The Mössbauer effect in ${\mathrm{Xe}}^{129}$ and ${\mathrm{Xe}}^{131}$ contained in Xe${\mathrm{F}}_4$ has been used to measure the ratio of electric quadrupole moments of the lowest ${\frac{3}{2}}^{+}$ states in the two isotopes. This is the ground state in ${\mathrm{Xe}}^{131}$ and the first excited state in ${\mathrm{Xe}}^{129}$. The value $\frac{Q\left({\mathrm{Xe}}^{129*}\right)}{Q\left({\mathrm{Xe}}^{131}\right)}=3.45\mathrm{}\pm 0.09$ is obtained. The sign of the moment in ${\mathrm{Xe}}^{129}$ is obtained by use of a source containing oriented crystals of K${\mathrm{I}}^{129}$${\mathrm{Cl}}_4$ which produces Xe${\mathrm{Cl}}_4$ in its beta decay. The quadrupole moment is found to be negative. The known quadrupole moment of the ground state of ${\mathrm{Xe}}^{131}$, -0.12b, then gives $Q\left({\mathrm{Xe}}^{129*}\right)=-0.41\mathrm{}$b. The large ratio of the moments is interpreted as due to the abrupt onset of a region of permanent deformation by analogy with a similar situation in the europium isotopes. Discussions of such a region appear in the literature. The linewidth observed in ${\mathrm{Xe}}^{131}$ yields a value for the first-excited-state lifetime, $T_{\frac{1}{2}}=0.504\mathrm{}\pm 0.017$ nsec, in good agreement with delayed-coincidence measurements. The measured value for the ratio of the moments removes a difficulty in understanding the structure of xenon fluorides generated by an earlier assumption that the ratio was unity.

• Received 17 April 1964

DOI:https://doi.org/10.1103/PhysRev.135.B1102