de-excitation strength of the fragment. Independent yields were deduced from these cross sections and fitted with standard formulations of the fragment charge and mass distributions. The results were used to study the transition from asymmetric to symmetric fission as a function of internal excitation energy of the fissioning system. This transition is interpreted in the context of the disappearance of shell structure at high excitation energies.