Nuclear Charge Radii across the N=Z=28 Shell Closure in Nickel Isotopes

Collinear laser spectroscopy enables the determination of nuclear ground-state charge radii and electromagnetic moments for short-lived isotopes. Within this work, spectroscopy was performed on neutron-deficient nickel isotopes in the region of the doubly magic 56Ni, and an improved charge-exchange cell for spectroscopy of palladium isotopes was developed. For numerous elements, laser spectroscopy is not possible from the ground state of the ion due to a lack of suitable transitions. For this reason, charge-exchange cells are often used and are typically operated with alkali metals. Spectroscopy on palladium would instead profit from an exchange on magnesium since the desired meta-stable state could then be resonantly populated. To make this possible, an advanced design of a charge-exchange cell for collinear laser spectroscopy has been realized. Compared to previous cells, it allows higher temperatures and operation at higher voltages. The new cell has been installed as part of the collinear spectroscopy setup at Argonne National Lab in Chicago. During commissioning measurements, the neutralization of a calcium-ion beam on magnesium vapor has been demonstrated and an atomic calcium resonance was recorded. Furthermore, neutron deficient and stable nickel isotopes have been investigated by collinear laser spectroscopy in the BECOLA experiment at the National Superconducting Cyclotron Laboratory in East Lansing. The spectra of 55,56,58,60Ni have been analyzed within this work. A King-plot method yielded the differential mean squared charge radii compared to the stable isotope 60Ni, and absolute nuclear charge radii were derived from these results. The measurements reveal a "kink" in the slope of the charge radii at the doubly-magic N=Z=28 shell closure. A comparison to nuclear theories shows good agreement for calculations using the Fayans density functional theory. Predictions from ab initio calculations for the nuclear charge radius of 56Ni could now, for the first time, be benchmarked to an experimental value. The comparison of different nuclear interactions via the coupled-cluster method reveals a significant scattering of the results and an incomplete uncertainty estimate complicates the interpretation. The latter is significantly improved by a recent method for Bayesian treatment of the uncertainties in the IMSRG framework, which yields results that are in good agreement with the experiment though less precise. In addition to the charge radii, the nuclear magnetic dipole moment of 55Ni was determined from the hyperfine structure. The result corrects the previous experimental value from a β-NMR measurement and is in good agreement with theory calculations that have proven reliable in other isotopes before