Modified continuum shell-model theory for (n, 2n) reactions

The modified continuum shell-model used in earlier papers for a consistent description of elastic and inelastic neutron scattering as well as neutron capture reactions has been extended to the case of (n,2n) reactions. Using a particle-core coupling basis, the resulting two-dimensional integral equation for the transition operator incorporates the important effects of core excitations and antisymmetrization. It is shown that the use of the modified continuum state reduces the numerical expenditure drastically as compared to the conventional continuum treatment. Generalizing common approximation schemes known from the shell model with one particle in the continuum, such as the neglect of the continuum-continuum coupling and the diagonal approximation, to the three-particle case yields a one-dimensional integral equation or allows even for a direct calculation of the transition operator. (orig.)