Measurement and analysis of the muonic x rays ofEu151andEu153

Monopole and quadrupole charge distributions of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$ were investigated by muonic atom $K$ and $L$ x-ray measurements. The model-independent Barrett charge radii $R_k$ and the isotope shift $\Delta R_k$ were measured, and the value of $\Delta 〈r^2〉=0.606\mathrm{}\left(18\right)\mathrm{}$ ${\mathrm{fm}}^2$ was deduced. This isotope shift is the largest known of all nuclear pairs. The isomer shift of the first excited state of ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$ is found to be close to zero, in contrast to the large isomer shifts observed in its neighbors: ${}_{}{}^{152}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Gd}$. The quadrupole moments of the first excited states were determined as $Q^{151}\left({\frac{7}{2}}^{+}\right)=1.28\mathrm{}\left(2\right)\mathrm{} e$ b and $Q^{153}\left({\frac{7}{2}}^{+}\right)=0.44\mathrm{}\left(2\right)\mathrm{} e$ b. The value for ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and its ground-state quadrupole moment of $Q^{151}\left({\frac{5}{2}}^{+}\right)=0.90\mathrm{}\left(1\right)\mathrm{} e$ b reported previously are several times larger than the respective single particle units. This fact shows that a fair amount of collectivity is involved in the ${\frac{5}{2}}^{+}$ ground state and in the ${\frac{7}{2}}^{+}$ first excited state of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$.