Changes in the mean square charge radii of neutron-deficient europium isotopes measured by the laser ion source resonance ionization spectroscopy

The laser ion source has been used for the study of the isotope shifts of neutron-deficient Eu isotopes. The extension of the region of applicability of the method by using the $\gamma $- and $\beta$-radiation detection is reported. We have measured the isotope shifts of the europium optical line 576.520 nm for $^{137\mbox{--}139,141,142m,143,144}{\rm Eu}$. To increase the laser ion source efficiency an axial magnetic field (350 gauss) was applied. Nearly a twofold rise of the ionization efficiency for Eu was observed. By using the effect of optical ion bunching an increase of the selectivity was obtained. The isotope shift data for $^{139,141,142m,143,144}{\rm Eu}$ are in agreement with the previously measured isotope shifts for these nuclides. The new data for $^{137}{\rm Eu}$ and refined data for $^{138}{\rm Eu}$ point to a gradual increase of the deformation for these isotopes. Comparisons with microscopic-macroscopic calculations and calculations in the framework of the Hartree-Fock model were performed