Quest of shape coexistence in Zr isotopes

Background: The mass region with A ≈100 and Z ≈40 is known to experience a sudden onset of deformation. The presence of the subshell closure Z =40 makes it feasible to create particle-hole excitations at a moderate excitation energy and, therefore, likely intruder states could be present in the low-lying spectrum. In other words, shape coexistence is expected to be a key ingredient to understand this mass region.

Purpose: The aim of this work is to describe excitation energies, transition rates, radii, and two-neutron separation energies for the even-even ^Zr-110₉₄ nuclei and, moreover, to obtain information about wave functions and deformation.

Method: The interacting boson model with configuration mixing will be the framework to study the even-even Zr nuclei, considering only two types of configurations: 0particle-0hole and 2particle-2hole excitations. On one hand, the parameters appearing in the Hamiltonian and in the E 2 transition operator are fixed trough a least-squares fit to the whole available experimental information. On the other hand, once the parameters have been fixed, the calculations allow to obtain a complete set of observables for the whole even-even Zr chain of isotopes.

Results: Spectra, transition rates, radii, ρ²(E 0 ) , and two-neutron separation energies have been calculated and a good agreement with the experimental information has been obtained. Moreover, a detailed study of the wave function has been conducted and mean-field energy surfaces and deformation have been computed too.

Conclusions: The importance of shape coexistence has been shown to correctly describe the A ≈100 mass area for even-even Zr nuclei. This work confirmed the rather spherical nature of the ground state of ^Zr-98₉₄ and its deformed nature for ^Zr-110₁₀₀ isotopes. The sudden onset of deformation in ¹⁰⁰Zr is owing to the rapid lowering of a deformed (intruder) configuration which is high-lying in lighter isotopes.