Can the 0 + states of even superfluid nuclei be described by anharmonic pairing vibrations?

Abstract

States in even nuclei in which no pairs are broken may be described by a seniority zero ( ν = 0) shell model calculation. It is generally believed that such 0 + states may also be understood in terms of anharmonic pairing vibrations. A microscopic description of such vibrations is given by the generator coordinate method (GCM) using the pairing gap as generator coordinate. We especially consider cases where the ground state is superfluid and we find that for this state the GCM description provides a good approximation to the result of a ν = 0 shell model diagonalization. For the excited states this is in general not the case. This is illustrated by results of two schematic models and the more realistic case of the Sn isotopes. We show that a much better approximation of the ν = 0 states is obtained by performing a number projected two-quasiparticle (2 q.p.) calculation. Therefore we believe that the nature of these excited 0 + states may be better understood from the 2 q.p. picture than from that of anharmonic pairing vibrations.