Session S13 - General Physics.
ORAL session, Monday afternoon, April 30
Room 12-13, Renaissance Hotel
Characteristic fission-fragment \gamma rays following the ^235U(n,f) reaction have been measured using GEANIE at LANSCE/WNR. These data have been used to study the disappearance of shell effects in the fissioning nucleus, as a function of internal excitation energy. Prompt \gamma-ray yields have been extracted for 206 transitions produced by 56 fission fragments as a function of incident neutron energy in the E_n = 1--250-MeV range. A subset of 29 ground-state transitions were used to extract fission yields for 22 fragments. These yields were fitted with well-established charge- and mass-distribution models, as a function of incident neutron energy, to map the phase transition from asymmetric to symmetric fission. The evolution of the fissioning nucleus from a quantum system, dominated by shell effects, to a classical fluid is explored based on the fission modes deduced from the GEANIE data.