The Threshold Anomaly of weakly bound projectiles from recent elastic scattering measurements around the Coulomb barrier

A thorough study of the energy dependence of the nuclear optical potential in reactions involving the weakly bound projectiles 8B, 7Be and 6Li on 58Ni is carried out by performing a χ2-analysis of recent measurements of elastic scattering cross sections for energies around the Coulomb barrier. For this purpose two different potential types are used: the double folding Sao Paulo potential and the Woods-Saxon potential. The calculations performed with these potentials show that the energy dependences of the real V and imaginary W parts of the polarization potential satisfy the dispersion relation. Besides, within the large uncertainties, the energy dependent function W(E) does not seem to decrease as the energy is lowered towards the barrier as expected for the threshold anomaly usually present in tightly bound systems. Actually, the imaginary W(E) seems to slightly increase for energies below the barrier. On the other hand, the real polarization potential V exhibits a repulsive character around the barrier. This behaviour corresponds to the so-called Breakup Threshold Anomaly, present in the scattering of several systems with small breakup threshold projectiles as the ones of the present work. However, a definitive conclusion about the presence of the BTA cannot be made in some cases of the present analysis due to the large uncertainties of the experimental data used.