Nuclear moments and charge radii of argon isotopes between the neutron-shell closures N = 20 and N = 28

We report the measurement of optical isotope shifts for 40–44 Ar relative to 38 Ar from which changes in the mean square nuclear charge radii across the 1 f 7 / 2 neutron shell are deduced. In addition, the hyperfine structure of 41 Ar and 43 Ar yields the spins, magnetic dipole and electric quadrupole moments, in particular the spin I = 5 / 2 for 43 Ar. The investigations were carried out by fast-beam collinear laser spectroscopy using highly sensitive detection based on optical pumping and state-selective collisional ionization. Mean square charge radii are now known from 32 Ar to 46 Ar, covering sd -shell as well as f 7 / 2 -shell nuclei. They are discussed in the framework of spherical SGII Skyrme-type Hartree–Fock calculations, semi-empirically corrected for quadrupole core polarization. The Zamick–Talmi formula excellently describes the charge radii across the f 7 / 2 neutron shell, as it does for the neighbouring elements. The new data on Ar, in addition to those on K, Ca and Ti, provide an extended overview of nuclear radii trends around Z = 20 and between N = 20 and N = 28 . Characteristic features are discussed, the most striking being the absence of any isotopic shell effect in the radii at N = 20 .