The nucleon-pair approximation for nuclei from spherical to deformed regions
Abstract
In this paper we model low-lying states of atomic nuclei in the nucleon-pair approximation of the shell model, using three approaches to select collective nucleon pairs: the generalized seniority scheme, the conjugate gradient method, and the Hartree-Fock approach. We find the collective pairs obtained from the generalized seniority scheme provides a good description for nearly spherical nuclei, and those from the conjugate gradient method or the Hartree-Fock approach work well for transitional and deformed nuclei. Our NPA calculations using collective pairs with angular momenta 0, 2, and 4 (denoted by SDG pairs) reproduce the nuclear shape evolution in the N=26 isotones, 46Ca, 48Ti, 50Cr, and 52Fe, and yield good agreement with full configuration-interaction calculations of low-lying states in medium-heavy transitional and deformed nuclei: 44-48Ti, 48Cr, 50Cr, 52Fe, 60-64Zn, 64,66Ge, 84Mo, and 108-112Xe. Finally, using the SDGI-pair approximation we describe low-lying states of 112,114Ba, cases difficult to reach by conventional configuration-interaction methods.