Nuclear structure of Cu isotopes studied with collinear laser spectroscopy

This work presents the nuclear spins, magnetic moments, quadrupole moments and differences in mean square radii of the Cu isotopes $^{58−75}$Cu. The collinear laser spectroscopy technique was used. The experiments were performed at the collinear laser spectroscopy setup COLLAPS at ISOLDE, CERN. A detailed overview of the setup and the analysis procedure of laser spectroscopy data is given in this work. A recent technical improvement was the installation of the RFQ beam cooler, which allowed to extend the measurements toward more exotic isotopes. The groundstate spin inversion for the odd-$A$ Cu isotopes from spin 3/2 to 5/2 is established to occur at $^{75}$Cu. This result presents a breakthrough in experimental and theoretical investigations in this region. The spin-parity of the odd-odd Cu isotopes $^{72−74}$Cu is determined to be 2$^−$. The resulting magnetic moments and quadrupole moments are compared with theoretical calculations in a $f_{5/2}pg_{9/2}$ model space and a pf shell model space. The magnetic moments of the odd-A Cu isotopes clearly show that excitations across the $Z$ = 28 and $N$ = 28 shell gaps need to be included in the model space to reproduce the experimental trend, providing another evidence of the softness of the $^{56}$Ni core. The odd-$A$ as well as the odd-odd $A$ Cu isotopes illustrate the sensitivity of the magnetic moment to the detailed composition of the wave function, which makes magnetic moments crucial parameters to evaluate shell model calculations. The quadrupole moments show a minimum in collectivity at $N$ = 40. However, this is not entirely related to the magnitude of the $N$ = 40 subshell gap but largely induced by the opposite parity of the $g_{9/2}$ orbit compared to the $pf$ shell orbits, which blocks single-particle excitations across $N$ = 40. The experimental quadrupole trend does not support an increase of collectivity beyond $N$ = 40. Theoretical calculations in a $pf$ model space appear to underestimate the experimental core polarization for the neutron-deficient Cu nuclei. The procedure of extracting the differences in mean square charge radii from the measured isotope shifts is given in detail. Comparison of the mean square charge radii with the droplet model prediction suggests a significant collectivity in the Cu isotope chain. A small structural effect at $N$ = 40 is observed.