Fission fragment mass-angle correlations and mass ratio distributions have been measured for the reactions 16 O + 186 Os, 24 Mg + 178 Hf, 34 S + 168 Er, and 48 Ti + 154 Sm, forming the 202 Po composite nucleus, at near barrier energies. Systematic analysis based on the expected dependence of the variance of the mass distribution on the angular momentum and temperature of the compound nucleus indicate that the two lighter systems evolve through true compound nucleus fission. Evidence of quasifission was observed for the two most mass-symmetric reactions, through strong mass-angle correlations for the 48 Ti + 154 Sm reaction and a broadened mass ratio distribution for the 34 S + 168 Er reaction. Furthermore, the increase in mass width at near barrier energies shows the influence of the alignment of statically deformed target nuclei.
Coincidence measurements of breakup fragments were carried out for the 7 Li + 144 Sm and 6,7 Li + 207,208 Pb, 209 Bi reactions at sub-barrier energies. Breakup modes in reactions of 6,7 Li were identified through the reaction Q values, and the time-scales of each process inferred through the relative energy of the breakup fragments. Breakup was found to be predominantly triggered by nucleon transfer, with p pickup leading to α + α coincidences being the preferred breakup mode for 7 Li, and n stripping leading to α + p for 6 Li. Breakup triggered by 2n stripping was also found to be prominent in the 7 Li + 144 Sm reaction. The breakup yields were separated into prompt and delayed components based on the relative energies of the breakup fragments. This enables the identification of breakup process important in the suppression of complete fusion of 6,7 Li at above-barrier energies.
The reaction 16 O + 208 Pb is a benchmark in nuclear reaction studies as it involves two doubly magic nuclei. New measurements of back-scattered projectile-like fragments at sub-barrier energies show that the probability of two-proton (2p) transfer is much larger than that of α-particle transfer. At energies around the fusion barrier the probability for 2p transfer is ∼10%, similar to that for one-proton transfer. The 2p transfer probabilities are enhanced by up to an order of magnitude compared to calculations based on an independent particle picture as simulated by the fully microscopic time-dependent Hartree-Fock model (TDHF). Since beyond mean-field correlations like nucleon pairing are not included in the TDHF model, the enhancement indicates strong pairing correlations between the transferred protons. 2p transfer leads to excitation energies (most likely in the target-like nucleus) up to ∼13 MeV, indicating that it may represent an effective doorway for the dissipation of energy and thus provide a microscopic mechanism toward understanding the inhibition of fusion and energies both above and below the barrier.
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