Collinear Laser Spectroscopy on exotic Ca isotopes towards new magic numbers N=32 and N=34

For more than a century physicists have been trying to understand the striking particularities of the atomic nucleus. Although several questions remain open for stable nuclei, our current interest for exploring the properties of exotic species has revealed new and unexpected aspects of nuclear structure. The study of nuclei at extreme conditions is not only relevant for nuclear physics, it can also provide answers to questions related to astrophysical processes such as the origin of elements in the universe and the limits of existence for nuclear matter. Besides the complexity of the nuclear many-body problem, nuclear structure properties exhibit regular patterns at the so called “magic” numbers of nucleons. The understanding of these apparently simple structures has motivated the development of some of the most elegant models of nuclear physics. Up to now, most of these models have been successfully applied to describe the properties of nuclei in specific regions of the nuclear chart. Even though some models might have a wide range of applicability, they can describe only part of the experimental data available, and generally fail to predict new observations for isotopes far away from stability. Understanding the properties of the atomic nucleus implies not only a reliable description of the nuclear force but also requires a proper treatment of the many-body problem. During the last few years, considerable progress has been made in both directions. Chiral effective field theory (ch-EFT) has allowed a systematic description of nuclear forces in terms of low-energy degrees of freedom, nucleons and pions, based on the symmetries of the underlying theory, quantum chromodynamics. It allows to explain naturally the hierarchy of many- body forces, and provides a consistent treatment of theoretical uncertainties. Thanks to the advances in ch-EFT, and the development of powerful many- body methods, nuclear physics has made important steps in the construction of an “effective theory of the atomic nucleus”. Ab-initio calculations are now available for medium and heavy nuclei, providing an accurate description of experimental properties of doubly- and semi-magic nuclei. Here, the importance of our experimental knowledge of “magic” nuclei to test the contemporary nuclear theories. Having two doubly-magic isotopes, $^{40}$Ca and $^{48}$Ca, the calcium isotopes are considered as a prime benchmark for nuclear structure, both from a theoretical and an experimental perspective. A renewed interest has been given to the neutron rich Ca isotopes, as their properties have revealed new aspects of the nuclear forces and many-body physics. Additionally, evidence of doubly-magic features in two new short-lived Ca isotopes has been recently reported in two exotic isotopes, $^{52}$Ca and $^{54}$Ca. By using high-resolution bunched-beam collinear laser spectroscopy (COLLAPS) at ISOLDE, CERN, this work presents the first measurements of the ground-state magnetic moments of $^{49,51}$Ca, the quadrupole moments of $^{47,49,51}$Ca, and the root-mean square charge radii of $^{49,51,52}$Ca. Additionally, the $^{51}$Ca ground-state spin I = 3/2 was determined in a model-independent way. Ground state electromagnetic moments are compared with state-of-the-art shell-model calculations using both phenomenological interactions and micro- scopic interactions derived from chiral effective field theory. The results for neutron-rich isotopes are in excellent agreement with predictions of interactions derived from chiral effective field theory including three-nucleon forces, while lighter isotopes illustrate the presence of particle-hole excitations of the $^{40}$Ca core in their ground state. Our measurements of charge radii are complemented by state-of-the-art density functional theory and ab-initio calculations. Ab-initio calculations can accurately reproduce the charge radii of $^{40,48}$Ca, but fail to predict the large increase observed beyond N = 28. This discrepancy between our unexpected experimental results and the different and theoretical predictions defy the doubly-magic character of $^{52}$Ca. This opens up new and intriguing question in our understanding of the atomic nucleus and the evolution of nuclear sizes for neutron-rich systems. Our findings highlight the importance of extending these experiments further away from stability, especially for the suggested doubly-magic $^{54}$Ca nucleus. With a production yield of ∼ 300 ions/s, the $^{52}$Ca (N = 32) isotope is at the limit of the optical-detection techniques. In order to extend the measurements up to $^{53}$Ca (< 100 ions/s) and $^{54}$Ca (< 10 ions/s), substantial modifications to the COLLAPS beam line were developed to implement an ultra-sensitive particle detection scheme. Ions initially in the ground state can be efficiently transferred to a metastable state by using multi-step optical pumping. The difference in neutralization cross section between the metastable and ground state is used to separate atoms and non-neutralized ions after passing the ion beam through a vapor cell. Finally, independent counting of atoms/ions (free of beam contaminants) by detecting the $\beta$-radiation of the decaying isotope. Therefore, the resonant signal on the laser-ion interaction is translated into a resonant on the $\beta$-detection. Numerical calculations of the laser-ion interaction and ion beam optics simulations were performed to choose the design of the different beam line components. The commissioning of the new experimental setup and the first experimental tests are presented.