Evolution of multimodal fission with energy in Np 238 populated by Li 6 + Th 232

Background: Nuclear fission is influenced by shell effects. Fission modes are a strong signature of the compound nucleus formation in heavy ion induced reactions. The evolution of the relative strengths of the fission modes with excitation energy is a matter of intense interest. Purpose: We investigate the signatures of fission modes in Np238 populated by the Li6+Th232 reaction through the mass-total kinetic energy distribution. Method: The mass-total kinetic energy distributions of fission fragments of the reaction Li6+Th232 are measured at four laboratory energies, Elab=28.5, 40, 45, and 62.5 MeV. Mass-total kinetic energy distributions of Li6+Th232 are described by the multimodal random neck rupture model. Results: Channel probabilities of different fission modes are obtained obtained through a two-dimensional fitting procedure. The contribution of the standard 1 (S1) mode is found to become ≈2% at Elab=40 MeV. The heavy fragments of S1 and standard 2 (S2) modes are found to be associated with Z≈52 and Z≈55 shells, respectively. The slope of the asymmetric to symmetric fission yields ratio with the excitation energy of Li6+Th232 is found to be similar to that of O18+Pb208 (previously reported). Conclusions: The analysis of mass-total kinetic energy distribution data reveals the presence of fission modes in Li6+Th232. The average kinetic energy release in fission obtained from Viola systematic matches well with the one of the of S2 mode. The liquid-drop-like broad symmetric (SL) mode is found to peak at a lower energy than predicted by Viola systematic. This is associated with the decrease of the total kinetic energy in the asymmetric fission mode due to the fading out of shell effects with increasing excitation energy.