Dependence of the prompt fission γ-ray spectrum on the entrance channel of compound nucleus: Spontaneous vs. neutron-induced fission

Prompt γ-ray spectra were measured for the spontaneous fission of 240,242Pu and the neutron-induced fission of 239,241Pu with incident neutron energies ranging from thermal to about 100 keV. Measurements were made using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments using a parallel-plate avalanche counter. The unfolded prompt fission γ-ray energy spectra can be reproduced reasonably well by Monte Carlo Hauser-Feshbach statistical model for the neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. The description of measurements and the discussion of results under the framework of a Monte Carlo Hauser-Feshbach statistical approach are presented.