(d, p−f) Angular-Correlation Study of Fission-Barrier Transition States: Energy Gap of a Highly Deformed Nucleus

The ($d, p$) reaction was used to produce fissioning nuclei with excitations from the fission threshold to 5 MeV above. The angular correlation of the fission fragments and the reaction protons has been measured as a function of excitation. A theoretical correlation function is developed to permit values of the parameter ${K_0}^2$ to be derived from the observed angular anisotropy. For a ${\mathrm{Pu}}^{239}$ target the values obtained for ${K_0}^2$ show a stepwise increase with increasing excitation. The largest step occurs at ∼2.6 MeV above the fission threshold, and is interpreted as the beginning of quasiparticle excitations of the highly deformed fissioning nucleus. On this basis the pairing energy gap appears to be significantly increased for large nuclear deformations. Also, ${K_0}^2$ is observed to change at ∼0.7 and ∼1.6 MeV above threshold. It is suggested that these energies are the onset of collective vibrational transition states through which fission occurs.