Very high spin states $(I=50\mathrm{}–60ħ)$ have been observed in the transitional nuclei ${}^{161}\mathrm{Er}$ and ${}^{162}\mathrm{Er}$ using the Euroball $\gamma$-ray spectrometer. In ${}^{161}\mathrm{Er},$ three bands are observed well above spin $50ħ.$ In the positive parity, positive signature $(+,+\frac{1}{2})$ band a discontinuity in the regular rotational behavior occurs at ${\frac{109}{2}}^{+}$ and a splitting into two branches occurs at ${\frac{97}{2}}^{-}$ in the negative parity, positive signature $(-,+\frac{1}{2})$ band. The $(-,-\frac{1}{2})$ band continues in a regular fashion to ${\frac{115}{2}}^{-},$ tentatively $\left({\frac{119}{2}}^{-}\right).\mathrm{}$ In ${}^{162}\mathrm{Er}$ the positive parity, even spin (+,0) yrast band is observed to continue smoothly up to ${58}^{+}$ ${(60\mathrm{}}^{+})$ and the negative parity, even spin $(-,0)$ and odd spin $(-,1)$ bands are extended from ${30}^{-}$ to ${34}^{-}$ and from ${31}^{-}$ to ${47}^{-}$ ${(49\mathrm{}}^{-}),$ respectively. The high spin experimental spectra are compared with both a simple model involving the occupation of specific single neutron states in the absence of neutron pair correlations and with more detailed cranked Nilsson-Strutinsky calculations in which both proton and neutron pairing correlations are neglected. The very high spin domain is found to comprise a series of unpaired rotational bands. Unpaired band crossings between bands with different neutron and proton configurations are identified in ${}^{161}\mathrm{Er}.$ There is no evidence for aligned oblate or terminating states being close to the yrast line in ${}^{161,162}\mathrm{Er}$ up to spin $\approx 60ħ$ in contrast to the lighter Er isotopes.

• Received 6 April 2000

DOI:https://doi.org/10.1103/PhysRevC.62.024321