Threshold anomaly in ¹²C+²⁰⁹Bi scattering

Abstract

Elastic scattering angular distributions have been measured for the system ¹²C+²⁰⁹Bi at ten energies between 58.9 to 87.4 MeV. Optical model analysis of these, together with existing data at 118 MeV, using both phenomenological and microscopic models, shows a pronounced energy dependence of the optical potential. Fusion cross sections have been deduced, by a barrier penetration model and by subtracting the direct reaction contributions from the reaction cross sections. Threshold anomaly of the optical potential has been explained both by dispersion relation and by coupled reaction channel (CRC) calculations. The CRC calculations, including inelastic excitation and transfer channels, reproduce simultaneously the elastic, inelastic, and transfer angular distributions measured at 61.9, 63.9, and 87.4 MeV. Comparison of threshold anomaly results for ⁴He, ¹¹B, ¹⁶O+²⁰⁹Bi systems along with the present case indicates that the degree of anomaly increases with the projectile mass.