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. Gamma-ray production cross sections for 29 ground-state-band transitions in 18 even-even fission fragments were obtained as a function of incident neutron energy, using the time-of-flight technique. Independent yields were deduced from these cross sections and fitted with standard formulations of the fragment charge and mass distributions to study the transition from asymmetric to symmetric fission. The results are interpreted in the context of the disappearance of shell structure at high excitation energies.