Search for entrance channel effects in sub-barrier fusion reactions

In order to explore the possibility that previously observed anomalous mean angular momenta are related to the mass asymmetry of the entrance channel, we have studied three systems, ²⁸Si + ¹⁴²Ce, ³²S + ¹³⁸Ba, and ⁴⁸Ti + ¹²²Sn, all of which lead to the same compound nucleus, ¹⁷⁰Hf. Gamma ray multiplicities for fusion have been determined using an electrostatic deflector and time of flight to identify the evaporation residues. Results have been obtained for bombarding energies from approximately 20% above the Coulomb barrier down to nearly 10% below the barrier. The mean angular momenta deduced from the gamma ray multiplicities are generally well reproduced by coupled channels calculations employing known deformation parameters and an estimate of the transfer strength and do not show any evidence of an unexpected effect related to the mass asymmetry of the entrance channel.