Ground-state electromagnetic moments of calcium isotopes

Abstract

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 $^{43\ensuremath{-}51}\mathrm{Ca}$ isotopes.Results: The ground-state magnetic moments of $^{49,51}\mathrm{Ca}$ and quadrupole moments of $^{47,49,51}\mathrm{Ca}$ were measured for the first time, and the $^{51}\mathrm{Ca}$ 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 $^{40}\mathrm{Ca}$ 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.