Application of the schematic model to four-quasiparticle states

The Tamm-Dancoff equation of motion for our BCS quasiparticles is derived. It is solved in the limit of degenerate energy levels using a separable interaction. The result is then applied to the coupling between collective states with two and four quasiparticles, which corresponds to a coupling between collective states with one and two phonons. A case of special interest involves the coupling of the collective two-quasiparticle state to the collective four-quasiparticle state. This interaction represents the leading-order anharmonic effect in the vibrational motion. Its effect on the energy level diagram and the strength of the crossover transition 22+ /yield/ 01+ are investigated for 106Pd, 108Pd, 110Pd, 74Se, 76Se, and 78Se. 21 refs., 9 figs., 4 tabs