R. Rafiei

Cluster transfer in the reaction 16 O + 208 Pb at energies well below the fusion barrier: A possible doorway to energy dissipation

The collision of the doubly-magic nuclei O+Pb is a benchmark in nuclear reaction studies. Our new measurements of back-scattered projectile-like fragments at sub-barrier energies show show that transfer of 2 protons (2p) is much more probable than α-particle transfer. 2p transfer probabilities are strongly enhanced compared to expectations for the sequential transfer of two uncorrelated protons; at energies around the fusion barrier absolute probabilities for two proton transfer are similar to those for one proton transfer. This strong enhancement indicates strong 2p pairing correlations in O, and suggests evidence for the occurrence of a nuclear supercurrent of two-proton Cooper pairs in this reaction, already at energies well below the fusion barrier.

Complete Characterization of Breakup of 9 Be by α-α Coincidence Measurements

The breakup mechanisms for 9Be have been studied through coincident detection of the breakup α particles at sub-barrier energies, where the probability of capture of a charged breakup fragment by the target nucleus is negligible. Combining the reconstructed reaction Q-value and the relative energy of the two coincident breakup α particles a complete picture of breakup dynamics has been obtained. The measurements reveal breakup processes which are fast enough (~ 10-22 s) to affect fusion, called prompt breakup. It is shown that prompt breakup is triggered predominantly by the transfer of a neutron -a two-step process. The prompt breakup probabilities are shown to have an exponential dependence on the surface-to-surface separation of the interacting nuclei, but are independent of the target nuclei, ranging from 144Sm to 209Bi. These results provide significant input towards a complete quantal model aiming to describe the breakup of weakly bound stable and halo nuclei.

Time-scales and mechanisms of breakup influencing fusion

Coincidence measurements of breakup of 6,7Li and 9Be incident on various high-Z targets have provided a complete picture of the physical mechanisms triggering breakup. The measurements identify those breakup processes that are fast enough (10−22) to affect fusion. The majority of these prompt breakup events are triggered by transfer of a neutron from 6Li, 9Be, and of a proton to 7Li. The dominance of breakup following transfer demonstrates that the reaction outcomes involving weakly bound nuclei depend not only on the properties of the two colliding nuclei, but also on the ground-state and excited state properties of their neighbours. This is a key insight for understanding and predicting reactions of weakly-bound nuclei near the limits of nuclear existence.

Fission and quasi-fission: insights into heavy element formation reactions

Mass‐angle distributions carry detailed information on the characteristics of quasi‐fission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussed.