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