Strong dependence of sub-barrier fusion on the nuclear hexadecapole deformation

Fusion cross-sections σ for the reaction ¹⁸⁶W + ¹⁶O have been measured with high precision at small energy intervals, for energies E spanning the barrier region. The distribution of fusion barriers, derived from the curvature of Eσ, has the shape expected classically from a target nucleus with a negative hexadecapole deformation. Comparison with the previously measured distribution for ¹⁵⁴Sm + ¹⁶O demonstrates the strong sensitivity of fusion to the hexadecapole deformation. The magnitudes of the deformation parameters for ¹⁸⁶W, extracted using a purely geometrical description, are different from those determined from non-fusion measurements. This is interpreted as reflecting the sensitivity of the excitation function to small additional couplings.