Emission of unstable clusters from hot Yb compound nuclei - art. no. 024611

Neutrons and isotopically resolved light charged particles have been detect ed in coincidence with evaporation residues produced in the reaction E/A = 11 MeV Ni-60 + Mo-100. Multiplicities of evaporated particle unstable clust ers have been determined from correlations in the emission of these light p articles. The decay of the short-lived He-5 and Br-8 (E* = 3.04 MeV) states was found to be affected by the Coulomb field of the compound nucleus in a ccordance with theoretical estimates. The contributions to the measured kin etic-energy distributions of stable fragments from the sequential decay of the unstable clusters was examined. Overall, the contributions from seconda ry fragments do not greatly influence the spectral shapes and specifically the location of the spectral peaks are not significantly shifted down in en ergy due to the presence of these secondary fragments. Therefore contrary t o the suggestion of Charity et al. [Phys. Rev. C 56, 873 (1997)], the lower peak energy of the experimental alpha -particle spectrum as compared to st andard statistical-model calculations cannot be attributed to sequential al pha particles from He-5 and other clusters. Only for the extreme "sub-barri er" regions of the alpha -particle, deuteron, Li-6.7, and Be-8 spectra was the sequential contribution found to be dominant. Statistical-model calcula tions incorporating large initial deformations are shown to provide enhance ments in the yield of low-energy fragments which are roughly appropriate fo r all the detected isotopes. This suggests that the origin of the sub-barri er enhancements may be a result of evaporation from highly deformed systems which are either produced dynamically during the fusion process or by ther mal shape fluctuations.