Microscopic calculation of absolute values of two-nucleon transfer cross sections

Arguably, the greatest achievement of many-body physics in the fifties was that of developing the tools for a complete description and a thorough understanding of superconductivity in metals. At the basis of it one finds BCS theory and the Josephson effect. The first recognized the central role played by the appearance of a macroscopic coherent field usually viewed as a condensate of strongly overlapping Cooper pairs, the quasiparticle vacuum. The second made it clear that a true gap is not essential for such a state of matter to exist, but rather a finite expectation value of the pair field. Consequently, the specific probe to study the superconducting state is Cooper pair tunneUng. Important progress in the understanding of pairing in atomic nuclei may arise from the systematic study of two-particle transfer reactions. Although this subject of research started about the time of the BCS papers, the quantitative calculation of absolute cross sections taking properly into account the fidl non-locaUty of the Cooper pairs (correlation length much larger than nuclear dimensions) is still an open question. In what follows we present residts obtained, within a second order DWBA framework, of two-nucleon transfer reactions induced both by heavy and light ions. The calcinations were carried out making use of software specifically developed for this purpose. It includes sequential, simidtaneous and non-orthogonality contributions to the process. Microscopic form factors are used which take into account the relevant structure aspects of the process, such as the nature of the single-particle wavefunctions, the spectroscopic factors, and the interaction potential responsible for the transfer. Overall agreement with the experimental absolute values of the differential cross section is obtained without any free parameter.