Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Purpose: Through extensive mass-angle distribution measurements, we map out the systematic dependence of quasifission characteristics as a function of the identity of the colliding nuclei. Methods: The Australian National University's Heavy Ion Accelerator Facility and CUBE spectrometer have been used to measure mass-angle distributions for 42 reactions forming heavy elements. Beam energies above their respective capture barriers were used to minimize the known influence of nuclear structure effects. Results: Different mappings of mass-angle distribution characteristics (including timescales) over the nuclear landscape show a systematic dependence on entrance channel and compound nucleus fissilities. Conclusions: The results provide an empirical baseline to assess effects of nuclear structure at lower beam energies, and motivate the testing and validation of complete dynamical models of heavy element fusion through comparison of mass-angle distributions.
The fusion cross section for 12 C+ 13 C has been measured down to Ec.m.=2.6 MeV at which the cross section is of the order of 20 nb. By comparing the cross sections for the three carbon isotope systems, 12 C+ 12 C, 12 C+ 13 C and 13 C+ 13 C, it is found that the cross sections for 12 C+ 13 C and 13 C+ 13 C provide an upper limit for the fusion cross section of 12 C+ 12 C over a wide energy range. After calibrating the effective nuclear potential for 12 C+ 12 C using the 12 C+ 13 C and 13 C+ 13 C fusion cross sections, it is found that a coupled-channels calculation with the Incoming Wave Boundary Condition (IWBC) is capable of predicting the major peak cross sections in 12 C+ 12 C. A qualitative explanation for this upper limit is provided by the Nogami-Imanishi model and level density differences among the compound nuclei. It is found that the strong resonance found at 2.14 MeV in 12 C+ 12 C exceeds this upper limit by a factor of more than 20. The preliminary result from the most recent measurement shows a much smaller cross section at this energy which agrees with our predicted upper limit.
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.
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