Nuclear charge radii of potassium isotopes beyond N=28N=28

We report on the measurement of optical isotope shifts for 38, 39, 42, 44, 46–51K relative to 47K from which changes in the nuclear mean square charge radii across the N=28N=28 shell closure are deduced. The investigation was carried out by bunched-beam collinear laser spectroscopy at the CERN-ISOLDE radioactive ion-beam facility. Mean square charge radii are now known from 37K to 51K, covering all νf7/2νf7/2-shell as well as all νp3/2νp3/2-shell nuclei. These measurements, in conjunction with those of Ca, Cr, Mn and Fe, provide a first insight into the Z   dependence of the evolution of nuclear size above the shell closure at N=28N=28.