Odd-even isotope shifts after closed neutron shells

The isotope shift of 91Zr (N=51) has been studied by means of a photoelectric recording Fabry-Perot spectrometer with digital data processing. From the field shift of a pure s-p transition the change in mean square nuclar charge radii was evaluated: deltasup(90.91)=0.123(24) fm2. This is just half the change deltasup(90.92). Thus, after the closure of the magic neutron shell with the neutron number N=50 no odd-even staggering was observed as is the case with Cr (N=28), Nd (N=82), and Pb (N=126). The odd-even staggering immediately after magic and non-magic neutron shells is reviewed and compared with current theories. (orig.)