Experimental determination of fission probabilities for proton-rich nuclei near the N=126 shell

In a study of Ar-induced fusion reactions, production cross-sections and Qsub(α) values for proton-rich evaporation residues in the region 84<=Z<=91 and N<=127 were determined. Particularly by use of the 176, ....,180Hf isotope chain as target nuclei different Th isotopes across the 126 neutron shell could be produced as evaporation residues. Though there is a pronounced shell effect of about 5 MeV observed in the ground-state masses, no enhancement in the measured cross-sections of evaporation residues around N=126 is observed. The lack of influence of the ground-state shell effect on the evaporation residue cross-sections is discussed in terms of the collective enhancement of the level density of deformed nuclei. - Moreover, surprisingly small cross-sections of very-neutron-deficient evaporation residues indicate that for these nuclei far from stability the fission barriers are overestimated by the droplet model. (author)