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, form ed at E almost-equal-to 54 MeV in the reactions O-16 + Sm-148 and 64N i + Mo-100 is investigated. Evaporation residue cross sections, entry state gamma-ray fold distributions as well as energy and angular distr ibutions of exit-channel selected charged particles were obtained for each reaction, using 4pi detection systems for both particles and gamm a rays. In the 64Ni-induced reaction, the center-of-mass alpha-particl e angular distributions were found to be symmetric around 90-degrees i ndicating 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 ch annel as reported previously for similar systems. These differences ca n 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 reaso n for the observed differences in the decay of 164Yb populated in the se reactions.