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 neutron-induced fission of $^{239,241}$Pu with incident neutron energy from thermal to about 100 keV and spontaneous fission of $^{240,242}$Pu using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. The unfolded prompt fission γ-ray spectra can be reproduced reasonably well by Monte Carlo Hauser-Feschbach statistical model for 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. A supportive evidence is provided by the unfolded 2-D spectrum of total γ-ray energy vs multiplicity where the γ-ray multiplicity distribution has a tail extended to higher multiplicity for neutron-induced fission channel.