On Applicability of the Random Phase Approximation to the Collective Excitation in Spherical Even Nuclei. II

Abstract

It is the main purpose of this paper to check the concept of the “2-phonon” states through an analysis of the characteristic features of the first order correction to the conventional “2-phonon” states under the RPA. Numerical results for Ni-isotopes (Ni⁶⁰ and Ni⁶²) and for the single j = 15/2 shell model show that the concept of the “2-phonon” states based on the RPA is in breakdown, in the sense that the correction is unexpectedly large in the region of the phonon energy which we are interested in. We have also another important physical situation to destroy the concept of the “2-phonon” from the fact that coupling between the “2-phonon” modes and other 2-quasi-particle excitation modes is essential in most actual nuclei.