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 -ray angular correlation patterns using the Spin Spectrometer. Measurements were made for the compound nuclear systems Sn*110(94 MeV), Sn*114(80 MeV), Nd*138(82 MeV), Yb*164(67 MeV), and Yb*170(135 MeV) at the indicated excitation energies as a function of the alpha-particle energy and -ray multiplicity. The anisotropy coefficients below the evaporation Coulomb barrier show distinct differences from Sn*110 to Yb*170. 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 ±-emission barriers is required. For these systems the ± emission below the barrier is a sensitive probe for deformation that samples a broad range of excitation energies in the decay sequence.