Nuclear Structure Effects in the Quasi-Continuum Deexcitation Process of 152Dy and 156Dy Nuclei Produced in Complete and Incomplete Fusion Reactions

Abstract

Intensities and energy dependent multiplicity distributions of γ-rays have been measured for 152Dy and 156Dy nuclei populated in (12C, xn) and (12C, αxn) reactions at E = 70-101 MeV. The relative population of the yrast states in 156Dy is found to be almost independent of the input angular momentum, which is not the case for 152Dy. In the latter nucleus the yrast states up to spin values close to the maximum input angular momentum are strongly populated whereas in the former the transition intensities decrease very rapidly with increasing level spin. Quasi-continuum feeding cascades in 156Dy are found to have considerably higher multiplicity than those in 152Dy at comparable input angular momentum. These features can be understood in 156Dy by assuming the existence of a number of collective bands which funnel the γ-ray deexcitation below the entry region away from the high spin yrast states whereas in 152Dy short γ-ray cascades feed directly the yrast states. The entry regions in 156Dy have been determined as a function of bombarding energy for the (12C, 6n) reaction. A decrease of the entry angular momentum above 90 MeV beam energy is found and explained by the existence of strong competing (12C, αxn) incomplete fusion reaction channels. Similar γ-ray sidefeeding patterns and multiplicity distributions for yrast states are observed in 12C induced complete (CF) and incomplete fusion (ICF) reactions leading to 156Dy. However, the data for quasi-continuum feeding multiplicites indicate a more narrow initial angular momentum distribution for ICF than for CF reactions. The mean initial angular momentum for ICF is found to be about 10 h higher than the one for CF and it is close to the critical angular momentum for CF as deduced from a simple reaction model.