State densities with linear momentum and their application to preequilibrium and photoabsorption reactions

We discuss the concept of state densities with linear momentum and describe their application to preequilibrium reaction theory as well as nuclear photoabsorption via the quasideuteron mechanism. An exciton model is presented for particle emission in nucleon-induced reactions in which linear momentum conversion is included. The nucleon emission contributions from the first two preequilibrium stages are calculated by determining exact particle-hole state densities with a specific energy and linear momentum in a Fermi-gas model of the nucleus. Angular distributions arise naturally from our treatment and do not have to be added in an ad hoc way. The angular distributions that we obtain from the first two preequilibrium stages are identical to those found using the Kikuchi-Kawai quasifree scattering kernel. Since many preequilibrium analyses are based upon an equidistant single-particle model of the nucleus, we also determine the state densities wit linear momentum (and hence angular distributions) in such a model. A no-parameter quasideuteron model of photoabsorption is presented in which the Levinger parameter and Pauli-blocking function are determined theoretically, using state densities with linear momentum. Comparisons with data are shown, and the temperature dependence of the photoabsorption cross section is calculated. 45 refs., 6 figs