SEARCH FOR ENTRANCE CHANNEL EFFECTS IN THE DECAY OF THE YB-164 COMPOUND NUCLEUS AT E-ASTERISK-APPROXIMATE-TO-54 MEV

The existence of entrance channel effects in the decay of 164Yb*, formed at E* almost-equal-to 54 MeV in the reactions O-16 + Sm-148 and 64Ni + Mo-100 is investigated. Evaporation residue cross sections, entry state gamma-ray fold distributions as well as energy and angular distributions of exit-channel selected charged particles were obtained for each reaction, using 4pi detection systems for both particles and gamma rays. In the 64Ni-induced reaction, the center-of-mass alpha-particle angular distributions were found to be symmetric around 90-degrees indicating emission from a fully equilibrated compound nucleus. However, the corresponding angular distributions in the O-16-induced reaction show a forward component which exclusively affects the population of the alphaxn residue channels. The gamma-ray fold distributions for the alphaxn and xn channels show differences depending on the entrance channel as reported previously for similar systems. These differences can be understood in terms, of contributions from the equilibrium decay of incompletely fused O-16 with Sm-148 and the mapping of the compound nucleus spin distributions, obtained from realistic fusion models, to the evaporation residue gamma-ray fold distributions. It is concluded that the initial population of the compound nucleus is the only reason for the observed differences in the decay of 164Yb* populated in these reactions.