The threshold anomaly in the 16O + 58,60,62,64Ni systems

Accurate angular distributions of 16O ions scattered by 58,60,62,64Ni have been measured at five energies between 60 and 120 MeV. An optical model analysis of these, together with existing data, indicates that, whereas there is a significant increase with A in the surface absorption, there is little, if any, isotopic dependence in the energy variation of the optical potentials, V(E) and W(E), near threshold (the threshold anomaly). Clear evidence is found for a hitherto unreported peak in W(E) just above threshold. CRC calculations, including inelastic excitation and transfer, reproduce the data but show no clear connection between the peak and a particular reaction channel.