Sub-barrier fusion enhancement in neutron-rich radioactive ¹³²Sn on ⁶⁴Ni

Evaporation residue cross sections have been measured using neutron-rich radioactive ¹³²Sn beams incident on a ⁶⁴Ni target in the vicinity of the Coulomb barrier. This is the first experiment using accelerated ¹³²Sn beams to study nuclear reaction mechanisms. The average beam intensity was 2 x 10⁴ particles per second and the smallest cross section measured was less than 5 mb. A large sub-barrier fusion enhancement was observed compared to evaporation residue cross sections for ⁶⁴Ni on stable even Sn isotopes. The enhancement cannot be accounted for by a simple barrier shift due to the change in nuclear sizes. Coupled-channel calculations including inelastic excitation underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of ¹³²Sn and ⁶⁴Ni.