Observation of superdeformation inHg191

The first observation of superdeformation in the mass region A≃190 is reported. A rotational band of twelve transitions with an average energy spacing of 37 keV, an average moment of inertia ${\mathrm{scrF}}^{\mathrm{}\left(2\right)\mathrm{}}$ of 110 ${ħ}^2$ ${\mathrm{MeV}}^{\mathrm{-}1}$, and an average quadrupole moment of 18±3 e b has been observed in ${}_{}{}^{191}{}_{}{}^{}\mathrm{Hg}$; this band persists at low rotational frequency. These results are in excellent agreement with a calculation that predicts an ellipsoidal axis ratio of 1.65:1 for the superdeformed shape in this nucleus.