Angular momentum limit of Hf isotopes produced in three fusion–evaporation reactions

Abstract

The compound nucleus 168 Hf was populated in three fusion–evaporation reactions with different beam–target mass asymmetries: 50Ti + 118Sn, 64Ni + 104Ru and 74Ge + 94Zr. Due to the large negative Q values of these reactions the compound nucleus is formed at low excitation energy. At three or four excitation energies for each reaction γ-ray spectra of the evaporation residues 166Hf to 163Hf, corresponding to the 2n to 5n exit channels, respectively, were recorded with the Ge detectors of the 8 π-spectrometer array. The γ-ray multiplicity and total energy were measured using the inner ball of BGO detectors. This data was used to determine the maximum angular momentum transferred to each evaporation channel, the γ-ray decay entry region and the relative cross sections. No differences are observed between the three reactions. This is explained by the very similar dependence of the excitation energy on the angular momentum of the compound nucleus. The relative cross sections for the 2n and 3n channels are observed to be significantly larger than predicted by statistical model calculations.