Spin and temperature dependence of nuclear deformation using alpha-gamma angular correlations

Alpha-particle angular distributions with respect to the spin direction of residual nuclei have been measured in heavy-ion fusion reactions. The spin direction was determined by measuring the ..gamma..-ray angular distributions, for each event, using the spin spectrometer. ..cap alpha..-particle anisotropies have been extracted for the compound nuclear systems: /sup 110/Sn*(94 MeV), /sup 114/Sn*(80 MeV), /sup 138/Nd*(82 MeV), /sup 164/Yb*(67 MeV) and /sup 170/Yb*(135 MeV) as a function of the ..cap alpha..-particle energy and ..gamma..-ray multiplicity. The results are compared with statistical model calculations using transmission coefficients from a spherically symmetric optical model potential. The trend of the anisotropy coefficients below the evaporation Coulomb barrier is consistent with spherical emitting shapes in the case of the Sn* isotopes. Small deformation effects are suggested by the /sup 138/Nd* and /sup 164/Yb* data. The /sup 170/Yb* data indicate a large deformation which increases considerably with increasing spin. These results are in agreement with findings for similar systems in which the decay of the giant resonances built on excited states have been studied. 16 refs., 5 figs.