Measurement and analysis of the muonic x rays ofEu151andEu153

Abstract

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 $\ensuremath{\Delta}{R}_{k}$ were measured, and the value of $\ensuremath{\Delta}〈{r}^{2}〉=0.606(18)$ ${\mathrm{fm}}^{2}$ 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 ${Q}^{151}({\frac{7}{2}}^{+})=1.28(2) e$ b and ${Q}^{153}({\frac{7}{2}}^{+})=0.44(2) e$ b. The value for $^{151}\mathrm{Eu}$ and its ground-state quadrupole moment of ${Q}^{151}({\frac{5}{2}}^{+})=0.90(1) e$ 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 $^{151}\mathrm{Eu}$.