Dynamics of heavy-ion fusion probed by d/p double ratios from a cross bombardment

The particle decay ensuing from the reactions 86.0 MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{148}{}_{}{}^{}\mathrm{Sm}$ and 239.1 MeV ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$ + ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ was studied. These reactions each form ${}_{}{}^{164}{}_{}{}^{}\mathrm{Yb}$ compound nuclei excited to ≊ 54 MeV. Particle decay from compound nucleus producing reactions was selected by gating on the gamma-ray fold and the angular region of the particle emission. While there are no discernable differences in the dominant decay channels between the two reactions, there are fewer deuterons from the more symmetric system. This difference can be interpreted two ways: as a suppression of the emission of energetically expensive clusters during the time required for shape equilibration (which is predicted to be longer for the more symmetric entrance channel), or as an enhancement of the emission of energetically expensive clusters from the more asymmetric system at the very early stage of the collision when the initial energy deposited is only available to a reduced number of nucleons. The first explanation is identical to that used in recent high energy photon work while the second could be identified as the result of the emission of clusters on the multistep compound branch leading to the fusion of the low energy heavy ions. If the first explanation is adopted, the observed suppression is larger than predicted by a standard statistical decay model coupled to a dynamical fusion model, but consistent with work using high energy photons as a probe of fusion dynamics.