Antisymmetrization effects in radiative capture reactions

The role played by the Pauli principle in low-energy radiative capture reactions is investigated through a comparison between a microscopic model-the generator coordinate method-and two of its approximations based on an orthogonality condition. The electromagnetic transition matrix element is shown to contain a dominant local term and a smaller residual-antisymmetrizations term. The validity of the approximation is studied for the 16O(α,γ)20Ne reaction under conditions of calculation leading to spectroscopic and elastic-scattering properties as close as possible to those of the microscopic model. Nonnegligible differences are found between the three calculations. The local approximation is not significantly less accurate than the more complicated nonlocal one. A simple criterion is proposed to determine the validity of both approximations for other radiative capture reactions. At very low energies, the variation of the astrophysical factor can be well reproduced by the extranuclear capture model. The slope of the S-factor at zero energy is explained qualitatively for different reactions by a simple analytical formula