Comparison of the energy and mass characteristics of the Pu239(nth,f) and the Pu240(sf) fragments

The energy and mass distributions and their correlations have been studied for the spontaneous fission of Pu240 and the thermal neutron induced fission of Pu239. A comparison of the Pu240(sf) and the Pu239(nth,f) results shows a narrower mass distribution, a much higher peak yield, a much lower symmetric fission yield, and a more pronounced fine structure for the spontaneous fission than for the neutron induced fission. The average total kinetic energy is 1.3 MeV higher for Pu240(sf) than for Pu239(nth,f), and also the energy-mass correlations behave differently in both cases. All these results are discussed and interpreted in the framework of the scission point model of Wilkins et al. Finally, the damping of the Pu240 fission mode below the barrier is demonstrated.