On the extended basis shell model and two-nucleon transfer

The enhancement of two-nucleon transfer reactions predicted by the extended basis shell-model (EBSM) calculations of Feng, Ibarra and Valliers and other theorists, is shown to be a higher order manifestation of the pairing effects in nuclei. A simple schematic model is introduced which illustrates the constructive interference of the contributions from the highly excited shell-model states included in the EBSM. The model can be used to estimate the effect of ground-state correlations of the closed-shell core. The results indicate that the assumption of an inert core in EBSM calculations introduced negligible error in cases in which the addition of a nucleon pair to a closed shell is treated. However, in the case of two-nucleon removal from a closed shell, including the effects of correlations may significantly enhance the cross section. A suggestion is made for including these effects in EBSM calculations. (Auth.)