Ground-State Electromagnetic Moments of Calcium Isotopes

Background: The neutron-rich calcium isotopes have gained particular interest as evidence of closed-shell structures has recently been found in two exotic nuclei, at N=32 and N=34. Additionally, the study of such neutron-rich systems has revealed new aspects of nuclear forces, in particular regarding the role of three-nucleon forces. Purpose: We study the electromagnetic properties of Ca isotopes around the neutron number N=32. Methods: High-resolution bunched-beam collinear laser spectroscopy was used to measure the optical hyperfine spectra of the Ca43−51 isotopes. Results: The ground-state magnetic moments of Ca49,51 and quadrupole moments of Ca47,49,51 were measured for the first time, and the Ca51 ground-state spin I=3/2 was determined in a model-independent way. Our experimental results are compared with state-of-the-art shell-model calculations using both phenomenological interactions and microscopic interactions derived from chiral effective field theory. Conclusions: The results for the ground-state moments of neutron-rich isotopes are in excellent agreement with predictions of interactions derived from chiral effective field theory including three-nucleon forces. Lighter isotopes illustrate the presence of particle-hole excitations of the Ca40 core in their ground state. Our results provide a critical test of modern nuclear theories, and give direct answer to the evolution of ground-state electromagnetic properties in the Ca isotopic chain across three doubly closed-shell configurations at N=20, 28, 32 of this unique system.