Reaction mechanism in the20Ne+59Co system at 3--7 MeV/nucleon, and observation of entrance-channel mass-asymmetry of the incomplete fusion fraction

Abstract

Incomplete fusion of $^{20}\mathrm{Ne}$ with $^{59}\mathrm{Co}$ has been investigated at 3--7 MeV/nucleon using the measurement and analysis of excitation functions. The recoil-catcher technique followed by offline gamma-ray spectroscopy has been employed. Evaporation residues are found to have contributions from precursor decays, which have been separated out from the measured cumulative cross sections of evaporation residues. Measured independent cross sections are compared with PACE-2 predictions. The PACE-2 calculations are carried out for evaporation residues formed in complete fusion (CF), and the parameters are optimized so as to reproduce the cross section of evaporation residues produced exclusively in CF, e.g., $\mathit{xn}$ and pxn products. With these parameters, the predicted CF cross sections for alpha emission products are calculated. Any substantial enhancement in the experimental cross section over the PACE-2 prediction is taken as a signature of incomplete fusion (ICF). The analysis indicates the occurrence of incomplete fusion involving the breakup of $^{20}\mathrm{Ne}$ into $^{16}\mathrm{O}$ $+$ $^{4}\mathrm{He}$ and/or $^{12}\mathrm{C}$ $+$ $^{8}\mathrm{Be}$(2\ensuremath{\alpha}) followed by fusion of one of the fragments with the target nucleus $^{59}\mathrm{Co}$. These data also suggest that the probability of incomplete fusion increases with the projectile energy. Moreover, the ICF probability is found to increase with entrance-channel mass-asymmetry of the projectile-target systems.