The elastic angular distributions for Ni+ "Al have been measured at five energies from E, 48.8 to 69.5 MeV. The optical model analysis assumes both phenomenological and folding model potentials. Regions of sensitivity, where the potential is well determined, are found to be difFerent for the real and the imaginary parts, and to vary with energy. The values of the real potential in the surface are weakly dependent on energy. PACS number(s): 25.70.z, 25.70.Bc I. INTRODU CTIONSome recent analyses of heavy-ion elastic scattering measurements have shown rapid variations with energy in the strengths of the nuclear optical potential in the vicinity of the top of the Coulomb barrier. There is a rapid increase in the absorptive strength, accompanied by a rapid decrease in the strength of the real potential, as the energy increases above the Coulomb barrier. This effect has been referred to as a threshold anomaly (see, for example, Ref.[1], for a review) and has been interpreted as due to the increase in importance of couplings to various reaction channels in this energy range. The variations with energy of the real and imaginary parts of the potential are expected to be correlated through a dispersion relation that arises from causality.The effect has been observed most clearly for 0+20sPb [1,2], where a decrease in the real potential strength of nearly a factor of 2 is seen as the energy rises to about 40 MeV above the Coulomb barrier. Weaker dependences on energy have been observed for other systems, such as S+ S [3], S+ Ca [4], and ss'srCI+24Mg [5], etc. In order to extend the variety of systems studied, we have measured the elastic scattering of Ni+ Al at Ave energies, up to 20 MeV in the c.m. above the Coulomb barrier. The scattering for the neighboring systems Si+ ' ' Ni has been measured by Sugiyama et al. [6, 7], who found an important isotopic dependence in their results [7]; an energy dependence was present for Ni, but not for Ni. This was associated with differences in the importance of neutron transfer reactions in the two cases. II. MEASUREMENTS The measurements were carried out with beams of Ni from the Holifield Heavy-Ion Research Facility 25-MV *Deceased. tandem at Oak Ridge National Laboratory (ORNL), at laboratory energies of 155, 160, 170, 185, and 220 MeV, which correspond (after energy losses in the 100 pg/cm thick Al target are taken into account) to center-of-mass energies equal to 48.8, 50.4, 53.6, 58.3, and 69.5 MeV, respectively. The Coulomb barrier for this system is about 50 MeV. Because of the reverse kinematics, c.m. angles between 30 and 165 were measured by covering a laboratory angular range between 7 and 51 . The nickelas well as the aluminum ejectiles were momentum analyzed by an Enge split-pole spectrograph equipped with a heavy-ion detection system [8] consisting of two positionsensing proportional counter wires and two ionization chambers. The ejectiles trajectories (angle of incidence at the detector front and position on the spectrograph focal plane) were calculated from the wire i...