Study of the role of complete fusion in the reaction of calcium-48 and iron-56 with cerium and terbium

48Ca and 56Fe beams from the SuperHILAC accelerator were used to irradiate thick metal foils of cerium and terbium. Product gamma-ray activities were detected off line, and individual products were identified by half-life gamma-ray energy, and gamma-ray abundances. The production cross sections were iteratively fit to charge and mass dispersions to allow correction for parent decay and calculation of mass yields. From the mass yield curves contributions from quasielastic transfer, deep inelastic transfer, and complete fusion reaction mechanisms were inferred. Complete fusion was made up of contributions from both evaporation residue and fusion-fission products for the 48Ca-induced reactions. However, only fusion-fission products were detected in the 56Fe-induced reactions. Critical angular momenta for fusion were found to be 82 +- 8 h-bar for 48Ca + 159Tb and 34 +- 5 h-bar for 56Fe + 140Ce, which can be compared with previous values of 53 +- 8 h-bar for 12C + 197Au and 86 +- 5 h-bar for 40Ar + 165Ho. All of these reactions lead to essentially the same compound nucleus, and seem to show the dramatic decline in complete fusion for heavy ions larger than 40Ar. The prediction of this decline was found to be beyond the model calculations of Bass and the critical distance approach of Glas and Mosel