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 {gamma}-ray angular correlation patterns using the Spin Spectrometer. Measurements were made for the compound nuclear systems {sup 110}Sn{sup *}(94 MeV), {sup 114}Sn{sup *}(80 MeV), {sup 138}Nd{sup *}(82 MeV), {sup 164}Yb{sup *}(67 MeV), and {sup 170}Yb{sup *}(135 MeV) at the indicated excitation energies as a function of the alpha-particle energy and {gamma}-ray multiplicity. The anisotropy coefficients below the evaporation Coulomb barrier show distinct differences from {sup 110}Sn{sup *} to {sup 170}Yb{sup *}. 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{sup *} data can be explained without invoking deformation effects other than the ones included in the experimental yrast lines. However, for the heavier Yb{sup *} systems, a considerable spin-dependent deformation in the {alpha}-emission barriers is required. For these systems the {alpha} emission below the barrier is a sensitive probe for deformation that samples a broad range of excitation energies in the decay sequence.