Evolution of nuclear structure of germanium isotopes around N = 40

Collinear laser spectroscopy measurements were performed on 68−74Ge isotopes (Z = 32) using the COLLAPS setup at ISOLDE/CERN. The hyperfine structure of the 4s24p2 3P1 → 4s24p5s 3P o 1 transition in the germanium atom was probed with laser light of 269 nm, produced by combining the frequency-mixing and frequency-doubling techniques. A detailed analysis of the hyperfine spectra of 69Ge, led to revised values of its magnetic and quadrupole moments, which deviate significantly from the results from an earlier study using the atomic beam magnetic resonance technique. The experimental moments of 69,71,73Ge agree well with the effective single-particle value for an odd neutron in respectively the νf5/2, νp1/2 and νg9/2 orbit, using $g^{eff}_s = 0.7 g^{free}_s , which might suggest rather pure configurations. However, a comparison with large-scale shell-model calculations using the JUN45 interaction, with 56Ni as a core and neutrons limited to N = 50, reveal rather mixed wave-functions. Through a comparison of the odd-N isotope magnetic and quadrupole moments with neighbouring isotones, the structural change from the single-particle nature of nickel to deformation in germanium is further investigated around N = 40. The experimental mean square charge radii of 68−74Ge are compared with the droplet model values revealing a weak subshell effect at N = 40 and an increased collectivity in the germanium isotopes, as compared to their lower-Z isotones. The odd-even staggering of the radii of the germanium isotopes is found to be larger than for the other isotopes in the region, with less than 40 neutrons. While only one value is available beyond N = 40, this value suggests a weakening of the odd-even staggering, but more neutron-rich data are needed to confirm this.