Extrapolation of the results of some optical model (OM) study of the $\alpha$-nucleus elastic scattering at incident energies up to 700 MeV implies that the $\alpha$-nucleus potential becomes repulsive above 910 MeV. To check this assumption, a detailed OM analysis of the elastic $\alpha {+}^{12}$C and $\alpha {+}^{40}$Ca scattering data at the incident energy of 1370 MeV has been performed using different forms of the optical potential. A realistic pattern of the near-side/far-side interference, required to reproduce the diffractive minima in the measured elastic cross sections, strongly suggests that the corresponding optical potentials are still attractive. The folding model prediction for the $\alpha {+}^{12}$C and $\alpha {+}^{40}$Ca optical potentials using the impulse approximation has been considered. This approximation, which uses the t-matrix parametrization of free nucleon-nucleon $\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$ interaction as the model input, was found to be inadequate to give a good account of the data. Consequently, the medium modifications of the $\mathrm{NN}$ interaction remain important at this energy. In addition, similar failure of a density dependent $\mathrm{NN}$ interaction (CDM3Y6), which works quite well at lower energies, stresses the need for a more appropriate effective $\mathrm{NN}$ interaction in this energy region.

• Received 29 October 2001

DOI:https://doi.org/10.1103/PhysRevC.65.024611