Institut für Kernphysik, Universität Mainz, D-6500, Mainz, Germany
Abstract:
Systematic corrections to the usual closure approximation are developed by introducing the notion of two (and more) mean excitation energies. These corrections depend on the energy moments of the distribution of transition strength. Application to simple solvable models in perturbation theory, muon capture and pion electroproduction indicate excellent convergence of the method as long as the kinematical constraints of energy conservation can be neglected.