Damping of unbound single-particle modes

Abstract

The (\ensuremath{\alpha}${,}^{3}$He-n) reaction has been investigated at 120 MeV incident energy on $^{64}\mathrm{Ni}$, $^{90}\mathrm{Zr}$, and $^{120}\mathrm{Sn}$ target nuclei. Neutrons in coincidence with $^{3}\mathrm{He}$ particles emitted at 0\ifmmode^\circ\else\textdegree\fi{} were detected using the multidetector array EDEN, in order to get information about the decay of single-particle states embedded in the (\ensuremath{\alpha}${,}^{3}$He) continuum. Neutron angular correlations, multiplicity values, and branching ratios to low-lying states of the 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 $^{91}\mathrm{Zr}$ deduced from this analysis are compared with the predictions of two nuclear structure models. At higher excitation energy, the decay characteristics of the (\ensuremath{\alpha}${,}^{3}$He) continuum are shown to be mainly statistical.