Threshold anomaly in ¹⁶O + ²⁰⁹Bi system

Abstract

Elastic-scattering angular distributions for the ¹⁶O + ²⁰⁹Bi system have been measured at laboratory energies of 80, 83, 90,95, 98 and 100 MeV. The present data along with the data available from the literature for this system, have been analysed using the optical model, employing both the phenomenological and the microscopic potentials. Evidence is found for a marked energy dependence of the real part of the optical potential - threshold anomaly - around the Coulomb barrier. The application of the dispersion relation proposed by Mahaux et al. to the energy variation of the imaginary part of the potential, reproduces the observed energy dependence of the real potential. Further it is demonstrated by a calculation that the "threshold anomaly" determined from elastic-scattering analysis is consistent with the enhancement of the fusion cross section observed at sub-barrier energies. It is also observed that the dispersive correction to the real potential already reported for ¹⁶O + ²⁰⁸Pb is similar to that found here for ¹⁶O + ²⁰⁹Bi which has an extra proton.