Hindered Prompt-Neutron Evaporation in Surrogate Reactions for $^{239}$Pu(n,f)

Abstract

Isotopic fission-fragment distributions of $^{240}$Pu have been measured, for the first time, as a function of the initial excitation energy, and the prompt neutron multiplicity has been derived from these data. The $^{240}$Pu fissioning system was produced through the two-proton transfer reaction between $^{238}$U and $^{12}$C, a surrogate reaction for the neutron-capture-induced fission $^{239}$Pu(n,f). The reaction was measured in inverse kinematics, allowing the fission fragments to be fully identified with the VAMOS Spectrometer. When compared to neutron-capture-induced reactions, the observed prompt neutron multiplicity shows a clear reduction in the surrogate two-proton transfer, revealing an unexpected influence of the entrance channel in the fission output. At the same time, fission-fragment yield distributions obtained in neutron-capture-induced reactions show a relative fission-fragment production in the symmetry region similar to that measured in this work. The discrepancy in neutron multiplicity is attributed to the additional angular momentum induced in the multi-nucleon transfer reactions, which excites the fissioning system to higher-spin states, increasing the probability of gamma emission that competes with neutron evaporation, in particular from the fission barrier to the scission point. This observation underlines the limitations in the utilisation of properties derived from surrogate reactions in nuclear technology and other applications of nuclear fission.