Reaction mechanism study for multinucleon transfer processes in collisions of spherical and deformed nuclei at energies near and above the Coulomb barrier: The O-16 + Sm-154 reaction

Background: Multinucleon transfer reactions at energies around the Coulomb barrier offer a vital opportunity to study the rich physics of nuclear structure and dynamics, e.g., single-particle level structure and quantum shells, mass and charge equilibration processes, energy dissipation, as well as secondary decays via particle emission or fission. Despite the continuous development in the field, we still have limited knowledge about how deformation-one of the representative nuclear structures-affects multinucleon transfer reactions. Purpose: To develop our understanding of the reaction mechanism and to shed light on the effect of deformation in multinucleon transfer processes, we study the O-16 + Sm-154 reaction at E-lab = 85 MeV (near the Coulomb barrier) and 134 MeV (substantially above the Coulomb barrier), where the target nucleus Sm-154 is a well-established, deformed nucleus. Methods: We have performed experiments on the O-16 + Sm-154 reaction at the BARC-TIFR pelletron-Linac accelerator facility, Mumbai, India, measuring angular distributions and Q-value spectra for various transfer products. The measured cross sections have been analyzed along with theoretical calculations based on the time-dependent Hartree-Fock (TDHF) theory, together with a statistical model for secondary deexcitation processes, GEMINI++. Results: Angular distributions for elastic scattering and for various transfer channels were measured over a wide angular range. The Q-value- and angle-integrated isotope production cross sections have been extracted from the measured angular distributions. We obtained production cross sections for various isotopes for E-lab = 85 MeV, while only for four isotopes could be deduced for E-lab = 134 MeV due to present experimental limitations. For the lower incident energy case, we find a reasonable agreement between the measurements and the TDHF calculations for a-few-nucleon transfer channels; whereas TDHF underestimates cross sections for many-nucleon transfers, consistent with earlier works. On the other side, we find that calculated cross sections for secondary reaction products for the higher incident energy case qualitatively explains the measured trends of isotopic distributions observed for the lower energy. The latter observation indicates possible underestimation of excitation energies in the present TDHF + GEMINI analysis. Although certain orientation effects were observed in TDHF results, it turns out to be difficult to disentangle them from the Q-value- and angle-integrated production cross sections. Conclusions: The present analysis highlights the deep-inelastic character of multinucleon transfer processes and importance of secondary deexcitation processes. We show that the orientation effect in multinucleon transfer processes in the O-16 + Sm-154 reaction is rather weak and hard to disentangle from the present measured data. Further systematic investigations, especially in the subbarrier energy regime, where the data would be more sensitive to single-particle properties, would be required to uncover effects of nuclear deformation on multinucleon transfer processes in low-energy heavy-ion reactions.