Exploring mechanisms inhibiting nuclear fusion

Abstract

Understanding the mechanisms leading to the inhibition of fusion seen at sub- and above-barrier energies has been one of the prime questions in nuclear physics. The complementary process to fusion is scattering, and an analysis of the scattered flux in heavy-ion collisions may give insight into answering this question. An analysis of the projectile-like fragments detected at backward angles in the reactions 16O+208Pb and 32S+ 208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ΔZ = 1 and ΔZ = 2 were extracted, indicating surprisingly large absolute transfer probabilities already at sub-barrier energies. Excitation energies in the projectile-like fragments up to ∼ 15MeV for the 16O and ∼ 25MeV for 32S-induced reactions show the population of highly excited states in the residual nuclei, and indicate substantial dissipation of kinetic energy into nucleonic degrees of freedom. A new phe-nomenological framework explores the effect of energy dissipation through these highly inelastic (large excitation energies) and complex (few-nucleon transfer) processes on the probability for fusion in the reaction 16O+208Pb. Calculations combining both couplings to low-lying excited states within the coherent coupled channels framework and energy dissipation show that the suppression of fusion at energies above the barrier in this reaction can be explained within the new framework. Results furthermore suggest that in reactions relevant to astrophysical scenarios such as 12C+ 12C, fusion hindrance may not be present.