Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of 48 Ca, 50 Ti, and 54 Cr with 208 Pb. Moving from 48 Ca to 54 Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured massangle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P CN (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for 50 Ti and 15 for 54 Cr in comparison to 48 Ca. The energy dependence of P CN indicates that cold fusion reactions (involving 208 Pb) are not driven by a diffusion process.

Background: Fission fragments from heavy ion collisions with actinide nuclei show mass-asymmetric and mass-symmetric components. The relative probabilities of these two components vary rapidly with beam energy with respect to the capture barrier, indicating a strong dependence on the alignment of the deformed nucleus with the partner in the collisions. Purpose: To study the characteristics of the mass-asymmetric quasifission component by reproducing the experimental mass-angle distributions to investigate mass evolution and sticking times. Methods: Fission fragment mass-angle distributions were measured for the 34 S + 232 Th reaction. Simulations to match the measurements were made by using a classical phenomenological approach. Mass ratio distributions and angular distributions of the mass-asymmetric quasifission component were simultaneously fit to constrain the free parameters used in the simulation. Results: The mass-asymmetric quasifission component-predominantly originating from tip (axial) collisions with the prolate deformed 232 This found to be peaked near A = 200 at all energies and center-of-mass angles. A Monte Carlo model using the standard mass equilibration time constant of 5.2 × 10 −21 s predicts more symmetric mass splits. Three different hypotheses assuming (i) a mass halt at A = 200, (ii) a slower mass equilibration time, or (iii) a Fermi-type mass drift function reproduced the main experimental features. Conclusions: In tip collisions for the 34 S + 232 Th reaction, mass-asymmetric fission with A ∼ 200 is the dominant outcome. The average sticking time is found to be ∼7 × 10 −21 s, independent of the scenario used for mass evolution.

Background:Reactions with stable beams have demonstrated a strong interplay between nuclear structure and fusion. Exotic beam facilities open new perspectives to understand the impact of neutron skin, large isospin, and weak binding energies on fusion. Microscopic theories of fusion are required to guide future experiments.Purpose: To investigate new effects of exotic structures and dynamics in near-barrier fusion with exotic nuclei.Method: Microscopic approaches based on the Hartree-Fock (HF) mean-field theory are used for studying fusion barriers in 40−54 Ca+ 116 Sn reactions for even isotopes. Bare potential barriers are obtained assuming frozen HF ground-state densities. Dynamical effects on the barrier are accounted for in time-dependent Hartree-Fock (TDHF) calculations of the collisions. Vibrational couplings are studied in the coupled-channel framework and near-barrier nucleon transfer is investigated with TDHF calculations.Results: The development of a neutron skin in exotic calcium isotopes strongly lowers the bare potential barrier. However, this static effect is not apparent when dynamical effects are included. On the contrary, a fusion hindrance is observed in TDHF calculations with the most neutron rich calcium isotopes which cannot be explained by vibrational couplings. Transfer reactions are also important in these systems due to charge equilibration processes.Conclusions: Despite its impact on the bare potential, the neutron skin is not seen as playing an important role in the fusion dynamics. However, the charge transfer with exotic projectiles could lead to an increase of the Coulomb repulsion between the fragments, suppressing fusion. The effect of transfer and dissipative mechanisms on fusion with exotic nuclei deserve further studies.

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In collisions of light, stable, weakly bound nuclides, complete fusion (capture of all of the projectile charge) has been found to be suppressed by ∼30% at above-barrier energies. This is thought to be related to their low thresholds for breakup into charged clusters. The observation of fusion suppression in the neutron-rich radioactive nucleus 8 Li is therefore puzzling: the lowest breakup threshold yields 7 Li + n which cannot contribute to fusion suppression because 7 Li retains all the projectile charge. In this work, the full characteristics of 8 Li breakup in reactions with 209 Bi are presented, including, for the first time, coincidence measurements of breakup into charged clusters. Correlations of cluster fragments show that most breakup occurs too slowly to significantly suppress fusion. However, a large cross section for unaccompanied α particles was found, suggesting that charge clustering, facilitating partial charge capture, rather than weak binding is the crucial factor in fusion suppression, which may therefore persist in exotic nuclides.