Shape Coexistence in the Entrance Channel of Heavy-Ion Fusion: Coupled-Channels Calculations for Prolate, Oblate, and Spherical Minima

Abstract

Heavy-ion fusion at energies near and below the Coulomb barrier provides one of the most sensitive probes of nuclear structure available in low-energy nuclear physics. The coupling between the relative motion of the colliding nuclei and the intrinsic degrees of freedom of target and projectile is known to enhance sub-barrier fusion cross sections by orders of magnitude and to redistribute a single barrier into a characteristic distribution of barriers. In a growing number of nuclei across the chart, low-lying prolate, oblate, and spherical minima coexist within an energy window of a few hundred keV, so that the colliding system does not possess a single well-defined intrinsic shape in the entrance channel. This article develops a coupled-channels description in which the entrance channel couples simultaneously to several coexisting intrinsic configurations, each carrying its own collective band, and in

 which configuration mixing transfers strength between them. The formalism is built on the standard eigenchannel reduction of the coupled-channels equations and is used to compute fusion excitation functions, barrier distributions, and mean angular momenta for representative medium-mass and heavy systems. The results demonstrate that prolate, oblate, and spherical entrance configurations leave distinct fingerprints on the barrier distribution, that configuration mixing shifts the effective barrier centroid downward and smooths the distribution, and that the largest sub-barrier enhancement is obtained when all three minima participate coherently. These findings establish entrance-channel shape coexistence as a measurable contributor to sub-barrier fusion dynamics and clarify how multi-configuration structure must be incorporated into quantitative fusion calculations.