DAMPING OF UNBOUND SINGLE-PARTICLE MODES

The (alpha, He-3-n) reaction has been investigated at 120 MeV incident energy on Ni-64, Zr-90, and Sn-120 target nuclei. Neutrons in coincidence with He-3 particles emitted at 0 degrees were detected using the multidetector array EDEN, in order to get information about the decay of the single-particle states embedded in the (alpha, He-3) continuum. Neutron; angular correlations, multiplicity values, and branching ratios to low-lying states of final nuclei have been compared with the predictions of the statistical decay model. Evidence for a significant nonstatistical decay branch has been observed in the three nuclei below about 15 MeV excitation energy. Direct branching ratios in Zr-91 deduced from this analysis are compared with the predictions of two nuclear structure models. At higher excitation energy, the decay characteristics of the (alpha, He-3) continuum are shown to be mainly statistical.