Deformation effects in the compound nucleus decay using the spin-alignment method.

Alpha-particle energy spectra and angular distributions with respect to the estimated spin direction of residual nuclei have been measured in heavy-ion fusion reactions. The spin direction was determined for each event by measuring the y-ray angular correlation patterns using the Spin Spectrometer. Measurements were made for the compound nuclear systems " Sn*(94 MeV), " Sn (80 MeV), '"Nd (82 MeV), '~Yb (67 MeV), and ' Yb (135 MeV) at the indicated excitation energies as a function of the alpha-particle energy and y-ray multiplicity. The anisotropy coefficients below the evaporation Coulomb barrier show distinct diS'erences from "Osn to " Yb. These results and the shapes of the alpha-particle spectra are compared with statistical model calculations that incorporate deformation effects in the optical model transmission coefficients. The Sn data can be explained without invoking deformation effects other than the ones included in the experimental yrast lines. However, for the heavier Yb systems, a considerable spin-dependent deformation in the a-emission barriers is required. For these systems the a emission below the barrier is a sensitive probe for deformation that samples a broad range of excitation energies in the decay sequence.