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 40–44Ar relative to 38Ar from which changes in the mean square nuclear charge radii across the 1f7/2 neutron shell are deduced. In addition, the hyperfine structure of 41Ar and 43Ar yields the spins, magnetic dipole and electric quadrupole moments, in particular the spin I=5/2 for 43Ar. 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 32Ar to 46Ar, covering sd-shell as well as f7/2-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 f7/2 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.