Moments and mean square charge radii of short-lived argon isotopes

We report on the measurement of optical isotope shifts for Ar-32-40 and for Ar-46 from which the changes in mean square nuclear charge radii across the N = 20 neutron shell closure are deduced. The investigations were carried out by collinear laser spectroscopy in fast beams of neutral argon atoms. The ultra-sensitive detection combines optical pumping, state-selective collisional ionization and counting of P-radioactivity. By reaching far into the sd shell, the results add new information to the systematics of radii in the calcium region (Z approximate to 20). Contrary to all major neutron shell closures with N greater than or equal to 28, the N = 20 shell closure causes no significant slope change in the development of the radii. Information from the hyperfine structure of the odd-A isotopes includes the magnetic moments of Ar-33 (I = 1/2) and Ar-39 (I = 7/2), and the quadrupole moments of Ar-35, Ar-37 (I = 3/2) and Ar-39. The electromagnetic moments are compared to shell-model predictions for the sd and fp shells, Even far from stability a very good agreement between experiment and theory is found for these quantities. The mean square charge radii are discussed in the framework of spherical SGII Skyrme-type Hartree-Fock calculations.