Angular momentum limit of Hf isotopes produced in three fusion-evaporationreactions

The compound nucleus Hf-168 was populated in three fusion-evaporation react ions with different beam-target mass asymmetries: Ti-50 + Sn-118, Ni-64 + R u-104 and Ge-74 + Zr-94. Due to the large negative e values of these reacti ons the compound nucleus is formed at low excitation energy. At three or fo ur excitation energies for each reaction gamma -ray spectra of the evaporat ion residues Hf-166 to Hf-163, corresponding to the 2n to 5n exit channels, respectively. were recorded with the Ge detectors of the 8 pi -spectromete r array. The gamma -ray multiplicity and total energy were measured using t he inner ball of EGO detectors. This data was used to determine the maximum angular momentum transferred to each evaporation channel, the gamma -ray d ecay entry region and the relative cross sections. No differences are obser ved between the three reactions. This is explained by the very similar depe ndence of the excitation energy on the angular momentum of the compound nuc leus. The relative cross sections for the 2n and 3n channels are observed t o be significantly larger than predicted by statistical model calculations. (C) 2001 Elsevier Science B.V. All rights reserved.