Shape coexistence in Sr isotopes

Sr isotopes are located in the mass region $A\approx 100$, where a very quick onset of nuclear deformation exists, being other notable examples of this area Yb, Zr, and Nb nuclei. The presence of the proton subshell closure $Z=40$ allows the existence of particle-hole excitations that produces low-lying intruder bands. Purpose: The goal of this work is the study of the nuclear structure of the even-even $^{92-102}$Sr 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, 0particle-0hole and 2particle-2hole 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 have been confirmed and its connection with the onset of deformation has been clarified. 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.