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Selected results recently obtained in studies of heavy ion transfer reactions at energies close to the Coulomb barrier are presented. In particular, the production of neutron-rich heavy nuclei via multinucleon transfer processes in 206Pb+118Sn, 197Au+130Te, and 94Rb+208Pb is discussed.

Multinucleon transfer reactions for the 206Pb +118 Sn system were measured at Elab = 1200 MeV using the PRISMA large solid-angle magnetic spectrometer. The experiment was conducted at laboratory angles around the grazing angle, covering an angular range of approximately 20°. The resulting differential and total cross sections, along with Q-value distributions for various neutron and proton pick-up and stripping channels, are presented. The Q-value distributions suggest a transition from quasi-elastic to deep inelastic collision processes, particularly in channels involving nucleon transfers. The experimental results have been compared with GRAZING code calculations, showing good agreement for few-nucleon transfer channels, while channels with large nucleon transfers are underestimated, indicating the involvement of more complex processes.

In recent years, a number of both theoretical and experimental investigations have been performed focusing on the zirconium isotopic chain. In particular, state-of-the-art Monte Carlo shell-model calculations predict shape coexistence in these isotopes. In this context, the 94Zr nucleus, which is believed to possess a nearly spherical ground state, is particularly interesting since the purported deformed structure is basedon the low-lying 02+ state, making it amenable for detailed study. In order to provide definitive conclusionson the shapes of the low-lying states, two complementary experiments to study 94Zr by means of low-energy Coulomb excitation were performed. This data will allow the quadrupole moments of the 21,2+ levels to be extracted as well as for the deformation parameters of the 01,2+ states to be determined and, thus, definitive conclusions to be drawn on the role of shape coexistence in this nucleus for the first time. The first experiment was performed at the INFN Legnaro National Laboratory with the GALILEO-SPIDER setup, which, for the first time, was coupled with 6 lanthanum bromide scintillators (LaBr3:Ce) in order to maximize the γ-ray detection effciency. The second experiment was performed at the Maier-Leibnitz Laboratory (MLL) in Munich and used a Q3D magnetic spectrograph to detect the scattered 12C ions following Coulomb excitation of 94Zr targets.

We present selected results recently obtained in the study of heavy ion transfer reactions at energies close to the Coulomb barrier by employing the large solid angle magnetic spectrometer PRISMA. We discuss the production of neutron-rich heavy nuclei via multinucleon transfer processes and the related effects of secondary processes, in particular nucleon evaporation, studied in a high resolution kinematic coincidence experiment. We also present the recent results in the studies of neutron-neutron correlations for closed shell and superfluid systems.

A detailed comparative study of the sub-barrier fusion of the two near-by systems 36S+50Ti,51V was performed at the National Laboratories of Legnaro (INFN). Aim of the experiment was the investigation of possible effects of the non-zero spin of the ground state of the 51V nucleus on the sub-barrier excitation function, and in particular on the shape of the barrier distribution. The results sh w that the two measured excitation functions are very similar down to the level of 20 - 30 μb. The same is observed for the two barrier distributions. Coupled-channels calculations have been performed and are in good agreement with the experimental data. This result indicates that the low-lying levels in 51V can be interpreted in the weak-coupling scheme, that is, 51V(I) = 50Ti(2+)⊗ p(1 f7/2).

Transfer reactions have always been of great importance for nuclear structure and reaction mechanism studies. So far, in multinucleon transfer studies, proton pickup channels have been completely identified in atomic and mass numbers at energies close to the Coulomb barrier only in few cases. We measured the multinucleon transfer reactions in the 40Ar+208Pb system near the Coulomb barrier, by employing the PRISMA magnetic spectrometer. By using the most neutron-rich stable 40Ar beam we could populate, besidesneutron pickup and proton stripping channels, also neutron stripping and proton pickup channels. Comparison ofcross sections between different systems with the 208Pb target and with projectiles going from neutron-poor to neutron-rich nuclei, as well as between the data and GRAZING calculations, was carried out.Finally, recent results concerning the measurement of the excitation function from the Coulomb barrier to far below for the 92Mo+54Fe system, where both proton stripping and pickup channels were populated with similar strength, will be discussed.

58Ni +64Ni is the first case where the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced, in a very well known experiment by Beckerman et al., by comparing with the two systems 58Ni + 58Ni and 64Ni+64Ni. Subsequent measurements on 64Ni + 64Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58Ni + 64Ni down to the measured level of 0.1 mb. In the present experiment the excitation function has been extended by two orders of magnitude downward. The cross sections for 58Ni + 64Ni continue decreasing very smoothly below the barrier, down to '1 µb. The logarithmic slope of the excitation function increases slowly, showing a tendency to saturate at the lowest energies. No maximum of the astrophysical S -factor is observed. Coupled-channels (CC) calculations using a Woods-Saxon potential and includinginelastic excitations only, underestimate the sub-barrier cross sections by a large amount. Good agreement is found by adding two-neutron transfer couplings to a schematical level. This behaviour is quite different from what already observed for 64Ni+ 64Ni (no positive Q-value transfer channels available), where a clear low-energy maximum of the S -factorappears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in 58Ni+ 64Ni in the measured energy range. This trend at deep sub-barrier energies reinforces the recent suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.

The multinucleon transfer reaction mechanism was employed to populate isotopes around the doubly-magic 208 Pb nucleus. We used an unstable 94 Rb beam on 208 Pb targets of different thickness. Transfer channels were studied via the fragment-γ and γ-γ coincidences, by using MINIBALL γ spectrometer coupled to a particle detector. Gamma transitions associated to the different Pb isotopes, populated by the neutron transfers, are discussed in terms of excitation energy and spin. Fragment angular distributions were extracted, andcompared with the reaction models.