Improved global α-optical model potentials at low energies

Many nuclear astrophysics applications involve radiative alpha-particle captures, alpha decays and alpha-particle transfer reactions. Theoretical estimates of the corresponding reaction rates within the framework of the statistical model of Hauser-Feshbach remain highly uncertain due to the poor knowledge of the alpha-nucleus optical model potential, especially at low energies far below the Coulomb barrier. In the present paper we propose a new global alpha-optical potential that takes into account the strong energy dependence and nuclear structure effects that characterize the alpha-nucleus interaction. The real part of the potential is calculated using a double-folding procedure over the M3Y effective nucleon-nucleon interaction. A Woods-Saxon potential is used for the imaginary potential where now a new parameterization is introduced to describe its energy dependence. The influence of purely volume absorption or volume plus surface absorption on the description of the experimental data is investigated. Finally, the dispersive relation is applied to relate the real and imaginary parts of the optical model potential and reduce the ambiguities in deriving the potential from experimental data. The three potentials considered are able to reproduce well the bulk of experimental data on (alpha, gamma), (alpha, n), (alpha, p) and (n, alpha) reactions as well as the existing elastic scattering data at energies of relevance to astrophysical applications. However, when considering reaction rates on experimentally unexplored targets, deviations within a factor of 10 are found. These uncertainties are principally due to the difficulty in constraining the diffuseness of the imaginary potential from analyses of existing experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.