Shape coexistence in Sr isotopes

Background: Sr isotopes are located in the mass region A approximate to 100, where a very quick onset of nuclear deformation exists; other notable examples of this area are Yb, Zr, and Nb nuclei. The presence of the proton subshell closure Z = 40 allows the existence of particle-hole excitations that produce low-lying intruder bands. Purpose: The goal of this work is the study of the nuclear structure of the even-even Sr92-102 isotopes through the accurate description of excitation energies, B(E2) transition rates, nuclear radii, and two-neutron separation energies. Method: The interacting boson model with configuration mixing will be the framework to calculate all the observables of the Sr isotopes. Only two types of configurations will be considered, namely, 0-particle-0-hole and 2-particle-2-hole excitations. The parameters of the model are determined using a least-squares procedure for the excitation energies and the B(E2) transition rates. Results: For the whole chain of isotopes, the value of excitation energies, B(E2)'s, two-neutron separation energies, nuclear radii, and isotope shifts have been obtained, with a good agreement between theory and experiment. Also, a detailed analysis of the wave functions have been performed and, finally, the mean-field energy surfaces and the value of the nuclear deformation, beta, have been obtained. Conclusions: The presence of low-lying intruder states in even-even Sr isotopes has been confirmed and its connection with the onset of deformation has been clarified. The lightest Sr isotopes present a spherical structure while the heaviest ones are clearly deformed. The rapid onset of deformation at neutron number 60 is due to the crossing of the regular and intruder configurations and, moreover, both families of states present an increase of deformation with the neutron number.