Interplay of spherical closed shells and N/Z asymmetry in quasifission dynamics

Background: Quasifission (QF) has gained tremendous importance in heavy-ion nuclear physics research because of its strong influence on superheavy-element synthesis. Collisions involving closed-shell nuclei in the entrance channel are found to affect the QF reaction mechanism. Hence, it is important to improve the understanding of their effect on QF. Apart from that, some recent studies show that the difference in N/Z of reaction partners influences the reaction dynamics. Since heavier doubly magic nuclei have different N/Z than lighter doubly magic nuclei, it is important to understand the effect of N/Z mismatch as well as the effect of shell closures. Purpose: To investigate the effect of entrance-channel shell closures and N/Z asymmetry on QF. The reactions were chosen to decouple these effects from the contributions of other entrance-channel parameters. Method: Fission fragment mass-angle distributions were measured using the CUBE fission spectrometer, consisting of two large area position-sensitive multi-wire proportional counters (MWPCs), for five reactions, namely, Cr50+Pb208, Cr52+Pb206,208, Cr54+Pb204,208. Result: Two components were observed in the measured fragment mass angle distribution, a fast mass-asymmetric quasifission and a slow mass-symmetric component having a less significant mass-angle correlation. The ratio of these components was found to depend on spherical closed shells in the entrance channel nuclei and the magnitude of the N/Z mismatch between the two reaction partners, as well as the beam energy. Conclusions: Entrance-channel spherical closed shells can enhance compound nucleus formation provided the N/Z asymmetry is small. Increase in the N/Z asymmetry is expected to destroy the effect of entrance-channel spherical closed shells, through nucleon transfer reactions.