The existence of isomeric states and the relative spacing of these levels, in the light barium isotopes associated with the population of $h_{\frac{11}{2}}$ and $s_{\frac{1}{2}}$ or $d_{\frac{3}{2}}$ states, could be an aid in establishing both the sign and the extent of the nuclear deformation, if interpreted in the spirit of the Nilsson levels or the approach of Kumar and Baranger. Isomerism has been observed in the cases of ${\mathrm{Ba}}^{125}$ and ${\mathrm{Ba}}^{127}$, with the unique characteristic that the high-spin states lie below the low-spin states of the isomeric pair. These isomers have been formed using heavy-ion reactions for direct formation of the high-spin states and from $\beta$ decay of their parents for the low-spin states. ${\mathrm{Ba}}^{125}$ decays from the high-spin state ($T_{\frac{1}{2}}=3.0\mathrm{}\pm 0.5$ min) by ${\beta }^{+}$ ($E_{\mathrm{ww}}=4.6\mathrm{}$ MeV) to levels at ∼20, 76, and 160 keV, while the low-spin state ($T_{\frac{1}{2}}=8\mathrm{}\pm 1$ min, $E_{\mathrm{ww}}=5.5\mathrm{}$ MeV) decays principally to the ground state and a state at <30 keV. ${\mathrm{Ba}}^{127}$, in turn, gives rise to $\gamma$ rays of 70, 110, 180, and 200 keV, with the high-spin state ($T_{\frac{1}{2}}=18\mathrm{}$ min) separated from the low-spin state ($T_{\frac{1}{2}}=10\mathrm{}\pm 1$ min) by 200 keV. This inversion of the Nilsson states from ${\mathrm{Ba}}^{129}$ would indicate that the deformation is oblate (negative $\beta$) and has a value of between 0.11, the point where $h_{\frac{11}{2}}$ crosses $d_{\frac{3}{2}}-s_{\frac{1}{2}}$, and ∼0.2, where the onset of a $\frac{7}{2}$— state would present an $E3\mathrm{}$ transition between isomers of short half-life.

• Received 30 November 1968

DOI:https://doi.org/10.1103/PhysRev.172.1176