Sub-barrier fusion of deformed nuclei in ⁶⁰Ni+¹⁵⁴Sm and ³²S+¹⁸²W reactions

We have investigated the sub-barrier fusion between spherical projectiles and well-deformed targets in ⁶⁰Ni+¹⁵⁴Sm and ³²S+¹⁸²W reactions, and both lead to the same compound nucleus ²¹⁴Th. In order to get direct evidence that the projectile really fuses with the deformed target, the fusion evaporation residues emitted to the beam direction were measured by using the JAERI recoil mass separator and identified on the basis of time- and position-correlated α decays. The angular distributions of fission fragments were also measured to obtain the total fusion cross section. The measured cross sections of the fusion fission and the fusion evaporation were compared with the results of the coupled channel calculation and the statistical model calculation, respectively. In the ³²S+¹⁸²W system, a good agreement between them was obtained for xn, pxn, and αxn channels. In the heavy projectile system of ⁶⁰Ni+¹⁵⁴Sm, on the other hand, the observed evaporation residue cross sections below the Bass barrier were much less than the calculated results whereas they were well reproduced above the barrier. It was found that this large hindrance could be reproduced by assuming an extra-extra push energy of around 20 MeV in the tip collision of the deformed target and nearly zero in the side collision.