Quasiparticle nature of excited states in random-phase approximation

Abstract

Quantum models of nuclear reactions require an effective way of taking into account the complex coupling of the components of different excited states as well as the nuclear residual interactions of the composite system. This is usually achieved with the use of distributions giving nonuniform weights for each of the modes involved. The response function of the quantum model permits the inclusion of the residual interaction and thus a unified description of collective and single-particle excitations. We analyze the particle-hole nature of the RPA modes of the response function in the context of multistep direct (MSD) nuclear reactions. The energy distribution of the particle-hole states that contribute to the response function is studied. Although many states make small contributions to the low-energy collective states, we show that the energy distribution of the higher-energy states is concentrated around the energies of the noninteracting components. The spreading energy of the strength function is determined for different nuclei, and a general fit accounting for both the collective and noncollective parts of the spectra is proposed. In addition we also test the randomness assumption commonly applied in models of MSD reactions.